United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

General Technical 

Report

PSW-GTR-240

November 2012

Proceedings of the Fourth 

International Workshop on 

the Genetics of Host-Parasite 

Interactions in Forestry:

Disease and Insect Resistance

in Forest Trees

July 31 to August 5, 2011 – Eugene, Oregon, USA
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Technical Coordinators

Richard A. Sniezko is center geneticist, U.S. Department of Agriculture Forest Service, Dorena Genetic Resource 

Center, 34963 Shoreview Road, Cottage Grove, OR 97424

(e-mail address: rsniezko@fs.fed.us). 

Alvin D. Yanchuk is senior scientist, British Columbia Ministry of Forests, Lands and Natural Resource 

Operations, Tree Improvement Branch, PO Box 9518, Stn Prov Govt, Victoria, BC, V8W9C2, Canada

(e-mail address: Alvin.Yanchuk@gov.bc.ca). 

John T. Kliejunas is regional forest pathologist (retired), U.S. Department of Agriculture Forest Service, Pacific 

Southwest Region, 1323 Club Drive, Vallejo, CA 94592-1110 (e-mail address: Kliejunas@comcast.net).

Katharine M. Palmieri is public information officer, California Oak Mortality Task Force, 163 Mulford Hall, 

University of California, Berkeley, CA 94720-3114 (e-mail address: kpalmieri@berkeley.edu). 

Janice M. Alexander is sudden oak death outreach specialist, University of California, Cooperative Extension, 

Suite 150-B, 1682 Novato Blvd, Novato CA 94947 (e-mail address: jalexander@ucdavis.edu).

Susan J. Frankel is plant pathologist, U.S. Department of Agriculture Forest Service, Pacific Southwest Research 

Station, 800 Buchanan Street, West Annex Building, Albany, CA 94710 (e-mail address: sfrankel@fs.fed.us).
i

Proceedings of the Fourth International Workshop on 

the Genetics of Host-Parasite Interactions in Forestry:

Disease and Insect Resistance in Forest Trees

July 31 to August 5, 2011 – Eugene, Oregon, USA

Richard A. Sniezko, Alvin D. Yanchuk, John T. Kliejunas,

Katharine M. Palmieri, Janice M. Alexander, and Susan J. Frankel

Technical Coordinators
ii

U.S. Department of Agriculture, Forest Service

Pacific Southwest Research Station

Albany, CA

General Technical Report PSW-GTR-240

October 2012

Conference Sponsors

International Union of Forest Research Organizations (IUFRO): 

- Working Party 7.03.11 Resistance to Insects 

- Working Party 2.02.15 Breeding and Genetic Resources of Five-Needle Pines

USDA Forest Service: 

- Western Wildland Environmental Threat Assessment Center (WWETAC)

- Eastern Forest Environmental Threat Assessment Center (EFETAC)

- Pacific Northwest Region, Forest Health Protection & Genetic Resource Programs

- Pacific Northwest Research Station (PNW)

- Pacific Southwest Research Station (PSW)

King Estate Winery

University of California Cooperative Extension

California Oak Mortality Task Force, California Forest Pest Council

Futuragene

Starker Forests

Cover illustration: The Elders, Sugar Pine in the Morning by Dean Davis.
iii

Technical Committee

Richard Sniezko, USDA Forest Service, Dorena Genetic Resource Center, USA

Alvin Yanchuk, British Columbia Ministry of Forests and Range, Canada

Dan Herms, Ohio State University, USA

Everett Hansen, Oregon State University, USA

Jeff Stone, Oregon State University, USA

Anna Schoettle, USDA Forest Service, Rocky Mountain Research Station, USA

Susan Frankel, USDA Forest Service, Pacific Southwest Research Station, USA

Dana Nelson, USDA Forest Service, Southern Research Station, USA

Fred Hebard, The American Chestnut Foundation, USA

Acelino Couto Alfenas, Universidade Federal de Viçosa, Brazil

Arnaud Dowkiw, French National Institute for Agricultural Research (INRA Orléans), France

Luis Sampedro, Centro de Investigación Forestal de Lourizán at Galicia, Spain

Alberto Santini, Istituto per le Protezione delle Piante – National Research Council (CNR), Italy

Charles G. “Terry” Shaw, USDA Forest Service, Western Wildland Environmental Threat

 Assessment Center, USA

Dan Quiring, University of New Brunswick, Canada

Fred Hain, North Carolina State University, USA

Brian Stanton, GreenWood Resources, Inc., USA

Bohun B. Kinloch, USDA Forest Service, Pacific Southwest Research Station, 

Institute of Forest Genetics (retired), USA

Fikret Isik, Research Associate Professor of Quantitative Genetics, NCSU Cooperative Tree

Improvement Program, North Carolina State University, Raleigh, North Carolina, USA

Local Organizing Committee

Richard Sniezko, USDA Forest Service, Dorena Genetic Resource Center, OR - Workshop Chair

Katie Palmieri, University of California, Berkeley; California Oak Mortality Task Force

Janice Alexander, University of California, Cooperative Extension, Marin County, 

California Oak Mortality Task Force

Susan Frankel, USDA Forest Service, Pacific Southwest Research Station, CA

Angelia Kegley, USDA Forest Service, Dorena Genetic Resource Center, OR

Sunny Lucas, USDA Forest Service, Dorena Genetic Resource Center, OR 

Michael Crawford, USDI Bureau of Land Management, OR

Larry Johnston, USDI Bureau of Land Management, OR
iv

Abstract

Sniezko, R.A.; Yanchuk, A.D.; Kliejunas, J.T.; Palmieri, K.M.; Alexander, J.M.; Frankel, S.J. 2012.

Proceedings of the Fourth International Workshop on the Genetics of Host-Parasite Interactions in 

Forestry: Disease and Insect Resistance in Forest Trees. General Technical Report PSW-GTR-240. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 372 p.

The Fourth International Workshop on the Genetics of Host-Parasite Interactions in Forestry: Disease and 

Insect Resistance in Forest Trees provided a forum for research and management options and successes 

which have occurred over the last 30 years (the previous workshop was held in 1980 in Wageningen, The 

Netherlands). Eighty-eight submissions from oral and poster presentations at the 2011 workshop held in 

Eugene, Oregon provide a worldwide, comprehensive update on many aspects of research and operational 

programs on genetic resistance to forest insects and diseases. Topics of concern to natural forest systems 

and intensively managed forests are discussed, including resistance mechanisms, durability of resistance, 

ecology and evolutionary biology of resistance and tolerance, pathogen evolution, molecular tools, shortterm screening assays for resistance and status of several applied forest tree resistance programs.

Key words: forest disease and insect resistance, evolutionary biology, climate change, durable resistance
v

Contents

1 Mechanisms of Resistance to Pests and Pathogens

3 Tree Breeding for Pest Resistance for the Next 50 Years: The Search for Cross

Resistance? 

Alvin D. Yanchuk

4 Multiple Resistances Against Diseases and Insects in a Breeding Population of

Pinus pinaster

 Alejandro Solla, María Vivas, Elena Cubera, Luis Sampedro, Xoaquín Moreira,

Esther Merl, Raúl de la Mata, and Rafael Zas 

5 Endophyte Mediated Plant-Herbivore Interactions or Cross Resistance to Fungi and Insect

Herbivores 

Kari Saikkonen and Marjo Helander

6 Insertion Site Selection and Feeding of the Hemlock Woolly Adelgid: Implications for

Host-Plant Resistance

K.L.F. Oten, A.C. Cohen, and F.P. Hain

11 Genetic Basis of Resistance in Eucalyptus spp. Pathosystems

Acelino Couto Alfenas, Lúcio Mauro da Silva Guimarães, and

Marcos Deon Vilela de Resende 

16 Genetic Variation of Lodgepole Pine Physical and Chemical Defenses Associated

With Each Step in Host Selection Behavior Sequence by Mountain Pine Beetle 

Kimberly F. Wallin,,

Daniel S. Ott, and Alvin D. Yanchuk

20 Stilbenes as Constitutive and Induced Protection Compounds in Scots Pine

(Pinus sylvestris L.) 

Anni Harju and Martti Venäläinen

27 Simultaneous Laurel Wilt Disease Biology and Resistance Research

Jason A. Smith and Randy C. Ploetz

29 Breeding for Resistance in a Changing Environment - Durable Resistance:

 Hopes, Pitfalls, and Management Strategies

31 Breeding Poplars with Durable Resistance to Melampsora larici-populina Leaf Rust:

A Multidisciplinary Approach to Understand and Delay Pathogen Adaptation

A. Dowkiw, V. Jorge, M. Villar, E. Voisin, V. Guérin, P. Faivre-Rampant, A.

Bresson, F. Bitton, S. Duplessis, P. Frey, B. Petre, C. Guinet, C. Xhaard,

B. Fabre, F. Halkett, C. Plomion, C. Lalanne, and C. Bastien
vi

39 Developing Hazelnuts (Corylus spp.) with Durable Resistance to Eastern Filbert Blight

Caused by Anisogramma anomala 

Thomas J. Molnar, John Capik, Clayton W. Leadbetter, Ning Zhang, Guohong Cai, and

Bradley I. Hillman

40 Breeding for Growth Improvement and Resistance to Multiple Pests in Thuja plicata

John H. Russell and Alvin D. Yanchuk

45 The New Zealand Douglas-fir Breeding Program: Proposed Adjustments for a

Changing Climate

Heidi Dungey, Charlie Low, Mark Miller, Kane Fleet, and Alvin D. Yanchuk

46 White Pine Blister Rust Resistance Research in Minnesota and Wisconsin

Andrew David, Paul Berrang, and Carrie Pike

54 Phenotypic Evidence Suggests a Possible Major-Gene Element to Weevil Resistance

in Sitka Spruce 

John N. King, René I. Alfaro, Peter Ott, and Lara vanAkker

65 Operational Program to Develop Phytophthora lateralis-Resistant Populations of

Port-Orford-cedar (Chamaecyparis lawsoniana)

R.A. Sniezko, J. Hamlin, and E.M. Hansen

80 Strong Partial Resistance to White Pine Blister Rust in Sugar Pine

Bohun B. Kinloch, Jr., Deems Burton, Dean A. Davis, Robert D. Westfall, Joan Dunlap,

and Detlev Vogler

92 An Investigation into Western White Pine Partial Resistance Against the Rust Pathogen

Cronartium ribicola Using In Vitro Screening Method

D. Noshad and J.N. King

93 Resistance Gene Management: Concepts and Practice

Christopher C. Mundt

95 Ecology and Evolutionary Biology of Resistance and Tolerance 

 – Natural Systems 

97 An Overview of Ecological and Evolutionary Research on Disease in Natural Systems:

An Annotated Reference List

Helen M. Alexander
vii

103 What Do We Know About Mechanisms for Tolerating Pathogens, and Can Tolerance Be

Applied to Managing Tree Diseases?

Bitty A. Roy

104 Approaches to Understanding the Impact of Life-History Features on Plant-Pathogen

Co-Evolutionary Dynamics

Jeremy J. Burdon, Peter H. Thrall, and Adnane Nemri

112 Integrating Regeneration, Genetic Resistance, and Timing of Intervention for the

Long-Term Sustainability of Ecosystems Challenged by Non-Native Pests – A Novel

Proactive Approach

A.W. Schoettle, J.G. Klutsch, and R.A. Sniezko

124 Plant Compensatory Growth in Aspen Seedlings: The Role of Frequency and Intensity

of Herbivory and Resource Availability

Nadir Erbilgin, David A. Galvez, and Bin Zhang

125 Trade-Offs Between Induced and Constitutive Resistance in Two Pine Species: Secondary

Chemistry, Effective Antiherbivore-Resistance, and Effect of Nutrient Availability

Luis Sampedro, Xoaquín Moreira, and Rafael Zas

126 Host Preference of the Vector Beetle, Host Resistance, and Expanding Pattern of Japanese

Oak Wilt in a Stand

Kazuyoshi Futai, Hiroaki Kiku, Hong-ye Qi, Hagus Tarn, Yuko Takeuchi, and

Michimasa Yamasaki

131 The Effect of Hemlock Woolly Adelgid Infestation on Water Relations of Carolina and 

Eastern Hemlock 

Laura Rivera, JC Domec, John Frampton, Fred Hain, John King, and Ben Smith

132 Genotype × Environment Interaction and Growth Stability of Several Elm Clones

Resistant to Dutch Elm Disease

Alberto Santini, Francesco Pecori, Alessia L. Pepori, and Luisa Ghelardini

133 Rapid Evolution of Introduced Tree Pathogens Via Episodic Selection and Horizontal

Gene Transfer

Clive Brasier

143 Dynamics of Surviving Ash (Fraxinus spp.) Populations in Areas Long Infested by

Emerald Ash Borer (Agrilus planipennis)

Kathleen S. Knight, Daniel Herms, Reid Plumb, Eileen Sawyer, Daniel Spalink,

Elizabeth Pisarczyk, Bernadette Wiggin, Rachel Kappler, Emily Ziegler, and Karen Menard

153 Coast Live Oak Resistance to Phytophthora ramorum

B.A. McPherson, David L. Wood, Sylvia R. Mori, and Pierluigi Bonello
viii

154 Relationship Between Field Resistance to Phytophthora ramorum and Constitutive Phenolic

Chemistry of Coast Live Oak

A.M. Nagle, B.A. McPherson, D.L. Wood, M. Garbelotto, A.O. Conrad,

S. Opiyo, and P. Bonello 

155 Modeling Elm Growth and Dutch Elm Disease Susceptibility

Alberto Santini and Luisa Ghelardini

156 Altered Distribution of Susceptibility Phenotypes Implies Environmental Modulation

of Genetic Resistance 

Thomas R. Gordon and Neil McRoberts

159 Latent Infection by Fusarium circinatum Influences Susceptibility

of Monterey Pine Seedlings to Pitch Canker 

Cassandra L. Swett and Thomas R. Gordon

162 Breeding for Resistance in Norway Spruce to the Root and Butt Rot

Fungi Heterobasidion spp. 

G. Swedjemark, A.K. Borg-Karlson, and B. Karlsson

167 Using Survival Analysis for Assessing Resistance to Phytophthora

lateralis in Port-Orford-Cedar Families

Sylvia R. Mori, Richard A. Sniezko, Angelia Kegley, and Jim Hamlin

169 Molecular and Genomics Tools in Resistance Programs

171 Integrating Molecular Tools and Conventional Approaches in the Oregon State University

Hazelnut Breeding Program

Shawn A. Mehlenbacher

174 High-Resolution Genetic and Physical Mapping of Eastern Filbert Blight Resistance

in Hazelnut 

Vidyasagar Sathuvalli and Shawn A. Mehlenbacher

176 Molecular and Genetic Basis for Partial Resistance of Western White Pine against

Cronartium ribicola

Jun-Jun Liu, Arezoo Zamany, and Richard Sniezko

177 Mapping Resistance to Phytophthora cinnamoni in Chestnut (Castanea sp.) 

Bode A. Olukolu, C. Dana Nelson, and Albert G. Abbott
ix

179 Evaluating Resistance

181 Methods for Screening Port-Orford-Cedar for Resistance to Phytophthora lateralis

Everett M. Hansen, Paul Reeser, Wendy Sutton, and Richard A. Sniezko

189 Resistance to Phytophthora cinnamomi in the Genus Abies

John Frampton, Fikret Isik, Mike Benson, Jaroslav Kobliha, and Jan Stejskal

190 Host Resistance Screening for Balsam Woolly Adelgid: A Comparison of Seedlings

From 12 Fir Species 

Leslie Newton, John Frampton, and Fred Hain

194 Resistance to Phytophthora cinnamomi Among Seedlings From Backcross Families

of Hybrid American Chestnut 

Steven N. Jeffers, Inga M. Meadows, Joseph B. James, and Paul H. Sisco

196 Screening for Resistance to Beech Bark Disease: Improvements and Results from

Seedlings and Grafted Field Selections

Jennifer L. Koch, Mary E. Mason, and David W. Carey

209 Breeding Resistance to Butternut Canker Disease 

James McKenna, Keith Woeste, and Michael Ostry

210 Towards the Development of a Laurel Wilt Screening Program in Redbay

(Persea borbonia)

Marc Hughes and Jason Smith

211 Breeding Eucalyptus for Disease Resistance

Edival A.V. Zauza, Acelino Couto Alfenas, Lúcio Mauro da Silva Guimarães, and

João Flávio da Silva

213 Fusiform Rust Resistance - Southern Pines 

215 Identification of Pathogen Avirulence Genes in the Fusiform Rust Pathosystem

John M. Davis, Katherine E. Smith, Amanda Pendleton, Jason A. Smith,

and C. Dana Nelson

216 Tandem Selection for Fusiform Rust Disease Resistance to Develop a Clonal Elite

Breeding Population of Loblolly Pine 

Steve McKeand, Saul Garcia, Josh Steiger, Jim Grissom, Ross Whetten, and Fikret Isik

217 Bulked Fusiform Rust Inocula and Fr Gene Interactions in Loblolly Pine

Fikret Isik, Henry Amerson, Saul Garcia, Ross Whetten, and Steve McKeand
x

218 Selection of Loblolly Pine Varieties Resistant to Fusiform Rust for Commercial Deployment

Andy Benowicz and Robert J. Weir

219 Resistance Breeding Programs

221 The American Chestnut Foundation Breeding Program 

F.V. Hebard

235 Breeding Strategies for the Development of Emerald Ash Borer - Resistant North

American Ash

Jennifer L. Koch, David W. Carey, Kathleen S. Knight, Therese Poland, Daniel A. Herms,

and Mary E. Mason

240 Breeding for Disease Resistance in Hevea spp. - Status, Potential Threats, and

Possible Strategies 

Chaendaekattu Narayanan and Kavitha K. Mydin

252 Developing a Disease Resistance Research Program for Tanoaks 

Katherine J. Hayden, Jessica W. Wright, Richard S. Dodd, and Matteo Garbelotto

253 Genetic Variation in Resistance to Pine Pitch Canker and Western Gall Rust in Monterey

Pine (Pinus radiata D. Don): Results From a Three-Country Collaborative Field Trial

A.C. Matheson, W.R. Mark, G. Stovold, C. Balocchi, N Smith, and C. Brassey

256 Genetic Selection in Coastal Douglas-fir for Tolerance to Swiss Needle Cast Disease

Keith J.S. Jayawickrama, David Shaw, and Terrance Z. Ye

262 White Pine Blister Rust Resistance in Pinus monticola and P. albicaulis in the

Pacific Northwest U.S. – A Tale of Two Species

Richard A. Sniezko, Angelia Kegley, and Robert Danchok

267 Shoot Winter Injury and Nut Cold Tolerance: Possible Limitations for American Chestnut

Restoration in Cold Environments? 

Thomas M. Saielli, Paul G. Schaberg, Gary J. Hawley, Joshua M. Halman, and

Kendra M. Gurney

269 Posters 

271 Screening Sitka Spruce for Resistance to Weevil Damage in British Columbia 

René I. Alfaro and John N. King

276 Developing Clones of Eucalyptus cloeziana Resistant to Rust (Puccinia psidii) 

Rafael F. Alfenas, Marcelo M. Coutinho, Camila S. Freitas, Rodrigo G. Freitas,

and Acelino C. Alfenas
xi

277 Screening for Resistance to Fusiform Rust in Southern United States Forest Trees

Josh Bronson

278 Variation in the Development of Current Season Needle Necrosis on Noble, Nordmann,

and Turkish Fir Christmas Trees in the United States Pacific Northwest

Gary A Chastagner, Kathy Riley, and Chal Landgren

281 Frequency of Hypersensitive-Like Reaction and Stem Infections in a Large Full-Sib

Family of Pinus monticola

Robert S. Danchok, R.A Sniezko, S. Long, A. Kegley, D. Savin, J.B. Mayo, J.J. Liu,

and J. Hill

286 Operational Disease Screening Program for Resistance to Wilt in Acacia koa in Hawaii 

Nick Dudley, Robert James, Richard Sniezko, Phil Cannon, Aileen Yeh, Tyler Jones,

and Michael Kaufmann

290 A First Look at Genetic Variation in Resistance to the Root Pathogen Phytophthora

cinnamomi using a Range-wide Collection of Pacific Madrone (Arbutus menziesii) 

Marianne Elliott, Gary Chastagner, Annie DeBauw, Gil Dermott, and Richard A. Sniezko

295 Range-wide Genetic Variability in Pacific Madrone (Arbutus menziesii): Examining

Disease Resistance, Growth, and Survival in a Common Garden Study 

Marianne Elliott, Gary A. Chastagner, Gil Dermott, Alan Kanaskie, Richard A. Sniezko,

and Jim Hamlin

301 Development of Screening Trials to Rank Pinus radiata Gentoypes for Resistance to

Defoliation by Monterey Pine Aphid (Essigella californica) 

Stephen Elms, Peter Ades, and Nick Collet

302 Tree-Mediated Interactions Between the Jack Pine Budworm and a Mountain Pine Beetle

Fungal Associate 

Nadir Erbilgin and Jessie Colgan

303 The Chapter Breeding Program of the American Chestnut Foundation

Sara Fitzsimmons, Kendra Gurney, William White, and Katy McCune

304 Needle Terpenoid Composition of Pinus halepensis (Mill.) Trees Infested by the Scale

Insect Marchalina hellenica (Genn.) in Greece

Athanassios Gallis, Carlos Arrabal, Aristotle C. Papageorgiou, and

Maria C. Garcia-Vallejo 

309 Genetic Variation of Piperidine Alkaloids in Pinus ponderosa From a Common Garden

Elizabeth A. Gerson, Rick G. Kelsey, and J. Bradley St. Clair
xii

310 Gene Expression in the Tanoak-Phytophthora ramorum Interaction 

Katherine J. Hayden, Matteo Garbelotto, Hardeep Rai, Brian Knaus, Richard Cronn, and

Jessica W. Wright

311 Blister Rust Resistance Among 19 Families of Whitebark Pine, Pinus albicaulis, From

Oregon and Washington – Early Results From an Artificial Inoculation Trial 

Angelia Kegley, Richard A. Sniezko, Robert Danchok, and Douglas P. Savin

316 Interaction of an Invasive Bark Beetle with a Native Forest Pathogen: Potential Effect of

Dwarf Mistletoe on Range Expansion of Mountain Pine Beetle in Jack Pine Forests

Jennifer Klutsch and Nadir Erbilgin

317 Using Dutch Elm Disease-Tolerant Elm to Restore Floodplains Impacted by Emerald

Ash Borer

Kathleen S. Knight, James M. Slavicek, Rachel Kappler, Elizabeth Pisarczyk,

Bernadette Wiggin, and Karen Menard

324 Quantitative Trait Loci for Resistance to Two Fungal Pathogens in Quercus robur

Cécile Robin, Amira Mougou-Hamdane, Jean-Marc Gion, Antoine Kremer, and 

Marie-Laure Desprez-Loustau 

325 Testing Resistance to Chestnut Blight of Hybrid Chestnuts 

Cécile Robin, Xavier Capdevielle, Gilles Saint-Jean, and Teresa Barreneche

326 The Italian Elm Breeding Program for Dutch Elm Disease Resistance 

Alberto Santini, Francesco Pecori, and Luisa Ghelardini

336 Patterns of Resistance to Cronartium ribicola in Pinus aristata, Rocky Mountain

Bristlecone Pine 

A.W. Schoettle, R.A. Sniezko, A. Kegley, R. Danchok, and K.S. Burns

337 Development of Methods for the Restoration of the American Elm in Forested Landscapes

James M. Slavicek

342 Generation of American Elm Trees with Tolerance to Dutch Elm Disease Through

Controlled Crosses and Selection 

James M. Slavicek and Kathleen S. Knight

347 Breeding for Resistance to Adelgids in Abies fraseri, Tsuga canadensis, and T. caroliniana

Ben Smith, Fred Hain, and John Frampton
xiii

348 Nine Year Survival of 16 Phytophthora lateralis Resistant and Susceptible

Port-Orford-Cedar Families in a Southern Oregon Field Trial 

Richard A. Sniezko, Jim Hamlin, Everett Hansen, and Sunny Lucas

356 White Pine Blister Rust Resistance of 12 Western White Pine Families at Three Field

Sites in the Pacific Northwest

Richard A. Sniezko, Robert Danchok, Jim Hamlin, Angelia Kegley, Sally Long,

and James Mayo

368 Are Needle Reactions in Resistance to Cronartium ribicola a Hypersensitivity Response?

Katarina Sweeney, Jeffrey Stone, Kathy Cook, Richard Sniezko, Angelia Kegley, and 

Anna Schoettle

372 The Potential of Breeding for Enhanced Inducibility in Pinus pinaster and Pinus radiata

Rafael Zas, Alejandro Solla, Xoaquin Moreira, and Luis Sampedro
xiv

Preface

Native and non-native pathogens, insects and animals continue to have serious negative impacts on forest 

ecosystems and planted forests worldwide. Climate change will alter host-pest relationships and may 

increase detrimental impacts from many biotic agents. Genetic variation in resistance within tree species 

is a key element to maintaining forest health. It is the primary natural method of tree defense and utilizing 

genetic resistance is one of the few viable management options available to combat the adverse impacts of 

pests. Resistance research programs, including resistance breeding, are vital since they increase the 

efficiency in utilizing genetic variation to restore forest health when mortality becomes unacceptably 

high. Resistance research and breeding has been underway in some forest tree species for over 50 years.

The Fourth International Workshop on the Genetics of Host-Parasite Interactions in Forestry: Disease and 

Insect Resistance in Forest Trees was convened to serve as the first major international gathering of 

researchers and technical specialists on this topic in the past 30 years. These Proceedings provide an 

update on many resistance programs at different stages throughout the world, with added insights and 

program examples from researchers in horticulture, plant genetics and evolutionary biology. Key past 

meetings that inspired this workshop, include:

Breeding Pest-Resistant Trees. 1964. University Park, Pennsylvania, USA.

Biology of Rust Resistance in Forest Trees. 1969. Moscow, Idaho, USA.

Resistance to Diseases and Pests in Forest Trees. 1980. Wageningen, The Netherlands.

Other related conferences:

Breeding Insect and Disease Resistant Forest Trees. 1982. Eugene, Oregon, USA.

Mechanisms and Deployment of Resistance in Trees to Insects. 2000. Iguassu Falls, Brazil.

An informal survey by the Food and Agriculture Organization of the United Nations (FAO 2008)

provides a snapshot of the level of world-wide activity in pest resistance breeding

(http://www.fao.org/forestry/26445/en/). In a recent review (Sniezko 2006), an update on four operational 

disease-resistance programs in the United States is presented. This Proceedings provides a much more 

comprehensive update. With climate change and continued invasion by non-native pathogens and insects, 

the contributions of resistance breeding to planted forests and natural ecosystems are needed more than 

ever. Long-term public support for these programs is essential to the success of these applied programs.

The purpose of this workshop was to advance progress in genetic resistance programs by fostering 

collaboration between scientists from throughout the world, and to provide an update to the forest 

community on resistance in forest trees. Updates on current status, issues and future plans for applied 

resistance programs, as well as research information and tools to fast-track the development and use of 

resistance in trees were presented and discussed. Presentations on the efforts in annual crops, as well as 

tree crops such as hazelnut (Corylus avellana) and rubber (Hevea brasiliensis), helped increase the 

breadth of understanding of the potential of genetic resistance in forest trees. The inclusion of topics in 

evolutionary biology broadened our perspectives. The conference field trip provided an opportunity to 

visit USDA Forest Service and USDI Bureau of Land Management facilities in Oregon to see some of the 

applied operational resistance programs that have been underway for decades, notably the white pine 

blister rust resistance work with Pinus monticola, P. lambertiana, P. albicaulis, P. flexilis, P. strobiformis

and P. aristata; the Phytophthora lateralis resistance program for Chamaecyparis lawsoniana ; and to see 

operational seed orchards of Pseudotsuga menziesii.

There are many people and groups to thank for the success of this workshop. The technical committee and 

local organizing committees provided essential inputs on planning and logistics. The generous support of 

the sponsors made possible the inclusion of some of the invited speakers. The early encouragement of 

Jerry Beatty and Charles G. Shaw and sponsorship of the USDA Forest Service, Western Wildland 
xv

Environmental Threat Assessment Center provided the impetus to undertake organization of this meeting.

The IUFRO Working Parties 7.03.11 (Resistance to insects) and 7.02.15 (Breeding and genetic resources 

of five-needle pines) provided scientific sponsorship. Volunteers Rob Mutch and Wesley Clark organized 

and implemented the live webcast of this meeting – providing the opportunity for those unable to attend 

to see and hear the presentations and discussions (http:ucanr.org/sites/tree_resistance_2011conference). A 

special thanks to the USDA Forest Service, Pacific Southwest Research Station for production of this 

Proceedings, and also my co-technical editors Susan Frankel, Katie Palmieri, Janice Alexander, John 

Kliejunas and Alvin Yanchuk, whose many hours made both the conference and proceedings come to 

fruition. Lastly, I would like to thank all of the presenters, moderators and other participants, who took 

time out of very busy schedules to participate in the workshop and share their work.

It is my hope that the 2011 workshop will provide impetus for a continued dialog – initial planning is 

underway for a possible follow-up meeting in 2015 in France. For further information and to stay engaged 

in this topic, subscribe to the Pest Resistance in Trees (Insect or pathogen resistance) mailing list at 

www.fs.usda.gov/goto/r6/dorena. Stay tuned!

RICHARD A. SNIEZKO

Coordinator, IUFRO Working Party 2.02.15 – Breeding and genetic resources of five-needle pines

http://www.iufro.org/science/divisions/division-2/20000/20200/20215/

Center Geneticist

USDA Forest Service - Dorena Genetic Resource Center

34963 Shoreview Road, Cottage Grove, Oregon, U.S.A.

July 31, 2012

Literature Cited

FAO. 2008. Selection and breeding for insect and disease resistance. www.fao.org/forestry/26445 (31 July 2012).

Sniezko, R.A. 2006. Resistance breeding against nonnative pathogens in forest trees — current successes in North 

America. Canadian Journal of Plant Pathology. 28: S270–S279.
xvi

Participants in the Fourth International Workshop on the Genetics of Host-Parasite Interactions in Forestry Disease and Insect Resistan

July 31 to August 5, 2011 in Eugene, Oregon USA. (Photo: Rob Mutch)
1

Mechanisms of Resistance to 

Pests and Pathogens

Weevil resistant Sitka spruce genotype 898 stands tall amidst weevil damaged trees

at the Pacific Forestry Centre in Victoria, BC.

(Photo: Justin Whitehill)

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

3

Tree Breeding for Pest Resistance for the Next 50 

Years: the Search for Cross Resistance?

Alvin D. Yanchuk1

Abstract

Research activities aimed at developing resistance to pests (insect, pathogens, mammals) in forest trees can be 

documented back over 5 decades. While a substantial body of research has been published on resistances in 

forest trees, not much of this work has made its way into applied tree improvement programs. There are several 

reasons for this, e.g.: (i) a new or interesting incursion is noticed, and studies are developed that work on 

materials not related to a breeding program; (ii) adequate infection or artificial inoculation techniques are too 

expensive and a large enough population cannot be screened; and (iii) the genetic gain in resistance may not be 

silviculturally useful or effective. However, a few notable exceptions are present, and important lessons should 

be taken from our experiences over the last 50 years.

What has changed recently is the concern that pest and disease occurrences (with notable outbreaks) are 

increasing and are likely to continue to increase. Moreover, it might be difficult to know what particular disease 

or pest may become the new threat, particularly with climate change and the increased movement of goods 

around the world leading to the introduction of exotic pests. We can go about tree breeding, in a re-active 

approach, as we have for the past 50 years, hoping that a genetic option can be developed within one cycle of 

selection and testing; however, it is not uncommon for this to take a decade or more. At some point soon, 

funding agencies and managers will start asking how useful is pest resistance breeding as a strategic and viable 

option for future forest health and biosecurity programs. The other approach may be that we more aggressively 

look for situations of cross resistance against future ‘classes of threats’ rather than the traditional ‘one-tree/onepest’ program(s) of the past. The risk we face here is that some pests (diseases in particular) will be quite host or 

genotype specific and we may provide no additional protection to future crops. While this is a risk, it seems 

that, particularly with a growing body of research showing that cross resistance is present, breeding trees which 

may have more ‘durable’ forms of resistance is probably a timely endeavor in forest tree breeding. At the very 

least, it should be seriously considered in particular programs.

 1 British Columbia Ministry of Forests, Lands and Natural Resources Operations, Tree Improvement Branch, 727 Fisgard 

Street, Victoria, BC Canada. V8W 9C1.

Corresponding author: alvin.yanchuk@gov.bc.ca.
GENERAL TECHNICAL REPORT PSW-GTR-240

4

Multiple Resistances Against Diseases and Insects 

in a Breeding Population of Pinus pinaster

Alejandro Solla,1 María Vivas,1 Elena Cubera,1 Luis Sampedro,2 Xoaquín 

Moreira,2 Esther Merlo,3 Raúl de la Mata,4 and Rafael Zas4

Abstract

The different plant defenses existing within a given taxon have been commonly assumed to trade-off among 

each other because of both evolutionary and physiological reasons. The higher the efficiency of a single 

defensive trait, the lower selective pressure for other redundant defenses expected. On the other hand, 

production of multiple defenses might be constrained by the resources required for maintaining, at a time,

increased growth and reproductive rates. However, different defensive traits do not need to always be 

redundant, and plants require the combination of multiple defenses to cope with all possible damages. This is 

particularly true in the case of forest trees, because their large size and long-life span make them targets for a 

wide range of enemies. Theory predicts that trees tend to evolve generalized defenses that are effective against a 

wide array of herbivores and pathogens, but empirical evidences of multiple resistances are lacking.

Objectives were to (1) compile data from several independent experiments of the same genetic material and 

explore whether resistances to an array of different pests and diseases were genetically related; (2) determine to

what extent resistances are genetically related with quantitative defensive traits; (3) check for possible trade-offs 

between resistances and other fitness-related traits, such as growth and cone production; and (4) identify 

genotypes with multiple resistances to be used for breeding. Plant material consisted of Pinus pinaster Aiton 

open-pollinated families from 39 maternal-plus trees of northwest Spain and control seedlings from unimproved 

material. The pathogens tested included Fusarium oxysporum, F. circinatum, and Armillaria ostoyae. The 

insects tested were Thaumetopoea pithyocampa, Dioryctria sylvestrella, and Hylobious abietis. Susceptibility to 

water stress was also assessed. Defensive traits included constitutive and induced levels of diterpene content, 

total phenolics, condensed tannins, starch, and soluble sugars. Inducibility of defenses was determined as the 

difference of the concentration of a given compound between methyl jasmonate( MeJA)-treated plants and 

untreated plants, i.e., the induced levels minus the constitutive levels. Resistances to diseases, insects, and water 

stress were normalized, and the relationships between these parameters together with the defensive traits were 

examined by Pearson’s correlations. 

Heritability values of P. pinaster were high enough to allow the studied pathogens and pests to be individually 

reduced through appropriate genetic strategies. However, breeding P. pinaster for resistance to a particular 

pathogen would enhance its susceptibility to another pathogen or pest. Although resistances to fungal pathogens 

were correlated with resistance to water stress, the Spanish breeding population of P. pinaster was not 

simultaneously resistant to a wide range of potential enemies. Moreover, all plus families were generally more 

susceptible than the unimproved control family. No trade-offs between resistances and plant growth or cone 

production was observed. Families showing multiple resistances were those showing low inducibility. Lack of 

multiple resistances appears to be the norm in this pine species, and would not be easily used for breeding.

 1 Universidad de Extremadura, Virgen del Puerto 2, 10600 Plasencia, Spain.

2 Centro de Investigación Forestal de Lourizán, Apdo 127, 36080 Pontevedra, Spain.

3 CIS Madeira, Avenida de Galicia, 32901 San Cibrao das Viñas, Ourense, Spain.

4 Misión Biológica de Galicia, CSIC, Carballeira 8, 36143 Pontevedra, Spain.

Corresponding author: rzas@cesga.es.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

5

Endophyte Mediated Plant-Herbivore Interactions or 

Cross Resistance to Fungi and Insect Herbivores

Kari Saikkonen1 and Marjo Helander2

Abstract

Endophytic fungi are generally considered to be plant mutualists that protect the host plant from pathogens and 

herbivores. Defensive mutualism appears to hold true particularly for seed-transmitted, alkaloid producing,

grass endophytes. However, we propose that the mutualistic nature of plant-endophyte interactions via 

enhanced plant resistance to pathogens and herbivores should be reconsidered when focusing on woody plants. 

We compared phenotypic and genetic frequency correlations for two endophytic fungal genera (Fusicladium

and Melanconium) and birch rust (Melampsoridium betulinum) with the performance of six invertebrate 

herbivores growing on the same half sib progenies of mountain birch Betula pubescens Ehrh. ssp. czerepanovii

(N. I. Orlova) Hämet-Ahti in two environments over a 3-year period. We found little support for causal 

association between fungal frequencies and performance of herbivore species. Instead, genetic correlations, 

particularly between autumnal moth (Epirrita autumnata) and birch rust, suggest that herbivore performance 

may be affected by (1) genetic differences in birch quality for fungi and herbivores, or (2) genetic differences in 

responses to environmental conditions. Genetic analysis of Venturia ditricha (anamorph Fusicladium betulae) 

revealed that (1) birch genotypes and environment influence the probability of infection by particular endophyte 

genotypes, (2) genetic variation correlated negatively with infection frequencies of the fungus, and (3) the 

susceptibility of the birch to a particular endophyte genotype may change when environmental conditions are 

changed (environment-host genotype interaction). 

Furthermore, statistical perusal of endophyte literature revealed clear contrasts between grass and tree 

endophytes. Grass endophytes appear to provide support for the hypothesis of defensive mutualism, whereas 

variability appears to be the nature of the tree endophyte mediated plant herbivore interactions.

We propose that: (1) the performance of endophytes, pathogens, and herbivores may be responses to genetically 

determined plant qualities rather than interconnected associations; (2) the observed genetic correlation structure 

may have importance for evolution of birch resistance to fungi and herbivores (e.g., negative correlation 

between birch resistance to rust fungus and autumnal moth may constrain birch population from reaching 

optimal species-specific resistance); this should be considered also in forest tree improvement attempting to 

overcome pests and pathogens; and (3) more knowledge about multispecies coevolution is necessary to fully 

understand bilateral interactions between plants and the organisms living on them.

 1 MTT Agrifood Research Finland, Plant Production Research, 31600 Jokioinen, Finland.

2 Section of Ecology, Department of Biology, University of Turku, 20014 Turku, Finland.

Corresponding author: kari.saikkonen@mtt.fi.
GENERAL TECHNICAL REPORT PSW-GTR-240

6

Insertion Site Selection and Feeding of the Hemlock 

Woolly Adelgid: Implications for Host-Plant 

Resistance

K.L.F. Oten,1 A.C. Cohen,1 and F.P. Hain1

The hemlock woolly adelgid (HWA), Adelges tsugae (Hemiptera: Adelgidae), is an invasive forest 

pest that threatens the existence of eastern hemlock (Tsuga canadensis (L.) Carr.) and Carolina 

hemlock (T. caroliniana Engelm.) in the eastern United States. It is a small, aphid-like insect with 

piercing-sucking mouthparts that it uses to penetrate its host plant and feed off the xylem ray 

parenchyma cells (Young et al. 1995). The hemlock woolly adelgid is native to eastern Asia and 

northwestern North America where it feeds on, but does not kill, its host (Annand 1924, Havill et al. 

2006). The mechanism of resistance and/or tolerance is unknown. The invasive population in the 

eastern United States was first detected near Richmond, Virginia in 1951 (Gouger 1971, Souto et al. 

1996). The range of HWA in the east has since spread through the range of hemlocks, now affecting 

18 states (USDA FS 2011). The mortality caused by HWA to eastern and Carolina hemlocks is 

tremendous. Approximately 80 to 90 percent of infested hemlocks native to the eastern United States 

have already vanished as a result of this exotic insect (Hale 2004, Townsend and Rieske-Kinney 

2006). Moreover, the production of hemlocks for the ornamental industry, valued at $34 million 

between Tennessee and North Carolina alone, has been virtually eliminated (Bentz et al. 2002).

Following initial infestation, a susceptible hemlock declines in health, evident by needle drop, bud 

abortion, and inhibition of new growth (McClure 1991). A healthy hemlock dies in as few as 4 years, 

but may survive beyond 10 (McClure 1987, 1991). In its native range, HWA is described as a minor 

pest and almost never kills its host (Bentz et al. 2002, Furniss and Carolin 1977, Keen 1938). For 

years, it was believed that eastern and Carolina hemlocks were entirely and exclusively susceptible to 

HWA. However, in the wake of widespread mortality, anecdotal evidence suggested surviving 

individuals or stands of eastern and Carolina hemlocks may be less susceptible. This has since been 

corroborated by research, and distinct populations have been selected and continue to be pursued as 

putatively resistant to HWA (Caswell et al. 2008, Ingwell and Preisser 2010, Kaur 2009, Oten et al. 

2011). In addition, Carolina hemlock has been identified as less susceptible to HWA than eastern 

hemlock (Jetton et al. 2008, Oten 2011).

The purpose of this research is to investigate host-plant selection and host use of HWA to further 

understand mechanisms of resistance that occurs both inter- and intra-specifically. Understanding this 

may facilitate progress toward a resistant hemlock by identifying characteristics that can be identified 

among individuals in natural stands for selection and/or to screen within a breeding program.

The crawler stage of HWA is the first stage and thus the host-seeking stage of the insect. Crawlers 

lack wings and compound eyes; consequently, transportation between host trees is entirely passive.

Dispersal occurs either by crawling to nearby branches, wind, phoresy on macro-vertebrates (e.g., 

birds, deer), or by human movement of infested stock (Butin et al. 2007, McClure 1990). Therefore, 

habitat and host location, the initial two steps in host-plant selection by phytophagous insects 

(Bernays and Chapman 1994), does not occur as part of the host selection behaviors of HWA. Using 

scanning electron microscopy (SEM), we studied the sensilla of HWA crawlers because it is this 

stage that must accurately determine feeding sites on hemlocks. The distal segment of the HWA 

labium possesses two fields of sensilla, including six long subapical sensilla trichodea and five shorter 

apical labial sensilla trichodea. Two apical sensilla are observed touching the stylet bundle in some 

 1 North Carolina State University, Raleigh, NC 27695.

Corresponding author: klfelder@ncsu.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry 

7 

images and are hypothesized to play a role in sensing the orientation and/or movement of the stylets 

during insertion site selection. In aphids, stylet penetration is initially based on substrate color and 

texture and chemoreception occurs after short probing of the host plant and internal gustation 

(Ibbotson and Kennedy 1959, Miles 1958, Powell et al. 2006). Given their biological similarities to 

aphids, we theorize that HWA use a similar host acceptance tool, and probes the plant briefly, sensing 

its host with internal contact chemoreceptors. However, this research should be extended to 

substantiate this conjecture. HWA tarsi are two-segmented and bear a claw on the distal end. A long 

pair of knobbed setae and four pairs of shorter, spatulate setae are observed. Such setae are present in 

other adelgids: a pair in the pine leaf adelgid (Pineus pinifoliae) and three per tarsus in the spruce gall 

adelgid (Adelges lariciatus). However, the authors of these studies did not speculate about the 

function of the setae (Cumming 1968, Underwood 1955). We suggest they may be involved in 

adhesion based on casual visual observations of surface contact during inverted walking and the 

morphological similarity to the adhesive tenant hairs of some springtails (Blottner and Eisenbeis 

1984). In comparison to the sessile adults, antennae of the same generation crawlers are welldeveloped and elongated. The scape and pedicel each have a pair of trichoid sensilla, seated in a pit 

and lacking an observable pore at its tip, indicating they likely function as mechanoreceptors. The 

flagellum has a long terminal sensillum and four secondary sensilla at the distal end. A sensorium 

lays subapically on the posterior edge of the flagellum and is surrounded by a cuticular fringe that 

may aid in keeping foreign particles out. The sensorium is likely analogous to the primary rhinaria of 

chemoreceptory function in aphids and is referred to by Stoetzel (1998) in alate female Adelgidae. 

Using whole-body homogenate of HWA, general protease enzyme activity was assayed using the 

EnzChek® protease assay kit (Invitrogen, Eugene, OR). Trypsin-like activity in HWA was present, 

although lower than in Lygus lineolaris, which was used as a positive control. The kinetic for HWA 

enzymatic activity reached its asymptote at 30 minutes, whereas the L. lineolaris comparison 

continued to increase after more than 1 hour (fig. 1). 

Figure 1—Enzyme kinetics of trypsin-like assay. 

Although this enzyme could be present in the salivary gland complex, midgut, or hemolymph of 

the insect, its occurrence is nevertheless revealing of the feeding habits of HWA. Generally, the 

presence or absence of specific enzymes is indicative of a consumer's ability to digest plant or prey 

materials (Agusti and Cohen 2000; Baptist 1941; Cohen 1990, 1998). The presence of trypsin-like 

enzyme activity in HWA confirms the ability of HWA to digest protein rather than depending upon 

free amino acids as is the case for sap-feeding insects such as aphids. If used extra-orally by HWA 

(i.e., injected into their host plant) the enzyme may be used to digest structural proteins of the plant 

that are otherwise insoluble (Hori 1971). In addition, the presence of protease may have implications 

for host response and host-plant resistance. The wound-response pathway, which may be triggered by 

the feeding of herbivorous insects, is one of these responses that may induce protease inhibitor genes 
GENERAL TECHNICAL REPORT PSW-GTR-240

8

(Dietrich et al. 1999). The systemic response of hemlock to HWA feeding, which is currently 

uncharacterized, implicates HWA feeding in inducing the systemic response (Radville et al. 2011, 

Walker-Lane 2009). The response could be triggered by mechanical wounding or physiological 

elicitors in the saliva. Future research should focus on this interaction to understand if HWA is 

triggering the wound-response pathway in hemlocks via mechanical or salivary wounding and the 

differences in the response between susceptible and resistant hemlocks.

Physical characteristics of hemlock surfaces were observed using low-temperature SEM to 

document differences and how they may relate to resistance. There were differences in trichome 

density, placement, and length on the species studied, but they did not relate to the degree of 

susceptibility to HWA and are likely not involved in host selection or resistance. Cuticle thicknesses 

were compared for various locations on the pulvinus: at the HWA stylet bundle insertion point, at the 

side of the needle where the HWA settles, and on the outside of the needle (fig. 2). We compared 

thicknesses to determine if cuticle was related to insertion site selection. Based on preliminary 

observations, there appears to be a slight trend. The cuticle thickness at the point on stylet insertion 

appears to be thinner than the other two locations on the pulvinus that were measured. Although this 

needs to be investigated more thoroughly, including a statistical analysis, this suggests that the 

specificity for stylet bundle insertion of HWA may be a result of accessibility to host tissues.

Figure 2—A) Settled hemlock woolly adelgid (HWA), showing adaxial side of the needle and pulvinus.

HWA, hemlock woolly adelgid; ins, insertion point; sty, stylet bundle. Scale, 100 μm. B) Locations 

measured on pulvini for cuticle thickness. ins, insertion point; out, outside. Scale, 200 μm.

Understanding the interactions between HWA and naïve hemlocks in the eastern United States

furthers our understanding of mechanisms involved in host-plant selection and resistance, aiding the 

progress towards a hemlock resistant to HWA. Ideally, a resistant hemlock will be a component of an

integrated pest management program to control populations of HWA in the eastern United States, 

allowing ecosystem restoration and restoring the hemlock ornamental industry.

Literature Cited

Agusti, N.; Cohen, A.C. 2000. Lygus hesperus and L. lineolaris (Meniptera: Miridae), phytophages, 

zoophages, or omnivores: evidence of feeding adaptations suggested by the salivary and midgut digestive 

enzymes. Journal of Entomological Science. 35: 176–186.

Annand, P.N. 1924. A new species of Adelges (Hemiptera: Phylloxeridae). The Pan-Pacific Entomologist. 1: 

79–82.

Baptist, B.A. 1941. The morphology and physiology of the salivary glands of Hemiptera-Heteroptera. 

Quarterly Journal of Microscopical Science. 83: 91–139.

Bentz, S.E.; Riedel, L.G.H.; Pooler, M.R.; Townsend, A.M. 2002. Hybridization and self-compatibility in 

controlled pollinations of eastern North American and Asian hemlock (Tsuga) species. Journal of 

Arboriculture. 28(4): 200–205.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

9

Bernays, E.A.; Chapman, R.F. 1994. Host-plant selection by phytophagous insects. NewYork: Chapman and

Hall. 312 p. 

Blottner, D.; Eisenbeis, G. 1984. Ultrastructure of long tibiotarsal spatula-hairs in Tomocerus flavescens 

(Collembola: Tomoceridae). Annales de la Société Royale Zoologique de Belgique. 114: 51–57.

Butin, E.; Preisser, E.; Elkinton, J. 2007. Factors affecting settlement rate of the hemlock woolly adelgid, 

Adelges tsugae, on eastern hemlock, Tsuga canadensis. Agricultural and Forest Entomology. 9: 215–219.

Caswell, T.; Casagrande, R.; Maynard, B.; Preisser, E. 2008. Production and evaluation of eastern hemlocks 

potentially resistant to the hemlock woolly adelgid. In: Onken, B., Rheardon, R., eds. Fourth symposium on 

the hemlock woolly adelgid in the eastern United States. FHTET-2008-01. Morgantown, WV: U.S.

Department of Agriculture, Forest Service, Forest Health Protection: 124–134.

Cohen, A.C. 1990. Feeding adaptations of some predaceous Hemiptera. Annals of the Entomological Society 

of America. 83(6): 1215–1223.

Cohen, A.C. 1998. Feeding biology of the silverleaf whitefly (Homoptera: Aleyrodidae). Chinese Journal of 

Entomology. 18: 65–82.

Cumming, M.E.P. 1968. The life history and morphology of Adelges lariciatus (Homoptera: Phylloxeridae). 

Canadian Entomologist. 100(2): 113–126. 

Dietrich, R.A.; Lawton, K.; Friedrich, L. [et al.]. 1999. Induced plant defense responses: scientific and 

commercial development possibilities. In: Chadwick, D.J.; Goode, J.A., eds. Insect-plant interactions and 

induced plant defence. Chichester, England: Wiley: 205–216. 

Furniss, R.L.; Carolin, V.M. 1977. Western forest insects. Miscellaneous Publication 1339. Washington, DC: 

U.S. Department of Agriculture, Forest Service, Government Printing Office.

Gouger, R.J. 1971. Control of Adelges tsugae on hemlock in Pennsylvania. Scientific Tree Topics. 3: 1–9.

Hale, F.A. 2004. The hemlock woolly adelgid: a threat to hemlock in Tennessee. SP503-G. Knoxville: The 

University of Tennessee, Agricultural Extension Service. 

Havill, N.P.; Montgomery, M.E.; Yu, G.; Shiyake, S.; Caccone, A. 2006. Mitochondrial DNA from hemlock 

woolly adelgid (Hemiptera: Adelgidae) suggests cryptic speciation and pinpoints the source of the 

introduction to eastern North America. Annals of the Entomological Society of America. 99(2): 195–203. 

Hori, K. 1971. Studies on the feeding habits of Lygus disponsi Linnavuori (Hemiptera: Miridae) and the injury 

to its host plants. I. Histological observations of the injury. Applied Entomology and Zoology. 6: 84–90.

Ibbotson, A.; Kennedy, J.S. 1959. Interaction between walking and probing in Aphis fabae. Journal of 

Experimental Biology. 36: 377–390.

Ingwell, L.L.; Preisser, E.L. 2010. Using citizen science programs to identify host resistance in pest-invaded 

forests. Conservation Biology. 25(1): 182–188.

Jetton, R.M.; Hain, F.P.; Dvorak, W.S.; Frampton, J. 2008. Infestation rate of hemlock woolly adelgid 

(Hemiptera: Adelgidae) among three North American hemlock (Tsuga) species following artificial 

inoculation. Journal of Entomological Science. 43(4): 438–442.

Kaur, N. 2009. Developing artificial rearing techniques for hemlock woolly adelgid, Adelges tsugae and 

balsam woolly adelgid, Adelges piceae; artificial infestation and epicuticular wax study of Carolina 

hemlock, Tsuga caroliniana, provenances. Raleigh, NC: North Carolina State University. 118 p. M.S. thesis.

Keen, F.P. 1938. Insect enemies of western forests. Misc. Pub. 273. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 

McClure, M.S. 1987. Biology and control of hemlock woolly adelgid. Bull. 851. New Haven: Connecticut 

Agricultural Station.

McClure, M.S. 1990. Role of wind, birds, deer, and humans in dispersal of hemlock woolly adelgid 

(Homoptera: Adelgidae). Environmental Entomology. 19(1): 36–43.

McClure, M.S. 1991. Density-dependent feedback and population cycles in Adelges tsugae (Homoptera: 

Adelgidae) on Tsuga canadensis. Environmental Entomology. 20(1): 258–264.

Miles, P.W. 1958. Contact chemoreception in some Heteroptera, including chemoreception internal to the stylet 

food canal. Journal of Insect Physiology. 2(4): 338–347. 
GENERAL TECHNICAL REPORT PSW-GTR-240

10

Oten, K.L.F. 2011. Host-plant selection by the hemlock woolly adelgid, Adelges tsugae Annand: sensory 

systems and feeding behavior in relation to physical and chemical host-plant characteristics. Raleigh, NC: 

North Carolina State University. 259 p. Ph.D. dissertation.

Oten, K.L.F.; Walker-Lane, L.N.; Jetton, R.M. [et al.]. 2011. Developing hemlocks resistant to the hemlock 

woolly adelgid. http://www.threatenedforests.com/research/wp-content/uploads/2011/08/NAFIWC2011.pdf. (11 August 2011).

Powell, G.; Tosh, C.R.; Hardie, J. 2006. Host plant selection by aphids: behavioral, evolutionary, and applied 

perspectives. Annual Review of Entomology. 51: 309–330. 

Radville, L.; Chaves, A.; Preisser, E.L. 2011. Variation in plant defense against invasive herbivores: evidence 

for a hypersensitive response in eastern hemlocks (Tsuga canadensis). Journal of Chemical Ecology. 37: 

592–597.

Souto, D.; Luther, T.; Chianese, B. 1996. Past and current status of HWA in eastern and Carolina hemlock 

stands. In: Salom, S.M.; Tignor, T.C.; Reardon, R.C., eds. Proceedings of the first hemlock woolly adelgid 

review. Morgantown, WV: U.S. Department of Agriculture, Forest Service: 9–15.

Stoetzel, M.B. 1998. Antennal and other characters useful in identification of the Aphidoidea (Homoptera). 

Proceedings of the Entomological Society of Washington. 100(3): 588–593.

Townsend, L.; Rieske-Kinney, L. 2006. Meeting the threat of the hemlock woolly adelgid. ENTFACT452.Lexington: University of Kentucky. Cooperative Extension Service.

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Phylloxeridae). Canadian Entomologist. 87(5): 201–209.

U.S. Department of Agriculture, Forest Service [USDA FS]. 2011. Counties with established HWA 

populations 2010. http://na.fs.fed.us/fhp/hwa/maps/2010.pdf. (24 August 2011).

Walker-Lane, L.N. 2009. The effect of hemlock woolly adelgid infestation on water relations of Carolina and 

eastern hemlock. Raleigh, NC: North Carolina State University. 61 p. M.S. thesis.

Young, R.F.; Shields, K.S.; Berlyn, G.P. 1995. Hemlock woolly adelgid (Homoptera: Adelgidae): stylet 

bundle insertion and feeding sites. Annals of the Entomological Society of America. 88(6): 827–835.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

11

Genetic Basis of Resistance in Eucalyptus spp.

Pathosystems

Acelino Couto Alfenas,

1

Lúcio Mauro da Silva Guimarães,

1

and Marcos Deon 

Vilela de Resende2

Introduction

Eucalyptus is the most widely planted hardwood crop in world-wide tropical and subtropical regions 

because of its high growth rate, broad adaptability, and multipurpose wood properties. Until the 1970s, 

the Eucalyptus plantations in Brazil were practically disease free. However, plantations have 

continued to expand into warmer and more humid areas that are more favorable to pathogen infection. 

Furthermore, the use of high-yielding genotypes with unknown disease resistance, the implementation 

of clonal forestry, and the introduction of new management techniques have favored the emergence of 

disease epidemics in recent decades. Presently, prominent diseases such as rust (Puccinia psidii), 

Ceratocystis wilt (Ceratocystis fimbriata), and canker (Chrysoporthe cubensis) are the most

damaging diseases in Eucalyptus plantations. During recent years, we have conducted a series of 

inoculation experiments with controlled-crossed progenies to better understand the genetic basis of 

resistance for the prominent Eucalyptus pathosystems.

Eucalyptus x Puccinia psidii Pathosystem

Under favorable conditions, Puccinia psidii infects juvenile organs of the plant, either in the nursery 

or field. Rust infection can reduce tree growth and lead to loss of apical dominance. In highly 

susceptible genotypes, rust can also induce malformations of the affected organs, necrosis, 

hypertrophy, mini-cankers, and death of growing tips (Alfenas et al. 2009, Coutinho et al. 1998, Glen 

et al. 2007). The wide inter- and intra-specific genetic variability for rust resistance allows selection 

of resistant clones, progenies, or species for planting (Carvalho et al. 1998, Xavier et al. 2007). 

Eucalyptus grandis (Hill) Maiden is one of the most planted eucalypt species, but it is also among the

most susceptible species. Thus, rust is currently the most damaging disease to eucalypt plantations in 

tropical regions because of the wide use of E. grandis (Zauza et al. 2010).In contrast, species such as

E. pellita F. Muell. can be considered an important reservoir of genes for rust resistance (Guimarães 

et al. 2010b). Resistance to rust is predominately expressed by a hypersensitive reaction (HR) (Xavier 

2002), which is a common type of response triggered when plants that contain resistance genes (R 

genes) are challenged by pathogens that contain avirulence genes (Avr genes).

In 2003, a major gene for rust resistance, Ppr-1, was identified in a control pollinated family of E. 

grandis and mapped near the AT9917 RAPD marker (Junghans et al. 2003). The locus Ppr-1 was 

positioned on a microsatellite reference genetic map for Eucalyptus on the linkage group 3 (Mamani

et al. 2010). Additionally, its position was validated by association genetics in one related and two 

unrelated background families. These results are consistent with the hypothesis that Ppr-1 controls a 

large proportion of the variation for rust resistance in E. grandis. However, in recent studies, analyses

of several inter-specific families, displaying different segregation patterns for rust resistance,

provided additional evidence that the genetic control of rust resistance in Eucalyptus is more complex

 1

Departamento de Fitopatologia/BIOAGRO, Universidade Federal de Viçosa (UFV), 36570-000, Viçosa, MG, Brazil. 2

Embrapa Florestas. Current address: Departamento de Engenharia Florestal, Universidade Federal de Viçosa, 36570-000, 

Viçosa, MG, Brazil.

Corresponding author: aalfenas@ufv.br.
GENERAL TECHNICAL REPORT PSW-GTR-240

12

and includes quantitative resistance traits (Alves et al. 2012).The recognition that both additive and 

non-additive genetic variation (epistasis) are important contributors to rust resistance in eucalypts 

reveals the complexity of this host-pathogen interaction and helps explain the success that breeding 

has achieved in selecting rust-resistant clones, where the additive and non-additive effects are readily 

captured.

Eucalyptus x Ceratocystis fimbriata Pathosystem

Due to its lethal nature, broad host range, and wide geographical distribution, Ceratocystis wilt caused

by Ceratocystis fimbriata is currently one of the most important diseases of eucalyptus plantations in 

Brazil (Alfenas et al. 2009).Genetic variability for resistance to Ceratocystis wilt in Eucalyptus

genotypes was confirmed by Zauza et al. (2004). Subsequently, Rosado et al. (2010) studied 

Ceratocystis wilt resistance by stem inoculations of E. grandis and E. urophylla S. T. Blake

genotypes, and estimated the heritability and gains of selection in families derived from controlled

interspecific crosses. In both species, some genotypes were highly resistant and others highly 

susceptible to Ceratocystis wilt. Estimates of individual narrow (50 percent) and broad (59 percent) 

sense heritability suggested a high degree of genetic control and low allelic dominance of the trait. A 

genetic gain inlesion size up to -74.4 percent was obtained from selection ofthe 50 best clones in the 

evaluated families.

Continuing the studies with this pathosystem, Rosado (2009) built a microsatellite genetic map for 

the inter-specific, full-sibling family DGxUGL [(E. grandis x E. dunnii Maiden) x (E. urophylla x E. 

globulus)] (127 individuals) and detected five QTLs for Ceratocystis wilt resistance. Marker analysis

showed that five markers located in the linkage groups 1, 3, 5, 8, and 10 were associated with

significant quantitative effects on resistance with heritabilities ranging from 9.6 to 34.2.

Eucalyptus x Chrysoporthe cubensis Pathosystem

The existence of inter- and intra-specific genetic variability for canker resistance in eucalypts 

(Alfenas et al. 1983, Ferreira et al. 1978), together with the development of large-scale cloning in the 

1980s, has led to the control of the disease through the selection and cloning of resistant genotypes 

(Alfenas et al. 2009). Recently, Guimarães et al. (2010a) evaluated resistance in E. grandis and E. 

urophylla trees, as well as in individuals from progenies derived from controlled crosses between the 

trees. Six-month-old plants were inoculated, and xylem and bark lesions were measured at 8 months 

post-inoculation. The results demonstrated that xylem lesions are strong indicators for the selection of 

resistant clones. The phenotypic analyses indicated the existence of high genetic variability for 

resistance in both species. Individual narrow and broad sense heritability estimates were 17 percent

and 81 percent, respectively, suggesting that canker resistance is quantitative and highly dependent on 

dominance and epistasis.

Due to the wide genetic variation for canker resistance in E. grandis and E. urophylla, the 

introduction of resistant parent trees into ongoing breeding programs may increase the chances of 

obtaining disease-resistant clones at the end of the selection program. The results of our study also 

reinforce the need for using artificial inoculation with C. cubensis, when selecting pathogen-resistant 

parent trees and progenies of Eucalyptus spp. Furthermore, these studies also demonstrate the 

importance of cloning resistant genotypes for disease control.

Breeding for Disease Resistance

Plantations of clones or elite-resistant varieties represent the most efficient strategy for disease 

control. Currently, most Brazilian commercial eucalyptus clones are based on inter-specific hybrids

(Assis and Resende 2011). However, artificial inoculations have shown that 80 percent of these 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

13

clones are susceptible to at least one important disease. In contrast to canker resistance, most eucalypt 

breeding programs have not targeted rust and Ceratocystis wilt resistance until recently. Rust and 

Ceratocystis wilt resistance has usually been assessed in the final stages of clonal trials, and

susceptible clones are recommended for areas with no history of epidemics or simply discarded. With 

the expansion of clonal plantations, epidemics have become more frequent. As a result, several 

tropical breeding programs now specifically include rust and Ceratocystis wilt resistance as a 

selection criterion.

Since forestry breeding programs are usually medium to long term in duration, it is essential to 

have disease-resistant parents. Thus, before the formation of a breeding orchard, it is recommended 

that the resistant parental materials are thoroughly evaluated and selected. Moreover, one should also 

evaluate the resistance clones selected in the breeding program. This evaluation should be performed 

after the selection of promising clones prior to implementation of expanded clonal trials. 

In reciprocal recurrent selection (RRS), evaluation of eucalyptus rust resistance should be made in 

the hybrid progeny before selection of parents for recombination, submitting all the families to mass

inoculation of the pathogen under controlled conditions. Only the parents of families with resistant 

plants must be recombined and thus participate in the RRS. A new screening test must be performed

in pure progeny test, selecting only plants that are resistant to rust (fig. 1). Evaluation of resistance to 

Ceratocystis wilt and canker, which require clonal replicates, should be performed in progeny tests of 

pure species. Resistant plants should be selected for the new breeding orchard (fig. 1).

Figure 1— Stages of the breeding program in the reciprocal recurrent selection, in which

assessments should be performed for disease resistance.

E. grandis

Parents x

Selection

Evaluation

Recombination of

the best

E. grandis

E. grandis

Second generation

Rating mass for rust resistance

Orchard hybridization

Hybrid Progeny Test

E. urophylla

Parents

Pure progeny test

E. urophylla

Selection

E. urophylla

E. urophylla

Second generation

Orchard hybridization

Pure progeny test

E. grandis

Evaluation of resistance 

to other diseases

Evaluation

Recombination of

the best
GENERAL TECHNICAL REPORT PSW-GTR-240

14

In addition to determining when disease resistance of the clones will be evaluated during the 

breeding program, other issues should be considered: (i) reliable inoculation protocols are needed for 

disease-resistance evaluations; (ii) determining the genetic basis and the mode of inheritance of 

disease resistance is an essential step to obtaining disease-resistant plant material for the breeding 

program; (iii) the introgression of genes from different Eucalyptus species has been suggested to 

broaden the genetic basis of commercial clones for resistance to the main diseases; and (iv) a better 

understanding of the population genetic structures of the pathogens is needed to determine the role of 

pathogen variability in disease resistance that is targeted by the breeding strategies.

Acknowledgments

We would like to thank Dr. Ned Klopfeintein (USDA Forest Service, USA) and Mee-Sook Kim (Kaokmin 

University, Seoul, Korea)for kindly reviewing the text and to CNPq, Fapemig, Suzano Papel, and Celulose,

Fibria, Veracel, Jari Celulose, Cenibra, Plantar, and Vale for supporting our research line on disease resistance 

of tree species.

Literature Cited

Alfenas, A.C.; Jeng, R.; Hubbes. M. 1983. Virulence of Cryphonectria cubensis on Eucalyptus species 

differing in resistance. European Journal of Forest Pathology.13: 197–205.

Alfenas, A.C.; Zauza, E.A.V.; Maffia, R.G.; Assis, T.F. 2009. Clonagem e doenças do Eucalipto. Viçosa.

Editora UFV. 2a edição. 500p.

Alves, A.A.; Rosado, C.C.G.; Faria, D.A.; Guimarães, L.M.G.; Lau, D.; Brommonschenkel, S.H.;

Grattapaglia, D.; Alfenas, A.C. 2012. Genetic mapping provides evidence for the role of additive and nonadditive QTLs in the response of inter-specific hybrids of Eucalyptus to Puccinia psidii rust infection. 

Euphytica. 183: 27–38. 

Assis, T.F.; Resende, M.D.V. 2011. Genetic improvement of forest tree species. Crop Breeding and Applied 

Biotechnology. S1: 44–49.

Carvalho, A.D.O.; Alfenas, A.C.; Maffia, L.A.; Carmo, M.G.F. 1998. Resistance of Eucalyptus species, 

progenies and provenances to Puccinia psidii. Pesquisa Agropecuária Brasileira. 33: 139–147.

Coutinho, T.A.; Wingfield, M.J.; Alfenas, A.C.; Crous, P.W. 1998. Eucalyptus rust: a disease with the 

potential for serious international implications. Plant Disease. 82: 819–825.

Ferreira, F.A.; Alfenas, A.C.; Freitas, A.L. 1978. Determinação da resistência de 16 procedências de 

Eucalyptus ao cancro do eucalipto causado por Diaporthe cubensis Bruner, no Vale do Rio Doce, MG. 

Revista Árvore. 2: 110–129.

Glen, M.; Alfenas, A.C.; Zauza, E.A.V.; Wingfield, M.J.; Mohammed, C. 2007. Puccinia psidii: a threat to 

the Australian environment and economy - a review. Australasian Plant Pathology. 36: 1–16.

Guimarães, L.M.S.; Resende, M.D.V.; Lau, D.; Rosse, L.N.; Alves, A.A.; Alfenas, A.C. 2010a. Genetic 

control of Eucalyptus urophylla and E. grandis resistance to canker caused by Chrysoporthe cubensis. 

Genetics and Molecular Biology. 33: 525–531.

Guimarães, L.M.S.;Titon, M.; Lau, D.; Rosse, L.; Oliveira, L.S.S.; Rosado, C.C.G.; Christo, G.G.O.;

Alfenas, A.C. 2010b. Eucalyptus pellita as a source of resistance to rust, Ceratocystis wilt and leaf blight. 

Crop Breeding and Applied Biotechnology. 10: 124–131.

Junghans, D.T.; Alfenas, A.C.; Brommonschenkel, S.H.; Oda, S.; Mello, E.J.; Grattapaglia, D. 2003. 

Resistance to rust (Puccinia psidii Winter) in Eucalyptus: mode of inheritance and mapping of a major gene 

with RAPD markers. Theoretical and Applied Genetics. 108: 175–180.

Mamani, E.M.C.; Bueno, N.W.; Faria, D.A.; Guimarães, L.M.S.; Lau, D.; Alfenas, A.C.; Grattapaglia, D.

2010. Positioning of the major locus for Puccinia psidii rust resistance (Ppr1) on the Eucalyptus reference 

map and its validation across unrelated pedigrees.Tree Genetics & Genomes 6: 953–962.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

15

Rosado, C.G.C. 2009. Genética da resistência à murcha de ceratocystis (Ceratocystis fimbriata) em Eucalyptus

spp. Viçosa, MG: UFV. Dissertação de MestradoemGenética e Melhoramento.

Rosado, C.C.G.; Guimarães, L.M.S.; Titon, M.; Lau, D.; Rosse, L.; Resende, M.D.V.; Alfenas, A.C. 2010. 

Resistance to ceratocystis wilt (Ceratocystis fimbriata) in parents and progênies of Eucalyptus grandis x E. 

urophylla. Silvae Genetica. 59: 99–106.

Xavier, A.A. 2002. Histoptologia da interaçãoPuccinia psidii e virulência de isolados do patógeno em espécies 

de Myrtaceae. Viçosa, MG: UFV.Tese de Doutorado em Fitopatologia.

Xavier, A.A.; Sanfuentes, E.V.; Junghans, D.T.; Alfenas, A.C. 2007. Resistência de Eucalyptus globulus e 

Eucalyptus nitens à ferrugem (Puccinia psidii). Revista Árvore. 31(4): 7317–7335.

Zauza, E.A.V.; Alfenas, A.C.; Harrington, T.C.; Mizubuti, E.S.; Silva, J.F. 2004. Resistance of Eucalyptus

clones to Ceratocystis fimbriata. Plant Disease. 88: 758–760.

Zauza, E.A.V.; Alfenas, A.C.; Old, K.; Couto, M.M.F.; Graça, R.N.; Maffia, LA. 2010. Myrtacea especies

resistance to rust caused by Puccinia psidii. Australasian Plant Pathology. 39: 406–411.
GENERAL TECHNICAL REPORT PSW-GTR-240

16

Genetic Variation of Lodgepole Pine Physical and 

Chemical Defenses Associated With Each Step in 

Host Selection Behavior Sequence by Mountain 

Pine Beetle

Kimberly F. Wallin,1,2,3 Daniel S. Ott,1,2 and Alvin D. Yanchuk2,4

Abiotic and biotic stressors exert selective pressures on plants, and over evolutionary time lead to the 

development of specialized adaptations and specific responses to stresses (Safranyik and Carroll 

2006, Wallin and Raffa 2002). In this way, the environment in which plants evolve shapes their life 

cycles, range, growth, reproduction, and defenses. Insects and diseases are particularly potent plant 

stressors. Insect herbivores exert pressure on plants by feeding on plant parts and using them to 

reproduce by forming galls and cysts. Some insect species carry mutualistic pathogens on their 

bodies, which exert pressure on plants in the form of disease.

The model system of mountain pine beetle Dendroctonus ponderosae (Coleoptera: Scolytidae 

(Curculionidae)) (MPB), in association with the blue stain fungus Ophiostoma clavigerum

(Grossimania clavigera), attacking lodgepole pine Pinus contorta Douglas ex Louden var. latifolia

Engelm. ex S. Watson trees is an example of how selective pressures construct changes in coevolution and trade-offs between defenses. In the process of identifying traits, it is important to 

understand where and when during attack defensive traits arise within trees. 

Colonization behavior by the MPB can be broken into several steps. The process begins when 

adult beetles emerge from their natal host and begin their dispersal flight in search of a suitable host. 

The mass attack of a tree by MPB is a dynamic process that involves both host chemical cues and 

pheromone compounds. Prior to, and during colonization, attacking beetles detect volatile secondary 

metabolites, such as kairomonal cues, to locate suitable hosts. Upon landing on a host tree, the female 

will either attempt to colonize the tree, or after testing the tree for levels of secondary metabolites or 

nutrients, she will leave in search of a more suitable host tree. When a female accepts and enters a 

host tree, she excavates a nuptial chamber and begins forming an egg gallery. At this time, the female 

begins producing aggregation pheromone that attracts other female beetles to the tree as well as males 

for mating. Due to the co- evolved association between MPB and its host tree species, the female 

MPB is able to use host chemicals as precursors for pheromone production. The MPB completes 

development within the host tree and must locate a new host upon emerging as an adult.

The first and primary line of physical and chemical-based defense that a tree employs is the 

physical exudation of oleoresin (resin) (Wainhouse et al. 1990). Oleoresin is a complex blend of 

water, nutrients, terpenoids (Martin et al. 2002), and phenolics. If tree resin volume and pressure are 

high during colonization, their attempt to enter the tree against the resin flow will be physically halted 

(Raffa and Berryman 1983, Ruel et al. 1998, Wallin and Raffa 1999). High volume of resin can expel 

an insect or fungal spores from a wound or bind and contain insects or pathogens (Franceschi et al. 

2005, Phillips and Croteau 1999). Constitutive oleoresin is stored in resin ducts; however, induced 

resin can also be produced in other locations, known as traumatic resin ducts, following wounding by 

an insect or pathogen (Martin et al. 2002, Phillips and Croteau 1999). The primary terpenoids found 

in conifer oleoresin are monoterpenes, sesquiterpenes, and diterpenes. Monoterpenes are composed of 

 1 University of Vermont, The Rubenstein School of Environment and Natural Resources, Burlington, VT 05405. 2 University of Northern British Columbia, Prince George, B.C. Canada. 3 USDA Forest Service, 705 Spear Street, South Burlington, VT 05404. 4 British Columbia Forest Service, Victoria, B.C. Canada.

Corresponding author: kwallin@uvm.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

17

two isoprene units; sesquiterpenes are composed of three isoprene units; and di-terpenes are 

composed of four isoprene units. Various side groups can/may be present, and terpenoids are often 

oxygenated. Terpenoids constitute a major class of secondary metabolites in conifer oleoresin; they 

play an important role in the relationship between conifers, insects, and pathogens. They may deter 

insect feeding or affect fungal growth; be induced to change in abundance in response to wounding; 

be precursors for pheromone components used in insect communication; and affect insect and fungal 

reproductive success. Raffa and Berryman (1987) found induced monoterpenes to be a major factor in 

tree resistance to mountain pine beetle colonization. 

The type and quantity of compounds in the oleoresin of plants influence herbivory (Zangerl and 

Berenbaum 2004). These same compounds or a combination thereof also affect pathogens that attack 

plant systems. Herbivores may be deterred or killed because of an inability to overcome these resin 

toxins or physical defenses (Raffa and Berryman 1983). Pathogens may be compartmentalized and 

contained within the tree, unable to spread. The mode of toxicity of a particular resin secondary 

metabolite to an invading pest varies. Some terpenoids interfere with insect digestion, while others 

affect insect reproduction, decreasing fecundity or future fitness. Secondary metabolites may also 

interfere with disease processes; for instance, some terpenoids inhibit germination of spores or growth 

of fungi (Klepzig et al. 1996, Six and Klepzig 2004). 

The third line of defense for lodgepole pine is through its secondary metabolites, including 

terpenoids (Franceschi et al. 2005). Constitutive terpenoids may be enough to overwhelm attacking 

beetles. If the constitutive defenses are not successful, induced terpenoids are produced and may stop 

the bark beetle and its associated fungi. These tree chemicals can be very toxic to the mountain pine 

beetle (Klepzig et al. 1996, Raffa and Smalley 1995, Reid and Robb 1999). Quantifying the 

relationships between concentrations of host compounds at the early phase of host colonization is an 

important step toward understanding how variability of host responses impacts the success of MPB 

attacks. This information will be used to improve indices of tree resistance and relate specific host 

properties to survival of MPB outbreaks. Studies have already demonstrated that the composition 

and/or concentration of monoterpenes in either constitutive or induced tissue can be linked to the 

likelihood of host mortality to subcortical complexes (Nebeker et al. 1992, Raffa and Berryman 

1982). 

While resin flow and tree chemistry affect MPB progression throughout its life cycle in the tree, a 

tree also responds to the fungi by allocating carbon-based defenses to the site in an attempt to stop the 

growth of the fungi (Krokene et al. 2000). The tree defense strategy against blue stain fungus is 

apparent as a dark, resinous lesion on the phloem and xylem (Raffa and Berryman 1983, Wallin and 

Raffa 2001), which may compartmentalize and stop fungal growth. When this occurs, the tree 

successfully halts the colonization attempt by the individual beetle or pair of beetles and their 

associated fungi. 

The goal of this research was to quantify responses during outbreak using replicated study plots of 

20-year-old lodgepole pine, which provided the opportunity to calculate heritabilities and correlate 

MPB behaviors and tree responses during colonization sequence. The trees used in this research were 

open pollinated, so the mother of each tree and her provenance are clearly known. Known pedigree of 

progeny will allow the comparison of defense-related phenotypical differences across a broad range 

of lodgepole pine families. An understanding of the heritability of defenses in lodgepole pines will 

provide guidance to tree breeders as they seek to develop more resistant strains of pines for planting 

future forests. 

The current outbreak of MPB has killed about 15 million ha of lodgepole pine in British 

Columbia. This is of great concern to the entire forestry industry and provided a framework to study 

genetic variation of lodgepole pine defense mechanisms against MPB. Based on a previous study, we 

selected 45 open-pollinated families from a population of 180 families, with the number of trees per 

family ranging from 16 to 26 (for a total of 887 trees) across two sites. During the summer of 2006, 

we quantified chemical and physical defensive responses to simulated attack by the MPB fungal 

complex. These responses included: physical exudation of resin, host compartmentalization response 
GENERAL TECHNICAL REPORT PSW-GTR-240

18

to fungal inoculation, and constitutive and induced terpenoids. During the summer and fall of 2007, 

we further assessed tree height, DBH (diameter at breast height, about 1.4 m), bark texture, tree 

mortality; and MPB presence, attack density, reproduction, and brood development, as well as the 

presence of hypersensitivity reactions by the tree to MPB. 

Our data suggests variation in terpenoid makeup pre- and post- induction of simulated MPB attack 

is significantly different between families. Traits in lodgepole pine that affect MPB colonization and 

tree mortality also vary among families. Estimates show many host tree traits that affect the MPB life 

cycle are significantly heritable. We calculated family mean correlations of traits with significant 

heritabilities. Results suggest that several specific lodgepole pine tree defenses are heritable. Some 

constitutive (pre- induction) terpenes were negatively correlated with family mean mortality. The 

frequencies of induced reactions were negatively correlated with MPB gallery production, 

reproduction, and brood development. These results may provide future direction for management 

groups trying to manage MPB outbreaks in lodgepole pine stands. 

Acknowledgments

This work was funded by a British Columbia Forest Service grant to K. F. Wallin and A. D. Yanchuk, with 

support from Oregon State University, the University of Vermont, and the USDA Forest Service to K. F.

Wallin. The authors thank laboratory and field crews for their assistance.

Literature Cited

Franceschi, V.R.; Krokene, P.; Christiansen, E.; Krekling, T. 2005. Anatomical and chemical 

defenses of conifer bark against bark beetles and other pests. New Phytologist. 167: 353–376. 

Klepzig, K.D.; Smalley, E.B.; Raffa, K.F. 1996. Combined chemical defenses against an insectfungal complex. Journal of Chemical Ecology. 22: 1367–1388. 

Krokene, P.; Solheim, H.; Langstrom, B. 2000. Fungal infection and mechanical wounding induce 

disease resistance in Scots pine. European Journal of Plant Pathology. 106: 537–541. 

Martin, D.; Tholl, D.; Gershenzon, J.; Bohlmann, J. 2002. Methyl jasmonate induces traumatic 

resin ducts, terpenoid resin biosynthesis, and terpenoid accumulation in developing xylem of 

Norway spruce stems. Plant Physiology. 129: 1003–1018. 

Nebeker, T.E.; Hodges, J.D.; Blanche, C.A.; Honea, C.R.; Tisdale, R.A. 1992. Variation in the 

constitutive defensive system in loblolly pine in relation to bark beetle attack. Forest Science. 38: 

457–466. 

Phillips, M.A.; Croteau, R.B. 1999. Resin-based defenses in conifers. Trends in Plant Science. 4: 

184–190. 

Raffa, K.F.; Berryman, A.A. 1982. Physiological differences between lodgepole pines resistant and 

susceptible to the mountain pine beetle and associated microorganisms. Environmental 

Entomology. 11: 486–492. 

Raffa, K.F.; Berryman, A.A. 1983. The role of host plant resistance in the colonization behavior and 

ecology of bark beetles (Coleoptera: Scolytidae). Ecological Monographs. 53: 27–49. 

Raffa, K.F.; Berryman, A.A. 1987. Interacting selective pressures in conifer-bark beetle systems: A 

basis for reciprocal adaptations? American Naturalist. 129: 234–262. 

Raffa, K.F.; Smalley, E.B. 1995. Interaction of pre-attack and induced monoterpene concentrations 

in host conifer defense against bark beetle-fungal complexes. Oecologia. 102: 285–295. 

Reid, M.L.; Robb, T. 1999. Death of vigorous trees benefits bark beetles. Oecologia. 120: 555–562. 

Ruel, J.J.; Ayres, M.P.; Lorio, P.L. Jr. 1998. Loblolly pine responds to mechanical wounding with 

increased resin flow. Canadian Journal of Forest Resources. 28: 596–602. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

19

Safranyik, L. and Carroll, A. L. 2006. The biology and epidemiology of the mountain pine beetle in 

lodgepole pine forests. Natural Resources of. Canada.1-66.

Six, D.L.; Klepzig, K.D. 2004. Dendroctonus bark beetles as model systems for studies on 

symbiosis. Symbiosis. 37: 1–26. 

Wainhouse, D.; Cross, D.J.; Howell, R.S. 1990. The role of lignin as a defense against the spruce 

bark beetle Dendroctonus micans: effect on larvae and adults. Oecologia. 85: 257–265. 

Wallin, K.F.; Raffa, K F. 1999. Altered constitutive and inducible phloem monoterpenes following 

natural defoliation of jack pine: implications to host mediated interguild interactions and plant 

defense theories. Journal of Chemical Ecology. 25: 861–880. 

Wallin, K.F.; Raffa, K.F. 2001. Effects of folivory on subcortical plant defenses: Can defense 

theories predict interguild processes? Ecology. 82: 1387–1400. 

Wallin, K.F.; Raffa, K.F. 2002. Density-mediated responses of bark beetles to host allelochemicals: 

a link between individual behaviour and population dynamics. Ecological Entomology. 27: 484–

492. 

Zangerl, A.R.; Berenbaum, M.R. 2004. Genetic variation in primary metabolites of Pastinaca 

sativa; Can herbivores act as selective agents? Journal of Chemical Ecology. 30: 1985–2002. 
GENERAL TECHNICAL REPORT PSW-GTR-240

20

Stilbenes as Constitutive and Induced Protection 

Compounds in Scots Pine (Pinus sylvestris L.)

Anni Harju1 and Martti Venäläinen1

Abstract

The goals of our studies are to describe the natural variation in the concentration of constitutive heartwood 

extractives; estimate the genetic parameters related to heartwood characteristics; determine whether there is a 

genetic connection between constitutive and inducible production of stilbenes; and, together with technical 

experts, to develop fast and reliable techniques to quantify stilbenes from wood samples and determine whether 

there are rapid ways to utilize the existing variation in silviculture. We are also collaborating with molecular 

geneticists to find the markers for pine stilbene biosynthesis that could be used in early selection.

The study material includes three generations of Scots pine (Pinus sylvestris L.): the first generation consists of 

grafted clones in seed orchards; their half-sib progenies growing in about 40-year-old progeny trials are the 

second generation; and the third generation includes seedlings in a nursery. The first and the second generations

provide heartwood samples for stilbene analysis and the third generation has been used to study mechanical 

induction of stilbenes. Moreover, after heartwood samples from a large natural stand have been surveyed for 

stilbenes and total phenolics (end of 2012), we will be ready to make experiments on their seedling progenies

and their biotic challengers.

We hypothesize that trees having strong inducible defense ability against biotic and abiotic stresses in their 

living tissues may also have high concentration of stilbenes in their heartwood when they mature. Such trees 

would be optimal to cultivate as timber with natural stilbene impregnation. One vision with practical importance 

is the exploitation of the genetic variation in stilbene production for the improvement of Scots pine regeneration 

material. For the evaluation of the possibilities and strategies to improve Scots pine trees to produce more 

durable heartwood, we need to estimate the interaction between genotype and environment as well as the 

genetic correlation between quality and growth traits.

Key words: Pinus sylvestris, stilbenes, protection compounds

Introduction

Because of its natural characteristics, Scots pine (Pinus sylvestris L.) heartwood is an important

timber resource in Nordic countries, the potential of which is currently not fully utilized. Due to its 

high content of durability providing extractives, Scots pine heartwood timber has been used in 

construction where high decay resistance was required. Today, after more than 100 years of using 

super effective pressure treated wood, the interest on the natural durability of Scots pine (Pinus 

Sylvestris L.) heartwood has increased due to the legal restrictions on the use of wood preservation 

chemicals. Natural decay resistance is a special and important wood quality trait of Scots pine 

heartwood; however, it is very laborious to measure and study.

Heartwood is dead tissue, where extractives such as stilbenes have accumulated constitutively 

under developmental inevitability. It has an important role in living trees (Taylor et al. 2002), and its

features continue to be important when heartwood serves as construction material. One of the most 

 1 The Finnish Forest Research Institute, Punkaharju Unit, Finlandiantie 18, FI-58450 Punkaharju, Finland.

Corresponding author: anni.harju@metla.fi.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

21

important goals of our studies has been to find indirect ways to grade heartwood material according to 

its natural decay resistance. Based on previous studies, it is evident that the concentration of 

extractives is an important factor in giving natural decay resistance to heartwood (Scheffer and 

Cowling 1966). Besides resin acids (Harju et al. 2002), stilbenes pinosylvin (PS) and its monomethyl 

ether (PSM) are the most important extractives in Scots pine heartwood (Venäläinen et al. 2004).

Laboratory decay tests with the brown-rot fungus Coniophora puteana (strain BAM Ebw. 15) of 

heartwood material have shown a strong negative correlation between the concentration of stilbenes

and total phenolics for mass loss (Harju and Venäläinen 2006, Heijari et al. 2005, Leinonen et al. 

2008). The result suggests that quantifying stilbenes or total phenolics might be used as an indirect 

measure of natural decay resistance.

The living tissues in the trunk of Scots pine—namely the sapwood, phloem, and the cambium 

located between them—are constitutively protected on the outside by the bark and on the inside by 

the heartwood (Franceschi et al. 2005). However, attacks by damaging agents such as insects and 

their fungal associates trigger additional defensive responses in the form of induced production of 

secondary compounds and changes in wood anatomy (Lieutier et al. 1989, Nagy et al. 2005, Raffa et 

al. 2005).

Scots pine seedlings have been shown to produce stilbenes in various types of tissue in response to 

different kinds of stress factors, such as ultraviolet light (Schoeppner and Kindl 1979), ozone 

(Rosemann et al. 1991), and infection by fungi (Bonello et al. 1993, Gehlert et al. 1990). In intact 

sapwood, stilbenes do not occur in detectable amounts. However, even the mere mechanical 

wounding of Scots pine sapwood (Nilsson et al. 2002) induces the production of chemical defenses. 

The discolored sapwood induced by mechanical wounding has a similar high-performance liquid 

chromatography (HPLC) chromatogram as natural heartwood (Nilsson et al. 2002). 

In this paper we describe the natural variation in the concentration of constitutive heartwood 

extractives and present estimates for the genetic parameters related to heartwood characteristics in 

Scots pine. We have used mechanical wounding to induce production of secondary compounds in the 

sapwood of 3-year-old seedlings and we present genetic parameters for some chemical and growth 

traits from that experiment.

Methods and Materials

Heartwood Studies

Study material consisted of two progeny trials with identical half-sib progenies, 53 of which were 

included in this study (10 trees/progeny). The first 5.4 ha progeny trial area is in Leppävirta (62°25'N, 

27°45'E) and the second 5.5 ha area in Savonranta (62°15'N, 29°00'E), both in eastern Finland. The 

trees were 43 and 45 years old, respectively, at the time of the increment core sampling of heartwood 

The original design for both trials was 12 randomized blocks, with three rows of three trees per plot,

at a spacing of 2 × 2 m (equivalent to 2,500 trees/ha).

The diameter at breast height (DBH) was measured, and 5 mm increment cores were drilled

radially through the stem at about breast height in Savonranta, and in Leppävirta at about 90 cm. The 

pith-including cores were placed in plastic tubes and stored in darkness at -20 °C. The boundary 

between the sapwood and heartwood was marked on the increment cores under UV light, and the 

radius of the heartwood was measured from both sides of the pith. From each increment core, sections 

covering annual rings five to eight counted from the pith were separated from both sides of the pith. 

The aim was to obtain heartwood annual rings with the same cambial age. One section was used for 

the Folin–Ciocalteu (FC) assay to measure the concentration of total phenolics, and the other section 

was used for gas chromatography – mass spectrometry (GC–MS) analysis of stilbenes. The 

heartwood samples for the chemical analyses were oven dried at 60 °C for 24 h, weighed, and stored 

in a desiccator prior to grinding with an analytical mill (Kinematica). The milled samples were stored 

in darkness in sealed test tubes and glass bottles at -20 °C.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Mechanical Wounding of the Seedlings

The seeds were collected in October 2000 from 18 unrelated trees from a progeny trial in Mäkrä 

(Kerimäki, Mäkrä, 61°50'N, 29°23'E), the heartwood of which had earlier been studied (Harju et al. 

2003, Tiitta et al. 2003, Venäläinen et al. 2004). Seeds from each known mother tree comprised a 

half-sib family, where the fathers originated from an unknown stochastic pollen pool. The seedlings 

were grown for 3 years in a greenhouse at the Punkaharju Research Unit (61°48’N, 29°19’E) of the 

Finnish Forest Research Institute.

A total of 10 seedlings from each half-sib family were wounded on April 2004, when elongation 

of the buds of the seedlings had just begun. Ten seedlings of each family were left unwounded as 

controls. When wounding, about 10 holes were drilled through the stem with a 2.5 mm drill at 

intervals of about 2 cm along the 2002 internode (2-year-old segment of the stem). After about 3

months of growth, the wounded internodes were cut apart, the bark and phloem removed, and the 

internodes frozen at -20°C. Similar sections were cut from the control seedlings. A longitudinal 5 mm

section from above and below each wound hole was sampled (fig. 1). The samples were prepared for 

chemical analysis using the same methods as used for the increment cores. The analysis of stilbenes,

resin acids, and lignans in the seedlings was carried out in accordance with the procedure for stilbenes 

described in Karppanen et al. (2007). More details of the experiment are found in Harju et al. (2009).

Figure 1—(left) Wounding of a 3-year-old Scots pine seedling by drilling holes through the stem, and

(right) longitudinal section of the wounded stem under UV light (wavelength 313 nm). The brightly 

fluorescent areas of the stem were sampled for chemical analysis.

Analysis of Stilbenes and Total Phenolics

The gas chromatography-mass spectrometry (GC–MS) analysis of stilbenes was carried out as 

described by Karppanen et al. (2007). However, slight changes were made to the procedure. For a 

detailed description of the accelerated GC–MS analysis used in the study, please contact Tarja 

Tapanila (tarja.tapanila@metla.fi). The Folin–Ciocalteu assay for measuring total phenolics is 

described in Harju and Venäläinen (2006). The absorptivity was measured after 20 to 90 min at 

735 nm against a mixture containing all reagents except the sample. Tannic acid (Merck; 

1.59446.0010) was used as a standard. Therefore, the results are expressed as milligram tannic acid 

equivalents (TAE) per gram of dry mass of wood.

Estimation of Genetic Parameters

Heritabilities were estimated using the formula 4 ˆ ( ˆ ˆ ) ˆ2 2 2 2

f f e h = σ σ +σ . True half-sibs and unrelated 

parents were assumed. Variance components were estimated using a model, where block effects were 

regarded as fixed and family effects as random effects. The model did not include interaction effect. 

A mixed procedure of Statistical Package for Social Sciences (SPSS) system with restricted 

maximum likelihood (REML) technique (SPSS Inc. 2006) was used to estimate the variance 

components. Genetic correlation between the traits in the same environment (Leppävirta) was 

estimated as rA= covxy /(Varx Vary )

0.5 and the genetic correlation between the two environments

was estimated applying the formula 22.8 in Lynch and Walsh (1998).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

23

Results and Discussion

Heartwood Studies

For the grading of existing timber or for tree improvement it is important to know the distribution of 

the stilbene concentration and the concentration of total phenolics in the heartwood among the trees 

within a single stand. In all the materials studied, the distribution was slightly skewed to the right, 

which is typical for many biological distributions.

We estimated heritability for some chemical and growth variables (table 1). Quantification of 

stilbenes is continuing for the progeny trial Savonranta. To date, the results show that for heartwood 

stilbenes and total phenolics, the prospects of breeding are really promising compared to other traits 

studied. The heritabilities were quite similar, but the coefficients of additive genetic variation were 

about half of the ones estimated by Fries et al. (2000) from a full-sib Scots pine progeny trial of about 

the same age as the trials in our study.

Table 1—Estimates for heritability in narrow sense and the coefficient of additive genetic 

variation (CVA) for the chemical and growth characteristics of Scots pine 

Heritability (CVA%)

Leppävirta Savonranta

2003 2009 2010

PSa 0.67 (35)c

PSM 0.41 (33)c

PS + PSM 0.56 (33)c

Total phenolics 0.71 (33)b 0.52 (26)c 0.54 (31)

Density 0.60 (6) 0.58 (7)c 0.26 (6)

DBH 0.15 (6) 0.28 (8)c 0.12 (5)

Heartwood proportion 0.23 (16) 0.16 (13)c 0.16 (11) a PS = pinosylvin, PSM = pinosylvin monomethyl ether. b Harju and Venäläinen (2006).

c Partanen et al. (2011).

Genetic correlation between the two environments (Leppävirta and Savonranta), which are located

about 100 km from each other, were ≥ 0.80 for the concentration of total phenolics, density, DBH,

and the proportion of heartwood. There were evident rank changes between the sites among the 

families, but there were many families that held their rank position in both environments.

For the progeny trial in Leppävirta, it was possible to estimate the genetic correlation between the 

measured traits. For the extractives mentioned in table 1, the genetic correlations were ≥ 0.75. Genetic 

correlation between the concentration of extractives and the proportion of heartwood was around 

0.20. Heartwood density and DBH had a negative genetic correlation of -0.35, and they both had 

unfavorable negative genetic correlation with the concentration of extractives varying from -0.06 to 

-0.33. However these negative genetic correlations could be handled during the thinning of the actual 

production stands by cutting the largest trees, which produce the poorest quality of wood.

Induced Stilbenes in Seedlings

In our study, besides the increased amount of resin acids (from the average of 3.4 mg/g in control 

seedlings to the average of 71.9 mg/g in wounded seedlings), typical heartwood compounds PS and 

PSM were found (fig. 2) from the brightly fluorescing areas of the mechanically wounded seedlings 

(Harju et al. 2009). Two lignans, nortrachelogenin (NTG) and matairesinol (MR), typical for Scots 

pine knot wood (Holmbom et al. 2003), were found as well (fig. 2).
GENERAL TECHNICAL REPORT PSW-GTR-240

24

Figure 2—Average concentration of pinosylvin , pinosylvin monomethyl ether , nortrachelogenin , and 

matairesinol found from the brightly fluorescing areas of the mechanically wounded seedlings.

We were especially interested in quantifying the variation among the seedlings and in the 

proportion of the heritable variation in response to the wounding. Except for the concentration of 

PSM and resin acids, the heritabilities in the narrow sense were surprisingly high for all the traits that 

were measured (table 2). The coefficient of additive genetic variation was higher for the chemical 

than for the growth traits.

Table 2—Estimates for the heritability in narrow sense ( ˆ 2 h ) and the coefficient of additive 

genetic variation (CVA) for growth and chemical traits induced by mechanical wounding of 3-

year-old seedlings (Harju et al. 2009)

Heritability in 

narrow sense,

Coefficient of additive 

genetic variation,

Variable n ˆ 2 h (SE) CVA, %

PSa 175 0.71 (0.36) 29

PSM 176 0.35 (0.25) 15

NTG 175 0.73 (0.37) 37

MR 171 1.03 (0.46) 52

Sum of resin acids 176 0.31 (0.24) 21

Height 180 0.84 (0.40) 10

Diameter of the sampled sectionb, mm 180 0.64 (0.34) 8 a PS = pinosylvin; PSM = pinosylvin monomethyl ether; NTG = nortrachelogenin; MR = matairesinol.

n = the number of seedlings. Number of half-sib families was 18. b Measured from the middle of the section.

In this material, there was no phenotypic correlation between the growth traits and the 

concentration of extractives. Because of the small number of studied families, it was not possible to 

estimate genetic correlation. However, correlation of family means was estimated. Except for resin 

acids, the family mean correlation between chemical and growth traits was negative, varying from 

-0.22 to -0.39 and did not differ statistically significantly from zero (n = 18). For resin acids, the 

correlation was positive, but did not differ statistically significantly from zero (Harju et al. 2009).

If mechanical wounding of the seedlings induces a reaction related to the extractive composition 

of their mother’s heartwood, then wounding could be utilized in developing an early testing method 

in the breeding of durable Scots pine heartwood. Thus, we also estimated heritability from the 

regression of the offspring mean values based on their mothers, i.e., regression of the induced 

concentration of extractives in the xylem of the seedlings on the concentration of extractives in the 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

PSM PS NTG MR

Concentration, mg/g d.w.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

25

heartwood of their mothers. The heritability value for PS was 0.31, which suggests that there might 

be a positive connection between the constitutive and inducible synthesis of PS (Harju et al. 2009).

We are now interested in the role of induced production of stilbenes for tree survival and we are 

looking for proper research tools to approach the topic. In terms of tree breeding, it would be 

profitable if the constitutive and induced resistance could be selected for simultaneously.

Acknowledgments

We greatly appreciate the contribution of the personnel in the Finnish Forest Research Institute for the research

described in this paper. This study was financed by the Finnish Forest Research Institute and by Tekes (the 

Finnish Funding Agency for Technology and Innovation) as part of the EffTech (Intelligent and ResourceEfficient Production Technologies) and EffFibre (Value through Intensive and Efficient Fibre Supply) research 

programs of the Forestcluster ltd.

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Harju, A.M.; Anttonen, S.; Viitanen, H.; Kainulainen, P.; Saranpää, P.; Vapaavuori, E. 2003. Chemical 

factors affecting brown-rot decay resistance of Scots pine heartwood. Trees. 17: 263–268.

Harju, A.M.; Kainulainen, P.; Venäläinen, M.; Tiitta, M.; Viitanen, H. 2002. Differences in resin acid 

concentration between brown-rot resistant and susceptible Scots pine heartwood. Holzforschung. 56:.479–

486.

Harju, A.M.; Venäläinen, M. 2006. Measuring the decay resistance of Scots pine heartwood indirectly by the 

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Harju, A.M.; Venäläinen, M.; Saranpää, P.; Laakso, T. 2009. Mechanical wounding of the Scots pine 

seedlings results in stilbene and lignan biosynthesis. Tree Physiology. 29(1): 19–25.

Heijari, J.; Nerg, A.M.; Kaakinen, S.; Vapaavuori; E.; Raitio, H.; Levula, T.; Viitanen, H.; Holopainen,

J.K.; Kainulainen, P. 2005. Resistance of Scots pine wood to brown-rot fungi after long-term forest 

fertilization. Trees. 19: 728–734.

Holmbom, B.; Eckerman, C.; Eklund, P.; Hemming, J.; Nisula, L.; Reunanen, M.; Sjöholm, R.; 

Sundberg, A.; Sundberg, K.; Willför, S. 2003. Knots in trees - a new rich source of lignans. 

Phytochemistry Reviews. 2: 331–340.

Karppanen, O.; Venäläinen, M.; Harju, A.M.; Willför, S.; Pietarinen, S.; Laakso, T.; Kainulainen, P. 

2007. Knotwood as a window to the indirect measurement of the decay resistance of Scots pine heartwood. 

Holzforschung. 61: 600–604.

Leinonen, A.; Harju, A.M.; Venäläinen, M.; Saranpää, P.; Laakso, T. 2008. FT-NIR spectroscopy in 

predicting the decay resistance related characteristics of solid Scots pine (Pinus sylvestris L.) heartwood. 

Holzforschung. 62: 284–288.

Lieutier, F.; Cheniclet, C.; Garcia, J. 1989. Comparison of the defense reactions of Pinus pinaster and Pinus 

sylvestris to attacks by two bark beetles (Coleoptera: Scolytidae) and their associated fungi. Environmental

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Inc.
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Nagy, N.E.; Krokene, P.; Solheim, H. 2005. Anatomical-based defense responses of Scots pine (Pinus 

sylvestris L.) stems to two fungal pathogens. Tree Physiology. 26: 159–167.

Nilsson, M.; Wikman, S.; Eklund, L. 2002. Induction of discolored wood in Scots pine (Pinus sylvestris). 

Tree Physiology. 22: 331–338.

Partanen, J.; Harju, A.M.; Venäläinen, M.; Kärkkäinen, K. 2011. Highly heritable heartwood properties of 

Scots pine: possibilities for selective seed harvest in seed orchards. Canadian Journal of Forest Research.

41(10): 1993–2000.

Raffa, K.F.; Aukema, B.H.; Erbilgin, N.; Klepzig, K.D.; Wallin, K.F. 2005. Interactions among conifer 

terpenoids and bark beetles across multiple levels of scale: an attempt to understand links between 

population patterns and physiological processes. In: Romero, J., ed. Chemical ecology and phytochemistry 

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Phytochemistry. 39: 79–118.

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stilbene biosynthesis in Scots pine (Pinus sylvestris L.) seedlings. Plant Physiology. 97: 1280–1286.

Scheffer, T.C.; Cowling, E. 1966. Natural resistance of wood to microbial deterioration. Annual Review of

Phytopathology. 4: 147–170.

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2003. Comparing the effect of chemical and physical properties on complex electrical impedance of Scots 

pine wood. Holzforschung. 57: 433-–439.

Venäläinen, M.; Harju, A.M.; Saranpää, P.; Kainulainen, P.; Tiitta, M.; Velling, P. 2004. The 

concentration of phenolics in brown-rot decay resistant and susceptible Scots pine heartwood. Wood 

Science and Technology. 38: 109–118.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

27

Simultaneous Laurel Wilt Disease Biology and 

Resistance Research

Jason A. Smith1 and Randy C. Ploetz2

Abstract

Laurel wilt (LW) is a devastating, emerging disease of native and non-native members of the Lauraceae family 

in the southeastern United States. Currently, the fungal pathogen (Raffaelea lauricola) and its vector (Xyleborus

glabratus) are found in Alabama, Florida, Georgia, Mississippi, and North and South Carolina. The wilt is 

decimating native stands of redbay (Persea borbonia (L.) Spreng.), and causing significant damage to planted 

and native sassafras (Sassafras albidum (Nutt.) Nees) and avocado (Persea americana Mill.). Research has 

addressed significant knowledge gaps that exist for the biology and management of this disease. To date, 

effective fungicidal management of LW has been limited to the expensive, preventive treatment of high-value 

landscape trees with systemic fungicides. Long-term management in landscape and avocado plantings may rely 

on a combination of sanitation practices and the use of disease-resistant germplasm. The susceptibility/tolerance 

of different taxa and avocado germplasm has been assessed in artificial inoculation work in the field. Host range 

experiments have assessed the response of 35 taxa in the Annonaceae, Fagaceae, Lauraceae, Magnoliaceae, 

Moraceae, and Sapindaceae, which include known hosts of the vector in southeast Asia and their relatives. In 

general, members of the Lauraceae that are native to the southeastern United States have been most susceptible, 

whereas those in the family from Asia and in other families have been resistant. Forty-one cultivars of avocado, 

representing the three races of the species (Guatemalan, West Indian, and Mexican) and hybrids thereof, have

been screened for disease response. Unfortunately, West Indian cultivars that predominate in Florida have been 

most susceptible. Host-pathogen interactions have been examined in greenhouse and field studies. Results from 

this work indicate that: (i) systemic colonization by the pathogen, without apparent internal or external 

symptom development, occurs in some hosts; (ii) wilting is associated with reduced hydraulic conductivity in 

the xylem; (iii) vascular dysfunction results from host responses, not occlusion of vessels by the pathogen; and 

(iv) the pathogen does not produce wilt-inducing toxins. In ongoing research, greater understandings are sought 

for how susceptible and tolerant/resistant host plants respond to this disease.

 1 School of Forest Resources and Conservation, University of Florida, Gainesville, Florida 32611. 2 Tropical Research and Education Center, Homestead, Florida 33031.

Corresponding author: jasons@ufl.edu.

29

Breeding for Resistance in a 

Changing Environment - Durable 

Resistance: Hopes, Pitfalls, and 

Management Strategies

 

Plantings of sugar pine (Pinus lambertiana) started in 1998 at Happy Camp, CA for 

white pine blister rust resistance testing.

(Photo: Deems Burton).

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

31

Breeding Poplars With Durable Resistance to 

Melampsora larici-populina Leaf Rust: A

Multidisciplinary Approach to Understand and Delay 

Pathogen Adaptation

A. Dowkiw,

1 V. Jorge,1 M. Villar,1 E. Voisin,1 V. Guérin,1 P. Faivre-Rampant,2 

A. Bresson,

2 F. Bitton,2 S. Duplessis,3 P. Frey,3 B. Petre,3 C. Guinet,3 

C. Xhaard,

3 B. Fabre,3 F. Halkett,3 C. Plomion,4 C. Lalanne,4 and C. Bastien1

Introduction

During the last decades, European poplar breeders learned the hard way that Melampsora laricipopulina (commonly abbreviated as Mlp, fig. 1) has an impressive adaptive potential (McDonald and 

Linde 2002). This fungal pathogen defeated all the deployed cultivars carrying qualitative (i.e.,

complete) resistances inherited from the American eastern black cottonwood Populus deltoides

Bartram ex Marsh. in less time than needed to grow a poplar tree.

Figure 1—Sporulating Melampsora larici-populina uredinia on the abaxial side of a Populus deltoides 

x P. nigra cv ‘Robusta’ poplar leaf. (Photo by A. Dowkiw)

Populus deltoides is a key species for poplar breeding and growing in Europe. As an illustration, 

90 percent of the cultivars sold by nurseries to growers in 2009 in France were interspecific hybrids 

of two types: P. deltoides x P. trichocarpa (i.e., interamerican hybrids) or P. deltoides x P. nigra (i.e.,

Euramerican hybrids, P. nigra being a European species) (French Ministry of Agriculture – DGPATT 

– 2009 national statistics of forest reproductive material sales). Of the three parental species involved, 

P. deltoides is the only one where we ever found qualitative resistances to Mlp. Moreover, hybrid 

vigor (i.e., positive heterosis for growth traits) is generally high in these cultivars. Unfortunately, 

most of the hybrid material deployed over the last 30 years exhibited high rust susceptibility once 

their one or two major resistance genes inherited from P. deltoides were defeated.

 1 INRA, UR0588 AGPF, 45075 Orléans Cedex 2, France. 2 INRA, UMR1165 URGV, 91057 Evry Cedex, France.

3 INRA UMR1136 IAM, 54280 Champenoux, France.

4 INRA, UMR1202 BIOGECO, 33612 Cestas Cedex, France.

Corresponding author: arnaud.dowkiw@orleans.inra.fr.
GENERAL TECHNICAL REPORT PSW-GTR-240

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The present situation is as follows: (i) all of the 44 clones found in the French registry of poplar 

cultivars, except two cultivars involving the non-host species P. alba, are susceptible to Mlp, and (ii) 

from 1990 to 2004, the proportion of immune clones in the P. deltoides breeding material grown in 

the nursery of INRA Orléans decreased from 45 percent to 2 percent (fig. 2).

Figure 2—Evolution of Mlp rust susceptibility from 1990 to 2004 in a breeding collection of 545 P.

deltoides clones evaluated under natural rust infection at the Institut National de la Recherche 

Agronomique in Orléans, France. 

Breeding for quantitative resistance (QR) was considered a reasonable option as it is often 

described as an a priori for a more durable strategy. Although significant genetic variability was 

found for slow rusting traits in laboratory inoculation experiments and for field resistance under 

natural rust pressure in the three considered poplar species and their hybrids (Dowkiw et al. 2003, 

Lefèvre et al. 1994, Lefèvre et al. 1998, Pinon 1992, Pichot and Teissier du Cros 1993), three major 

results raised doubts on this optimistic point of view. First, elucidating the genetic determinism of QR 

in P. deltoides x P. trichocarpa hybrid progenies essentially yielded a few loci with major effects 

(Dowkiw and Bastien 2004) while small effect QTLs were either rare or hard to detect and often 

exhibited strain specificity (Jorge et al. 2005, fig. 3a). Second, quantitative and qualitative resistances 

did not appear to be completely independent as most identified defeated qualitative resistances 

inherited from P. deltoides happened to have statistical (possibly residual) effects on QR against 

virulent strains of the pathogen (Dowkiw and Bastien 2007, fig. 4). Third, Mlp strains able to 

completely defeat a major QR factor inherited from P. richocarpa have been identified before any 

commercial deployment of this resistance factor (Dowkiw et al. 2010).

Based on these considerations, poplar breeders and pathologists developed a broad holistic 

approach to understand the factors governing the durability of resistance, from genes to landscape, in 

order to conceive new breeding and deployment strategies.

Functional Genomics of the Poplar-Mlp Interaction

Following Eenink (1976), we believe that “the stability (i.e., the durability) of resistance is 

determined by the genetics of host-parasite relationships and not by the genetics of resistance. 

Quantity as well as quality of resistance and pathogenicity genes may be important. Monogenic and 

polygenic resistances can be stable or unstable.” Consequently, many efforts are made to elucidate the 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

33

functions of both qualitative and quantitative resistances and plant and rust geneticists try to bring the 

knowledge about the host-pathogen interaction at the same level for both protagonists.

Because genes with strong effects are the easiest ones to study and to follow among pedigrees, an 

integrated approach combining quantitative genetics, transcriptomics, and proteomics was conducted 

on three major loci: R1 and Mer, two qualitative resistances inherited from P. deltoides defeated by 

virulences 1 and 7 of the pathogen, respectively, and RUS, a major QR factor inherited from P.

trichocarpa with strong effect on uredinia size. RUS and R1 were identified at INRA (Dowkiw and 

Bastien 2004) whereas Mer was identified by a Belgian team (Cervera et al. 1996). All outputs from 

this combined approach led to the same conclusion of similarities between qualitative and quantitative 

resistances.

Figure 3—Mlp resistance loci described in the litterature (Bresson et al. 2011, Cervera et al. 1996, 

Cervera et al. 2001, Jorge et al. 2005, Lescot et al. 2004): A: Minor QTLs described in Jorge et al.

(2005) and detected in P. deltoides (D) and P. trichocarpa (T) after inoculation in controlled conditions 

with 7 Mlp strains followed by QR assessments (UN: uredinia size; LP: latent period; UN: uredinia 

number) or after natural rust infection in the field (MAX). Numbers in parentheses represent the 

percentage of clonal variance explained by each QTL. See Jorge et al. (2005) for further details. B: 

Localization of three major resistance factors on a consensus genetic map and alignment on the P. 

trichocarpa genome sequence for chromosome 19 (v2.0). Genetic distances in cM are indicated on 

the left of the linkage groups. The gene models are designated by their number without the prefix 

POPTR_0019s. Resistance gene analog domains are abbreviated as follows: B for BEAF and DREF 

DNA-binding finger (BED), L for leucine-rich repeat (LRR), n for nuclear localization sequence (NLS), 

N for nucleotide-binding site (NBS) and T for Toll interleukin 1 receptor (TIR). STK stands for serine 

threonine kinase. Position of the Mer locus (Cervera et al. 2001) is deduced from marker RGAm4-1 

designed from a P. deltoides BAC sequence developed for positional cloning of Mer (Lescot et al. 

2004). For marker details, see Bresson et al. (2011).

Although RUS maps to a clearly distinct area, all three loci map on the same chromosome (XIX, 

fig. 3b) which happens to be particularly rich in NBS-LRR resistance gene analogs (Kohler et al.

2008).

A combined transcriptomic and proteomic approach has been carried out in two contrasted groups 

of poplar half-sib genotypes possessing the resistant RUS allele (i.e., heterozygous RUSrUS genotypes) 

or deprived from it (i.e., rUSrUS genotypes) to determine molecular markers associated to the RUSmediated QR. Interestingly, whereas almost no genes were induced at 2 and 4 days after inoculation 
GENERAL TECHNICAL REPORT PSW-GTR-240

34

in the rUSrUS bulk compared to mock-inoculated leaves, the RUSrUS bulk was characterized by a strong 

induction of many marker genes typically associated to qualitative resistance (Rinaldi et al. 2007), 

suggesting a delay in defense reaction activation for QR. Several of these “marker” genes were 

confirmed by proteomic and by RTqPCR expression profiling (e.g., thaumatin-like protein, 

glutathione S-transferase). 

Figure 4—Family means (+/- associated standard errors) for three epidemiological components in 12 

P. deltoides x P. trichocarpa F1 families involving three distinct P. deltoides mothers of different 

origins (ILN, TNS) after leaf-disk inoculation with an Mlp strain able to sporulate in presence of any 

complete resistance factor inherited from P. deltoides segregating in this material. For each family, a 

distinction is made between genotypes carrying such qualitative resistance factor (in black) and 

genotypes lacking it (in grey). Presence of segregating qualitative resistance factors was revealed 

using incompatible stains of the pathogen. See Dowkiw and Bastien (2007) for further details.

Regarding the genetics of the pathogen’s side of the interaction, not much is known yet, even on 

the genetic determinism of virulence (i.e., the pathogen’s matching piece to qualitative resistance). 

One reason is that progenies are difficult to obtain due to the heteroecious life cycle of the pathogen 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

35

(i.e., the need to perform its sexual stage on a different host plant, here larch) and to its strict 

biotrophic status. However, recent sequencing of the Mlp genome opens the way for new genomics 

tools and makes the popular rust interaction a model pathosystem for forest pathology (Duplessis et 

al. 2011a). Most pathogenicity effectors described so far in fungal biotrophic pathogen encode small 

cysteine-rich secreted proteins (SSP) (Stergiopoulos and de Wit 2009). The Mlp genome contains a 

total of 1,184 SSP-encoding genes, which are mostly specific to this rust fungus (i.e., no homologs 

are found in other fungi, including the wheat stem rust; Duplessis et al. 2011b). More than 50 percent

of these SSP genes are expressed during the successful colonization of poplar leaves representing 

candidate pathogenicity factors (Duplessis et al. 2011b; Hacquard et al., in press)

Thinking at Larger Scales

One of the reasons for the rapid spread of newly adapted strains of Mlp lies within the spatial and 

temporal organization of host diversity over the country. Poplar is a perennial host that is essentially 

cultivated as monoclonal stands, and less than 10 cultivars are being significantly used by growers. In 

this context, we investigated to what extent the population genetic structure of Mlp can be impacted 

by the deployment of resistant poplar cultivars over the country.

Since 1982, the interamerican hybrid poplar cv. 'Beaupré' carrying the rust resistance gene Mer

was broadly planted in France and remained immune for 12 years, but once overcome by virulence 7 

of the pathogen, severe rust outbreaks occurred (Xhaard et al. 2011). Using both phenotypic (i.e., 

virulence profiles) and genotypic (i.e., microsatellites) markers, we showed that Mlp isolates carrying 

virulence 7 were widely distributed all across France and displayed a specific genetic signature 

consistent with a history of selection and drastic demographic changes resulting from the resistance 

breakdown (Xhaard et al. 2011, fig. 5). This study illustrates how poplar cultivation has influenced 

the spatial and genetic structure of the pathogen, and has led to the spread of virulence alleles in most 

pathogen populations. As a consequence, resistance management should certainly be thought about at 

a continental scale in order to maximize its sustainability (McDonald and Linde 2002).

Alternative Breeding Strategies

Urged by growers to deliver new cultivars, breeders explore multiple (possibly combinable), very 

pragmatic, strategies to delay pathogen adaptation.

Pure P. deltoides genotypes, although defeated, always show much lower field susceptibility to the 

pathogen in our nursery in Orléans than their interspecific hybrid progenies. This may result from 

favorable gene associations that happen to be broken in these hybrids. Constitutive resistance traits 

related to leaf anatomical characteristics may be particularly affected by hybridization with P.

trichocarpa and P. nigra. As a short-term solution, pure P. deltoides cultivars selected for 

productivity in northern climatic conditions under traditional cultivation practices will be released in 

2013. They all carry defeated qualitative resistances but repeatedly showed high QR levels under 

natural rust infections. Backcrossing interamerican F1 hybrids to the P. deltoides species is also being 

considered.

Medium-term solutions will rely on a more careful exploration of the genetic variability available 

in the European species P. nigra that co-evolved with the pathogen and on the simultaneous release of 

several unrelated cultivars to generate host diversity at regional scale. A collection of 2,300 P. nigra

genotypes, most of them originating from different French natural populations, but also from Italy, 

Germany, and the Netherlands, have recently been screened for rust resistance under natural and 

controlled infection. Additional breeding traits like avoidance and tolerance (i.e., the ability to 

maintain growth despite being susceptible to the disease) are also being evaluated using fungicide 

treated vs. untreated field experiments.
GENERAL TECHNICAL REPORT PSW-GTR-240

36

Figure 5—Spatial interpolations for the frequency of Mlp strains. A: belonging to the “cultivated” 

genetic group (i.e., “group 1” in Xhaard et al. 2011), and B: possessing virulence 7.

Acknowledgments

The research on genetic variation for rust resistance in the Populus host species received funding from the 

European Community’s Seventh Framework Program (FP7/ 2007-2013) under the grant agreement n 211868 

(Noveltree Project), from INRA under the ECOGER Program (AIP 00295-298, INTERPOPGER Project) and 

from the French Ministry of Agriculture under grant agreements GIS n E05/08 and E05/09. The research on 

population genetics of Mlp was supported by INRA under the ECOGER Program (AIP 00295-298, 

INTERPOPGER Project), the Agence Nationale de la Recherche (ANR 07/BDIV-003, EMERFUNDIS 

Project), and INRA (projet innovant EFPA). S.D. was supported by junior scientist grants from INRA and 

Région Lorraine. Physical mapping of the resistance factors was supported by INRA and the French Bureau des 

Ressources Génétiques (BRG). C.X. and A.B. were supported by PhD fellowships from the French Ministry of 

Education and Research (MESR). B.P. and C.G were supported by joint PhD fellowships from INRA and 

Région Lorraine. We would like to thank Axelle Andrieux, Béranger Bertin, Christine Géhin, I. Le Clainche,

and the INRA Orléans Experimental Unit GBFOR for their technical help.

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rust fungus Melampsora larici-populina: evidence for isolation by distance in Europe and recent founder 

effects overseas. Infection Genetics and Evolution. 8(5): 577–587.

Bresson, A.; Jorge, V.; Dowkiw, A.; Guerin, V.; Bourgait, I.; Tuskan, G.A.; Schmutz, J.; Chalhoub, B.;

Bastien, C.; Rampant, P.F. 2011. Qualitative and quantitative resistances to leaf rust finely mapped within 

two nucleotide-binding site leucine-rich repeat (NBS-LRR)-rich genomic regions of chromosome 19 in 

poplar. New Phytologist. 192(1): 151–163.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

37

Cervera, M.T.; Gusmao, J.; Steenackers, M.; Peleman, J.; Storme, V.; Vanden Broeck, A.; Van Montagu, 

M.; Boerjan, W. 1996. Identification of AFLP molecular markers for resistance against Melampsora laricipopulina in Populus. Theoretical and Applied Genetics. 93: 733–737.

Cervera, M.T.; Storme, V.; Ivens, B.; Gusmão, J.; Liu, B.H.; Hostyn, V.; Van Slycken, J.; Van Montagu, 

M.; Boerjan, W. 2001. Dense genetic linkage maps of three populus species (Populus deltoides, P. nigra

and P. trichocarpa) based on AFLP and microsatellite markers. Genetics. 158: 787–809.

Dowkiw, A.; Bastien, C. 2004. Characterization of two major genetic factors controlling quantitative resistance 

to Melampsora larici-populina leaf rust in hybrid poplars: strain specificity, field expression, combined 

effects, and relationship with a defeated qualitative resistance gene. Phytopathology. 94(12): 1358–1367.

Dowkiw, A.; Bastien, C. 2007. Presence of defeated qualitative resistance genes frequently has major impact 

on quantitative resistance to Melampsora larici-populina leaf rust in P. xinteramericana hybrid poplars. 

Tree Genetics and Genomes. 3(3): 261–274.

Dowkiw, A.; Husson, C.; Frey, P.; Pinon, J.; Bastien, C. 2003. Partial resistance to Melampsora laricipopulina leaf rust in hybrid poplars: genetic variability in inoculated excised leaf disk bioassay and 

relationship with complete resistance. Phytopathology. 93(4): 421–427.

Dowkiw, A.; Voisin, E.; Bastien, C. 2010. Potential of Eurasian poplar rust to overcome a major quantitative 

resistance factor. Plant Pathology. 59: 523–534.

Duplessis, S.; Cuomo, C.A.; Lin, Y.C.; Aerts, A.; Tisserant, E.; Veneault-Fourrey, C.; Joly, D.L.;

Hacquard, S.; Amselem, J.; Cantarel, B.L.; Chiu, R.; Coutinho, P.M.; Feau, N.; Field, M.; Frey, P.;

Gelhaye, E.; Goldberg, J.; Grabherr, M.G.; Kodira, C.D.; Kohler, A.; Kues, U.; Lindquist, E.A.;

Lucas, S.M.; Mago, R.; Mauceli, E.; Morin, E.; Murat, C.; Pangilinan, J.L.; Park, R.; Pearson, M.;

Quesneville, H.; Rouhier, N.; Sakthikumar, S.; Salamov, A.A.; Schmutz, J.; Selles, B.; Shapiro, H.;

Tanguay, P.; Tuskan, G.A.; Henrissat, B.; Van de Peer, Y.; Rouze, P.; Ellis, J.G.; Dodds, P.N.; Schein, 

J.E.; Zhong, S.B.; Hamelin, R.C.; Grigoriev, I.V.; Szabo, L.J.; Martin, F. 2011a. Obligate biotrophy 

features unraveled by the genomic analysis of rust fungi. Proceedings of the National Academy of Sciences 

of the United States of America. 108(22): 9166–9171.

Duplessis, S.; Hacquard, S.; Delaruelle, C.; Tisserant, E.; Frey, P.; Martin, F.; Kohler, A. 2011b.

Melampsora larici-populina transcript profiling during germination and timecourse infection of poplar 

leaves reveals dynamic expression patterns associated with virulence and biotrophy. Molecular plantmicrobe interactions : MPMI. 24: 808–818.

Eenink, A.H. 1976. Genetics of host-parasite relationships and uniform and differential resistance. Netherlands

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Hacquard, S.; Joly, D.L.; Lin, Y.C.; Tisserant, E.; Feau, N.; Delaruelle, C.; Legué, V.; Kohler, A.;

Tanguay, P.; Petre, B.; Frey, P.; Van de Peer, Y.; Rouzé, P.; Martin, F.; Hamelin, R.C.; Duplessis, S. 

[In press]. A comprehensive analysis of genes encoding small secreted proteins identifies candidate 

effectors in Melampsora larici-populina (poplar leaf rust). Molecular Plant-Microbe Interactions.

Jorge, V.; Dowkiw, A.; Faivre-Rampant, P.; Bastien, C. 2005. Genetic architecture of qualitative and 

quantitative Melampsora larici-populina leaf rust resistance in hybrid poplar: genetic mapping and QTL 

detection. New Phytologist. 167(1): 113–127.

Kohler, A.; Rinaldi, C.; Duplessis, S.; Baucher, M.; Geelen, D.; Duchaussoy, F.; Meyers, B.C.; Boerjan, 

W.; Martin, F. 2008. Genome-wide identification of NBS resistance genes in Populus trichocarpa. Plant 

Molecular Biology. 66: 619–636.

Lefèvre, F.; Goué-Mourier, M.C.; Faivre-Rampant, P.; Villar, M. 1998. A single gene cluster controls 

incompatibility and partial resistance to various Melampsora larici-populina races in hybrid poplars. 

Phytopathology. 88(2): 156–163.

Lefèvre, F.; Pichot, C.; Pinon, J. 1994. Intra- and interspecific inheritance of some components of the 

resistance to leaf rust (Melampsora larici-populina Kleb.) in poplars. Theoretical and Applied Genetics. 88: 

501–507.

Lescot, M.; Rombauts, S.; Zhang, J.; Aubourg, S.; Mathe, C.; Jansson, S.; Rouze, P.; Boerjan, W. 2004.

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novel class I and class II transposable elements. Theoretical and Applied Genetics. 109(1): 10–22.
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McDonald, B.A.; Linde, C. 2002. Pathogen population genetics, evolutionary potential, and durable resistance. 

Annual Review of Phytopathology. 40: 349–379.

Pichot, C.; Teissier du Cros, E. 1993. Susceptibility of P.deltoides Bartr. to Melampsora larici-populina and 

M.allii-populina. Silvae Genetica. 42: 4–5.

Pinon, J. 1992. Variability in the genus Populus in sensitivity to Melampsora rusts. Silvae Genetica. 41(1): 25–

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Rinaldi, C.; Kohler, A.; Frey, P.; Duchaussoy, F.; Ningre, N.; Couloux, A.; Wincker, P.; Le Thiec, D.;

Fluch, S.; Martin, F.; Duplessis, S. 2007. Transcript profiling of poplar leaves upon infection with 

compatible and incompatible strains of the foliar rust Melampsora larici-populina. Plant Physiology. 144: 

347–366.

Stergiopoulos, I.; de Wit, P. 2009. Fungal effector proteins. Annual Review of Phytopathology. 47: 233–263.

Xhaard, C.; Fabre, B.; Andrieux, A.; Gladieux, P.; Barrès, B.; Frey, P.; Halkett, F. 2011. The genetic 

structure of the plant pathogenic fungus Melampsora larici-populina on its wild host is extensively impacted 

by host domestication. Molecular Ecology. 20: 2739–2755.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

39

Developing Hazelnuts (Corylus spp.) With Durable 

Resistance to Eastern Filbert Blight Caused by 

Anisogramma anomala

Thomas J. Molnar,1 John Capik,1 Clayton W. Leadbetter,1 Ning Zhang,1

Guohong Cai,1 and Bradley I. Hillman1

Abstract

Eastern filbert blight (EFB) is a devastating fungal disease of European hazelnut, Corylus avellana L., and is 

considered to be the primary reason hazelnuts have not been developed as a commercial crop in the eastern 

United States. The pathogen, Anisogramma anomala, is native to a wide area east of the Rocky Mountains, 

where it is harbored by the wild hazelnut C. americana Marshall. While C. americana is tolerant of the disease, 

EFB causes stem cankers, dieback, and death of C. avellana, the hazelnut of commerce. The absence of EFB, 

along with a mild climate, allowed for the development of a commercial hazelnut industry in the Pacific 

Northwest (PNW), which thrived for nearly a century. However, despite quarantine efforts, EFB was discovered 

in Washington in the late 1960s and has spread throughout the Willamette Valley of Oregon, where 99 percent 

of the United States crop is produced, causing significant economic losses. To combat this disease, a major 

investment in research and breeding was made by Oregon State University (OSU), the U.S. Department of 

Agriculture, and the Oregon hazelnut industry. Over the last 30 years, these efforts have resulted in the 

development of a better understanding of A. anomala as well as effective disease management protocols and 

improved resistant cultivars, leading to a recent revival of the industry.

Building on advances made in Oregon, a hazelnut research and genetic improvement program was initiated at 

Rutgers University in New Jersey in 1996. Realizing the major limiting factor to production of commercialquality hazelnuts in this region was EFB, the initial focus was to evaluate known sources of resistance to the 

disease in both C. avellana and other Corylus species, along with investigating pathogenic variation of the 

fungus, which was unknown at the time. In support of this work, close collaboration was developed with the 

OSU hazelnut program to evaluate their plant material when exposed to A. anomala isolates originating from 

across its native range; it is believed a limited diversity of the fungus is present in the PNW due to its 

occurrence likely having arisen from a single point introduction. In addition, wide germplasm collections were

made in Europe to search for novel sources of resistance and to identify plants expressing other traits important 

in the eastern United States, such as cold hardiness and nuts that fall free from the husk at maturity. Concurrent 

with this work, controlled hybridizations were made annually to incorporate genes for EFB resistance from a 

diversity of backgrounds with the superior nut quality and yield expressed by OSU cultivars and breeding 

selections. Today, over 25,000 hazelnut seedlings are under evaluation in greenhouses and field nurseries at 

Rutgers.

An overview of the objectives and progress to date of the Rutgers breeding program were provided, including 

resistance screening protocols, findings on the pathogenic variation of A. anomala, the identification of novel 

sources of resistance, and the development of molecular biology tools to assess genetic diversity and population 

structure of the pathogen and to assist in its early detection for disease management and efficient breeding.

 1 Plant Biology and Pathology Department, Rutgers University, New Brunswick, NJ 08901.

Corresponding author: molnar@aesop.rutgers.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Breeding for Growth Improvement and Resistance 

to Multiple Pests in Thuja plicata

John H. Russell1 and Alvin D. Yanchuk1

Introduction

Western redcedar (Thuja plicata Donn ex D. Don), a member of the Cupressaceae, is an important 

commercial species in British Columbia (BC) and the Pacific Northwest (PNW), which is prized for 

its heartwood and its performance as a naturally durable outdoor building material. Although a

genetic improvement program has been ongoing for only 15 years in BC, substantial progress has 

been made owing in part to its unique genetic and biological properties. These include early precious 

flowering and short generations (Russell and Ferguson 2008), the ability to self with minimal 

inbreeding depression (Russell et al. 2003), and ease of vegetative propagation and clonal 

deployment.

Initial population improvement focused on growth and adaptability as well as heartwood 

durability–an important wood quality trait for second growth western redcedar. The heartwood of this 

species contains secondary extractives implicated in rot resistance to a number of fungal species 

(Daniels and Russell 2007, Maclean and Gardner 1956). Tropolones are a group of extractives that 

are present in relatively small concentrations, but are highly fungitoxic to a suite of fungal rot species

in living trees. These extractives are readily leached out of wood in service, but lignans, another 

group of secondary compounds (e.g., plicatic acid) are not. Lignans are mildly fungitoxic as 

compared to the tropolones, but are in greater concentrations in the heartwood (Morris and Sterling 

2010). Individual tropolone and lignin compounds have moderate to high heritibilities (h2

ns = 0.25 to 

0.58) and additive coefficients of variability (CVadd >68 percent) (Russell and Daniels 2010).

In addition to fungal rots, black-tailed deer (Odocoileus hemionus columbianus) and cedar leaf 

blight (Didymascella thujina) have increasingly caused western redcedar plantation failures and 

reduced growth, and resulted in longer times to reach free to grow status. Black-tailed deer have been 

shown to avoid browsing trees that are high in volatile foliage monoterpenes (Kimball et al. 2012, 

Vourc’h et al. 2002). Similar to heartwood secondary extractives, foliage monoterpenes have 

moderate to high heritibilities (h2

ns = 0.48 to 0.60) and additive coefficients of variability (CVadd 24 to 

66 percent) (Russell, unpublished data)

Cedar leaf blight (CLB) occurs throughout western redcedar’s range, but is especially virulent in 

humid, warm environments typical of the highly productive maritime ecosystems for western 

redcedar (Kope and Trotter 1998). There is significant genetic variation in resistance to CLB across 

moderately to heavily infected sites (h2

ns=0.18; CVadd=14 percent) (Russell et al, 2007) and this 

variation is strongly correlated to population-origin climate with parents from wetter, milder sites

having greater resistance.

As climates change and pests, known and unknown, become more abundant, breeding strategies 

need to adapt to better accommodate shifts in environmental stresses. Developing breeding 

populations that are resilient to multiple pests, as well as maintaining adaptability and growth, is 

imperative to ensure current and future forest health.

 1 Research Scientists, Forest Genetics, British Columbia Forest Service, Canada.

Corresponding author: John.Russell@gov.bc.ca.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

41

Initial Population Improvement 

Western redcedar has been under domestication since the late 1990s. Two main populations were 

used for initial selections and breeding: 1) 1,000 wildstand parent trees selected from throughout the 

species coastal range from northern California to northern BC, and grafted into clonebanks at 

Cowichan Lake Research Station (CLRS), and 2) BC range-wide provenance trials with openpollinated family structure. The four main traits described above had the following improvement to

date:

1) Growth and CLB resistance. The above wildstand parent trees were polycrossed with a 

common 20 parent mix, and progeny established in 45 field tests across seven annual series 

throughout BC between 2000 and 2008. Top breeding value parents for predicted volume 

production at 60 years varied from 18 to 35 percent, over wildstand seedlots based on 7 to 10

year heights. On individual sites highly infected with CLB, an increase in the disease impacted

height and diameter growth significantly with genetic correlations from -0.58 to -0.88 at age 

10 years (fig. 1); however, type B genetic correlations between CLB and height on sites with 

no CLB are not significantly different from zero. Backward selections that are both CLB 

resistant and high volume producers have been established in a breeding orchard and are 

currently being mated using partial diallels with assortative mating. Approximately 350 fullsib families have been completed to date with an anticipated 700 families in total when 

completed.

Figure 1—Relationship between western redcedar parental breeding values (BV) for cedar 

leaf blight (CLB) and height at a British Columbia maritime low elevation test site. Positive 

CLB BVs indicate susceptibility.

2) Heartwood rot-resistant population. Approximately 350 of the wildstand parent trees were old 

enough (20 years) to estimate heartwood extractives. Inner and outer heartwood from these

trees have been profiled for tropolones (implicated in resistance of living heartwood) and 

lignans (responsible for durability of wood in service) using multiple ramets from clonebanks 

and seed orchard trees. Sixty backward selections have been made based on total thujaplicins 

and plicatic acid. These backward selections are currently in a breeding orchard for advanced 

generation breeding.
GENERAL TECHNICAL REPORT PSW-GTR-240

42

3) Deer browse-resistant population. All 1,000 wildstand parent trees were profiled for individual

monoterpene compounds using foliage from multiple ramets in clone banks and seed orchards.

In addition, foliar monoterpenes were estimated from 2,200 open-pollinated progeny from the

range-wide provenance trial mentioned above, at age 6 years. Approximately 120 clonal and 

forward selections were made from both populations based on deer browse intensity and total 

monoterpenes. These parents were bred using partial diallels with assortative mating including 

selfs. Approximately 400 families were sown and forward selections made based on total 

foliage monoterpenes from one-year-old seedlings in the greenhouse (target age for deer 

browse resistance). First generation selections averaged around 35,000 ppm total 

monoterpenes and second generation selections averaged over 75,000 ppm. These forward 

selections are established in a breeding orchard and are currently available for 3rd generation 

breeding.

Advanced Population Improvement

Our objective for western redcedar is to develop a durable advanced generation breeding population 

with potential cross resistance. Developing breeding populations that are resilient to multiple pests 

may not only give protection against the current target pests, but potentially against future unknown 

ones. Selections for both increased volume production and CLB resistance are readily achieved for 

deployment on sites that have CLB, which is currently the majority of productive western redcedar 

forests. Genetic correlations between secondary extractives in the foliage and growth rate (fig. 2), and 

secondary extractives in heartwood and foliage (fig. 3), although not strong, are positively low to 

moderate. This is a simplistic measure and assessment of complex chemical pathways, but it does 

give us a potential indication that there is minimal competition in chemical resources between foliage 

and heartwood extractives. We have no information on CLB resistance mechanisms at this time, but 

as in many leaf disease studies, foliar monoterpenes have played a significant role (e.g., Wallis et al. 

2011). 

Figure 2—Relationship between western redcedar breeding values (BV) for foliage monoterpene and 

growth in a BC coastal open-pollinated progeny trial.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

43

Figure 3—Relationship between western redcedar clonal values for foliar monoterpenes and 

heartwood tropolones in a 20-year-old clonal trial (blue box = independent culling).

Advanced generation breeding strategy will involve breeding groups by traits and matings performed 

within and between groups using partial diallels (fig. 4) with assortative mating within groups.

Growth CLB Foliage terpenes Hdwd extractives

Growth

X X X X X X X X X

X X X X X

X

CLB

X X X X X X X

X X X X

X

Foliage terpeness

X X X X X

X X X

X

Hdwd extractives

X X X

X X

X

Figure 4—Western redcedar advanced generation breeding strategy for multiple pests and growth.
GENERAL TECHNICAL REPORT PSW-GTR-240

44

This new population will be tested in environments conducive to the respective stresses. Because 

of the partial confounding of population with selection objectives in the first generation of screening,

we expect it would be beneficial to do further field testing in each of the growth/CLB and the deer 

resistance populations prior to make final selections for advanced generation breeding population.

For the growth/CLB selections, advanced generation full-sib matings from backward selections 

have already been done. Seed from these families will be grown in the nursery and tested for foliar 

monoterpenes at age one year. The top selections for total monoterpenes will then be cloned and 

tested for growth/CLB in short-term tests. For the deer resistance population, families from second 

generation breeding have already been tested for foliar monoterpenes and forward selections cloned 

for production hedges. These clones will be established in short-term tests in environments conducive 

to good growth and cedar leaf blight infection. Clones from both of these additional trials will be 

selected based on the respective target traits for the advanced generation durable breeding population.

The final population (Ne~100) will be composed of: 1) 50 third generation forward selections 

from the deer resistant population further clonally selected for growth and CLB resistance; 2) 50 first 

generation parental selections based on clonal values for total heartwood extractive content with

independent culling for the other target traits; and 3) 50 second generation forward selections for 

volume further selected for deer resistance. Currently, mechanisms for CLB resistance are unknown,

but being researched. It may be appropriate to make additional CLB selections based on future 

potential resistance mechanisms.

Literature Cited

Daniels, B.; Russell, J.H. 2007. Analysis of western redcedar (Thuja plicata Donn) heartwood components by 

HPLC as a possible screening tool for trees with enhanced natural durability. Journal of Chromatographic

Science. 45(5): 281–285.

Kimball, B.A.; Russell, J.H.; Ott, P. 2012. Phytochemical variation within a single plant species influences

foraging behavior of deer. Oikos. 121: 743–751.

Kope, H.H.; Trotter, D. 1998. The use of degree days to establish biological events for Didymascella thujina, 

a foliar fungal leaf blight of Thuja plicata seedlings. Proceedings of the 7th International Congress of Plant 

Pathology. Oxford, UK: Elsevier.

Maclean, H; Gardner, J.A.F. 1956. Distribution of fungicidal extractives (thujaplicin and water-soluble 

phenols) in western red cedar heartwood. Forest Products Journal. 6(12): 510–516.

Morris, P.I; Stirling, R. 2011. Western redcedar extractives associated with durability in ground 

contact. Wood Science and Technology. DOI: 10.1007/s00226-011-0459-2.

Russell, J.H.; Burdon, R.B.; Yanchuk, A.D. 2003. Inbreeding depression and variance structures for height 

and adaptation in self- and outcross Thuja plicata families in varying environments. Forest Genetics. 10(3): 

171-184.

Russell, J.H.; Daniels, B. 2010. Variation in western redcedar heartwood extractives. In: Harrington, C., (ed.). 

A tale of two cedars – international symposium on western redcedar and yellow-cedar. Gen. Tech. Rep. 

PNW-GTR-828. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station: 83–86.

Russell, J.H.; Ferguson, C.F. 2008. Preliminary results from five generations of a western redcedar (Thuja 

plicata) selection study with self mating. Tree Genetics and Genomes. 4(3): 509–518.

Russell, J.H.; Kopes, H.; Ades, P.; Collinson, H. 2007. Genetic variation in Didymascella thujina resistance 

of Thuja plicata. Canadian Journal of Forest Research. 37(10): 1978–1986.

Vourc’h, G.; Russell, J.H.; Martin, J.L. 2002. Linking deer browsing and terpene production among genetic 

identities in Chamaecyparis nootkatensis and Thuja plicata (Cupressaceae). Journal of Heredity. 93(5):

370–376.

Wallis, C.M.; Huber, D.P.W.; Lewis. K.D. 2011. Ecosystem, location and climate effects on foliar secondary 

metabolites of lodgepole populations from central British Columbia. Journal of Chemical Ecology. 37: 607–

621.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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The New Zealand Douglas-fir Breeding Program:

Proposed Adjustments for a Changing Climate

Heidi Dungey,1 Charlie Low,1 Mark Miller,1 Kane Fleet,1 and Alvin D. Yanchuk1

Abstract

Genetic improvement of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in New Zealand was initiated in 

1955 with large provenance trials established in the late 1950s. These trials showed that material of Oregon and 

Californian origin was growing faster than other provenances. Additional collections were made to further 

evaluate provenance performance from these two areas, and in 1996 additional trials were established at four 

low-altitude sites across New Zealand.

Genotype ×environment (GxE) interaction between these sites was found to be important for diameter at breast 

height (DBH), less important for stem straightness and malformation, and not important for outerwood acoustic 

velocity (a surrogate for wood stiffness). Heritabilities were low to moderate for all growth traits, and very low 

for malformation. Heritability for Swiss needle cast caused by Phaeocryptopus gaeumannii, measured as needle 

retention, was moderate at one site (0.37) with a high infection rate, and was likely a major factor creating GxE

interactions for growth among sites. The heritability of wood acoustic velocity was moderate to high at 

individual sites (0.26 to 0.74) and across sites (0.49). Individual-trait selection revealed the potential for good 

genetic gains to be made when selecting the top 20 families for diameter growth (an average of 10.7 percent), 

straightness (an average of 11.5 percent), and acoustic velocity (an average of 7.0 percent).

When we examined how gains were distributed when selecting for needle retention and/or DBH, we found that 

selecting for needle cast at the affected site did not compromise DBH gains at that site. Selecting for genotypes 

with low needle cast at the affected site did, however, effect gains for DBH estimated across all sites and would 

not be an ideal scenario. In order to maximize gains across the current Douglas-fir growing estate, a division of 

growing sites between those affected by needle cast and those not affected would seem sensible, particularly 

given recent work into climate change scenarios indicating that Swiss needle cast will become more important 

in the South Island and even more destructive in the North Island of New Zealand. We therefore suggest 

addressing differences in site through the development of separate deployment populations.

 1 Scion, Te Papa Tipu Innovation Park, 49 Sala Street, Private Bag 3020, Rotorua, 3046, New Zealand.

Corresponding author: Heidi.Dungey@scionresearch.com.
GENERAL TECHNICAL REPORT PSW-GTR-240

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White Pine Blister Rust Resistance Research in 

Minnesota and Wisconsin

Andrew David,1 Paul Berrang,2 and Carrie Pike3

Abstract

The exotic fungus Cronartium ribicola causes the disease white pine blister rust on five-needled pines 

throughout North America. Although the effects of this disease are perhaps better known on pines in the 

western portion of the continent, the disease has also impacted regeneration and growth of eastern white pine 

(Pinus strobus L.), especially in the upper Great Lakes region of Minnesota, Wisconsin, Michigan, and Ontario. 

This paper summarizes some of the early white pine blister rust research in Minnesota and Wisconsin, 

particularly the Moose Fence site near Tofte, Minnesota, the consistently high resistance level of genotype 

P327, and how the Minnesota Tree Improvement Cooperative (MTIC) housed at the University of Minnesota

and the USDA Forest Service Oconto River Seed Orchard (ORSO) are working together to advance the state of 

blister rust resistance in eastern white pine.

Introduction

At the turn of the 20th century, the exotic fungus Cronartium ribicola was brought to North America 

on seedling stock imported from Europe. Although there were multiple introductions across the 

continent, the results were similar; five-needled pines could become infected and die wherever the 

fungus entered an ecosystem that contained these pines and Ribes species that could act as an 

alternate host (Van Arsdel and Geils 2011). Forest pathologists came to recognize the life cycle of 

Cronartium ribicola or white pine blister rust in its new environment and combating the disease took 

on a two-pronged approach. One option was to physically remove individual Ribes plants. This 

method was cumbersome as it required a large, committed workforce to survey vast acreages on a 

repeated basis. The second option was to search for genetic resistance to the new disease in the native 

population.

One of the first plant pathologists to search for and find eastern white pine trees with putative 

resistance to white pine blister rust was A.J. Riker at the University of Wisconsin. By the mid-1930s,

Riker recognized that, while most young white pines were susceptible to the new disease, there were 

rare individuals (roughly 1 in 400) that remained healthy. In the late 1930s, he identified 163 young 

white pine trees in Wisconsin that exhibited no external signs of infection despite coexisting with 

Ribes and the blister rust fungus for at least 15 to 20 years (Riker and Kouba 1940). In the early 

1940s, Riker and his team artificially inoculated the progeny of these trees in a nursery setting and 

found differences between the offspring of selected and unselected trees (Riker et al. 1943). Although

not complete resistance, this research was among the earliest evidence of natural occurring genetic 

resistance in eastern white pine and it offered hope that a genetic basis for full resistance was 

possible. These results also spawned the development of an active rust research group in the Lake 

States that included Cliff and Isabel Ahlgren at the Quetico-Superior Wilderness Research 

Foundation, Bob Patton at the University of Wisconsin, and Gene Van Arsdel also of the University 

of Wisconsin and U.S. Department of Agriculture, Forest Service (USDA FS), as well as many more 

 1 Department of Forest Resources, University of Minnesota, North Central Research and Outreach Center, 1861 Highway 

169 East, Grand Rapids, MN 55744. 2 USDA Forest Service, 626 E. Wisconsin Avenue, Milwaukee, WI 53202. 

3 Department of Forest Resources, University of Minnesota, Cloquet Forestry Center, 175 University Avenue, Cloquet, MN 

55720. 

Corresponding author: adavid@umn.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

47

applied state and federal employees. The methodology established by Riker also set a precedent for 

the rust screening efforts by the USDA FS in Wisconsin and in the western states.

However, the efforts of these researchers notwithstanding, a high level of heritable resistance 

remained elusive in eastern white pine. Unlike the western five-needled pines where outdoor nursery 

screening techniques identified a stably inherited source of major gene resistance (MGR), time and 

time again eastern white pine researchers found increased levels of resistance, but not complete 

resistance. Due to the apparent lack of MGR in eastern white pine, researchers began to see the need 

for a large-scale, long-term experiment that would expose the most promising genotypes to a high 

level of fungal inoculum. Such an experiment, if allowed to continue long enough, would create a 

population of eastern white pine with a higher than average level of genetic resistance.

Moose Fence Planting Near Tofte, Minnesota

In the 1960s, Cliff and Isabelle Ahlgren proposed a large-scale, long-term experiment and began 

collecting cones from putative rust resistant white pine along the north shore of Lake Superior and in

the Boundary Waters Canoe Area Wilderness. By the late 1960s, the Quetico-Superior Wilderness 

Research Foundation, the USDA FS, and the University of Minnesota had entered into an agreement 

to establish an eastern white pine blister rust disease garden trial in a high blister rust risk area. The 

goals of the project were to provide a source of eastern white pine with higher than average rust 

resistance that could be used as a source of seed for reforestation purposes, and could serve as a 

source of advanced generation breeding material. This idea came to fruition when a total of 43,176

open pollinated 3-year-old eastern white pine seedlings from 873 families were planted in two stages 

(1972 and 1974) at a site near Tofte, Minnesota. The 873 mother trees came from 22 sites in two 

general areas in northern Minnesota: the Boundary Waters Canoe Area Wilderness and the north 

shore of Lake Superior, with a 23rd site in northern Wisconsin (Merrill et al. 1984). The 8.9 ha

“Moose Fence” site was fenced to keep out moose (Alces alces) (hence the name) and interplanted 

with various Ribes species, the obligate alternate host, to increase the spore load and the chance for 

disease incidence.

In the fall of 1984, after these seedlings had spent 11 or 13 years in the field (14 or 16 years from 

seed), each tree in the 10 tree row plots was scored for survival and evidence of previous infection by 

white pine blister rust. The results of this initial screening indicated that overall survival ranged from 

38 percent (1972 planting) to 41 percent (1974 planting). The frequency of seedlings with no 

evidence of previous infection was very low at 1.0 percent and 0.5 percent (1972 and 1974 

respectively). Furthermore, in the 1972 cohort, the two best families had 14 percent non-infected 

seedlings while the two best families in the 1974 cohort had a 6 percent non-infected rate. This 

information suggested two things: the frequency of uninfected individuals occurred at a very low 

frequency in the natural population, and the data do not support the existence of a major gene for 

resistance (MGR) among the maternal parents (Merrill et al. 1984).

Subsequent to the 1984 assessment, surviving trees were evaluated for vigor and rust and 888 noninfected trees with high vigor were selected and permanently tagged with aluminum tags on nylon 

rope tied at the base of a lower branch. These 888 non-infected trees represented 2.1 percent of the 

total number of seedlings planted and were two to four times higher than the estimate of non-infected 

seedlings from the 1984 assessment. This difference is likely the result of stringent initial scoring in 

1984 which used the terminology, “no evidence of previous infection” before a seedling was 

considered non-infected. This would have resulted in seedlings with marks on stems or branches 

(potential hail or insect damage) being left out of the non-infected category. Additionally, when the 

trees were tagged, their larger size relative to the 1984 scoring undoubtedly made visualization of the 

entire bole and all stems more difficult, potentially resulting in missed infections. 

After the 1993 growing season, at approximately 20 years of age, the tagged trees at the Moose 

Fence site were scored for survival, vigor, and evidence of rust. Rust incidence was scored on a 1 to 4 

scale, with 4 representing a tree free of rust, 3 a tree with inactive canker(s), 2 a tree with active 
GENERAL TECHNICAL REPORT PSW-GTR-240

48

canker(s), and 1 indicating a tree dead from rust. Vigor was scored on a 1 to 4 scale with 4 being the 

most vigorous and 1 representing dead. Only 802 of the 888 tagged trees could be found because, at 

roughly 20 years of age, a combination of factors were rendering the monumentation in the planting 

difficult to use. The serpentine layout of the 10 tree family plots, small size of the aluminum stakes, 

considerable mortality, and non-linear rows due to 1.5 m by 1.5 m spacing on shallow, rocky soils, all 

contributed to making navigation in the plantation extremely difficult. Of the 86 missing trees, 67 

were never found, while 19 trees were found in future assessments. These 67 lost trees represent trees 

that either died or lost their tags due to breakage or self-pruning of their lower branches where the 

aluminum tags were attached. Results of this 1993 scoring indicated that 747 (93.1 percent) of the 

802 assessed trees were uninfected while 4.6 percent had inactive cankers and 2.2 percent had active 

cankers. There were no dead trees among the 802 assessed (table 1), although there may have been 

dead trees among the 67 that could not be found.

Table 1—Number of select eastern white pine trees by rust score category (4 = uninfected, 3 = 

inactive canker(s), 2 = active canker(s), 1 = dead) and average rust score at the 20, 30, and 37 

year assessmentsa

Rust Score Rust score 

Year Age 4 3 2 1 Not found Average N =

<1993 11-19 years 888 888

1993 20 years 747 37 18 0 86 3.9 802

2003 30 years 516 191 95 0 0 3.5 802

2010 37 years 442 55 255 47 3 3.1 802

2003 30 years 552 194 97 0 0 3.5 843

2010 37 years 465 59 266 48 5 3.1 843 a 888 trees were initially selected and tagged sometime between 1984 and 1993 of which only 802 could be found in 1993. 

The 843 trees in 2003 and 2010 represent the 802 tagged trees plus the inclusion of 32 uninfected trees likely among the 86 

lost trees in 1993 and 9 trees found from the original 888.

Prior to the 30 year assessment in 2003, an additional 32 non-infected trees (rust score = 4) were 

tagged permanently with numbers from 900 to 931. These trees were most likely from the group of 67 

trees that had lost their original tags prior to the 1993 assessment. The remaining 35 trees from the 

group of 67 were unaccounted for and were either infected and not tagged or died of unknown causes, 

most likely blister rust. The 2003 assessment recorded rust scores on the same 1 to 4 scale for 843 

trees (table 1). The percentage of uninfected trees dropped approximately one-third between age 20 

and age 30. Despite this drop in the average health of the population, it should be noted that the

majority dropped just one classification, while approximately 70 trees dropped two classifications.

None of the tagged individuals died between the 20 and 30 year assessments.

Despite the lack of mortality among tagged trees, by age 30 it was obvious that blister rust was

actively working in the planting. Large, standing dead trees could be seen throughout the plantation 

while other trees had fantastically contorted trunks due to multiple cankers. The frequency of tagged 

trees in the plantation appeared to be increasing over time as non-tagged trees died at a faster rate.

Surprisingly, some trees that appeared to be growing vigorously were infected, but had not been 

tagged. Presumably these trees were infected before the tags were put on the original 888 uninfected 

trees. Therefore, these vigorous but infected trees had existed with the disease longer than any of the 

selected tagged trees and, borrowing the parlance from western North American breeding programs,

we began to think of these trees as potential “slow rusters”.

In 2010, a complete census of the plantation was performed, placing the remaining live trees into 

one of three categories: 1) previously tagged, 2) not previously tagged, with cankers but appearing 

healthy, and 3) not previously tagged, with cankers and appearing unhealthy and/or likely to die 

within the next year. The census results indicated that there were a total of 1,274 trees alive after 37 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

49

years, a survival rate of 2.9 percent. The 1,274 surviving trees were categorized as follows: 794 were 

previously tagged, 346 had cankers but appeared otherwise healthy, and 134 were cankered, appeared 

unhealthy, and/or were likely to die within the next year (table 2).

Table 2—Thirty-seven year census results for the Moose Fence eastern white pine blister rust 

trial near Tofte, Minnesota

Category Description Number alive

1 Previously selected 794

2 Possible slow-rusting genotypes 346

3 Severely cankered or likely to die within 1 year 134

Total 1,274

After the 2010 census, all the category 1 trees and the best of the category 2 trees were evaluated 

for vigor (very good, good, fair, poor), rust score (1 to 4 scale as previous), the number and type of 

cankers (round or linear), presence or absence of callus at the canker edge, and the percentage of the 

stem alive at the most infected point. This process identified 114 trees (84 category 1 trees and 30 

category 2 trees) that are considered potential slow rusters and will be followed over time. Of these 

114 potential slow rusters, 52 were selected based on percent bole alive (> 80 percent), with

preference given to linear vs. round cankers, two or fewer cankers, rust score = 3, and the subjective 

“this looks like a good slow ruster”. All selections had callus formation around the canker edges.

Scions from these 52 trees were collected and grafted in 2011 and were outplanted in spring 2012.

These 52 trees represent 43 category 1 trees and 9 category 2 trees.

The trial at Tofte has provided some very valuable long-term research results. It is obvious from 

the rust score numbers in table 1 that a wave of mortality is working through the tagged trees. 

Because most of these trees are dying due to cankers that are located within 3.0 m of the ground 

(personal observation), we know that these trees were at least 3.0 m tall before they were infected.

Since it is taking at least 10 to 20 years for them to die from blister rust, there is a period of time 

where the tree is still functional while the pathogen is a) moving through the branch towards the bole 

or b) latent in the branch and/or bole. As the remaining trees have few lower branches to become 

infected and the incidence of blister rust infection is partially ontologically controlled (Patton 1961),

we expect that the rate of new infections will decrease in the future. The health of currently infected 

trees, whether the infection is latent or not, is expected to decrease over time. The census indicated 

that after 37 years, 2.9 percent of the trees were still alive, although that is expected to drop to 2.6

percent by the 40th year in the field as the category 3 trees succumb to the disease. In retrospect, this 

2.6 percent survival rate is remarkable considering there is no evidence for MGR in the population, 

the trial was established in a high blister rust risk area, and Ribes spp. were interplanted among the 

seedlings. 

By comparing survival rates between selected trees and the population as a whole it is clear that 

early selection for uninfected trees with high vigor during the second decade was successful in this 

population. The initial census of the trial (Merrill et al. 1984) indicated total survival was 

approximately 40 percent at 12 years. After 37 years, total survival was down to 2.9 percent with the 

selected trees representing 62.3 percent of the survivors. Furthermore, the fact that the selected trees 

had an 89.4 percent survival rate (794 of 888 trees) at age 37 and that 52.7 percent of the selected 

trees (468 of 888) were still considered uninfected roughly 20 years after selection, is additional 

evidence that early selection was effective in this population.

After close to 40 years of constant selection under real world conditions, it is clear that the Moose 

Fence planting has tremendous practical value. The remaining trees represent the largest genetically 

diverse source of potentially resistant eastern white pine anywhere in North America and most likely 

the world. As such, Moose Fence is a unique site that provides locally adapted seed and genotypes 

with increased genetic resistance to white pine blister rust that can be used for reforestation purposes 

and advanced generation breeding. 
GENERAL TECHNICAL REPORT PSW-GTR-240

50

The Genotype P327

The Moose Fence planting at Tofte is not the only source of genetic material being used in Minnesota 

or Wisconsin. Dr. Patton at the University of Wisconsin oversaw a large white pine blister rust 

selection and screening program. He and his team scoured woods and plantings throughout 

Minnesota, Wisconsin, and Michigan for potential rust resistant trees and tested their scion and open 

pollinated progeny at several locations throughout Wisconsin. The most famous of these Patton 

selections is P327 which was identified in a planting of unknown origin outside of Duluth, 

Minnesota. Because P327 is consistently one of the last families to reach 25 percent, 50 percent, and 

75 percent mortality levels in greenhouse screening trials, it is often used as a positive control.

Jurgens et al. (2003) used histological techniques to identify a potential mechanism of resistance 

among a portion of open pollinated P327 seedlings. Using various stains he was able to differentiate 

the fungal mycelium from mesophyll cells as the hyphae moved into and through the needle of 

infected white pine seedlings. Figure 1 compares infected needles from progeny of P327 and a 

susceptible control H109. What is clear from the figure is that mesophyll cells in P327 collapse just 

ahead of the fungal hyphae slowing down the spread of the fungus which requires live cell tissue to 

survive. Conversely, needles of the very susceptible H109 are packed with mycelium allowing the 

hyphae to progress through the needles to twigs and branches.

More recent research (Smith et al. 2006) has shed some light on stomatal status in P327.

Environmental scanning electron microscopy (eSEM) images demonstrate that needles of P327 have 

occluded stomates (fig. 2). These occlusions were shown to be similar to the waxes that envelope the 

needles and are hypothesized to act as a physical barrier that prevents fungal hyphae from entering 

the needles. In theory, the more susceptible clones like H111 lack occluded stomates, allowing blister 

rust hyphae an opportunity to enter the needles following spore germination on the needle surface.

 

Figure 1—Needle sections stained with periodic acid-Schiff’s reagent reveal (A) lysed 

mesophyll cells ahead of blister rust hyphae in an open pollinated seedling of P327, and 

(B) densely packed mycelium surrounding the vascular bundle in an open pollinated 

seedling of the more susceptible H109.

A B
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

51

 

Figure 2—Environmental scanning electron microscopy image of secondary needles 

showing (A) open stomates in susceptible H111, and (B) wax occluded stomates in 

more resistant P327.

USDA Forest Service Oconto River Seed Orchard

The USDA FS’s Oconto River Seed Orchard (ORSO) near Langlade, Wisconsin serves as the source 

of seed for white pine regeneration efforts in Region 9. The bulk of this seed comes from an interim 

seed orchard comprised of 30 Heimberger and five Patton selections. Future plans call for the 

establishment of at least one new clonal seed orchard of local origin where the increased resistance to 

white pine blister rust is well documented. At ORSO there is also a grafted clone bank (roughly 1,200

selections) from northern Minnesota, northern Wisconsin, and the upper peninsula of Michigan.

Clonal material of non-infected and slow rusting Tofte genotypes has been kept separate to facilitate 

tracking known origins.

One of ORSO’s mandates is to screen for blister rust resistant white pine. This is accomplished

using an indoor system that was refined (starting in 2004) using suggestions from Paul Zambino

(research pathologist; formerly at the USDA FS Rocky Mountain Forest Research Station). A typical 

example would use open pollinated seedlings with a mixture of primary and secondary needles from 

mother trees in the orchards or clone bank and expose them to blister rust spores at a spore load of 

approximately 10,000 spores/cm2

. The seedlings are sown in the greenhouse in February or March 

and inoculated in November of the first year. In May of the second year they are scored for percent of 

seedlings with foliar symptoms and the severity of symptoms before being placed outside. In midsummer of the second year, the seedlings are scored for the percent of seedlings with stem cankers 

and their severity. Ideal families would have many seedlings with foliar symptoms, indicating 

exposure and initial infection, but a decided lack of stem symptoms indicating some level of 

resistance. The seedlings are followed for an additional 2 to 3 years while they are evaluated for 

evidence of resistance mechanisms, although at this time there is no formal, definitive end to the 

evaluation period. Initial work has shown that the 10,000 spores/cm2 appears to provide sufficient 

inoculum to produce foliar symptoms across all families without killing all seedlings. The 10,000 
GENERAL TECHNICAL REPORT PSW-GTR-240

52

spores/cm2 level also demonstrates a definite dosage effect associated with susceptible and resistant 

families. In the most recent results ORSO has identified four to five additional genotypes out of 34 

tested that show promise for elevated levels of blister rust resistance. 

University of Minnesota’s Tree Improvement Cooperative

The Minnesota Tree Improvement Cooperative (MTIC) is based on the industry-governmental 

organization-university model and is headquartered at the University’s Cloquet Forestry Center. It is 

comprised of 14 full members and six supporting members who focus their efforts on five conifer 

species native to Minnesota, including white pine. The MTIC’s white pine program has been active 

since the mid-1980s. Among the cooperators with full membership, there are five 1st generation 

clonal seed orchards totaling 3.4 ha. These seed orchards are comprised of genotypes selected by 

earlier researchers for increased blister rust resistance in nursery and early field trials. Sources 

include: USFS ORSO, University of Wisconsin, and the Quetico-Superior Wilderness Research 

Foundation. The MTIC has been collecting open pollinated cones from individuals in these seed 

orchards and at Tofte in anticipation of establishing a progeny trial in 2014.

The MTIC also manages two other long-term white pine plantings. One site, the Moose Fence trial 

mentioned earlier, is jointly managed with the USDA FS, Superior National Forest, Tofte Ranger 

District while the second planting is a white pine breeding arboretum located at the Cloquet Forestry 

Center in Cloquet, Minnesota. As of August 2011, the white pine breeding arboretum has three 

ramets each of 214 Tofte genotypes and 94 non-Tofte genotypes; primarily selections from Minnesota 

and Wisconsin. In the spring of 2012, grafts of the 52 potential slow rusting genotypes from Tofte 

were planted at Grimsbo Field, increasing the total to over 350 individual genotypes in the breeding 

program. Current plans are to breed these selections both for testing increased resistance using the 

ORSO method and to establish more traditional field based progeny tests. 

In summary, MTIC and ORSO have programs that have benefited from previous white pine blister 

rust research. As a result of this early work, both organizations have been able to establish seed 

orchards and provide their stakeholders with eastern white pine seed for reforestation needs. Early 

selections such as P327, which consistently produces progeny that outlive other families in blister rust 

screening trials, have formed the bulk of this early research. However, recognition is growing that, to 

support white pine reforestation efforts across Minnesota and Wisconsin, there is a need for locally 

adapted germplasm of known origin and more genetic diversity than existed in the early selections.

The value of the Moose Fence site at Tofte is that over the past 40 years there has been active 

selection for increased blister rust resistance in a population that is genetically diverse and adapted to 

growing conditions that are found in northern Minnesota and northern Wisconsin. As a result, both 

the MTIC and ORSO have been working cooperatively to critically evaluate individual trees within 

Moose Fence and determine the genetic value of both uninfected and slow rusting phenotypes.

Acknowledgments

The authors thank Jim Blanchard, Kathy Haiby, Egon Humenberger, Sam Krueger, Carrie Sweeney, Jim 

Warren, and Keith Webb for assistance in the field at Moose Fence; Jim Warren specifically for the GIS map of 

Moose Fence selections; and Bro Kinloch for thoughtful comments in the field on identifying slow rusting 

phenotypes. Recognition and thanks also to Minnesota Tree Improvement Cooperative, University of Minnesota 

Agricultural Experiment Station, University of Minnesota Department of Forestry, USDA Forest Service Gene 

Conservation Program, USDA Forest Service Region 9, and the Hubachek Wilderness Research Foundation 

(formerly the Quetico-Superior Wilderness Research Foundation) for previous and continued support of 

research to decrease the impact of white pine blister rust on eastern white pine. This manuscript was improved 

through the thoughtful comments of Thomas Saielli and Joseph Zeleznik.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

53

Literature Cited

Merrill, R.E.; Mohn, C.A.; Ahlgren, C.E. 1984. Survival and white pine blister rust infection in a Minnesota 

white pine screening study. Misc. Journal Series Article No. 2049. St. Paul, MN: Univ. Minn. Ag. Exp. 

Station. 13 p. 

Patton, R.F. 1961. The effect of age upon susceptibility of eastern white pine to infection by Cronartium 

ribicola. Phytopathology. 51: 429–434. 

Jurgens, J.A.; Blanchette, R.A.; Zambino, P.J.; David, A.J. 2003. Histology of white pine blister rust in 

needles of resistant and susceptible eastern white pine. Plant Disease. 87(9): 1026–1030. 

Riker, A.J.; Kouba, T.F. 1940. White pine selected in blister rust areas. Phytopathology. 30(1): 20.

Riker, A.J.; Kouba, T.F.; Brener, W.H.; Byam, L.E. 1943. White pine selections tested for resistance to 

blister rust. Journal of Forestry. 41(10): 753–760. 

Smith, J.A.; Blanchette, R.A.; Burnes, T.A.; Gillman, J.H.; David, A.J. 2006. Epicuticular wax and white 

pine blister rust resistance in resistant and susceptible selections of eastern white pine (Pinus strobus). 

Phytopathology. 96(2): 171–177. 

Van Arsdel, E.P.; Geils, B.W. 2011. Blister rust in North America: what we have not learned in the past 100 

years. In: Fairweather, M., comp. Proceedings of the 58th Annual Western International Forest Disease 

Work Conference. Flagstaff, AZ: U.S. Department of Agriculture, Forest Service, Forest Health Protection.
GENERAL TECHNICAL REPORT PSW-GTR-240

54

Phenotypic Evidence Suggests a Possible MajorGene Element to Weevil Resistance in Sitka Spruce

John N. King,

1, René I. Alfaro,2 Peter Ott,1 and Lara vanAkker2

Abstract

The weevil resistance breeding program against the white pine weevil, Pissodes strobi Peck (Coleoptera: 

Curculionidae), particularly for Sitka spruce (Picea sitchensis (Bong.) Carr), is arguably one of the most 

successful pest resistance breeding programs for plantation forest species, and it has done a lot to rehabilitate 

this important western conifer. Nearly all planting stock currently comes from this breeding program and so far 

the resistance seems effective, durable, and stable. We have used this program to also study causes behind this 

resistance, including: various hindrance mechanisms including induced and constitutive resin cells, sclereid or 

stone cells, and terpene defenses. All of them appear factors in resistance, but none singly is strongly predictive 

to resistance - the strongest are sclereid cells. All of these factors are in their nature complex, multifaceted, and 

appear to offer some partial solution that is likely controlled by complex multigenic systems. We do note,

however, that we have very strongly expressed and complete resistance in some individuals. This and some 

preliminary data investigation indicates that there may also be a major gene element in our observed resistance. 

Such elements are well described against rusts and other pathogens in forestry, but are also well described for 

insects and nematodes in crop breeding. Particularly interesting is the Hessian fly in wheat which has a similar 

life strategy to the weevil. We describe here the elements that suggest this conjecture and how we might go 

about proving this. Understanding the genetic elements behind this observed resistance has implications for the 

overall strength and durability to resistance against the white pine weevil.

Keywords: Pissodes strobi, white pine weevil, Sitka spruce, resistance breeding program

Introduction and Background

White pine weevil (Pissodes strobi Peck) is one of the most devastating pests of young spruce (Picea

spp.) and pines (Pinus spp.) in North America. The weevil is a native insect that occurs across Canada 

and the northern United States. In eastern North America, it is a major pest of eastern white pine 

(Pinus strobus L.) and introduced Norway spruce (Picea abies (L.) Karst.), but in the west it mainly 

attacks spruce species; Sitka spruce (Picea sitchensis (Bong.) Carr.) is particularly susceptible. Adult 

weevils lay eggs in the bark on tree leaders, emerging larvae mine down under the bark, consuming the 

phloem, severing the cambial layer, girdling and eventually killing the leader. Damage is so severe 

that young plantation trees often become stunted and bushy as terminal leaders are repeatedly killed 

and young trees fail to achieve apical dominance. Because of the weevil, Sitka spruce has been 

avoided as a re-forestation species in many parts of its former range.

A resistance breeding program initiated in the early 1990s is relatively unique for insect pests in 

forest trees, but it has been successful and has done a lot to rehabilitate this important western conifer 

(Alfaro and King these proceedings, King and Alfaro 2009). Nearly all planting stock currently 

 

1 British Columbia Ministry of Forests and Range, Research Branch, P.O. Box 9519, Stn. Prov. Govt., Victoria, British 

Columbia, Canada V8W 9C2. 2 Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, 506 W. Burnside Rd., Victoria, British 

Columbia, Canada V8Z 1M5.

Corresponding author: King.forgen@gmail.com.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

55

comes from this breeding program and, to date, the resistance seems effective, durable, and stable.

We have also used this program as a case study for mechanisms of resistance in conifers against shoot 

infesting insects (Alfaro et al. 2002). To date, various hindrance mechanisms including: induced and 

constitutive resin cells, sclereid or stone cells; and terpene defenses have been studied (Alfaro 1995,

Alfaro et al. 1997, King et al. 2011, McKay-Byun et al. 2006, Robert et al. 2010). All of these appear 

factors in resistance, but none singly is strongly predictive to resistance - the strongest to date appears 

to be sclereid or lignified stone cell (King et al. 2011). All of these factors are in their nature complex,

multifaceted, and appear to offer some partial solution that is most likely controlled by complex 

multigenic sytems. We do note, however, that we have very strongly expressed, complete, or total 

resistance in some individuals. 

Major gene, or R-gene, resistance is characterized as “total resistance” and is well documented for 

pathogens in forest trees (Kinloch et al. 2007, Kubisiak et al. 2005), but is also well characterized for 

insects in many crop plants, especially phloem feeding insects such as is the white pine weevil 

(examples include aphids, leaf hoppers, and fly larvae). Particularly interesting are the midges, such 

as the Hessian fly in wheat, which, as a phloem feeder (Harris et al. 2003), has a similar life strategy 

to the weevil. 

In this paper, we review the evidence to date for the hypothesis of “total resistance” in the spruce -

insect pest system. We present features associated with this type of resistance and describe what we 

know about these highly resistant individuals. We suggest methods of confirming this type of 

resistance and discuss the implications of deploying major gene resistance.

Marked Distribution of Resistance in Native Populations

Early provenance trials indicated strong geographic regions of resistance: the hybrid area of northwest 

British Columbia and two provenance sources within the high hazard region of southwest British 

Columbia (Haney and Big Qualicum) (Ying 1991). Screening included clonal selections of 

individuals from these two provenances as well as a series of open-pollinated (OP) collections from 

this high-hazard region (King and Alfaro 2009). Results from the first of these OP series indicated 

weevil resistance was not widely distributed through this region, but had a marked area of resistance 

close to the original provenances (King et al. 2004). Figure 1 incorporates results from all four of the 

OP family series trials and graphically demonstrates the distribution of naturally occurring resistance 

found in the high-hazard zone of southwest British Columbia. The dots indicate provenance sources 

where trees were sampled in this region. Three to 30 trees per source were included (with over 500 

families screened overall) – the relative resistance of these sources is shown by color coding with red 

expressing higher resistance and blue susceptible. The strongly defined resistance boundaries for the 

Fraser Valley (Haney) and East Vancouver Island (Big Qualicum) regions are evident. They 

transition over relatively short distances to susceptible regions to the north (Salmon River and Amor 

de Cosmos, North Vancouver Island), and south (Duncan and Saanich, South Vancouver Island, and 

Puget Sound, Washington). 
GENERAL TECHNICAL REPORT PSW-GTR-240

56

Figure 1—Resistant provenances by mean annual attack (MAA) level and zones of high and low 

natural resistance; MAA is expressed as a percentage. The resistant region is shown in red, the 

susceptible region in blue shades (intermediate in yellow).

The strong geographic demarcation noted is also demonstrated clearly by the plots of the 

frequency distributions based on families from the first OP series families that most widely sampled 

this high-hazard zone (King et al. 2004). In fig. 2, the distribution f0 represents the hypothesis that 

there is one underlying distribution within the high-hazard zone (broad-based resistance). The 

trendline, fR, represents the families from the resistant Big Qualicum sources, and fS represents 

families from other sources within the high-hazard zone, but outside the resistant (red) region 

described in fig. 1. The trendline, fC, represents the combined distribution of fR and fS, and fits 

significantly better than the single distribution f0, with a likelihood ratio statistic of 42.5 (P <0.0001).

Mean annual attack rates of the Haida Gwaii (Queen Charlotte Islands) families used also in this first 

OP series range from 0.28 to 0.40 (the archipelago is free of the weevil), the resistant Big Qualicum 

families (range 0.05 to 0.23) did not overlap at all. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

57

Figure 2—Frequency distributions of weevil attack. f_0 represents the null hypothesis that highhazard mean families belong to one distribution. f_q depicts families from the high resistant Big 

Qualicum source ( fig. 1). f_s depicts families from the other sources in the high hazard zone. f_c 

shows the combined frequency distribution (f_q + f_s). Bars show frequency of actual data points −

the red shaded are those for f_q and blue for f_s.

Observation of Segregation within Families

Besides analyzing the distribution of family means within the high-hazard zone, within family 

segregation can also offer insights. Simple statistical evaluation techniques to detect segregation

within families have been widely used especially in human and animal genetics (Fain 1978, Uimari et 

al. 1996). Such techniques have recently also been used in forest trees (Zeng and Li 2003). These 

tests assume that the presence of major genes with large effects will inflate the variance within the 

families in which the genes are segregating. This segregation may also cause non-normality of the

phenotypic data, producing significant skewness and/or kurtosis. One method to explore this withinfamily phenotypic effect is to regress the offspring variance on the sib or family means (Fain 1978).

With no inbreeding there should be no relationship between the family variance and its phenotypic 

value in traits influenced by many genes with small effects. When a character is controlled by genes 

with large effects, individuals with the most extreme phenotypes are likely to be homozygotes 

(extreme means and low variance), whereas intermediate phenotypes are likely to be heterozygotes 

(intermediate means and high variance). This quadratic (concave down) relationship can be described 

by a polynomial regression of offspring variance on phenotypic value:

( ) 2 var i 0 1 i 2 i z = β + β z + β z (1)

Where var (zi) is the phenotypic variance within the i

th offspring family, and zi is the family mean 

(Fain 1978). 

0

2

4

6

8

0.00 0.08 0.14 0.20 0.26 0.32 0.38 0.44 0.50

Number of families

Estimated annual attack rate r

f0

fQ

fS

fC
GENERAL TECHNICAL REPORT PSW-GTR-240

58

This analysis is designed for continuous normally distributed data, and although major gene 

segregation will distort the normal distribution, this test is considered robust to non-normality of the 

data. Our data (the number of attacked leaders divided by number of years of observation) follows 

more correctly a Poisson distribution, characterized by a single parameter where the variance equals 

the mean. Specific study of this variance requires a generalization, so an overdispersed Poisson 

distribution (McCullagh and Nelder 1989) was fitted to the 20 observations within each family. Under 

this distribution, the expected number of attacked leaders per observation is ( )i i E X = rt where i x

denotes the number of attacked leaders over ti years for the i

th plot, and r denotes the annual attack 

rate for the family. The variance is ( )i i Var X = φ ⋅rt , where φ represents an overdispersion 

parameter − the degree to which the nominal Poisson variance should be inflated or deflated as 

dictated by the data (i.e., φ = 1 indicates no overdispersion). The scaled residual deviance was used to 

estimate φ.

Results from fitting the Fain model to the estimated Poisson parameters rˆ and φ

ˆ are shown in fig.

3 for the resistant Big Qualicum families. Figure 3 shows a good, significant curvilinear fit (R2

= 

0.63, n = 23) to the quadratic term in the model (P <0.0001). This same model fit poorly to the 

susceptible families (R2 = 0.37, n = 44), and the quadratic term was not significant (P = 0.5389).

Using simply the average attack rate and (log) variance as in Equation (1) instead of the Poisson 

distribution, a curvilinear regression was also highly significant for the Big Qualicum families (R2

= 

0.86, P <0.0001 for the quadratic term), and the susceptible families again fit poorly (R2 = 0.02 and P 

= 0.5377 for the quadratic term).

Figure 3—Plot of Big Qualicum families and fitted quadratic (Fain 1978) model (R2 = 0.63, n = 23).

Overdispersion and annual attack rate are estimated from an overdispersed Poisson distribution.

Studies of Strongly Expressed Phenotypic Resistant Individuals

A series of studies focused on gaining insight into weevil resistance has been conducted on one 

particular tree, parent clone 898, since this highly resistant individual was first noticed at the Sayward 

0.65

0.80

0.95

1.10

1.25

0.025 0.075 0.125 0.175 0.225

Estimated annual attack rate r

Estimated overdispersion 

φ
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

59

(Bigtree Creek) trial (Alfaro and Ying 1990, King and Alfaro 2009). This individual has been clonally 

replicated in many field trials, archives, and nursery experiments. Over a decade of observations have 

been made in screening trials containing close to 100 898 clonal ramets, and in this time only two 

trees of this clone have been noted to have leader kill – this compared to observations of leader kill in 

100 percent of individuals, many times repeatedly, in many other clones (King and Alfaro 2009). In 

addition to being weevil resistant, the 898 clone is easily identified in a plantation by its exceptional 

growth form. Its vigorous growth produces long, straight internodes that typically cause the tree to 

stand out among its neighbors and make it highly desirable for forestry (fig. 4).

Figure 4—Clonal blocks of 898 (and other immune clone 1210) with susceptible seedlots in 

foreground. At this site (Hisnit – King and Alfaro 2009) under very heavy attack, not a single 898 

individual was successfully attacked.

In efforts to gain insight into the resistance strategy of the 898 clone, feeding and oviposition 

behavior have been observed through caging experiments over the years (fig. 5). Where weevils had a 

choice, they preferred not to colonize 898. However, when caged on ramets of clone 898, weevils 

preferred not to feed on the leader, but rather on the one-year-old internode. Again, in caged (no 

choice) experiments, gravid females would lay eggs that hatched, but the larvae died at first instar and 

females that were not gravid did not produce eggs (Tony Ibaraki, personal communication).
GENERAL TECHNICAL REPORT PSW-GTR-240

60

Figure 5—Weevil caging studies showing some of the results of forced caging in different positions on 

the crown.

Studies of the resin canal systems of Sitka spruce revealed 898 has high bark constitutive resin cell 

density as well as high sclereid cell density, but most of the trees from the resistant region (East 

Vancouver Island – Big Qualicum or Fraser Valley – Haney) show this compared to individuals from 

the more susceptible regions. Interestingly, although 898 is high for both of these traits it is not ‘off 

the chart’ (King et al. 2011) which suggests other mechanisms may also be present. The traumatic 

resin response in Sitka spruce has not been thoroughly explored. However, observations to date

indicate that this trait does not seem to be as well expressed in Sitka spruce as it is in white spruce 

(O’Neill et al. 2002). This perhaps accounts for the particular susceptibility of Sitka spruce and why it 

acts in such an unstable manner to this endemic pest. Our work to date on traumatic resin with 898 

was inconclusive.

Various biochemical studies have been made of 898, but none so far have shown anything unique 

for this clone (McKay-Byun et al. 2006, Robert et al. 2010). One interesting hypothesis is that the 

resistant 898 clone inhibits the expression of the vitellogenin gene in weevils after they feed. 

Vitellogenin is an egg-yolk protein precursor necessary for the maturation of eggs in reproductively 

active female weevils (Leal et al. 1997). Weevils that fed on resistant ortets or cuttings of the 898 

clone had fewer eggs form in their ovaries, and ovaries of P. strobi females that fed on this clone 

were less likely to mature. Levels of ovarian growth and expression of the vitellogenin gene were also 

reduced in weevils that fed on severed leaders of the resistant 898 clone relative to those that fed on 

severed leaders of a susceptible family (Sahota et al. 2001). However, as we noted earlier, gravid 

females can successfully lay eggs in 898, although they would not prefer it.

It needs to be noted that 898 is not the only ‘highly resistant’ or immune individual, but it has been 

the most extensively studied. An F1 generation breeding program was instituted (King and Alfaro

2009) and some preliminary investigation has begun. Seven of the ten most resistant crosses in the F1

generation had 898 as either parent (Moreira et al. 2012). A full segregation analysis has yet to be 

conducted on the F1 generation, but it should be noted that these trials, primarily made with resistant x 

resistant crosses, have far fewer attacks, thus the difficulty with getting robust screening compared to

the 1st generation OP trials. It is also possible at this time to get to a backcross generation – this would 

really help us to confirm the segregation – which should be unambiguous compared to either the F1 or 

OP generation.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

61

Major Gene Resistance in Other Plant – Insect Systems

Some of the best documented insect resistance systems involving R genes are gall midges 

(Cecidomyiidae) in both wheat and rice. Harris et al. (2003) detailed the genetic and biochemical 

interactions between gall midge avr genes and plant R genes. In the wheat-Hessian fly system, close 

to 30 major genes in the host interact with various biotypes of the pest that display virulence to 

specific R genes in typical gene-for-gene fashion (Ratcliffe and Hatchett 1997). Although antibiosis 

appears to be the primary mechanism, often leading to the death of the first-instar larvae within a few 

days of establishment, the nature of this antibiosis is only beginning to be understood. Although 

hypersensitive responses (HR) have been reported in this system (Grover 1995) this may not be the 

only or even key defense response to these piercing/sucking type of insects (Kaloshian 2004). Harris 

et al. (2006) investigated with detailed microscopy the compatibility interactions between larvae and 

plant. In the compatible interaction in these gall-forming insects, a virulent larva promotes gall 

formation in the host. This gall does not have any noticeable macroscopic structure, but is defined as 

aberrant tissue structure resulting from stimulation from a foreign organism, in this case the fly larva 

(Harris et al. 2003). It had been proposed that injection of salivary substances by the larvae during the 

first instar might be part of the attack process (Hatchett et al. 1990, Ollerston et al. 1990) and piercing 

marks by larva mandibles were indicated in the microscopy investigation (Harris et al. 2006). It is 

hypothesized that avr gene products from the larva are secreted into epidermal cells and activate a 

galling process (Harris et al. 2003). Part of the ensuing aberrant tissue structure (in this compatible 

interaction) is enhanced protein synthesis combined with breakdown of cell walls – the cells rupturing 

with ‘nutritive tissue’ providing the larva with a diet rich in soluble amino acids and sugars (Harris et 

al. 2006). The virulence genes that aid the larva in successfully attacking a susceptible plant would 

act as avirulence genes in a plant defended by R genes (incompatible reaction) (Harris et al. 2003). In 

fact, it appears the interaction of R gene product and avr effectors can involve complex recognition 

and response interactions (Kaloshian 2004). In a defended plant this gall-induced nutritive tissue is 

not formed and the larva starves. It has been noted that when virulent and non-virulent biotypes 

colonize the same plant the nutritive tissue can be enough to also sustain the non-virulent larvae 

(Grover et al. 1989) which has implications for fitness and survival in the evolution of fly virulence 

(Harris et al. 2003).

This type of galling response, associated with nutritive tissue has been noted in many plant 

defense responses to insects (Rohfritsch 1992). There is also a phenotypic difference in the expression 

of these incompatible reactions between resistant genotypes (Harris et al. 2003). Classic HR may also 

be observed as part of a gall midge attack (e.g., Salix viminalis to the gall midge Dasineura 

marginemtorquens (Ollerston et al. 2002). But there appears to be substantial variability in the 

manifestation of R-gene mediated plant defenses in host-insect interactions. HR may be a feature but 

not always and different forms of limitation to phloem feeding have been observed (Kaloshian 2004). 

Summary and Conclusions

There are important hallmarks of large genetic effects for resistance to the weevil: marked geographic 

distribution, and some evidence of within family segregation. This does not necessarily indicate 

classic major gene or R- gene resistance and it could be explained by relatively few genes controlling

the hindrance traits, resin canals, and sclereid cells already investigated. Also, the induced or 

traumatic resin cell investigation that appears so effective in interior spruce has so far not been 

managed in Sitka spruce. Genotypes showing ‘total resistance’ in Sitka spruce with respect to the 

white pine weevil and indication of life-history parallels with similar phloem feeding insects – e.g. the 

starvation of first instar larvae manifested in R-gene defense in the Hessian fly, clearly though point 

to the need for follow-up investigations. Although investigative work to date has concentrated on one

particular cloned individual – other individuals have been identified but as yet have not been followed 

through in such detail. F1 breeding has been carried out with these individuals (resistant x resistant, 
GENERAL TECHNICAL REPORT PSW-GTR-240

62

resistant x susceptible, susceptible x susceptible) (Moreira et al. 2012) and it would be very feasible 

to make backcross segregating lines (with clonal emblings). Developing such segregating lines would 

certainly help in any further investigation and would set up a good experimental population in further

understanding the phenotypic and biochemical natures of this resistance. Molecular genetic surveys

based on linked molecular markers could also help in identifying the genetic architecture underlying 

resistant phenotypes. The molecular and proteomic signatures for R-gene resistance can be also be 

identified (Liu et al 2004) and structured experimental populations and structured experimental 

pedigrees would help in this investigation.

Although the genetic variation governing plant-insect interactions is usually observed as 

continuously variable, and most likely controlled by the segregation of multigenic quantitative trait 

loci, this does not preclude major genes as important mechanisms of resistance. Certainly for phloem 

feeding insects that attack crop plants, R genes are widely used as key plant defense mechanisms. We 

do know that long term stable and manageable resistance is possible by the nature of the resistance we 

see in white spruce in Eastern North America, where it is not a major problem and acts more like a 

classic endemic system. An understanding of the genetics behind this resistance is necessary to assess

the durability and effectiveness of the resistance found so far in Sitka spruce. 

Acknowledgments

We acknowledge the many people who have worked on this weevil resistance program over the decades in the 

British Columbia Forest Service, Canadian Forest Service, University of British Columbia, and Simon Fraser 

University; and the Forest Industries and Forest Genetics Council. We would like to thank Ward Strong and 

Alvin Yanchuk of the British Columbia Forest Service and Timothy Sexton of the University of British 

Columbia for their thoughtful reviews, and Richard Sniezko and organizers of this conference for providing a 

place to publish this valuable information.

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Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

65

Operational Program to Develop Phytophthora

lateralis-Resistant Populations of Port-Orford-cedar

(Chamaecyparis lawsoniana)

R.A. Sniezko,

1 J. Hamlin,2 and E.M. Hansen3

Abstract

Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murr.) Parl.) (POC) is a long-lived conifer native to

northwestern California and southwestern Oregon. It has been widely used in horticulture in both western North

America and elsewhere. The accidental introduction of the non-native pathogen Phytophthora lateralis into

North America and Europe has raised concerns about the future viability of POC in some forest ecosystems, in

managed forests, and in horticultural settings. Fortunately, some level of natural genetic resistance to P.

lateralis exists in POC. Utilizing this genetic resistance in reforestation and restoration of POC offers

considerable benefits without the negative side-effects of alternatives. In 1997, the USDA Forest Service and

USDI Bureau of Land Management began a large operational program to develop populations of POC with

genetic resistance to P. lateralis. With essential ongoing pathology support from Oregon State University, this

resistance program has advanced rapidly. The program goal includes developing orchards for production of

resistant seed, while maintaining genetic variation and adaptability within the species. Using classical selection

and testing techniques, over 12,600 initial field selections have been made, resistance screening protocols

refined, 13 breeding zones delineated, field trials established, eight seed orchards started, and resistant seed

produced and being used for some breeding zones. Early, short-term testing in the greenhouse of orchard

seedlots shows a 30 percent or higher survival than woods-run seedlots. In greenhouse testing, survival among

individual susceptible and resistant families varies from 0 to 100 percent, and at least two types of resistance are

apparent. Field trials have been established and early results are encouraging, but much longer monitoring is

needed to examine a full range of sites for efficacy and durability of resistance under different environments

and a changing climate. The recent finding of P. lateralis in Europe and Taiwan warrants further investigation

into the pathogenicity of these isolates on resistant POC. The biology of POC and the concerted efforts of all

those involved have made this fast moving resistance program one of the few involving non-native pathogens to

produce seed for restoration and reforestation. Planting the less susceptible seedlings will increase the frequency

of resistance in areas affected by P. lateralis and help to establish a new balance in affected ecosystems.

Key words: Port-Orford-cedar, Chamaecyparis lawsoniana, Phytophthora lateralis, resistance 

program

Introduction

Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murr.) Parl.) (POC) is a large, long-lived conifer

species native to southwestern Oregon and northwestern California (figs. 1 and 2) (Betlejewski et al.

2011; USDA FS and USDI BLM 2003, 2004; Zobel et al. 1985). It has also been widely utilized in

urban plantings in North America, Europe, and elsewhere, where it is often called Lawson’s cypress.

However, the introduction of a root disease, caused by the non-native pathogen Phytophthora

lateralis (Tucker and Milbrath), in North America and Europe has caused high mortality in some

forest ecosystems and has limited POC’s use in forest plantings and in horticulture (Betlejewski et al.

2011, Casavan et al. 2003, Robin et al. 2011, Sansford 2011, Zobel et al. 1985). Phytophthora

lateralis is spread primarily via water and soil. All sizes and ages of POC can be killed by P.

lateralis. POC is the only species in North America currently known to be highly susceptible to P.

 1 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon. 2 USDA Forest Service, Umpqua National Forest, Roseburg, Oregon. 3 Oregon State University, Department of Botany and Plant Pathology, Corvallis, Oregon.

Corresponding author: rsniezko@fs.fed.us.
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66

lateralis, although Pacific yew (Taxus brevifolia Nutt.) is occasionally killed (DeNitto and Kliejunas

1991).

Figure 1—Port-Orford-cedar (a) tree in urban area, (b) large, healthy tree in southern Oregon, (c)

dead trees in forest along a creek in southwest Oregon, (d) dead and dying trees along Highway 101

in southwest Oregon, (e) resistant parent tree (510015) surrounded by dead Port-Orford-cedar. 

(Photo credits: Richard Sniezko, a to d; Chuck Frank, e)

The native range of POC encompasses lands managed by many governmental and private

organizations and individuals. In this region, POC occurs widely on lands managed by the USDA

Forest Service (USFS) and USDI Bureau of Land Management (BLM). The USFS and BLM have

implemented an integrated strategy to manage POC ecosystems on their lands in the presence of P.

lateralis, including: measures to exclude the pathogen from areas not yet affected, sanitation of some

infested areas, and minimizing planting sites of highest hazard, such as wet, low-lying areas

(Betlejewski et al. 2011, USDA FS 2004, 2006; USDI BLM 2004). However, genetic resistance is a

species’ natural line of defense against potentially damaging biotic and abiotic agents, and resistance

to P. lateralis is necessary to retain POC in some ecosystems and to help ensure its continued use in

managed forests and horticulture. Fortunately, early work examining possible genetic resistance in

POC (Hansen et al. 1989, Sniezko et al. 2003) showed enough promise to begin an operational

program to develop resistant populations of POC. The production of seed from seed orchards

containing genetically-resistant parents offers land managers the opportunity to continue to use POC 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

67

in reforestation and restoration. Until recently, however, few operational programs have been 

undertaken to develop genetic resistance to a non-native pathogen and even fewer are currently 

producing resistant seed (FAO 2008, Sniezko 2006). In this paper, we provide an update on the 

substantial progress made since 1997 in the P. lateralis resistance program for Port-Orford-cedar.

Figure 2—Map showing the 13 breeding zones in the natural range of Port-Orford-cedar and the

geographic distribution of field selections being tested for Phytophthora lateralis resistance.

Overview

The USFS and BLM program to develop populations of Port-Orford-cedar with genetic resistance to

P. lateralis expanded to an operational level in 1997. Earlier work at Oregon State University (OSU)

and by the USFS and BLM focused on confirming the presence of genetic resistance, selecting the

first several hundred candidate trees displaying reduced susceptibility, refining screening techniques

for resistance, and developing tree improvement protocols necessary to implement an operational

program for this species. The USFS provides overall design for the resistance program and oversees

most operational phases, including coordination of field trial establishment, control pollinations for

breeding to enhance resistance, data analysis, and seed orchard development for producing resistant

seed for reforestation and restoration. Oregon State University provides essential pathology support,

including work on biology of the pathogen as well as establishment and monitoring short-term

resistance screening trials. The USFS and BLM provide genetics expertise, funding and personnel for
GENERAL TECHNICAL REPORT PSW-GTR-240

68

the resistance program, sites for field trials, and have identified most of the field selections. The BLM

has established and maintains a gene conservation clone bank. Other federal, state, and private

cooperators have supported the program by providing tree selections for resistance testing and sites

for field trials. Summaries of genetic and resistance work in the operational program through to 2001

are reported elsewhere (Kitzmiller et al. 2003, Sniezko 2004; Sniezko and Hansen 2000, 2003;

Sniezko et al. 2003), but significant updates on delineation of breeding zones, seed orchard

development, seed availability, testing protocols, and understanding of resistance have occurred over

the intervening 10 years.

Integrated Strategy

Developing populations of POC with resistance to P. lateralis will be key to increasing success in

reforestation and restoration efforts. At this stage, to help maximize the progress in disease resistance,

the POC program has focused only on genetic resistance and has not selected for increased growth

rate or other traits that may be of interest in reforestation.

POC is a long-lived species that spans very diverse environments (e.g., soils, climate) (Zobel et al.

1985). It is therefore essential to include both genetic diversity and adaptability considerations in the

implementation of the resistance program to ensure resulting seedlings will be adapted to current

conditions, as well as to buffer the species against a changing climate and other possible insect or

pathogen problems.

To help ensure adaptability of future reforestation and restoration plantings, the natural range of

POC is currently divided into five breeding blocks, and a total of 13 breeding zones (fig. 2). A

breeding block designates the geographic area that envelops a number of breeding zones. The

preliminary breeding blocks and zones serve to guide seed transfer and associated breeding activities,

where genetic reproductive materials (such as seed, seedlings, and cuttings) are procured or produced

in a breeding program, and then subsequently deployed (seeded or planted). The preliminary breeding

blocks and zones were updated in 2005 by the BLM and USFS, and were delineated on the basis of

various short-term and long-term studies and data. Breeding zones are represented by an elevation

band within a respective breeding block. Further analyses of existing long-term common garden

studies (fig. 3) that supplement the earlier analysis of 7-year-old data by Kitzmiller (2006), or other

future studies, will be used to refine the breeding blocks/zones over time. Seedlots may also be

delineated in the future within any respective breeding zone on the basis of source soil type. The tree 

species occurs on both ultramafic (serpentine) and other soil types (Zobel et al. 1985). A number of

plant species have differentially adapted to these distinct soil types (Linhart 1995), but the degree to

which POC is specifically adapted is not known.

Since 1997, over 12,600 field selections have been made to help maintain genetic diversity during

the subsequent testing and selection process for parents for the seed orchards and for future breeding.

Many of the selections have been made in areas where mortality from P. lateralis is evident, while

others represent random selections in areas not yet impacted. Selections have been made throughout

the range of POC, with more selections in the areas where the species is more prominent (fig. 2).

Field selections are tested in several steps for resistance (Hansen et al., Methods for screening PortOrford-cedar for resistance to Phytophthora lateralis, these proceedings; Sniezko et al. 2003) with the

final selections placed in seed orchards or breeding orchards. Based on their rating in the stem dip

test, approximately 10 percent of the original field selections have been retained for further resistance

evaluation using a root dip testing method (Hansen et al., Methods for screening Port-Orford-cedar

for resistance to Phytophthora lateralis, these proceedings; Sniezko et al. 2003). The root dip testing

of much of this subset of parental selections (or their progenies) has been completed. Results from

this test are used to determine which materials are placed into orchards and used for breeding.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

69

Figure 3—Port-Orford-cedar: (a) measuring height in 2011 in1996 POC provenance trial in northern

California; (b) different color and form for multiple copies of two different parent trees in the Tyrrell

clone bank; (c) clone bank. (Photo credits: Chuck Frank, a; Richard Sniezko, b and c)

In general, the disease-resistance program involves a sequence of steps to identify resistant parent

trees and to propagate those parent trees in seed orchards to provide seed for reforestation and

restoration (fig. 4). In the first cycle of selection and testing, candidate parent trees are selected from

forest stands within the range of POC (fig. 2). These selections are tested for resistance in short-term

tests (for methods, see Hansen et al., Methods for screening Port-Orford-cedar for resistance to

Phytophthora lateralis, these proceedings; Sniezko et al. 2003) and resistant parents or progeny are

retained for orchard development and breeding. Control crosses (selfs and outcrosses) are used to

generate progenies for testing resistance and for further selection to increase resistance. A project to

examine the genetic diversity within the orchard populations using molecular markers is underway.

Resistance Testing

Efficient, short-term resistance testing is a key step in any operational resistance program. It permits

the rapid identification of parent trees to be included in the production orchards or for further

breeding. Several short-term tests have been used (Hansen et al., Methods for screening Port-Orfordcedar for resistance to Phytophthora lateralis, these proceedings; Sniezko et al. 2003), but a

greenhouse root dip test of seedling families or of rooted cuttings of parent trees provides the most

information on resistance. The seedling test is preferred over that of rooted cuttings of older parent

trees which sometimes give anomalous results. The root dip testing of seedlings can identify

differences in survival among families with qualitative resistance, as well as differences in time of

mortality among families and the substantial within-in family variation in resistance when

quantitative resistance is present (Sniezko 2004). In the POC program, these differences became

much more apparent beginning with the 2005 testing, when the duration of the assessment period for

the root dip test was expanded from less than a year to up to 3 years.
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Figure 4—Selection and breeding cycles for developing Phytophthora lateralis resistant populations

of Port-Orford-cedar for reforestation and restoration.

Both wind-pollinated (especially if collected from forest stands) and control-crossed families are

used in the root dip testing. Many POC are somewhat self-fertile (Sniezko et al. 2003), and selfpollinated families have been used routinely for resistance testing in the last few years. In the shortterm root dip test, families can vary in survival from 0 to 100 percent (fig. 5) (Sniezko 2004). One or

more high-susceptible controls are normally included to provide a baseline to compare all families.

The susceptible control families usually reach 100 percent mortality in 100 to 150 days after artificial

inoculation (fig. 5). Current efforts are primarily focused on finishing the root dip resistance testing of

the forest selections and adding these parents (or their progenies) to the orchards. Other efforts

include increasing resistance through breeding, gaining further understanding of the types of

resistance and their inheritance, and examining the level of genetic resistance present in windpollinated seedlots from the orchards.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

71

Figure 5—Percent mortality in root dip testing for a susceptible control Port-Orford-cedar family and

two resistant families.

The greenhouse root dip test has provided a relatively quick assay to identify resistance in POC. In

the root dip test, it appears that at least two types of resistance are present, a qualitative one that

identifies some families with high survival (fig. 5) and a quantitative one in which there is often very

wide variation within a family for the rate of progress of P. lateralis in reaching the root collar

(Sniezko, unpublished data). Expansion of the assessment period of the root dip test in 2005 from

approximately 9 months to up to 3 years more clearly identifies those families with qualitative

resistance, quantitative resistance, or both. Families with each type of resistance are now under field

test. Breeding to combine resistances is also underway.

Research on a small subset of resistant and susceptible families from the operational program was

undertaken to examine the underlying nature of the resistance in POC. Symptom development on

resistant trees included cell wall thickening, cell collapse, and deposition of electron-dense materials

around hyphae; all reactions consistent with a hypersensitive response in the host (Oh and Hansen

2007, Oh et al. 2006). Further work to discern the number of resistance mechanisms, their

inheritance, the underlying nature of resistance, and its durability is warranted.

Durability of Resistance

Genetic variability within North American isolates of P. lateralis is small, likely due to a genetic

bottleneck from the original introduction of this pathogen from elsewhere (Goheen et al. 2003), and

this may bode well for durability of resistance. Recorded survival for a decade or more of many of the

initial field selections in areas of high P. lateralis-caused mortality is encouraging (fig. 1e). These

trees could serve as permanent plots to help monitor resistance. In addition to the root dip testing,

short-term testing using raised beds infested with P. lateralis has been used to test resistance (Hansen

et al., this proceedings). No change in efficacy of resistance is apparent in continued use of these

raised beds for more than a decade (Sniezko, unpublished data). However, the relatively recent

findings of P. lateralis in Europe (Sansford 2011) and Taiwan (Brasier et al. 2010, Webber et al.

2011) raise concerns about potential increased pathogenicity from new introductions or the
GENERAL TECHNICAL REPORT PSW-GTR-240

72

evolutionary potential of current isolates. Testing the efficacy of POC resistance to these different

isolates is planned.

Long-term field testing of a subset of the resistant selections is essential to assay resistance under

a variety of field conditions as well as to continue to monitor durability of resistance. An extensive set

of field trials have been established, and early results from the first ones are encouraging (Sniezko et

al. 2006; Sniezko et al., Nine Year Survival of 16 Phytophthora lateralis resistant and susceptible

Port-Orford-cedar families in a southern Oregon field trial, these proceedings). However, it is

essential that these trials continue to be monitored to assess the efficacy and durability of resistance in

long-lived POC.

Seed Orchards

Resistant seed can be collected from selected trees in the forest and from seed orchards. Seed

orchards offer several potential advantages, including increased genetic resistance and relatively easy

production of large quantities of resistant seed from a diverse array of parents. For POC, seedlings

from wind-pollinated containerized seed orchards (CSOs) are the principal source of reproductive

materials for reforestation and restoration (fig. 6). The biology of POC is very conducive to the use of

CSOs for the production of large quantities of seed at an early age (Elliott and Sniezko 2000, Sniezko

et al. 2003). Branches from resistant trees or tops clipped from 1-year-old resistant seedlings are used

to produce rooted cuttings which are then placed in pots in a greenhouse orchard. The CSO

technology for POC has allowed for a much shorter time interval between identification of resistant

trees and operational production of seed. With the ability to induce seed cones and pollen cones, seed

can be produced within 5 years of orchard establishment. For some breeding zones, resistant seed has

been available since 2003. In addition, breeding POC to enhance resistance can be done within

several years of identifying resistant trees. Seed orchards have been started for eight of the 13

breeding zones (table 1). In these orchards, rooted cuttings of resistant clones have been propagated

(a) (b)

(c)

Figure 6—Containerized seed orchards at Dorena Genetic Resource Center: (a) seed orchard, (b)

abundant cone production on orchard tree, (c) control pollinations. (Photo credits: Richard Sniezko)
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

73

and placed together by breeding zones in the greenhouses. The current number of parents per orchard

varies from five to 115 (table 1). Ongoing priorities include screening more forest selections for

potential inclusion into orchards for additional breeding zones and to increase the number of parent

trees in existing orchards as well as to retest some of the early orchard selections to confirm their

resistance. The utilization of containerized seed orchards allows for easy updating of orchards as

resistance screening results become available. Rooted cuttings of many of the most resistant trees

have also been used to establish a clone bank for long-term preservation (fig. 3).

Table 1—Number of Phytophthora lateralis-resistant Port-Orford-cedar parents in

containerized seed orchards (CSOs) for eight breeding zones as of 2011

Breeding zonea Number of parents

110 60

125 115

210 10

225 9

325 16

340 68

440 5

450 8 a See fig. 1 for delineation of 13 breeding zones; No CSOs for BZs 315, 350, 425, 545, 550.

The first-generation seed orchards are wind-pollinated and produce a genetically heterogeneous

mix of seed with an intermediate level of resistance. A short-term greenhouse root dip trial,

inoculated in 2004, of several orchard breeding zones, showed 30 percent or higher survival in many

zones over woods-run seed (fig. 7) (Sniezko et al. 2005). Results from short-term tests like the

greenhouse root dip test probably represent an upper limit to resistance from the current generation of

orchards, and confirmation of the level under field conditions is needed. Field tests on sites of

moderate to extreme disease hazard are essential to determine any limits on the use of resistance and

the level of survival to be expected over time. Seedlots from several orchards have been included in

recent field tests and results are pending. This field data will be essential information to guide

activities of land managers interested in utilizing resistant POC. Early results from some of the first

field trials using individual half-sib and full-sib families show encouraging results (Sniezko et al.

2006; Sniezko et al., Nine year survival of 16 Phytophthora lateralis-resistant and -susceptible PortOrford-cedar families in a southern Oregon field trial, these proceedings), but as with any long-lived

species, these trials need continued monitoring. Information on the disease hazard of sites would also

be useful to managers planning on utilizing resistant POC. Until more field results are available,

planting sites should be chosen carefully, avoiding the highest risk sites such as low-lying wet areas

down slope from infested areas. The impact of changing climate on the spread of P. lateralis or the

efficacy of resistance is unknown and warrants investigation. The level of genetic resistance in

advanced-generation orchards should increase with breeding and subsequent testing and selection.

The containerized orchards serve to meet seed needs for both the Forest Service and BLM. In

addition, the Oregon Department of Forestry (ODF) has acquired resistant seed for its seed bank

(Oregon Forest Tree Seed Bank) to serve the needs of other landowners. The ODF has filled requests

for seed from nurseries and non-federal landowners from 2003 to 2011 (Larry Miller, Oregon 

Department of Forestry, personal communication). Over 3.85 kg of seed (385,000 to 452,000 seed per

kg, approximately 60 percent filled seed) has been dispersed (table 2). Most of the seed need has been

for the western most portions of Oregon (Breeding Zones 110 and 125, fig. 1; table 2).
GENERAL TECHNICAL REPORT PSW-GTR-240

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Figure 7—Port-Orford-cedar survival of orchard seedlings versus ‘woodsrun’ seedlings for five

breeding zones (from 2000 zones) in a 2004 greenhouse root dip test for Phytophthora lateralis

resistance.

Resistant seed is now being used by federal and non-federal land managers for reforestation and

restoration (figs.8 and 9; table 2). Due to changing management practices on some federal lands,

including reduced timber harvest, opportunities for using resistant POC seed in restoration and

reforestation are now more limited, and the use of the highest level of resistance possible would aid

efforts to retain POC in ecosystems while allowing resistance genes to spread as trees reach

reproductive age. On USFS and BLM lands, plantings are often in areas that have been burned by

wildfires (USDA FS 2010). Plantings on non-federal lands will facilitate more diversely managed

forests, while also setting up opportunities via natural pollen and seed dispersal for increasing the

spread of resistance genes to adjacent USFS or BLM lands. Increasing the frequency of genetic

resistance over the landscape will help ensure that POC remains a viable component in forest

ecosystems impacted by P. lateralis.

With the discovery of a number of POC parent trees with resistance to P. lateralis, there is revived

interest in POC in the horticultural sector. The only option currently available, and limited in use, is

grafting established cultivars onto the one resistant parent that has been made available for this

purpose. However, the resistance screening of parent trees and their progenies from throughout the

range of POC and crosses among them offers additional possibilities. In order to meet the different

objectives for horticulture, materials should be carefully evaluated for the range of usage anticipated.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Figure 8—Volunteers from several community groups and a USDA Forest Service biologist planting

900 Port-Orford-cedar (POC) along Grayback Creek in southern Oregon in 2008. This planting

project will help restore a riparian area used by salmon and steelhead and also serve as a test of

resistant P. lateralis-resistant POC. (Photo credit: Scott Kolpak)

Figure 9—Port-Orford-cedar technical committee visiting 2011 planting of resistant POC along trail at

South Slough National Estuarine Research Reserve in western Oregon. (Photo credit: Richard

Sniezko)
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Table 2—Seed (grams) requested by non-federal landownersa

Year

Breeding Zonesb

110 115 125 130 210 215 430 Total

2003 - 172 - - - - - 172

2004 - 340 - - - - - 340

2005 - 458 - 254 - - - 712

2006 - 481 - - - - 73 554

2007 - 503 - - - 50 - 553

2008 - - - - 45 - - 45

2009 68 - 340 - - - - 408

2010 - - 340 - - - - 340

2011 386 - 340 - - - - 726

Total 454 1954 1020 254 45 50 73 3,850 a Data provided by Larry Miller, Oregon Department of Forestry. b Breeding zones were reduced and reorganized in 2005 (see fig. 1 for current zones).

Other Possible Future Activities

If funding permits, other components of the program could be refined:

(1) The seedling assay is currently the most favored, but it can take several years to fully

distinguish the different types of resistance. The development of chemical biomarkers or

genomic tools for selecting for resistance would help increase our understanding of resistance

and its genetic control.

(2) The recent development of microsatellite markers for POC has allowed a start on further

investigation of genetic variation patterns across the range of the species. This work will

complement previous isozyme and common garden studies and help demarcate potential

additional areas for gene conservation.

(3) Research is needed to understand the evolutionary potential of P. lateralis and its potential

impact on durability of resistance in POC. Investigations into the genetics and pathogenicity

of isolates of P. lateralis from outside North America will expand our knowledge base.

(4) More work is needed to understand the limits to resistance in POC, particularly mortality and

any fall down in resistance on the sites of highest risk: wet, low-lying areas.

Summary

Genetic variation within a species is the key to helping it evolve and adapt to changes in its 

environment, even to an unexpected threat such as a non-native pathogen. In an extreme case, such as 

the one that occurred with Port-Orford-cedar and the introduction of P. lateralis, humans can help reestablish a more balanced ecological equilibrium through active measures such as a resistance 

selection program. Such a program harnesses natural genetic variation within POC and would not 

have the negative side effects that other management activities could have. The program to develop P. 

lateralis resistance in POC has made significant progress in a relatively short period of time. Aspects 

of the program have involved not only USFS, BLM, and OSU, but also a wide array of other land 

managers. It is one of few operational resistance programs to non-native pathogens to produce 

significant levels of seed for potential restoration and reforestation. A large number of field selections 

have undergone the first rounds of resistance testing, breeding zones and a large genetic base has been 

established, seed orchard establishment is well underway, protocols for short-term resistance testing 

have been refined, field trials are underway, and the planting of resistant seedlings has started. Future 

efforts will focus on completing the development of first-cycle orchards, breeding to increase 

resistance and monitoring field trials for efficacy and durability of resistance. Genetic resistance is an 

invaluable tool to complement other management activities that prevent or reduce the spread of P. 

lateralis.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

77

Acknowledgments

We thank Leslie Elliott, Scott Kolpak, Heather May, Sunny Lucas, Pete Angwin, Katy Mallams, Don Goheen,

Kirk Casavan, Rod Stevens, Frank Betlejewski, Joe Linn, Clinton Armstrong, Paul Reeser, Wendy Sutton, the

nursery crew at Dorena GRC, and the many others for their key roles in the operational elements of the

resistance program. Stable funding and key personnel support from both USFS and BLM have been key in the

program development. We also thank all of the other governmental, industrial, and private landowners who

have supported the program; and Pete Angwin, Steve Lee, Sunny Lucas, Larry Miller, and Greg Filip for their

reviews of an earlier version of this paper.

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GENERAL TECHNICAL REPORT PSW-GTR-240

80

Strong Partial Resistance to White Pine Blister Rust 

in Sugar Pine

Bohun B. Kinloch, Jr.,

1 Deems Burton,2 Dean A. Davis,2 Robert D. Westfall,3

Joan Dunlap,

4 and Detlev Vogler1

Abstract

Quantitative resistance to white pine blister rust in 128 controlled- and open-pollinated sugar pine families was 

evaluated in a “disease garden”, where alternate host Ribes bushes were interplanted among test progenies. 

Overall infection was severe (88%), but with great variation among and within families: a 30-fold range in 

numbers of infections per tree, and a 10- fold range in trees with completely inactive, or aborted infections. 

Inheritance of these partial resistances was strong, with significant amounts of both additive and non-additive 

genetic variance evident. They will be most useful in combination with major gene resistance (MGR) by 

reducing the probability of infections virulent to MGR.

Key words: Cronartium ribicola, Pinus lambertiana, partial resistance

Introduction

Sugar pine (Pinus lambertiana Dougl.), one of the most susceptible of white pines to white pine 

blister rust (Cronartium ribicola), has a major gene (Cr1) that confers virtual immunity to the disease, 

but is vulnerable to specific virulence from a complementary gene (vcr1) in the pathogen. Partial 

resistance (PR; also known as slow rusting, SR) is a suite of traits that reduces the amount and rate of 

infection through lowered receptivity to infection and incompatibility reactions in the host when

infections do occur. Partial resistance has been shown to be effective and durable to blister rust in 

white pines (King et al. 2010, Kinloch et al. 2008). Combining the two types of resistance in synthetic 

lines could be mutually reinforcing by sanitizing all inoculum lacking vcr1 with Cr1 and by reducing 

the probability of infection by vcr1 with PR. 

Methods

To assess the degree and inheritance of PR, 128 sugar pine progenies with different pedigrees were 

exposed to the pathogen in a plantation at Happy Camp (HC) in northern California. Happy Camp has 

been used since 1958 as the principal site for breeding, testing, and evaluating sugar pine and other 

white pines by the Genetic Resources Unit of the Pacific Southwest Region, USDA Forest Service, 

with the objective of developing durable resistance to blister rust. Here, resistant parent selections 

were grafted in a clone bank and interbred when reproductively mature, and different cohorts of test 

progenies established every year. Seedlings are naturally challenged by blister rust inoculum 

(enhanced by interplanting alternate host Ribes sp. among pines in the test plots). Surviving seedlings 

are allowed to remain in place indefinitely, and many reach reproductive maturity. Although many 

parent trees were known to carry Cr1, this gene was effectively neutralized by the overwhelming 

prevalence of vcr1 inoculum on the site, allowing other kinds of resistance to express.

Test progenies consisted of four main groups: 1) a full-sib and half-sib matrix of 75 families 

controlled-pollinated (CP) among select parents; 2) 14 families of the same select parents that were 

wind or open-pollinated (OP) by the ambient pollen cloud on the site (_xW(HC)); 3) 33 families from 

 1 Institute of Forest Genetics, Pacific Southwest Research Station, Albany, CA and Placerville, CA. 2 Genetics Resource Unit, USDA FS Pacific Southwest Region, Klamath National Forest, Yreka, CA. 3 USDA FS Pacific Southwest Research Station, Albany, CA. 4 Genetics Resource Unit, USDA FS Pacific Southwest Region, Camino, CA.

Corresponding author: bkinloch@earthlink.net.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

81

cone bearing parents of uncertain genotype that had survived the chronic epidemic at HC, also 

pollinated from pollen on site (?xW(HC); and 4) four multi-tree seed lots, each mixed and bulked 

from four different natural stands. 

The full- and half-sib group was made from parents a) known from previous experience to 

transmit relatively high resistance (SR) or susceptibility (fast rusting, FR) to their offspring; b) F1s of 

some of the selects; and c) F1s of other trees on the site of unknown or uncertain resistance genotypes 

that had survived many years of the chronic epidemic. Controlled crosses were made among the 16 

parents in this group in all available combinations, emphasizing known SR x SR, SR x FR, and FR x 

FR genotypes (see tables 1 and 2).A mixed general linear model (GLM) was used to account for 

repeated measurement of families. The SAS (v. 9.2) MIXED procedure was used to perform paired ttests with the Satterthwaite adjusted degrees of freedom and the Bonferroni’s level of significance 

adjustment.

All parents were SR phenotypes, by virtue of identified characteristics or survival under chronic 

high-hazard exposure to the disease. This included two that were known to produce highly susceptible 

(FR) offspring (i.e., were FR genotypes, though SR phenotypes). Because crowns damaged by rust 

rarely bear cones, adequate numbers of FR phenotypes are difficult to find.

We hoped to make enough crosses for a balanced partial diallel, but this goal was frustrated 

because of sugar pine’s notoriously erratic reproductive behavior. Many crosses were not successful, 

resulting in a severely unbalanced mating design. The shortage of susceptible control genotypes was 

particularly conspicuous. Nevertheless, we were able to extract two factorials of unrelated seed and 

pollen parents from the matrix of crosses to make estimates of additive and non-additive variances.

We expected that F1 parents would generally transmit the greatest amount of SR to offspring, 

followed by SR parent (P1) selects. We also expected on-site pollen to have a major effect. Since this 

pollen derived overwhelmingly from both select SR and other parents surviving the chronic epidemic, 

we presumed it would carry strong resistance, especially if additive genetic variance is predominant. 

In effect, it would simulate pollen in a seed orchard of inter-pollinating selected parents, and allow 

estimates of realized heritability. Thus, we expected the open-pollinated progenies from the select SR 

parents would perform well; less than resistant selects, but probably somewhat better than the other 

33 open-pollinated seed parents. We expected the FR crosses to depress expression of SR and we

assumed the bulked seed lots would represent average susceptibility in the wild population. 
Table 1—Mean number of infections per tree on sugar pine families

A) Controlled- and wind-pollinated families: 

Parents: Pollen 

Seed K71 K86 K44 K23 K33 K72 K14 K36 K71 x 

K73

K86 x 

K44

K71 x 

K44

K70 x 

K43

K87 x 

K19

K36 x 

K73

K71 2.2 1.5 1.6 3.3 1.0 2.1 2.8 

K86 1.2 1.3 2.4 1.1 1.7 3.2 1.7 1.1 1.0 2.0 

K44 

K23 2.7 2.1 1.0 2.6 

K33 2.2 1.7 2.6 1.1 1.4 

K72 

K14 3.6 3.3 4.1 5.9 6.1 11.8 3.1 5.4 10.2 

K36 2.7 3.9 2.8 

K71 x K73 1.6 3.4 3.2 5.4 

K70 x K43 1.4 1.4 2.7 3.6 0.4 2.8 

K87 x K19 1.2 5.4 4.3 1.0 

K36 x K73 3.8 3.8 3.3 8.3 4.2 

K16 x K17 2.1 3.3 1.6 3.0 2.8 2.3 0.7 2.2 1.5 1.5 4.2 

K36 x K17 2.8 2.8 5.5 1.7 

Parent mean1 2.3 2.0 2.7 2.7 1.7 3.1 5.8 5.0 3.3 1.4 1.6 1.9 2.7 4.5 

Half-sib mean: 3.0

Full-sib range: 0.4 - 

Half-sib parent mean/W(HC) mean r p 

x W (wild stand)

82
B) Group means:

No. 

families

in type

Seed Pollen Mean Range

Rank

Legend: Obs. E

SR select parent 12 1.8 1.1 2.7 2 

F1 of SR select parent 9 1.9 1.0 3.4 3 

F1 survivor 20 2.3 1.0 3.8 5 

FR select parent 9 4.0 2.7 6.1 8 

0 - - - - 

8 2.2 0.4 5.5 4 

3 3.7 2.8 5.4 7 

5 2.8 1.5 4.2 6 

8 6.1 2.7 11.1 9 

1 11.8 - - 10 

75 4.06 rp = 0

33 ? x W (HC) 5.00 2.1 10.5

4 Bulked mix x W (wild) 4.79 4.2 5.6

1 includes both seed and pollen entries.

83
Table 2—Percent of sugar pine families with no active infections from blister rust

A) Controlled- and wind-pollinated families: 

Parents: Pollen

Seed K71 K86 K44 K23 K33 K72 K14 K36 K71 x 

K73

K86 x 

K44

K71 x 

K44

K70 x 

K43

K87 x 

K19

K36 x 

K73

K71 41.4 54.7 35.7 17.8 73.0 44.4 30.7 

K86 55.3 37.5 36.2 51.4 34.1 16.7 37.8 67.1 32.8 40.0 

K44 

K23 27.0 15.8 52.1 20.6 

K33 24.1 38.1 25.4 44.1 29.0 

K72 

K14 26.8 16.2 9.6 8.2 6.9 0.0 23.5 12.9 0.7 

K36 20.6 13.5 41.5 

K71 x K73 50.7 4.4 9.6 13.6 

K70 x K43 36.6 34.0 10.5 22.0 73.2 23.5 

K87 x K19 42.4 2.9 16.0 58.7 

K36 x K73 20.3 12.9 17.8 6.1 8.7 

K16 x K17 45.2 15.2 21.3 25.0 14.1 23.4 64.4 46.6 26.9 29.6 10.8 

K36 x K17 20.3 18.1 20.0 30.8 

Parent Mean1 37.8 36.0 24.7 26.1 31.1 22.2 11.2 15.1 22.3 51.2 56.9 32.0 26.9 17.1 

Half-sib mean: 28.70

Full-sib range: 0 - 73

C-P parent mean:xW(HC) mean: rp= 

* x W (wild)

84
B) Group means:

No. families

in type Seed Pollen Mean Range

R

Legend: Obs. 

SR select parent 12 37.6 16 - 55 4 

F1 of SR select parent 9 40.5 4 - 73 3 

F1 survivor 20 28.9 13 - 45 5 

FR select parent 9 15.1 8 - 27 8 

0 - - - 

8 41.6 16 - 46 2 

3 26.2 14 - 42 6 

5 19.9 9 - 30 7 

8 8.8 1 - 22 9 

1 0.0 - 10 

75 24.3 rp = 

33 ? x W (HC) 11.90 2 - 25

4 bulk lots Bulked mix x W (wild) 7.7 3 - 11

1 includes both row and column entries

85
GENERAL TECHNICAL REPORT PSW-GTR-240

86

Progenies were established in 2006 with an average of 57 trees each in single-tree plots in 75

blocks. In all, 7,545 trees in 128 controlled- or open-pollinated families were planted and 

evaluated. Disease was assessed in 2010 and 2011, after overall infection was nearly complete on 

susceptible controls. Separate infections were counted on each seedling in 2010, usually in their 

incipient stages (fig. 1A), and determined to be either typical (normal) for the disease or reactive. 

Reactions were of two main kinds. The first, bark reactions (BRs), caused infections to abort or 

‘cork out’ (Struckmeyer and Riker 1951), leaving a necrotic scar beneath and surrounding the 

infection site (fig. 1B). BRs ranged in size from “micro-“ necrotic lesions surrounding the base of 

the infection court of the short shoot to lesions many cm long. The other reaction was a blighting 

(BL) of the infected shoot, from the point of infection distal to the end of the shoot (fig. 1C). Both 

reactions often occur on the same tree, and reactions of both types often expressed only partially: 

death and collapse of most diseased tissue, but with some activity remaining on the margins of 

the lesion (see Kinloch and Davis 1996 for further details). Trees with typical infections, or those 

with incomplete reactions, were classed as normal/susceptible; trees with BRs, BLs, or both were 

classed as reactive, but only if disease symptoms were in complete remission and the tree had no 

active infections (NAI).

Results

The epidemic started early, with 3 successive wave years from 2007 to 2009. By spring 2011, 88 

percent of all trees were infected and 9 percent were dead from rust. Individual families ranged 

from 44 to 100 percent infected and 0 to 51 percent dead.

The most useful indexes of resistance were low receptivity to infection (LRI), measured by 

numbers of separate infections on individual trees, and the percent of trees in individual families 

with no active infections (percent NAI). Mean number of infections for all families (CP and OP) 

was 3.7 and ranged from 0.39 to 11.80. Within families, there was great variability among trees, 

many having no infections and ranging as high as 92. For percent NAI, the mean of all families 

was 22.1, and ranged from 0.0 to 81.1. Correlation between the two indexes was -0.71.

Tables 1A and 2A summarize data for numbers of infections and percent NAI, respectively, 

for the matrix of individual full-sib families of select parents; half-sib means of select parents; 

and select xW(HC) means. Part B of tables 1 and 2 compare families grouped according to 

parental generation (P1, F1) or parental genotype (SR, FR) with each other, and with the 33 

families that were open-pollinated from the ambient pollen cloud at HC (?xW(HC)), and the four 

seed lots bulked from wild stands. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

87

Figure 1—Reactions of sugar pine seedlings to infection by blister rust. A) Typical incipient 

infections, susceptible phenotype; note pink discolorations, swelling at base of short shoots; four 

separate infections are visible, but will soon coalesce. B) Micro bark reactions on slow rusting 

(SR) phenotype. About 10 are visible. C) Blight reactions on SR phenotype resulting from several 

separate infections. 

A B

C
GENERAL TECHNICAL REPORT PSW-GTR-240

88

For both indexes the general ranking of the four main group means, from most resistant to 

most susceptible, was: the CP matrix of full/half-sib select parents > OP select parents > OP nonselect parent survivors > OP bulk seed lots from wild stands. None of OP family group means 

were significantly different from one another (figs. 2 and 3). 

The greatest variation was among individual full-sib families, which ranged (resistant to 

susceptible) from 0.39 to 11.8 infections/tree and from 73.0 to 0.0 percent NAI. Half-sib family 

means from P1 and F1 selects ranged from 1.4 to 5.8 infections/tree, and from 56.9 to 11.2 

percent NAI. The 14 families from open-pollination of the P1 and F1 parents ranged from 2.0 to 

7.2 infections/tree, and 31.0 to 5.1 percent NAI. Correlations between these OP families and their 

CP counterparts were 0.82 for infections/tree and 0.60 for percent NAI.

The nine full-sib family groups generally ranked as expected, with correlations between 

observed and expected values of 0.90 and 0.95 for numbers of infections and percent NAI, 

respectively (tables 1B and 2B). Half-sib means varied greatly, with strong resistance from 

families with parents K86 and/or K44 in their pedigrees, and the greatest from the F1 (K86xK44).

But this performance was not always consistent; for example, K44, as pollen parent, performed 

relatively poorly with another F1 select (K71xK73; table 2). None of the groups was statistically 

different from others, with the sole exception of the single FR x FR family (K14xK36), which 

was significantly more susceptible than all other groups (figs. 4 and 5). The negative influence of 

the FR parents was pervasive: groups with FR, taken as a whole (coded red, figs. 4 and 5), were 

significantly different from groups lacking FR (coded black, figs. 4 and 5). 

Discussion

For a species so generally susceptible to an introduced disease, sugar pines show remarkable 

variability in resistance to blister rust: a 30-fold range in mean numbers of infections among fullsib families, and a ten-fold range in percent of trees with no active infections (tables 1 and 2). The 

modest correlation (-0.71) between the two indexes indicates that they can be selected for 

separately or simultaneously for development of stable resistance.

Although the expression and underlying causes are different, both mechanisms are highly 

effective, either in reducing the probability of infection occurring (LRI) or the probability of an 

infection becoming lethal (percent NAI). Both are strongly inherited; several of the best select 

parents in this test have shown consistent performance in several previous trials (Kinloch and 

Davis 1996, Kinloch et al. 2008). Still, modes of inheritance remain unclear. From the two 

factorials, we estimated additive effects to consist of 78 and 58 percent of the total genetic 

variance in LRI and percent NAI, respectively, but both estimates have large standard errors of 

the estimated means. In a related pathosystem, Isik et al. (2003) estimated that epistatic effects 

were nearly equal to additive effects for resistance to fusiform rust in loblolly pine, and we have 

observed that certain sugar pine parents (e.g., K71, K36) sometimes exhibit specific rather than 

general combining abilities to blister rust resistance (Kinloch and Davis 1996, Kinloch et al. 

2008). Although most quantitative genetic traits are additive, partial resistance to disease can be 

controlled by specific interactions with pathogens (Krenz et al. 2008) or even gene-for-gene 

interactions (Antonovics et al. 2011). 

The lack of a stronger effect of on-site pollen (_xW(HC)) on progeny performance was 

contrary to expectations, implying that pollen from the ambient cloud at HC does not carry 

particularly strong or abundant genes for SR that combine additively. Neither of the two openpollinated groups, select xW(HC) or ? xW(HC), were significantly different from controls 

represented by the four bulk seed lots from natural stands or from each other (figs. 2 and 3). Seed 

parents of the ? xW(HC) group, on the other hand, showed wide variation in resistance (tables 1B 

and 2B). Taken together, the data suggests that both additive and non-additive effects are 

important in controlling resistance.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

89

Figure 2—Means and 95 percent confidence intervals for numbers of infections/family in four 

main open-pollinated groups of sugar pine progenies. Groups with the same letter are not 

statistically different; the six pairwise comparisons were tested using a Bonferoni’s adjusteda=.05/60.0082to attain an experimentwise error rate a=0.05.

Figure 3—Means and 95 percent confidence intervals for percentage of trees with no active

infections/family (percent NAI) in four main open-pollinated groups of sugar pine progenies.

Groups with the same letter are not statistically different; the six pairwise comparisons were 

tested using a Bonferoni’s adjusted-a=.05/6=0.0082 to attain an experimentwise error rate 

a=0.05.

0

1

2

3

4

5

6

7

Sel. Parent half-sibs Sel parent xW(HC) ? xW(HC) Bulk mix

No. infections/tree mean (95% CI)

a

b

 b

ab
GENERAL TECHNICAL REPORT PSW-GTR-240

90

Figure 4—Means and 95 percent confidence intervals for numbers of infections/family in nine fullsib groups of sugar pine progenies. (cf., table 1 or 2 for group color codes. The mean of the 

groups in red is significantly different than the mean of groups in black).

Figure 5—Means and 95 percent confidence intervals for percentage of trees with no active

infections/family ( percent NAI) in nine full-sib groups of sugar pine progenies. (cf., table 1 or 2 for 

group color codes. The mean of the groups in red is significantly different than the mean of 

groups in black).

Regardless of the type of inheritance of PR, there is increasing consensus that major resistance 

genes are more effective when deployed in a background containing quantitative resistance traits 

(Brun et al. 2010, Wulff et al. 2011). Quantitative resistance genes can work additively with R

genes, prolonging their durability, probably through reduction in pathogen effective population 

0

2

4

6

8

10

12

14

G-G G-Y G-O G-R Y-O Y-R O-O O-R R-R

Mean no. infections/tree (95% CI)

GxG GxY GxO GxR YxO YxR OxO OxR RxR
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

91

size and especially the number of virulent mutants that could be selected by R-genes (McDonald, 

2010). Accordingly, the highest value of PR in breeding sugar pine is to protect Cr1 from vcr1 in 

synthetic populations with combined pedigrees. Cr1 will protect against all ambient inoculum 

except that containing vcr1. Concentrated PR genes will sanitize a great proportion of incoming 

inoculum, reducing the probability of vcr1, which is rare in natural populations, from becoming 

epidemic (Kinloch and Davis 1996, Kinloch et al. 2008). The prospect of developing more 

durable resistance in sugar pine is good.

Acknowledgments

We thank John Gleason and Sara Wilson for assistance with field assessments, Sylvia Mori for statistical 

advice, and Nancy Gillette for helpful suggestions on an earlier draft.

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blister rust in white pines. Forest Pathology. 40: 292–313.

Kinloch, B.B., Jr.; Davis, D. 1996. Mechanisms and inheritance of resistance to blister rust in sugar pine. 

In: Kinloch, B.B., Jr.; Marosy, M; Huddleston, M., eds. Sugar pine: status, values, and roles in 

ecosystems: proceedings of a symposium presented by the California sugar pine management 

committee. Publication 3362. Davis, CA: University of California, Division of Agriculture and Natural 

Resources: 125–132.

Kinloch, B.B., Jr.; Davis, D.A; Burton, D. 2008. Resistance and virulence interactions between two white 

pine species and blister rust in a 30-year field trial. Tree Genetics & Genomes. 4: 65–74.

Krenz, J.E.; Sackett, K.E.; Mundt, C.C. 2008. Specificity of incomplete resistance to Mycosphaerella

graminicola in wheat. Phytopathology. 98: 555–561.

McDonald, B. 2010. How can we achieve durable disease resistance in agricultural ecosystems? New 

Phytologist. 185: 3–5. 

Struckmeyer, E.B.; Riker, A.J.R. 1951. Wound periderm formation in white pine trees resistant to blister 

rust. Phytopathology. 41: 276–281.

Wulff, B.B.H.; Horvath, D.M.; Ward, E.R. 2011. Improving immunity in crops: new tactics in an old 

game. Current Opinion in Plant Biology. 14: 468–476.
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An Investigation into Western White Pine Partial 

Resistance Against the Rust Pathogen Cronartium 

ribicola Using In Vitro Screening Method

D. Noshad1 and J.N. King1

Abstract

Cronartium ribicola is one of the most destructive forest pathogens of North American white pines. The 

pathogen infects pine trees through their stomata, colonizes the stem, and produces stem cankers the following 

growing season. In this research, we collected samples from different white pine populations across Canada and 

the United States to develop an efficient ex vitro and in vitro screening method for phenotype expression of 

western white pine (Pinus monticola Douglas ex D. Don) following inoculation with C. ribicola. A major group 

of partial resistant plants in British Columbia, described as difficult-to-infect (DI), presented significant 

resistance in our field trials. We developed a disease assessment index, based on both in vitro and ex vitro 

techniques, to evaluate specific reactions to the pathogen of the DI plants. A disease progression index (DPI) for 

each category has been established. Ribes leaves from 2-year-old plants have been successfully cultured and 

maintained on the half-strength Murashige and Skoog (MS) medium supplemented with 0.02μM NAA+0.1μM

IBA. These in vitro inoculated leaves produced urediospores after 2 weeks and teliospores after 6 weeks in 

culture. The preliminary results from our DI screening experiments indicated a significant difference in the 

number of successful infections between DI and control populations. Further morphological investigation into 

the mechanism(s) responsible for these variations with electron microscopy revealed a considerable difference 

in the morphology of stomata. Also, the amount of epicuticular wax on the stomata of the resistant populations 

was significantly higher than the control plants. These adaptations could provide a greater structural defense

system against white pine blister rust.

 1 BC Forest Service, PO Box 9519, Stn Prov Govt, Victoria, BC V8W 9C2 Canada.

Corresponding author: dnoshad@interchnage.ubc.ca.
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Resistance Gene Management: 

Concepts and Practice

Christopher C. Mundt1

Abstract

There is now a very long history of genetics/breeding for disease resistance in annual crops. These efforts have 

resulted in conceptual advances and frustrations, as well as practical successes and failures. This talk will 

review this history and its relevance to the genetics of resistance in forest species. All plant breeders and 

pathologists are familiar with boom-and-bust cycles of single major resistance genes. Though there may be 

occasional situations where a single-gene approach to resistance is reasonable, most interest is in finding 

alternatives to this approach. This is especially true of long-lived species, for which durability of resistance 

becomes essential.

Combinations or “pyramids” of major genes can greatly increase durability of resistance and have provided 

some major success stories in annual crops. The mechanisms by which pyramids contribute durability, however, 

still are not entirely clear, and this is of importance to potential for success of this approach. Accumulating 

multiple resistances in a single genotype is complicated by epistatic effects of major genes, though both 

phenotypic and marker approaches can reduce this difficulty to some degree. Deploying major genes in mixed 

populations has been highly successful in some cases, including for some woody perennials. There is 

considerable variation in the epidemiological effectiveness of mixtures, however, and much is yet to be learned 

about the mechanisms behind this variability. Concern about selection for “superraces” in host mixtures has 

likely been greatly overstated, and the mechanisms by which selection for multiple virulences occur are much 

more complex than a simplistic evaluation of “virulence costs.”

Once firmly established in a breeding program, the value of quantitative resistance (QR) is difficult to argue 

against. Though erosion due to pathogen adaptation may be possible, QR has a solid history of durability. Some 

have avoided QR because of its perceived complexity of inheritance. Research has shown, however, that QR is 

much less complex than initially thought. In recent years, many QTL studies have been conducted, and this 

information may be useful in both augmenting field selection for QR and in avoiding “Vertifolia effects” in the 

presence of effective major genes. A significant challenge, however, will be in identifying QTL that express in 

multiple genetic backgrounds of the host.

Theoretical considerations aside, the best approach to breeding for resistance also will depend strongly on 

availability of resistance sources and on other traits being addressed in the breeding program. Flexibility and 

forethought must also be used in preparing for changes in silvicultural practices, introduction of new pathogen 

populations, and climate change that may influence disease. Combining different approaches to resistance 

should increase both the epidemiological effectiveness and durability of resistance.

 1 Dept. of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis, OR 97331-2902.

Corresponding Author: mundtc@science.oregonstate.edu.

95

Ecology and Evolutionary Biology

of Resistance and Tolerance -

Natural Systems

Whitebark pine (Pinus albicaulis) at Crater Lake National Park in southern Oregon.

(Photo: Robert Mutch) 

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An Overview of Ecological and Evolutionary 

Research on Disease in Natural Systems: An 

Annotated Reference List

Helen M. Alexander1

Background

The Fourth International Workshop on the Genetics of Host-Parasite Interactions in Forestry (July 31-

August 5, 2011) included a session on “Ecology and Evolutionary Biology of Resistance and 

Tolerance, Natural Systems.” Within this session, I gave a talk entitled “An overview of ecological 

and evolutionary research on disease in ‘natural’ systems” that reviewed research on disease that is 

primarily in ecology and evolutionary biology journals (as opposed to forestry and plant pathology 

journals). Several people requested that I provide details of the references listed in that talk, and this 

annotated reference list is the result.

To reduce the length of this document, I note that several of the references in my talk were cited in 

a recent review article (Alexander 2010). Thus, instead of writing the full reference, I have simply 

listed “PD” next to the names of the authors for citations included in this review.

Overview and a Brief Timeline of Studies of Disease by 

Ecologists/Evolutionary Biologists

Most work on plant disease has been done by plant pathologists working in agriculture or forestry. In 

contrast, ecologists and evolutionary biologists working in less applied settings have historically

largely ignored disease (but see Haldane 1949, PD). Harper, a well-established British ecologist, was 

one of the first to focus attention on disease when he included a chapter on pathogens in his classic

book on plant population biology (Harper 1977 PD). The 1970s and 1980s were also a time when

many researchers compared disease levels in crops and wild plants (e.g., Thresh 1981 PD). Burdon’s

(1987 PD) book on the interactions between pathogens and the ecology and genetics of plant 

populations stimulated considerable work. Since the 1980s, studies of plant disease by ecologists and 

evolutionary biologists in natural/unmanaged systems have become increasingly common.

The reference to plant disease in “natural” systems is potentially confusing. Realistically, there are 

no plant communities on earth that are not in some way impacted by humans and, of course,

indigenous people have always interacted with plant communities. It is useful, however, to recognize 

that there is a continuum of landscapes from “managed” on one hand to “unmanaged/natural” on the 

other hand. An example of the former is row crops in western agriculture where there is reduced plant 

species diversity, high uniformity within plant populations (low genetic diversity, even-age age 

structure, regular plant spacing), and a lack of linkage in plant dynamics from year to year (i.e., plants 

growing at a site are not the result of seeds produced at the site in the previous year). In contrast, 

unmanaged systems typically have higher species diversity, more complex structure within a plant 

population, and a linkage in plant dynamics from year to year. Unmanaged systems can include 

apparently pristine settings, but also apply equally well to disturbed environments such as roadside 

weed communities. 

 1 Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045.

halexander@ku.edu.
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Overview of Talk: Studies of Disease in Relation to Plant 

Population Dynamics, Evolutionary Interactions Between Plants 

and Pathogens, Plant Community Composition, and Global Change 

and Ecosystem Function

Within these areas there are biases, with more research on the effects of pathogens on plants than vice 

versa and more emphasis on fungal pathogens than other types of pathogens.

Plant Population Dynamics 

Research in this area examines how pathogen populations affect the size and spatial distribution of 

plant populations. A variety of approaches are used; excellent examples include experimental 

approaches (such as pesticide applications; Roy et al. 2011) and long-term surveys of plant and 

pathogen numbers (research on anther-smut disease; Antonovics 2004 PD, Antonovics et al. 1998

PD). Both of these examples also have a strong modeling component. Other examples of research in 

disease/plant population dynamics include:

a. Effects of disease on plant age/stage composition (Davelos and Jarosz 2004).

b. Effects of pathogens on seed populations and seed bank persistence (Beckstead et al. 2010, Eviner 

and Chapin 2003 PD, Meyer et al. 2007 PD).

c. Density-dependent effects of disease and the degree to which plants and plant populations can 

compensate for disease losses (Alexander and Mihail 2000 PD, Lively et al. 1995 PD).

d. Examining plant-pathogen interactions on landscape spatial scales; effects on plant metapopulation 

dynamics (Antonovics 1999 PD, Antonovics 2004 PD, Antonovics et al. 1994 PD).

e. Temporal and spatial variation in disease (19–year-study over large number of Swedish islands;

Smith et al. 2011).

Other examples include: Alexander et al. 2007 PD, Antonovics 2004 PD, Antonovics et al. 1994 

PD, Augspurger and Kelly 1984 PD, Carlsson et al. 1990, Carlsson-Granér and Thrall 2002 PD,

Koslow and Clay 2010, Laine and Hanski 2006, Reinhart and Clay 2009, Smith et al. 2003 PD, Thrall 

et al. 2001 PD.

Evolutionary Interactions Between Plants and Pathogens: Genetic

Variation in Resistance, Virulence, and Tolerance 

Studies of genetic interactions between plants and pathogens have a long history, and were the 

primary focus of talks by B. Roy and J. Burdon at this conference. The references below provide 

examples of this kind of work: Antonovics et al. 2010, Barrett et al. 2007 PD, Barrett et al. 2009,

Burdon and Thrall 2009, Burdon et al. 2002 PD, Carr et al. 2003, Carr et al. 2006, Dinoor 1977 PD,

Gilbert and Parker 2010, Harry and Clarke 1986, Inglese and Paul 2006 PD, Jarosz and Burdon 1991,

Koslow and Clay 2007, Laine 2004 PD, Laine et al. 2011, Roy 1993 PD, Roy and Bierzychudek 

1993, Roy and Kirchner 2000 PD, Roy et al. 2000 PD, Roux et al. 2010, Simms 1993, Springer 2007 

PD, Thompson and Burdon 1992 PD, Thrall et al. 2002 PD.

Of particular importance is research that links ecology and genetics; examples include:

a. Links between resistance structure and disease levels and plant dynamics: Alexander et al. 1996 

PD, Thrall and Burdon 2000 PD, Thrall and Jarosz 1994 PD, Laine 2004 PD, Springer 2007 PD.

b. Passive resistance – heritable traits other than resistance genes affect field disease levels:

Alexander 1989 PD, Alexander et al. 1993, Biere and Antonovics 1996 PD, Giles et al. 2006 PD.

c. Links between community ecology and population genetics/phylogeny

1. Host shifts: Antonovics et al. 2002 PD, López-Villavicencio et al. 2005 PD, Roy 2001 PD.
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2. Use of metagenomics to determine viral prevalence (70 percent and 25 percent of samples 

across tropical forest and prairie communities, respectively, have viral RNA): Muthukumar et 

al. 2009 PD, Roossinck et al. 2010.

3. Role of phylogeny in host range: Gilbert and Webb 2007 PD, Webb et al. 2006.

Plant Community Composition

Research in this area includes work examining the direct and indirect effects of pathogens on the 

number of plant species in an area and their relative frequency. Many of the classic studies in this area 

come from work in forest systems, such as research on mortality centers caused by Phellinus weirii in 

mountain hemlock forests (Hansen and Goheen 2000 PD) and work on sudden oak death in California 

forests (Rizzo and Garbelotto 2003 PD, Rizzo et al. 2005 PD).

Three subdisciplines of community ecology have been of major interest in recent years:

1. Interactions of pathogens with multiple hosts: apparent competition and pathogen spillover.

Generalist pathogens may lead to the development of “apparent competition” where increases in 

pathogen populations on tolerant hosts may “spill over” and lead to increased disease on less tolerant 

hosts (Holt 1984 PD, Power and Mitchell 2004 PD). In addition to forestry examples (e.g., sudden 

oak death), another classic example of this phenomenon is work on barley yellow dwarf virus in 

California, where increases in virus prevalence and aphid vector numbers on exotic grasses can lead 

to adverse viral impacts on nearby native grasses (Malmstrom et al. 2005 PD). Recent work by 

Cronin et al. (2010) suggests that host physiological phenotypes may be predictive of pathogen 

reservoir potential. 

2. Soil microbial community: feedback studies, Janzen-Connell studies. Several models and research 

groups have examined similar questions such as the following: Consider plant species “A:” Does the 

presence or high density of plant species A contribute to the development of a soil microbial 

community that leads to low survival for individuals of species A (but not species B, C, D, etc.)?.

Such processes may be important in the maintenance of plant species diversity. Research examples 

include: Bagchi et al. 2010, Bever 2003 PD, Bever et al. 1997 PD, Diez et al. 2010, Holah and 

Alexander 1999, Klironomos 2002 PD, Mangan et al. 2010, Mills and Bever 1998 PD, Packer and 

Clay 2000 PD, Petermann et al. 2008 PD, Reinhardt et al. 2003 PD, Reynolds et al. 2003 PD, Swamy 

and Terborgh 2010, Van der Putten et al. 1993 PD. Many of the studies provide general support for 

the theories noted above, but there are major gaps in our knowledge. In particular, there is a tendency 

in previous work to treat the microbial community as a “black box” in most experiments. Further, 

most theory in this area focuses on specialist pathogens, yet most soil pathogens are generalists.

3. Invasion ecology and disease. A major issue in ecology and conservation biology is the 

unprecedented movement of species around the world due to both intentional and nonintentional 

introductions. One mechanism that may be important is the idea of “enemy release,” where pathogens 

regulate plant populations in native sites, but the reduced pathogen presence or pressure in introduced 

sites allow for an expansion of the plant population. There is supporting evidence for this mechanism, 

but results are variable and can depend on the system being studied (Agrawal et al. 2005, Diez et al.

2010, Mitchell et al. 2006 PD, Mitchell et al. 2010, Parker and Gilbert 2007 PD, Reinhart et al. 2003 

PD, Roy et al. 2011, van Kleunen and Fisher 2009 PD). Current work on invasion ecology and 

disease have focused on a) plant demography and dynamics (Chun et al. 2010, Roy et al. 2011), b) 

role of generalists versus specialist enemies (Halbritter et al. 2012), c) variation among species in 

enemy release (Blumenthal et al. 2009), and d) accumulation of pathogens on invasive plants and 

evolutionary responses (Diez et al. 2010, Gilbert and Parker 2010, Mitchell et al. 2010).

Global Change and Ecosystem Function

In recent years, there is increasing emphasis on how global change variables may affect pathogen 

populations and communities, and in turn, how pathogens may alter the functioning of ecosystems.

Due to time limitations in my talk, I did not cover this area deeply. Examples of work, however,

include:
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a. Effects of temperature, carbon dioxide, and nutrients on plant disease: Mitchell et al. 2003 PD,

Nordin et al. 1998 PD, Nordin et al. 2009, Roy et al. 2004 PD, Strengbom et al. 2002 PD, Strengbom 

et al. 2006, Tylianakis et al. 2008.

b. Genetic variation in pathogen populations in response to environmental change: Laine 2007 PD.

c. Effects of disease on ecosystem function: Eviner and Likens 2008 PD, Mitchell 2003 PD, Lovett et 

al. 2006 PD.

Conclusions

As we contemplate the future, it is important to emphasize that natural areas are increasingly 

fragmented and occur within a mosaic of human impacted biomes (see concept of “anthropogenic 

biomes,” Ellis and Ramankutty 2008 PD). An area worthy of increased emphasis is studies of disease 

at the interface between agriculture/forestry and “natural” habitats (Burdon and Thrall 2008 PD,

Fabiszewski et al. 2010 PD).

Literature Cited

As noted earlier, to save space on references, many citations above have the letters “PD” after them. 

This means that the full reference can be found in a recent review article in the journal Plant Disease

(Alexander 2010); such PD references are not listed below.

Agrawal, A.A.; Kotanen, P.M.; Mitchell, C.E. [et al.]. 2005. Enemy release? An experiment with congeneric 

plant pairs and diverse above- and belowground enemies. Ecology. 86: 2979–2989. 

Alexander, H.M. 2010. Disease in natural plant populations, communities, and ecosystems: insights into 

ecological and evolutionary processes. Plant Disease. 94: 492–503.

Alexander, H.M.; Antonovics, J.; Kelly, A.W. 1993. Genotypic variation in disease resistance – physiological 

resistance in relation to field disease transmission. Journal of Ecology. 81: 325–333.

Antonovics, J.; Thrall, P.H.; Burdon, J.J. [et al.]. 2010. Partial resistance in the Linum-Melampsora host 

pathogen system: does partial resistance make the red queen run slower? Evolution. 65: 512–522.

Bagchi, R.; Swinfield, T.; Gallery, R.E. [et al.]. 2010. Testing the Janzen-Connell mechanism: pathogens 

cause overcompensating density dependence in a tropical tree. Ecology Letters. 13: 1262–1269.

Barrett, L.G.; Thrall, P.H.; Dodds, P.N. [et al.]. 2009. Diversity and evolution of effector loci in natural 

populations of the plant pathogen Melampsora lini. Molecular Biology and Evolution. 26: 2499–2513.

Beckstead, J.; Meyer, S.E.; Connolly, B.M. [et al.]. 2010. Cheatgrass facilitates spillover of a seed bank 

pathogen onto native grass species. Journal of Ecology. 98: 168–177. 

Blumenthal, D.; Mitchell, C.E.; Pysek, P. [et al.]. 2009. Synergy between pathogen release and resource 

availability in plant invasion. Proceedings of the National Academy of Sciences. 106: 7899–7904.

Burdon, J.J.; Thrall, P.H. 2009. Coevolution of plants and their pathogens in natural habitats. Science. 324: 

755–756.

Carlsson, U.; Elmqvist, T.; Wennstrom, A. [et al.]. 1990. Infection by pathogens and population age of host 

plants. Journal of Ecology. 78: 1094–1105.

Carr, D.E.; Murphy, J.F.; Eubanks, M.D. 2003. The susceptibility and response of inbred and outbred 

Mimulus guttatus to infection by Cucumber mosaic virus. Evolutionary Ecology. 17: 85–103.

Carr, D.E.; Murphy, J.F.; Eubanks, M.D. 2006. Genetic variation and covariation for resistance and 

tolerance to Cucumber mosaic virus in Mimulus guttatus (Phrymaceae): a test for costs and constraints.

Heredity. 96: 29–38. 

Chun, Y.J.; van Kleunen, M.; Dawson, W. 2010. The role of enemy release, tolerance, and resistance in plant 

invasions: linking damage to performance. Ecology Letters. 13: 937–946.

Cronin, J.P.; Welsh, M.E.; Dekkers, M.G. [et al.]. 2010. Host physiological phenotype explains pathogen 

reservoir potential. Ecology Letters. 13: 1221-1232.
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Davelos, A.L.; Jarosz, A.M. 2004. Demography of American chestnut populations: effects of a pathogen and a 

hyperparasite. Journal of Ecology. 92: 675–685.

Diez, J.M.; Dickie, I.; Edwards, G. [et al.]. 2010. Negative soil feedbacks accumulate over time for nonnative plant species. Ecology Letters. 13: 803–809.

Gilbert, G.S.; Parker, I.M. 2010. Rapid evolution in a plant-pathogen interaction and the consequences for 

introduced host species. Evolutionary Applications. 3: 144–156.

Halbritter, A.H.; Carroll, G.C.; Güsewell, S. [et al.]. 2012. Testing assumptions of the enemy release 

hypothesis: generalist versus specialist enemies of the grass Brachypodium sylvaticum. Mycologia. 104:34–

44.

Holah, J.C.; Alexander, H.M. 1999. Soil pathogenic fungi have the potential to affect the co-existence of two 

tallgrass prairie species. Journal of Ecology. 87: 598–608. 

Harry, I.B.; Clarke, D.D. 1986. Race-specific resistance in groundsel Senecio vulgaris to the powdery mildew 

Erysiphe fischeri. New Phytologist. 103: 167–176.

Jarosz, A.M.; Burdon, J.J. 1991. Host-pathogen interactions in natural populations of Linum marginale and 

Melampsora lini II. Local and regional variation in patterns of resistance and racial structure. Evolution. 45: 

1618–1627.

Koslow, J.M.; Clay, K. 2007. The mixed mating system of Impatiens capensis and infection by a foliar rust 

pathogen: patterns of resistance and fitness consequences. Evolution. 61: 2643–2654.

Koslow, J.M.; Clay, K. 2010. Spatial and temporal patterns of rust infection on jewelweed (Impatiens 

capensis). International Journal of Plant Sciences. 5: 529–537.

Laine, A.L.; Hanski, I. 2006. Large-scale spatial dynamics of a specialist plant pathogen in a fragmented 

landscape. Journal of Ecology. 94: 217–226. 

Laine, A.L.; Burdon, J.J.; Dodds, P.N. [et al.]. 2011. Spatial variation in disease resistance: from molecules 

to metapopulations. Journal of Ecology. 99: 96–112.

Mangan, S.A.; Schnitzer, S.A.; Herre, E. [et al.]. 2010. Negative plant-soil feedback predicts tree-species 

relative abundance in a tropical forest. Nature. 466: 752–755.

Mitchell, C.E.; Blumenthal, D.; Jarosik, V. [et al.]. 2010. Controls on pathogen species richness in plants’ 

introduced and native ranges: roles of residence time, range size, and host traits. Ecology Letters. 13: 1525–

1535.

Nordin, A.; Strengbom, J.; Forsum, A. [et al.]. 2009. Complex biotic interactions drive long-term vegetation 

change in a nitrogen enriched boreal forest. Ecosystems. 12: 1204–1211.

Reinhart, K.O.; Clay, K. 2009. Spatial variation in soil-borne disease dynamics of a temperate tree, Prunus 

serotina. Ecology. 90: 2984–2993.

Roossinck, M.J.; Prasenjit, S.; Wiley, G.B. [et al.]. 2010. Ecogenomics: using massively parallel 

pyrosequencing to understand virus ecology. Molecular Ecology. 19: 81–88.

Roux, F.; Gao, L.; Bergelson, J. 2010. Impact of initial pathogen density on resistance and tolerance in a 

polymorphic disease resistance gene system in Arabidopsis thaliana. Genetics. 185: 283–291.

Roy, B.A.; Bierzychudek, P. 1993. The potential for rust infection to cause natural selection in apomictic 

Arabis holboelli (Brassicaceae). Oecologia. 95: 533–541.

Roy, B.A.; Coulson, T.; Blaser, W. [et al.]. 2011. Population regulation by enemies of the grass 

Brachypodium sylvaticum: demography in native and invaded ranges. Ecology. 92: 665–675.

Simms, E.L. 1993. Genetic variation for pathogen resistance in tall morning glory. Plant Disease. 77: 901–904.

Smith, D.L.; Ericson, L.; Burdon, J.J. 2011. Co-evolutionary hot and cold spots of selective pressure move in 

space and time. Journal of Ecology. 99: 634–641.

Strengbom, J.; Englund, G.; Ericson, L. 2006. Experimental scale and precipitation modify effects of 

nitrogen addition on a plant pathogen. Journal of Ecology. 94: 227–223. 
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Swamy, V.; Terborgh, J.W. 2010. Distance-responsive natural enemies strongly influence seedling 

establishment patterns of multiple species in an Amazonian rain forest. Journal of Ecology. 98: 1096–1107. 

Tylianakis, J.M.; Didham, R.K.; Bascompte, J. [et al.]. 2008. Global change and species interactions in 

terrestrial ecosystems. Ecology Letters. 11: 1351–1363.

Webb, C.O.; Gilbert, G.S.; Donoghue, M.J. 2006. .Phylodiversity dependent seedling mortality, size 

structure, and disease in a Bornean rain forest. Ecology. 87: S123–S131.
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What Do We Know About Mechanisms for Tolerating

Pathogens, and Can Tolerance Be Applied to 

Managing Tree Diseases?

Bitty A. Roy1

Abstract

The terms ‘‘resistance’’ and ‘‘tolerance’’ have been used by different scientists to refer to different things, and 

they have often been measured (and thus operationally defined) in ways that confuse the two concepts with each 

other. In keeping with the emerging consensus on resistance and tolerance, the following conceptual distinction 

is useful: resistance refers to traits that prevent infection or limit its extent, and tolerance refers to traits that do 

not reduce or eliminate infection, but instead reduce or offset its fitness consequences. Thus, resistance and 

tolerance can both improve host fitness; resistance does so by reducing infection, whereas tolerance does so by 

reducing the fitness loss under infection. In this review, I will briefly set up the differences between resistance 

and tolerance, then discuss what we know about mechanisms for tolerance and what is known about tolerance in 

relation to tree diseases. 

 1

Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, OR.

Corresponding author: bit@uoregon.edu.
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Approaches to Understanding the Impact of LifeHistory Features on Plant-Pathogen Co-Evolutionary 

Dynamics

Jeremy J. Burdon,1 Peter H. Thrall,

1 and Adnane Nemri1

Abstract

Natural plant-pathogen associations are complex interactions in which the interplay of environment, host, and 

pathogen factors results in spatially heterogeneous ecological and epidemiological dynamics. The evolutionary 

patterns that result from the interaction of these factors are still relatively poorly understood. Recently, 

integration of the appropriate spatial and temporal context of the metapopulation has permitted an essential 

conceptual advance stressing the importance of these factors in evolutionary interactions. Yet, a major aspect of 

host and pathogen biologies that still needs considerable attention is that of the impact of particular life-history 

traits of both host and pathogen and how these may influence patterns of disease incidence and prevalence and 

the evolution of host resistance and pathogen infectivity. Here we outline the importance of both empirical 

comparisons and simulation modelling as a means of uncovering the full impact of host and pathogen life 

history traits on co-evolutionary dynamics.

Key words: empirical comparisons, simulation models, co-evolutionary dynamics

Introduction

Individual plant species frequently have wide geographic ranges that cover considerable latitude and 

distinctly different climatic zones. Yet, within particular regions their distribution is usually patchy 

and individual demes are often associated with local micro-environments showing specific moisture, 

temperature, and humidity regimes. Over the course of evolutionary history this mixture of broad and 

narrow scale environmental variation has provided a range of selective pressures across the spacetime continuum. In turn this has generated a complex mosaic of genetic variation in the plant with 

differentiation within and among regions in a wide range of morphological (e.g., leaf shape and 

thickness), phenological (e.g., timing of bud-break, flowering), biochemical (e.g., lignin/terpene 

content), and other life-history characters (e.g., growth rate, life span, mating system). In a similar 

way, the biology and life-history characteristics of pathogens, particularly their responses to 

environmental cues, often show differentiation among and within regions. Among those traits, host 

specificity, transmission mode, life cycle complexity, and dispersal ability can have major influence 

on spread, diversity, and persistence of pathogen populations (Barrett et al. 2008a). At a broad level, 

these traits control the range of environments, and hence host populations in which pathogens may be 

found, the genetic structure of pathogen populations and, as a result, the extent to which they exert 

selective pressure on their host (Barrett et al. 2009).

Pathogens form associations with their hosts that cover part or all of the host’s range (although, in 

some situations, a pathogen’s range can exceed that of an individual host species if, as is the case for 

many necrotrophic species, they are capable of attacking/parasitizing more than one host). The 

epidemiological and evolutionary interaction that develops between host and pathogen in these 

associations is influenced by the detailed interplay of life-history differences at the individual patch 

and metapopulation level. Clearly, as the spatial scale of interactions expands across the full 

geographic distribution of the host, opportunities for differences in the nature of epidemiological 

outcomes occur both in different individual population associations and in different parts of the 

 1 CSIRO-Plant Industry, GPO Box 1600, Canberra, ACT. 2601 Australia.

Corresponding author: jeremy.burdon@csiro.au.
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overall geographic range of the interaction. Given the potential for stochastic colonization/extinction 

processes in partially isolated local populations and hence differences in the intensity and persistence 

of consequent selective pressures, this raises the possibility of the development of divergent longerterm evolutionary trajectories in different parts of the geographic range of individual host-pathogen 

associations. 

Assessing the Impact of Host-Pathogen Life-History Features on 

the Epidemiological and Evolutionary Trajectory of Specific 

Interactions

How can we start to tease out whether or not life-history features affect the epidemiological and 

evolutionary trajectories of host-pathogen associations, and, if they do, the role of specific life-history 

characters? This would be an essential first step towards developing a more general conceptual 

framework. Importantly, the integration of information on host and pathogen life history has the 

potential to advance our predictive ability with regard to disease outbreak and emergence (Barrett et 

al. 2008a).

Thrall and Burdon (1997) developed a framework based on three realistic scenarios which 

illustrate the link between simple host and pathogen life history combinations and spatial 

considerations (spatial scale of associations; temporal predictability), and how these together are 

likely to influence the longer-term evolutionary trajectory of associations. Those scenarios recognized 

the existence of associations in which the scale of pathogen dispersal was likely to be (a) substantially 

less than that of its host (e.g., soil-borne pathogens); (b) roughly comparable with that of its host (e.g., 

seed borne viruses, vector-borne floral smuts); and (c) substantially greater than that of its host (e.g., 

wind-dispersed rusts, persistent aphid-borne viruses). Subsequently, they used simulation modelling 

to illustrate the importance of spatial scale and the occurrence of multiple demes showing varying 

degrees of interconnectedness in the longer-term ecological and evolutionary dynamics of hosts and 

their pathogens (Thrall and Burdon 1999, 2002).

Within pathogens themselves there is also a major life-history dichotomy between biotrophic and 

necrotrophic fungi. The former essentially rely on living host material for growth and, in its absence, 

surviving as relatively inert long-lived spores or other protected structures. Necrotrophic pathogens, 

on the other hand, exploit host tissue that they kill through toxin release. Unlike biotrophs, during offseason periods these pathogens may continue active growth by saprophytic exploitation of dead plant 

remains. This alternation between parasitic and saprophytic growth phases typically results in 

switches in the intensity, direction, and indeed the traits under selection – a situation that is markedly 

contrast to the patterns of selection experience by typical biotrophic pathogens.

Despite these differences, the most apparent way in which to assess the impact of life-history 

features is to characterize their influence on the epidemiology of the host-pathogen interaction and 

through this ultimately the reciprocal selective pressures applied by host and pathogen. The longerterm evolutionary consequences of such selective pressures are ultimately manifest in a variety of 

ways, but perhaps the clearest is through changes in the incidence of resistance in the host and 

pathogenicity in sympatric pathogen populations. Indeed, plants possess a range of different 

resistance mechanisms whereby they minimize their susceptibility to pathogens and the extent of 

damage they incur. Broadly speaking, these mechanisms can be categorized into qualitative or major 

gene resistance that is typically controlled by single genes with major phenotypic effects, and 

quantitative resistance that is usually controlled by many loci each with small phenotypic effects. In a 

classic genetic sense, qualitative resistance is characterized by Mendelian responses while 

quantitative resistance is typified by continuous variation. In real world host-pathogen systems, some 

plant populations may be protected against particular pathogens by one or other, or by a combination 

of these mechanisms.
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Heuristic arguments have been developed (Burdon et al. 1996) as to the circumstances that may 

favor the evolution of one or other of these genetic mechanisms (largely reflecting the predictability 

of infection and its selective intensity), but as yet no concerted effort has been made to assess their 

incidence in natural systems and little is known about the selective and evolutionary forces that 

determine their balance. There are two clear ways in which to explore these possibilities. First, 

empirically through comparative investigation of situations where various combinations of host and 

pathogen species are assessed and contrasted, and second, through the development of simulation 

models that help develop a general conceptual framework for predicting outcomes in broad classes of 

plant-pathogen interactions.

Empirical Comparisons

In the first of these approaches, a number of combinations can be assessed. Thus:

One Host – One Pathogen; Distinctly Different Environments 

Within single host-pathogen associations, correlations between parameters of pathogen dynamics 

(e.g., prevalence, severity, predictability) and the structure and nature of resistance occurring in 

contrasting host populations provide insights into the way spatio-temporal variation in selective 

intensity and predictability can shape host and pathogen population structure and life history features. 

An example of the insights provided by this sort of comparison is found in the Populus – Melampsora

interaction occurring on the eastern and western sides of the Rocky Mountains. This interaction is one 

involving an annual host target (deciduous leaves) and hence one in which the pathogen tends to 

cycle through massive amplitudes in population size over short periods of time. Because the pathogen 

cannot survive on this host during the winter season, it is repeatedly forced to re-establish through 

migration from more benign environments or from alternate hosts. Under these circumstances, racespecific resistance genes which thwart initial entry of pathogen isolates carrying corresponding 

avirulence genes may provide adequate long-term resistance for the host. On the dry, cold eastern side 

of the Rocky Mountains in western Canada, this pattern of major gene resistance appears to 

predominate (Hsiang and Chastagner 1993). In contrast, west of the mountains, the more temperate 

winter environment means that the pathogen is constantly present, and only multigenic race nonspecific resistance is found.

A similar potential direct effect of the environment on pathogen dynamics leading to varying 

selection pressures on associated hosts is seen in the interaction between Avena fatua (wild oats) and 

the rust fungus Puccinia coronata in Australia. In that situation, host and pathogen population pairs 

occurring in southern and northern New South Wales (NSW) respectively exhibit marked differences 

in the phenology of their interaction. In northern NSW, winters are warmer than further south and 

pathogen populations have the opportunity to develop earlier in the host’s vegetative stage. As a 

consequence, earlier, more damaging disease epidemics are likely to ensue with the potential for 

greater selective impact on their hosts than in the south (Burdon et al. 1983). Consistent with this 

prediction, surveys reveal a marked increase in the incidence of resistance with major phenotypic 

effects in the more benign north.

One Host – One Pathogen; Distinctly Different Major Life History Feature Contrast

Mating system is possibly the life-history character that has the biggest single impact on populations 

of all organisms. It is at the core of the structure of plant populations (Barrett 2002; Barrett et al.

2008) and has been implicated as a determinant of the success of biological control programs to use 

insect herbivores and fungal pathogens to manage invasive weeds (Burdon and Marshall 1981). 

Indeed, it has long been hypothesized that differences in recombination systems provide hosts with 

quite different strategies for controlling their pathogens (cf., the Red Queen hypothesis; Hamilton 

1980). 

In some cases, lack of the opportunity for completing a full life cycle can present major 

impediments to the development of pathogen populations, which in turn influences the geographic 

distribution of resistance in host populations. For example, the rust fungus Cronartium comptoniae is 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

107

a full-cycle heteroecious rust fungus that naturally alternates between its telial and aecial hosts 

(Myrica gale and Pinus contorta respectively). Where the range of these two plant species overlaps, 

the pathogen is also present and significant levels of resistance are found in P. contorta populations. 

However, the geographic range of P. contorta is considerably greater than that of M. gale and outside 

their sympatric range, the pathogen is rarely found; populations of P. contorta are essentially 

susceptible (Hunt and Van Sickle 1984). 

The presence or absence of sexual recombination may also have a marked effect on pathogen 

populations. Evidence for this is seen in two distinct examples – one from agriculture, the other from 

a naturally occurring interaction. In the first case, the contrast found between populations of wheat 

stem rust (P. graminis f.sp. tritici) in the Pacific Northwest and the Great Plains of the United States 

clearly demonstrates the contribution that sexual recombination makes to population diversity. In the 

Pacific North West, sexual recombination occurs annually leading to the generation of an enormous 

number of incipient clonal lineages, each with a distinctive genotype for both neutral (isozyme) and 

selected (pathogenicity) traits. A sample of 427 isolates collected from that population in 1975 

contained 100 distinct pathotypes (Roelfs and Groth 1980), while among a sub-sample of 92 of those 

isolates (Burdon and Roelfs 1985), no two isolates with the same isozyme multilocus phenotype had 

the same combination of avirulence genes. When the possibility of sexual recombination is removed 

from such an interaction, population diversity falls dramatically and the way in which this diversity is 

distributed also changes markedly. Thus, further east in the Great Plains, the virtual eradication of 

barberry (the alternate host required to complete the sexual cycle) during the late 1920s generated 

significant genetic changes. On a yearly average, the number of distinct pathotypes fell by more than 

half while those that did occur could be grouped into a number of distinct clonal lineages that differ 

from one another by an average of 11 virulences and various isozyme markers (Roelfs and Groth 

1980; Burdon and Roelfs 1985).

A not dissimilar effect is seen in the interaction between the herbaceous perennial Linum 

marginale and an associated rust pathogen (Melampsora lini) where differences in the extent of 

sexual recombination are found between montane and plains populations of both species. In montane 

regions the rust pathogen is trapped in an asexual cycle (Burdon and Roberts 1995), where it 

undergoes severe population crashes each winter; its sympatric hosts also show a high level of 

selfing. In contrast, on the plains, host populations show significant levels of outcrossing (~30

percent; Burdon et al. 1999). Although their sympatric pathogen populations face a summer survival 

bottleneck, this is ameliorated by the pathogen’s ability to survive via telia and subsequently undergo 

sexual recombination (Barrett et al. 2008b). Comparisons of the resistance and avirulence structure of 

host and pathogen populations respectively showed significant differences in the diversity of 

pathogen populations between the two regions and the way in which resistance genes were distributed 

within and among host populations.

Two Closely Related But Different Hosts – One Pathogen

While the examples above give hints of the potential impact of mating system on host and pathogen 

diversity, and have strength as a result of their focus on the same host and pathogen species, their 

value is simultaneously weakened by the lack of a common environment. In contrast, analogous 

examples in a common environment could be achieved by comparing patterns of resistance and 

pathogenicity in two closely related, co-occurring host species which differ markedly in a particular 

life history feature. 

To our knowledge, no such comparisons have yet been made, although a number of potential 

species comparisons exist. One such example occurs in Australia and New Zealand where the two 

closely related beach strandline species Cakile edentula (tight inbreeder) and C. maritima (obligate 

outcrosser) are both attacked by the necrotrophic pathogen, Alternaria brassicicola, making this an 

ideal system to investigate the evolution of resistance in individual host populations with contrasting 

recombination constraints (Cousens and Cousens 2011). Furthermore, data already exists for the C. 

maritima – A. brassicicola component of this potential comparison that clearly demonstrates the 
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occurrence of severe pathogen epidemics, and the existence of variation in both host resistance and 

pathogen pathogenicity (Oliver et al. 2001; Thrall et al. 2001) - both necessary pre-conditions for 

pathogens to have a detectable role in the evolution of their hosts.

A not dissimilar example is found in the work of Sicard and her colleagues (2007) who assessed 

evidence for local adaptation in infectivity and aggressiveness in the pathogen Colletotrichum 

lindemuthianum attacking three native Mexican populations of Phaseolus vulgaris. This interaction 

was elaborated with a second association in which the pathogen was the same and the host closely 

related (P. coccineus – Colletotrichum lindemuthianum). The two host species differed in a number of 

life-history features – perenniality, population size and stability, and degree of outcrossing – but,

while the pathogen showed evidence of local adaptation in its interaction with the annual P. vulgaris,

it did not when attacking the longer-lived P. coccineus which is also predominately out-crossing.

One Host – Two Contrasting Pathogens 

Biotrophic and necrotrophic pathogens exhibit distinct differences in their relationship with their 

hosts, especially with respect to survival. Biotroph populations (e.g., many rusts and mildews) 

typically undergo periodic seasonal crashes with individual demes within a metapopulation going 

extinct or being subject to marked genetic drift from one year to the next. Necrotroph populations, on 

the other hand, are usually far less affected by environmental change as they typically can survive in a 

saprophytic phase on dead plant material or generate persistent infections of perennial parts of the 

plant. These different life history strategies have been predicted to generate different epidemiological 

patterns and impose distinctly different selective regimes on hosts (Burdon et al. 1996) and their 

pathogens. Indeed, as demonstrated in the barley scald pathogen Rhynchosporium secalis, selection 

pressures may be quite different between parasitic and saprophytic phases of the life cycle (Abang et 

al. 2006). 

Examples of this category of host–pathogen interaction abound and could include attack by 

different leaf pathogens [e.g., Meadowsweet (Filipendula ulmaria) attacked by Melampsora

(biotrophic rust) and Ramularia (necrotrophic leaf blight)] or by pathogens attacking different organs 

[e.g., Poplar (Populus spp.) attacked by Melampsora (leaf rust) and Hypoxylon (stem canker)]. Thus,

in the latter comparative set of host and pathogens, depending on the environment, both race-specific 

and more broadly based non-specific resistance has been detected in the interaction between Populus

and Melampsora (Hsiang and Chastagner 1993, Prakash and Thielges 1987; see discussion above). In 

contrast, once established, the interaction between Populus and the stem canker fungus Hypoxylon is 

protected from environmental fluctuations, and progress through the tree is primarily determined by 

characters which are controlled through the combined action of multiple genes (French and Manion 

1975).

Simulation Modelling and Meta-Analyses

In contrast to animal host-parasite and agriculturally based plant-pathogen interactions, there has been 

relatively little use of theoretical models to explore aspects of the impact of life-history characters on 

the epidemiological or evolutionary trajectory of natural plant-pathogen associations. However, the 

development of a general conceptual framework is critical for understanding links between different 

systems. For example, much of the current general theoretical work on animal host-parasite dynamics 

has been based on models that differentiate between two broad classes of parasites: micro- and 

macroparasites (Anderson and May 1981). Not only are there qualitative dynamical differences that 

emerge from these models, but this work has led to the recognition that different factors must be 

considered for these two broad classes of host-parasite systems. 

A perceived lack of comparable generalizing principles with respect to natural plant-pathogen 

dynamics is probably due, in part, to a combination of the great range of plant pathogens and their 

diverse life histories (e.g., multiple hosts, free-living stages, vectors, vertical transmission, mortality 

vs. fecundity effects, epidemic vs. endemic) and the fact that study of plant-pathogen interactions has 

tended to arise from agriculture and forestry which focused more on traditional plant pathology 

approaches than on the tools of population biology. However, similar to the useful sets of 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

109

generalizations that have been made for animal diseases (Lockhart et al. 1996), it seems likely that 

plant pathogens can also be divided into broad categories recognizing fundamentally different effects 

that are likely to transcend taxonomic categories. Thus even very general comparisons between 

associations involving (i) systemic diseases of plants with varying degrees of vegetative reproduction 

(Wennström and Ericson 1992); (ii) sporulation and infection characteristics in airborne pathogens 

(Sache and de Vallavielle Pope 1995); (iii) pathogen dispersal scale (Thrall and Burdon 1997); or 

differences in host fecundity and mortality effects (Burdon 1993) all illustrate the potential for life 

history differences to have consequences for co-evolutionary dynamics. 

Indeed, an early comparative study assessing the success of biological control programs aimed at 

reducing the size of invasive weed populations demonstrated that the reproductive mode of the target 

species was an important factor in the success of biological control with species with low or no 

recombination being more vulnerable to pathogens than those with high levels of recombination 

(Burdon and Marshall 1981). More recently, some simple comparative models have been developed 

to generate insights into the most appropriate types of pathogen to deploy in biological control of 

weeds. Using a spatially explicit simulation modelling approach built around simple single lifehistory difference comparisons such as whether a pathogen has its impact solely through host 

fecundity or host mortality, Thrall and Burdon (2004) found that even with such broad caricatures, 

there were profound impacts on host population size and persistence, and the potential for evolution 

of host resistance. Importantly, the effect of these pathogen traits was itself mediated by host traits 

such as longevity. Other simulation studies, supported by some data from natural plant-pathogen 

interactions, also predict that increasing host longevity and connectivity of host patches is likely to 

select for pathogen strains with lower transmission rates as well as for hosts with higher resistance 

(Carlsson-Granér and Thrall 2002, 2006).

Many host and pathogen life-history traits are likely to have at least some impact on the intensity 

of ecological and evolutionary forces affecting individual associations. However, the nature and sign 

of those effects is often less than intuitively obvious, especially when such interactions are considered 

in realistic ecological settings involving spatially patchy, small, and ephemeral host populations in 

which genetic drift, extinction, and re-colonization are relatively frequent occurrences. In these 

circumstances, simulation modelling is a very powerful means of exploring the impact of variation in 

key host and pathogen life history features on patterns of disease incidence and prevalence and likely 

changes in the intensity of selective pressures leading to evolution of variation in resistance and 

virulence. Overall, a simulation modelling approach, particularly one that incorporates both 

demographic and genetic processes, will help understanding of how interactions between host and 

pathogen life-histories influence: 

a) pathogen invasion and spread and the degree to which dynamics are endemic vs. epidemic; 

b) levels of impact on host demography and fitness; 

c) conditions favoring the evolution of different genetic systems of resistance and pathogenicity; 

d) the maintenance of polymorphisms; and

e) correlated life histories of hosts and pathogens that are likely to favor long-term associations.

Conclusions

Individually empirical comparisons and simulation models both have their strengths and weaknesses. 

While comparisons in which one or more life history traits can be held constant provide powerful 

clues to the interactive effects of life history characters and host spatial heterogeneity in the long-term 

development of co-evolutionary trajectories of specific associations, at the same time, empirical 

testing of the causal relationships between, and polarity of, particular life-history attributes and the 

structure of host and pathogen populations is always going to be very difficult. Indeed, ultimately, 

unless sufficient comparisons are available to allow rigorous meta-analyses to be conducted, such 

studies will remain open to criticism that observed patterns are due to nothing more than chance or 

uncontrolled variation in other key factors. Stimulation of a broader range of empirical studies on 
GENERAL TECHNICAL REPORT PSW-GTR-240

110

natural host-pathogen interactions and the development of rigorous meta-analyses represent clear 

research challenges for the immediate future.

Similarly, simulation modelling can be extremely powerful in setting up direct comparisons 

between specific life-history traits and in uncovering counter-intuitive interactions. However, at the 

same time such models also suffer from an inevitable lack of biological reality, rapidly becoming 

unwieldy and complex as more variables are added.

The two approaches we advocate here – empirical comparisons and simulation modelling – are 

essential companion components in developing a strong conceptual framework grouping pathogens 

with similar ecologies. Doing this will stimulate the development of theory, provide testable 

hypotheses for empirical studies, and place the relative differences between long-term coevolved 

systems, recent exotic invasions, novel emerging pathogens, and biological control programs into a 

generally understandable context. 

Acknowledgments

This research was supported by the National Institutes of Health (NIH grant 5RO1 GM074265-01A2).

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Integrating Regeneration, Genetic Resistance, and 

Timing of Intervention for the Long-Term 

Sustainability of Ecosystems Challenged by NonNative Pests – a Novel Proactive Approach

A.W. Schoettle,

1 J.G. Klutsch,1 and R.A. Sniezko2

Abstract

Global trade increases the likelihood of introduction of non-native, invasive species which can threaten native 

species and their associated ecosystems. This has led to significant impacts to forested landscapes, including 

extensive tree mortality, shifts in ecosystem composition, and vulnerabilities to other stresses. With the 

increased appreciation of the importance of healthy ecosystems for watershed protection, wildlife habitat, and 

aesthetics, we present a new management approach specifically designed for the long-term sustainability of 

ecosystems challenged by non-native pests for continued non-timber ecosystem services. Sustaining host 

population resilience in the presence of a non-native pest requires maintenance of the population’s recovery 

capacity after disturbance, adaptive capacity over time, and multi-generational persistence. Therefore, the 

management approach must incorporate a long-term and evolutionary perspective which also incorporates

continued adaptation to climate change. Management to promote self-sustaining host populations to support 

ecosystem processes and services can be implemented (1) in degraded ecosystems as reactive restoration 

management or (2) in threatened ecosystems not yet impacted as proactive intervention. When threatened 

ecosystems can be identified, we can choose to act proactively by gathering baseline genetic and ecological 

characteristics of the species and ecosystems to design timely interventions to increase resilience. Such a

proactive approach herein referred to as the Proactive Strategy, can be contemplated for any threat that is 

anticipated to impact critical ecosystems. In this paper, we use the white pine blister rust – high elevation pine 

pathosystem as an example application. Genetic resistance is an essential management tool for both restoration 

and proactive management, yet the Proactive Strategy can also manipulate the timing of (1) resistance 

deployment and (2) landscape management of the mosaic of stand age class structures. These two additional 

tools provide further opportunities to sustain the host population and genetic diversity and mitigate the 

development of impacts over time. Populations having little disease resistance may need to be enhanced by

direct planting. However, in other cases, manipulation of the age-class structure of stands across the landscape 

in populations in which heritable resistance is present at high enough frequency, can facilitate selection and 

accelerate the evolution of resistance after the pathogen eventually invades. Therefore, management that 

generates a mosaic of patches of different stand ages across a landscape can effectively generate patches with 

different rates of selection upon white pine blister rust invasion, thereby mitigating the effect of mortality in any 

one cohort or stand on overall ecosystem function of the greater landscape. Positioning ecosystems for greater 

resilience before invasion may avert impaired ecosystem conditions after invasion, when ecosystem services 

would be compromised, and reduce the need for later restoration. Regeneration is a stabilizing factor in 

populations; how and when it is managed in coordination with gene frequencies for resistance will play an 

important role in the establishment of a new sustainable condition for the host population and its ecosystems in 

the presence of a non-native pest.

Key words: genetic resistance, disease impacts, proactive strategy, Cronartium ribicola, invasive species

Proactive vs. Reactive Strategy

Global trade increases the likelihood of introduction of non-native, invasive species which can 

threaten native species and their associated ecosystems. This has led to significant forested landscape

 1 USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO. 2 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, OR.

Corresponding author: aschoettle@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

113

impacts, including extensive tree mortality, shifts in ecosystem composition, and vulnerabilities to 

other stresses.

Until recently, the use of genetic resistance to mitigate disease impacts in forest trees was focused 

primarily on plantation species, principally because of their obvious economic importance and the 

cost involved. However, as invasive species continue to decimate ecosystems beyond the traditionally 

managed forests, there is now an ever-increasing appreciation of the importance of sustaining healthy 

ecosystems that provide services such as snow capture, watershed protection, wildlife habitat, and 

aesthetics. As with the plantation species, there will be a cost involved, but we suggest that in some 

cases, it may be time to consider novel approaches specifically designed for the long-term 

sustainability of ecosystems challenged by non-native pests for continued non-timber ecosystem 

services. We present a proactive strategy that offers options for addressing a potential problem early 

and focuses on sustaining healthy ecosystems in the presence of an invasive species.

In some cases, non-native species spread slowly enough through a host species range to allow

early-affected ecosystems to be studied and to identify threatened, but not-yet-impacted, ecosystems. 

For threatened ecosystems, we have a choice between acting proactively to increase resilience of the 

threatened ecosystem or waiting until it is degraded before doing restoration management (fig. 1). If 

ecosystems have been invaded and are already heavily impacted by a non-native species, the 

Restoration Strategy pathway (upper pathway, fig. 1) is the only option for restoring ecosystem 

function. In ecosystems threatened, but not yet heavily impacted by a non-native species, the 

Proactive Strategy (lower pathway, fig. 1) can be followed to minimize impaired ecosystem 

conditions and sustain ecosystem function during naturalization of the pest.

Both approaches have uncertainty. While genetic resistance is an essential management tool for 

both, the Proactive Strategy can also manipulate the timing of resistance deployment and the mosaic 

of stand age-class structures across the landscape to utilize natural processes to further support 

resilience over time. These two additional tools provide further opportunities to sustain populations 

and genetic diversity and mitigate the development of impacts over time (Schoettle and Sniezko 

2007). Additional opportunities of the Proactive Strategy include (1) gathering baseline genetic and 

ecological characteristics of the species and intact ecosystems, including the frequency of resistance 

to the non-native invader; (2) minimizing genetic bottlenecks through gene conservation and 

management efforts; (3) timing interventions relative to invasion for optimal benefit; and (4) 

manipulating natural processes to promote further intervention effectiveness. 

Proactive Strategy approaches can be developed for many ecosystems challenged by invasive 

organisms, climate change, or other anthropogenic stress. For the purposes of this paper, the 

discussion will focus on its application to invasions by Cronartium ribicola, the non-native pathogen 

that causes white pine blister rust (WPBR), in North American five-needle pine (Family Pinaceae,

Genus Pinus, Subgenus Strobus) ecosystems. The fungus was introduced to North America in the 

early 1900s and continues to spread. All of the North American five-needle pines have at least low 

frequencies of genetic resistance, yet high mortality is unavoidable (Sniezko et al. 2011a). The 

southern Rocky Mountains and Great Basin are currently at the expansion front for WPBR in the 

western United States. These landscapes are susceptible to invasion, and microclimate analyses 

suggest that invasion of the pathogen over time is inevitable (Howell et al. 2006). This paper provides 

an overview of some of the key elements of the Strategy and highlights some measures currently 

being implemented (e.g. gene conservation, screening seedling progenies for genetic resistance, stand 

dynamics studies, and modeling) as an integrated program in the southern Rocky Mountains.

The rust disease has caused high mortality in five-needle pine species in the northern Rocky 

Mountains (Tomback and Achuff 2010). The western white pine (Pinus monticola Douglas ex D. 

Don) timber industry collapsed and the incidence of this species is now a small fraction of its historic 

occurrence (Fins et al. 2001). As a consequence of WPBR, mountain pine beetle, and climate change 

pressures, whitebark pine (Pinus albicaulis Engelm.), a non-timber species, has recently been listed as 

a candidate species for endangered status under the Endangered Species Act in the United States, and 
GENERAL TECHNICAL REPORT PSW-GTR-240

114

is listed along with limber pine (Pinus flexilis James) as Endangered under the Alberta Wildlife Act in 

Canada. With early intervention, we can position the threatened populations of five-needle pines so 

that they do not follow the same trajectory as whitebark pine. 

Figure 1—A schematic of pathways for facilitating transitions from native ecosystems threatened by a 

non-native species to functional ecosystems where the invader is present. 

Shifting to an Evolutionary Perspective

Historically, the context for management of genetic resources to mitigate disease impacts has been 

plantation forestry. However, the management goals for non-timber species differ from those for 

species for extracting timber resources. Restoration efforts for timber species focus primarily on 

managing tree growth to merchantable size and are supported by intensive resistance breeding 

programs (e.g., Kearns et al. 2012, Schwandt et al. 2010). Five-needle pines that are not timber 

resources, but are important components of natural ecosystems, such as whitebark, limber, and Rocky 

Mountain bristlecone (Pinus aristata Engelm.) pines, and in some areas western white pine, provide

ecosystem services (e.g., watershed protection and wildlife habitat) that depend not just on the health 

of the pines, but the overall ecosystem (Schoettle 2004, Tomback and Achuff 2010). These pines and 

their ecosystems are traditionally not managed and are in remote, high-elevation areas, including 

wilderness. They are also considered foundation and keystone species and are often the only tree 

species that can tolerate the extreme environmental conditions at treeline. As a consequence, other

species are not available to replace the trees’ ecological functions, should the five-needle pines be lost 

from the ecosystem (Tomback et al. 2011). The management goal for these ecosystems is promotion 

of self-sustaining pine populations in the presence of WPBR and other stresses to support ecosystems 

processes and services (Keane and Schoettle 2011, Schoettle and Sniezko 2007). Therefore, success is 

evaluated on multi-generation outcomes of the treated landscape in contrast to the stand-based, 

generation-by-generation, outcomes for plantation forestry.

Sustaining population resilience requires maintenance of populations’ (1) recovery capacity after 

disturbance, (2) genetic diversity to support adaptive capacity over time, and (3) multi-generational 

persistence. Therefore, the management approach must incorporate a long-term and evolutionary 

perspective which also incorporates adaptation to climate change. Maintaining a functioning 

Time

Natural 

Ecosystem

Functional Ecosystem

in the Presence of 

non-native pest

Early 

Invasion Restoration Impaired 

Ecosystem

Recovering

Ecosystems

Restoration Strategy

Modified Natural 

Informed Ecosystem 

Intervention

Late 

Invasion

Proactive Strategy

Data 

gathering
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

115

regeneration cycle is essential as it is the engine that supports post-disturbance recovery and enables 

selected traits to accumulate within populations (fig. 2 – solid narrow arrows). Unfortunately for the 

high-elevation, non-timber five-needle pines, generation time is very long. The species are not 

prolific seed producers and establishment of seedlings after disturbance is protracted in the harsh 

high-elevation habitats and is constrained by competition by other species on more moderate sites 

(e.g., Coop and Schoettle 2009). Once established, high-elevation five-needle pine seedlings mature 

very slowly and only begin to produce full cone crops after 100 years under natural conditions. The 

mature trees can be long-lived, commonly reaching ages of 1,000 years. These species are tolerant of 

stresses under which they have evolved, but are not well equipped, without additional regeneration 

opportunities, for rapid adaptation to novel stresses such as those imposed by the introduction of C.

ribicola in a changing climate.

Figure 2—Flow diagram of the forest regeneration cycle and the points of interaction with white pine 

blister rust and management. White pine blister rust can cause impacts (ovals) at all life stage. Broad 

arrows depict intervention options for increasing blister rust resistance and population resiliency. 

(Redrawn from Schoettle et al. 2009)

Managing Host Demography to Offset Disease-Caused Mortality

In some cases, the regeneration cycle can be managed to increase the resilience of the host population 

to mortality from non-native disease even without genetic disease resistance. Cronartium ribicola

impacts the regeneration cycle of the pines at each life stage; rapidly killing young seedlings, 

reducing cone production in infected mature trees, and later causing mature tree mortality (fig. 2 –

ovals) (Schoettle and Sniezko 2007). At locations where infection is high, the number of survivors 

may be insufficient to sustain populations. Maintaining a functioning regeneration cycle requires trees 

from multiple age classes distributed across landscapes.

Using a population infection model parameterized for high–elevation, five-needle pines and 

WPBR, Field et al. (2012) reveal that pine populations with high seedling recruitment are sustainable 

under moderate levels of WPBR infection (table 1). In contrast, in a situation where regeneration is 

suppressed by inter-specific competition, the model predicts that WPBR-caused mortality and interspecific competition further depress regeneration and the population enters a downward spiral.

Alternatively, if regeneration is stimulated by removing inter-specific competing trees prior to rust 

Maturing 

Trees

Reproductive 

Forest

Established 

Seedlings

Reduced density

Dispersal limitation

Top-kill

Mortality

Top-kill

Less seed production

Mortality

Mortality

Reduced density

Regeneration Cycle

Introduce adaptive traits 

via seedling outplanting

Stimulate establishment 

via site preparation

Seed
GENERAL TECHNICAL REPORT PSW-GTR-240

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invasion, the population remains sustainable under the same infection conditions. The greater the 

regeneration capacity of a population, the greater is its recovery ability and tolerance of the 

population to mortality under moderate disease pressure. Therefore, management to promote 

regeneration in low to moderate rust hazard sites will contribute to populations’ resilience upon C. 

ribicola invasion (fig. 2 – lower broad arrow). Under conditions of greater infection, regeneration can 

slow, but not offset, population decline (table 1). These model runs are conservative as they do not 

include other important disturbance agents such as bark beetles that further compromise the 

population and impose demographic imbalances by selectively reducing mature cohorts at their peak 

of fecundity. 

Table 1—Model predictions of population projections for three high-elevation, five-needle pine 

populationsa 

a The initial stage structure for the no inter-specific competition and high inter-specific competition are at equilibrium 

densities (trees per hectare at zero infection probability) and the initial conditions for the high to low competition condition 

are equilibrium five-needle pine densities with inter-specific competition and then competitors are removed at time zero. 

White pine blister rust is introduced (except for the “zero infection probability” case) at time zero, and projections are for 

200 years. The density (per hectare) predictions include mature adults (MA), total population (TotPop) at 200 years. See 

Field et al. (2012) for more details. Data adapted from Field et al. (2012).

The ability of a population to respond to site manipulation for increased seedling establishment is 

time dependent in the rust-pine pathosystem. Because WPBR kills trees of all ages, and therefore 

directly impacts the pine regeneration cycle, intervention to increase seedling establishment is best 

accomplished before or during the early stages of WPBR invasion. Attempts to stimulate regeneration 

in whitebark pine forests heavily impacted by WPBR have failed to produce established seedlings,

even after more than a decade (Keane and Parsons 2010). Therefore, this intervention option is a tool 

unique to the Proactive Strategy.

The model suggests that management of regeneration can favorably affect the outcome of a 

population under low to moderate rust hazard conditions. We know regeneration capacity varies 

among habitats (Coop and Schoettle 2009). Integrating this understanding into risk and hazard models 

can provide further guidance for the spatial design and prioritization of treatment areas and improved

outcome projections. Regeneration capacity information can be gained through field surveys and 

studies of healthy ecosystems and is much more difficult to ascertain in ecosystems already impaired 

by disease.

Managing the Regeneration Cycle for Increased Disease 

Resistance

Due to the harsh sites that high-elevation, five-needle pines occupy, protracted regeneration dynamics

on those sites, and persistence of introduced pathogens, management of demographic structure and 

distribution alone will not sustain all pine populations in the presence of WPBR. Reducing the effect

of disease on survival and fecundity by increasing heritable disease resistance is essential to 

sustaining many of these populations and ecosystems services. Fortunately, some level of natural 

genetic resistance to this non-native pathogen is present in each North American five-needle pine 

species, and many trials are underway to discern geographic patterns of resistance and identify parent 

Competition

Zero Infection 

Probability

Mod. Infection 

Probability

High Infection

Probability

MA TotPop MA TotPop MA TotPop

None 354 624 171 815 70 165

High 200 354 59 145 38 60

High  Low 404 472 176 746 42 99
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

117

trees and populations for protection and from which seed collections can be made (Schoettle et al. 

2011b; Sniezko et al. 2007, 2011a). Ultimately, we need to increase the frequencies of resistance 

genes over landscapes to help establish a new equilibrium from which pine species and associated 

ecosystems will have the best opportunity to exist and function in the face of the permanent residence

of C. ribicola. We can also learn from studying ecosystems where the disease is native. Resistance, 

life history traits and disturbance dynamics of white pine species that have co-evolved with C. 

ribicola, such as in Russia and other parts of Asia, provide optimism as well as insights into possible 

equilibrium population outcomes for North American species.

Early deployment of genetic resistance through artificial regeneration and stimulating natural 

regeneration to facilitate selection of genetic resistance are key tools for preparing a threatened 

landscape for invasion (Schoettle and Sniezko 2007). The natural frequency of resistance will 

determine if supplemental resistance is required through outplanting (fig. 2 – upper broad arrow) or if 

the natural frequency of resistance can be increased sufficiently via natural regeneration (fig. 2 –

lower broad arrow). 

Benefits of Estimating Baseline Resistance Frequencies

Estimates of disease resistance frequencies and their geographic distributions in threatened native 

pine populations provide valuable information for designing, prioritizing, and evaluating proactive 

management options. Similar to assessing variation in regeneration capacity, estimating the 

frequencies of disease resistance in the host populations is best accomplished in non-invaded 

populations where sampling is not biased by disease-caused tree mortality or reduced cone 

production. Likewise, collections of seed, pollen, buds/scion, and DNA in intact ecosystems offer 

opportunities for gene conservation of the full genetic diversity of the host before it is reduced by the 

disease.

Information on resistance mechanisms and their frequencies helps identify the populations that are 

least and most susceptible to disease-caused mortality. For example, the proportion of seedlings from 

different seedlots collected from healthy populations of Rocky Mountain bristlecone pine that were

disease-free 3 years after heavy artificial inoculation with C. ribicola ranged from 17 to 60 percent,

demonstrating both high levels of natural resistance and high variability across the range of this host 

species (Schoettle et al. 2011b). In combination with mapping disease risk and regeneration potential 

of the site, the types and geographic distribution of genetic resistance can be used to prioritize and 

implement proactive management options (Schoettle et al. 2011a). North American white pine 

species show a number of types of resistance to WPBR (Hoff and McDonald 1980, Kegley and 

Sniezko 2004, Kinloch and Davis 1996, Kinloch and Dupper 2002, Kinloch et al. 2003, Sniezko and 

Kegley 2003, Sniezko et al. 2007, 2008). Early identification of areas with simply-inherited resistance

mechanisms (e.g., R genes) versus those with complex polygenic inheritance also offers insight into

resistance durability which should be included in management planning. 

Supplementing Resistance Through Artificial Regeneration

If resistant stock is planted in a mosaic across the landscape before or at the time of WPBR invasion, 

young resistant trees will be maturing as older, diseased trees are dying (Schoettle and Sniezko 2007).

Not only would these resistant trees be providing ecosystem services in the eventually diseaseimpacted landscape, they would also provide pollen, and eventually seed, for the flow of resistance

genes to neighboring native populations. Therefore, the earlier resistant seedlings can be planted 

before WPBR invasion the shorter the window of time the recovery capacity (i.e., ability to produce 

sufficient seed to regenerate) of the overall population is compromised upon invasion. Planting also 

offers opportunity to provide ‘assisted migration’ if needed to help counter-balance the projected 

impacts of climate change. With a proactive deployment of genetic resistance, the amount of time that 

ecosystem services are at risk for interruption by the disease is reduced.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Development of disease-resistant planting material for proactive deployment requires (1) transport 

of seed from resistant parent trees identified from distant areas already impacted by the disease or (2) 

identification of resistant seedtrees in local, healthy host populations. In populations already invaded, 

seeds are traditionally collected from individual trees that appear disease-free or have less severe 

disease symptoms than most of the other trees in the population. The progeny from these putatively 

resistant trees are then screened for heritable resistance under controlled conditions and further tested 

under natural growing conditions (Sniezko et al. 2011a). In combination with genecological studies, 

the consequences of seed movement (i.e., using non-local seed sources) can be evaluated and 

appropriate seed sources identified. The development of seed orchards is one option to maximize the 

level of, and availability of, resistant seed. However, because maturation of high-elevation, fiveneedle pines is slow, orchards will not provide seed in the near future. For immediate and near future 

needs, the only option is to utilize seed collections from parent trees in natural ecosystems. To 

identify resistant seed trees in populations not yet invaded by the pathogen, pre-selection of seed trees 

for putative resistance traits is not possible, so trees are essentially random samples from the 

population. This may appear to be inefficient compared to identifying putatively resistant seedtrees in 

impacted areas; however the benefits of early resistance identification and deployment for sustained 

ecosystem benefits could be substantial and warrant the extra effort. Fortunately, rust screening 

facilities can evaluate progenies from 100s to 1,000s of parent trees in a relatively short time. With 

proper sampling, these studies also offer added significant benefits for gene conservation and 

providing estimates of the baseline population frequencies of disease resistance, allowing a forecast 

of the likely direct impacts of the disease before it happens and enabling managers to make more 

informed decisions (see above; Schoettle et al. 2011b,Sniezko et al. 2011a, 2011b). 

In the southern Rocky Mountains, genetic resistance to WPBR has been identified in healthy 

limber pine and Rocky Mountain bristlecone pine populations (Schoettle et al. 2011b) and summaries

of the geographic distribution of some resistances are underway. Screening studies allow us to discern 

(1) if there is resistance, (2) what level of resistance is present, and (3) how it is distributed over 

landscapes. They also provide some first ideas on how resistance might be inherited (which 

influences strategies) and how many types of resistance are present and whether they would be 

durable under different sets of circumstances. This provides a foundation for further work on the 

underlying mechanisms and the genes involved. This early detection of resistance has enabled in situ

protection of WPBR-resistant seed trees and populations from the recent mountain pine beetle 

(Dendroctonus ponderosae) epidemic, collection of resistant seed stocks for outplanting and gene 

conservation, and refinement of management plans (Schoettle et al. 2011a).

Facilitating Selection for Resistance Through Landscape 

Management

Planting resistant stock will be needed in populations with little disease resistance. However, in 

populations in which heritable resistance is present at high enough frequency, manipulation of the age 

class structure of stands across the landscape can facilitate selection and therefore accelerate the 

evolution of resistance throughout the population once WPBR invades (Schoettle and Sniezko 2007).

After infection with WPBR, young trees are killed more quickly than older, larger trees, so selection 

for resistance proceeds more rapidly in younger cohorts. The model of Field et al. (2012) 

demonstrates high mortality of young seedlings even when all age cohorts have the same probability 

of infection (see fig. 11 in Field et al. 2012). When an R gene for WPBR resistance is incorporated 

into the model, it shows accelerated increase in the allele frequency in younger cohorts (Schoettle et 

al., unpublished information). Consequently, selection against susceptible genotypes (i.e., diseasecaused mortality) will occur more rapidly in young stands than older stands. Therefore, management 

that generates a mosaic of patches of different stand ages across a landscape can effectively generate 

patches with different rates of selection upon WPBR invasion, thereby mitigating the effect of 

mortality in any one cohort or stand on overall ecosystem function of the greater landscape.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

119

A landscape of diverse age class structures will also reduce the susceptibility of the populations to 

future impacts by mountain pine beetle. Likewise, in other host-pest systems, a diversified host ageclass structure may affect not only the distribution of mortality, but also susceptibility to infection or 

infestation by those pests that preferentially infest one host age or size over another. The scale of the 

landscape, the patch sizes, and the portion of the landscape that is managed can be adjusted to 

optimize gene flow and management objectives. At a minimum, establishment of some refugia 

populations with resistance and genetic diversity would serve as progenitors of future generations and 

re-establishment. The landscape perspective can help manage the effects of mortality and retain 

overall population resilience and ecosystem services.

Continued Management to Promote Further Adaptive Capacity

Whether the genetic resistance was in the native population or added via artificial regeneration, 

maintaining a diverse mosaic of stand structures across the landscape will sustain the benefits of 

initial interventions, facilitate further adaptation to changing climate conditions, and reduce 

susceptibility to mountain pine beetle impacts. Future estimates of resistance frequencies in 

populations can be compared to original baseline resistance frequencies to assess management 

efficacy and detect changes in durability of resistance. Likewise, changes in environmental condition 

with climate change may also affect the expression or efficacy of resistance.

The introduction of a non-native pathogen changes many dynamics for host species (McDonald et 

al. 2005). The disturbance regime to best promote adaptation to this novel stress is not likely to be the 

same as the historic pattern for the species (Coop and Schoettle 2011); monitoring and research is 

needed on these new landscapes as they develop to optimize the mosaic for adaptation and 

sustainability of the pine and ultimately naturalization of the rust.

Applying the Proactive Strategy – Southern Rockies Example

In 2008, the USDA Forest Service Rocky Mountain Research Station (RMRS), Rocky Mountain 

National Park (RMNP), USDA Forest Service, Rocky Mountain Region Forest Health Management, 

several National Forest districts, and USDA Forest Service Dorena Genetic Resource Center initiated 

an application of the Proactive Strategy to conserve and develop management plans for limber pine in

Rocky Mountain National Park and northern Colorado. Approximately 42,000 ha of limber pine 

occur in this area. Most are threatened and not yet impacted by WPBR.

The objectives of the cooperative program to conserve and sustain limber pine on the northern 

Colorado landscape are five-fold: (1) protect limber pine from the mountain pine beetle epidemic so 

seed collections can immediately be made for WPBR resistance tests, genetic conservation, and 

research; (2) screen seedlings for WPBR resistance to determine the frequency of resistance across 

the landscape and to identify resistant parent trees and populations for further seed collections; (3) 

estimate population differentiation along the elevation and latitudinal gradient to refine seed transfer 

guidelines; (4) survey forest health, biotic damage incidence, regeneration capacity, and advanced 

regeneration condition to project persistence of these populations after MPB invasion; and (5) prepare 

proactive management plans for northern Colorado. The program is described in detail by Schoettle et 

al. (2011a).

The Northern Colorado Limber Pine Conservation Program contributes site-specific scientific data 

and tools for decision making about the need for and/or trade-offs of intervention to promote 

sustainability of limber pine populations in the presence of multiple stressors (WPBR and mountain 

pine beetle). Some of the information, tools, and activities, such as estimating rust resistance 

frequency, understanding natural regeneration dynamics, and capturing the pine’s full genetic

diversity via seed collections, can only be taken advantage of in healthy forests that have not yet had 

their processes disrupted by these stressors. Common garden studies are also underway to refine seed 

transfer guidelines. The early availability of information facilitates justification and direction for 
GENERAL TECHNICAL REPORT PSW-GTR-240

120

interventions if prescribed and permits the inclusion of science-based information in prioritizing sites 

for strategic planning. It also provides the opportunity for early implementation of some management 

options, such as stimulating regeneration in populations with high frequencies of rust resistance, that 

are best implemented when those forests are still healthy. Gathering and using this information before

the loss of ecosystem functions allows land managers the widest range of management options to 

sustain limber pine populations and mitigate future impacts in these ecologically important 

ecosystems.

The Proactive Strategy framework can be adapted for other five-needle pine species threatened by 

WPBR and/or mountain pine beetle and possibly for other pathosystems. It is currently also being 

applied in Rocky Mountain bristlecone pine and Great Basin bristlecone pine (Pinus longaeva D.K. 

Bailey) ecosystems and is being adopted for some remaining healthy whitebark pine ecosystems. An 

intensive location-based program, such as that implemented in northern Colorado, is especially 

appropriate for administrative units that want to use local genotypes as much as possible, such as 

national parks. Understanding the interaction of the species’ life history traits and ecology with 

resistance mechanisms will highlight factors that limit the species’ sustainability in the presence of 

the pathogen. Focusing timely management on maintaining genetic diversity and a functional 

regeneration cycle will promote sustained adaptive capacity and ecosystem resiliency. Early 

intervention activities also allow better potential maintenance of overall genetic diversity within the 

species which provides the foundation for the species to continue to evolve in a changing 

environment.

Summary

Managing threatened populations and ecosystems that are not yet impacted by a non-native pest can 

position the population or ecosystem for greater resilience. The Proactive Strategy is especially 

appropriate for long-lived, slow-maturing species. Utilization of genetic tools in conjunction with 

timely management of natural demographic processes can facilitate the transition of the native 

ecosystems to a new and sustainable condition in the presence of the introduced pest. Early research 

on healthy populations enables regeneration capacity and disease resistance frequencies and their 

geographic distributions to be estimated, enabling more accurate forecasting of impacts and providing 

valuable information for designing, prioritizing, and evaluating management options. Early 

intervention allows management to (1) increase regeneration to increase genetic combinations and 

offset disease-caused mortality; (2) deploy resistance in a temporal and spatial array to promote 

population persistence and gene flow; (3) diversify the age class structures on the landscape to affect

selection rates, mitigate impacts of mortality, and promote further adaptive capacity; and (4) preserve 

genetic diversity with in situ and ex situ conservation. Positioning the ecosystems for greater

resilience before invasion may avoid the impaired ecosystem condition after invasion when 

ecosystem services are compromised and reduce the likelihood of the need for restoration later.

Information learned while conducting the Proactive Strategy can likewise help inform restoration 

efforts in impacted ecosystems. Effective implementation of the strategy requires a concerted effort 

by researchers, forest health professionals, and land managers working together.

The existing genetic management programs (including development of disease resistance) for the 

timber species provide infrastructure and a solid foundation of knowledge about genetic disease 

resistance and management from which to more efficiently build management plans for ecosystems 

that deliver essential ecosystem services. Building on this foundation and shifting from a stand-based 

plantation perspective to a landscape evolutionary perspective requires considerations, and provides 

opportunities for managing host demographics and timing of intervention to achieve multigenerational sustainability of populations that can maintain ecosystem functions. Regeneration is a 

stabilizing factor and is an important factor in determining if a new sustainable condition is 

established or the population spirals into decline in the presence of the disease.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

121

Gathering information in threatened, but not-yet-impacted, ecosystems also allows time to explore

the interactions of life-history traits of the host with (1) pathogen infection, (2) regeneration capacity, 

(3) heritable resistance, (4) cost of infection on survival and fecundity, and (5) transitory population 

dynamics (Field et al. 2012). All of these factors affect the outcome of invasion on the populations,

and through modeling, insights can be gained to optimize the type and timing of management 

interventions for the best outcomes. Co-evolution dynamics between the host and pest are also 

important to consider when implementing either proactive or restoration approaches. As we learn 

more about these interactions for specific pathosystems and develop silvicultural prescriptions to 

manipulate regeneration dynamics, it will enable us to further address these and other key questions:

(1) What frequency of resistance(s) is enough to sustain a population under an array of conditions?

(2) What spatial arrangement of structural and genetic diversity on the landscape is best to optimize 

selection, regeneration, gene flow, and adaptive capacity for long-term host population sustainability?

(3) When during the invasion process does intervention achieve optimal ecological effectiveness? 

(4) What will be the characteristics of the new population condition and how will it behave in relation 

to natural disturbances after pathogen naturalization? (5) What are the public preferences and 

economic and ecological trade-offs of different management options implemented at different times 

during the invasion process? 

The recent national-level directive on the management of invasive species across aquatic and 

terrestrial areas of the U.S. National Forest System (U.S. Department of Agriculture, Forest Service 

2011) includes an aspiration to sustain healthy native ecosystems through proactive detection and 

mitigation of ecosystems threatened or impacted by non-native species. High-elevation pine forests, 

under the threat of multiple stressors, serve as an excellent flagship to lead the paradigm shift away 

from crisis management and toward proactive management for ecosystem resilience. The Proactive 

Strategy is a novel approach to providing information and technologies to make informed decisions to 

better sustain mountaintop ecosystems. The components of the strategy are also amendable to other 

biotic and abiotic agents that will impact forest ecosystems.

Acknowledgments

We would like to thank Mike Antolin, Stu Field, Kelly Burns, Jeff Connor, Betsy Goodrich, Angelia Kegley, 

Bill Jacobi, Jonathan Coop, and Phyllis Pineda-Bovin for their collaborations and many discussions regarding 

this topic. We also thank Paul Zambino and John King for their helpful reviews of an earlier version of this 

manuscript.

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Phytopathology. 92: 278–280. 

Kinloch, B.B., Jr.; Sniezko, R.A.; Dupper, G.E. 2003. Origin and distribution of Cr2, a gene for resistance to 

white pine blister rust in natural populations of western white pine. Phytopathology. 93: 691–694.

McDonald, G.I.; Zambino, P.J.; Klopfenstein, N.B. 2005. Naturalization of host-dependent microbes after 

introduction into terrestrial ecosystems: Evolutionary epidemiology of white pine blister rust. In: Lundquist, 

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landscapes. St. Paul, MN: American Phytopathological Society Press: 41–57.

Schoettle, A.W. 2004. Ecological roles of five-needle pines in Colorado: potential consequences of their loss. 

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2011a. The proactive strategy for sustaining five-needle pine populations: an example of its implementation 

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Mountains (USA) in the presence of Cronartium ribicola and Dendroctonus ponderosae in a changing 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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climate. In: Noshad, D.; Noh, E.; King, J.; Sniezko, R., eds. Breeding and genetic resources of five-needle 

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rust resistance screening tests of Pinus flexilis and Pinus aristata. In: Keane, R.E.; Tomback, D.F.; Murray, 

M.P.; Smith, C.M., eds. The future of high-elevation, five-needle white pines in western North America: 

proceedings of the high five symposium. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station: 265–269. 

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perspectives. In: Stone, J.; Maffei, H., comps. 50th Annual Western International Forest Disease Work 

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Disease Service Center: 111–119.

Sniezko, R.A.; Kegley, A.J.; Danchok, R.S.; Long, S. 2007. Variation in resistance to white pine blister rust 

among 43 whitebark pine families from Oregon and Washington—early results and implications for 

conservation. In: Goheen, E.M.; Sniezko, R.A., tech. coords. Whitebark pine: a Pacific Coast perspective.

R6-NR-FHP-2007-01. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Region: 82–97. http://www.fs.fed.us/r6/nr/fid/wbpine/papers/2007-wbp-wpbr-resist-sniezko.pdf. (05 May 

2012).

Sniezko, R.A.; Kegley, A.J.; Danchok, R. 2008. White pine blister rust resistance in North American, Asian, 

and European species – results from artificial inoculation trials in Oregon. Annals of Forest Research. 51: 

53–66. http://www.e-afr.org. (05 May 2012).

Sniezko, R.A.; Mahalovich, M.F.; Schoettle, A.W.; Vogler, D.R. 2011a. Plenary paper: Past and current 

investigations of the genetic resistance to Cronartium ribicola in high-elevation five-needle pines. In: 

Keane, R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. The future of high-elevation, five-needle 

white pines in western North America: Proceedings of the high five symposium. RMRS-P-63. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 246–264.

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Sniezko, R.A.; Schoettle, A.W.; Dunlap, J.; Vogler, D.; Conklin, D.; Bower, A.; Jensen, C.; Mangold, R.;

Daoust, D.; Man, G. 2011b. Ex situ gene conservation in high elevation white pine species in the United 

States-a beginning. In: Keane, R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. The future of highelevation, five-needle white pines in Western North America: Proceedings of the high five symposium. 

RMRS-P-63. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station: 147–149. http://www.fs.fed.us/rm/pubs/rmrs_p063/rmrs_p063_147_149.pdf. (05 May 2012).

Schwandt, J.W.; Lockman, I.B.; Kliejunas, J.T.; Muir, J.A. 2010. Current health issues and management 

strategies for white pines in the western United States and Canada. Forest Pathology. 40: 226–250.

Tomback, D.F.; Achuff, P. 2010. Blister rust and western forest biodiversity: ecology, values and outlook for 

white pines. Forest Pathology. 40: 186–225.

Tomback, D.F.; Achuff, P.; Schoettle, A.W.; Schwandt, J.W.; Mastrogiuseppe, R.J. 2011. Plenary paper: 

The magnificent high-elevation five-needle white pines: ecological roles and future outlook. In: Keane, 

R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. The future of high-elevation, five-needle white pines 

in western North America: Proceedings of the high five symposium. RMRS-P-63. Fort Collins, CO: U.S. 

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policy. Federal Register. Vol. 76 (233): 75860-75866.
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Plant Compensatory Growth in Aspen Seedlings:

The Role of Frequency and Intensity of Herbivory 

and Resource Availability

Nadir Erbilgin,1 David A. Galvez,2 and Bin Zhang3

Abstract

Plant ecologists have debated the mechanisms used by plants to cope with the impact of herbivore damage for 

more than a century. During that time, plant resistance mechanisms, which reduce the amount of herbivore 

damage before and during herbivory, have received most of the attention, while plant tolerance mechanisms, 

which may minimize the impacts of damage after herbivory, have been less studied. The aim of this 

presentation is to bring the topic of plant compensatory responses, especially compensatory growth, to the front 

of research in plant ecology and plant-herbivore interactions. We conducted a greenhouse experiment to 

evaluate how carbon sink-source relationships and compensatory plant growth operate under different 

intensities and frequencies of simulated defoliation of aspen seedlings, with or without N-enriched media. We 

found large variations in plant responses, ranging from undercompensatory to overcompensatory growth,

depending on the resource availability and defoliation intensity and frequency. We developed a new predictive 

model, the Frequency and Intensity of Herbivory and Resource Availability (FIRA), based on carbon sinksource relationships. Our model incorporates the interactions between frequency and intensity of herbivory and 

resource availability as modulators of plant compensatory responses. We concluded by discussing the results of 

this and earlier studies in the context of the FIRA model, and elaborated the intricate relationship between 

resource availability and compensatory growth following herbivory.

 1 University of Alberta, Department of Renewable Resources, 4-42 ESB, Edmonton, Alberta, T6G2E3, Canada. 2 University of Alberta, Edmonton, Alberta, Canada.

3 Southwest University, Chongqing, P.R. China.

Corresponding author: erbilgin@ualberta.ca.
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Trade-Offs Between Induced and Constitutive 

Resistance in Two Pine Species: Secondary 

Chemistry, Effective Antiherbivore-Resistance, and 

Effect of Nutrient Availability

Luis Sampedro,1Xoaquín Moreira,2 and Rafael Zas2

Abstract

Constitutive chemical defenses, always expressed in the plants, and plastic defensive responses, those mobilized 

in response to plant injury or other cues or herbivory risk, differ in their benefits in terms of fitness for longlived plants. Induced defenses are considered to be less expensive than constitutive preformed defenses since 

the cost is realized only when required. Plant defense theory predicts that, as secondary metabolism is costly for 

the plant, presenting effective levels of constitutive defenses and the ability of expressing efficient inducible 

defenses by a plant are two resource-related attributes that are not likely to be maximized at the same time.

Furthermore, selective pressure favoring the expression of induced responses is likely to be lower in those 

lineages well defended constitutively, as they would be less subjected to herbivore attacks. A negative, nonspurious genetic correlation between constitutive and inducible defenses illustrates this classical trade-off. The 

existence of these evolutionary conflicts has been suggested often in the literature and sometimes reported for 

angiosperms, but rarely in conifers and not yet in pine trees. Besides, the emergence of this genetic constraint

could be hidden by environmental factors affecting growth potential, such as nutrient availability.

In this paper we present the results from three independent experiments aimed to explore the existence of this 

trade-off in 2-year-old seedlings of Maritime pine (Pinus pinaster Aiton) and Monterey pine (P. radiata D. 

Don), native from similar climate regions in Europe and North America. We used 22 mM methyl-jasmonate 

(MJ), a fitohormone involved in the biosynthetic pathways of chemical defenses, to induce pine defensive 

responses. In the first experiment, we analyzed the secondary chemistry of constitutive (control) and MJinduced P. pinaster seedlings belonging to 18 half-sib families from the Atlantic population of Galicia 

(northwest Spain). One month after induction, we performed an in vivo feeding bioassay with a generalist insect 

herbivore (the large pine weevil, Hylobius abietis) to check how the expressed defenses reflected the ability to 

resist the attack. In the second experiment, we reproduced the same design with 34 half-sib families belonging 

to P. radiata. In the third experiment, we analyzed this trade-off in a wider collection of P. pinaster genetic 

entries (33 half-sib families) grown either in low- or in high-phosphorus availability, to test if this trade-off 

could be dependent on environmental context. We explored the existence of trade-offs regressing the difference 

in mean resistance levels between experimentally MJ-induced individuals and control individuals from a given 

family (induced-control) against the family means of control treatment. Correlations were checked not to be 

spurious by means of Monte-Carlo analysis. 

We identified, in both species, strong negative genetic correlations between induced and constitutive 

concentrations of total polyphenolics, resin content, and effective resistance against weevil and caterpillar

damage, with R2 ranging from 0.38 to 0.80 (all relationships significant at P < 0.001). Negative genetic 

correlations observed under complete fertilization in P. pinaster appeared to be likely spurious, providing 

evidence that this trade-off is context dependent. 

 1

Centro de Investigación Forestal de Lourizán – UA MBG-CSIC, Apdo 127, 36080 Pontevedra, Galicia, Spain. 2

Misión Biológica de Galicia, MBG-CSIC, Apdo. 28, 36080 Pontevedra, Galicia, Spain.

Corresponding author: lsampe@uvigo.es.
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Host Preference of the Vector Beetle, Host 

Resistance, and Expanding Patterns of Japanese 

Oak Wilt in a Stand

Kazuyoshi Futai,1 Hiroaki Kiku,1 Hong-ye Qi,1 Hagus Tarn,1 Yuko Takeuchi,1

and Michimasa Yamasaki2

Introduction

Since the early 1980s, an epidemic forest disease, Japanese Oak Wilt (JOW), has been spreading from 

coastal areas along the Sea of Japan to the interior of Honshu island and has been devastating huge 

areas of forests by killing an enormous number of oak trees in urban fringe mountains, gardens, and 

parks. The disease is caused by a fungus, Raffaelea quercivora, and is spread by the vector beetle, 

Platypus quercivorus (Coleoptera: Platypodidae). On the thorax of female P. quercivorus, are several 

holes, so called mycangia, where several species of yeasts are kept as the beetles’ food. Together with 

such yeasts, the pathogenic fungus R. quercivora exists in the mycangia and is thereby transferred to 

the next tree. The spreading pattern of this disease is, therefore, determined by the searching and host 

preference behavior of the vector beetle. To control epidemic spreading of JOW, it is essential to

understand the spread dynamics of JOW in infected trees. The object of this study was to analyze the 

spread dynamics of JOW in infected trees in relation to the host selection behavior of vector beetles in 

a stand.

Materials and Methods

Study Site

We have been studying the spreading pattern of this disease at a nearby low elevation mountain (109 

m, 35°01 ́N, 135°47 ́E) in Kyoto city. The experimental site established at the mountain in the spring 

of 2009 is 1.36 ha and involved 288 living oak trees (Quercus serrata Murray) with DBH ranging 

from 10 to 90 cm. 

Categorization of Host Damage

Based on the quality of frass produced from beetle tunnels, the density of beetle attacks (= number of 

entrance holes per given area of trunk surface), exudates from the trunks, and symptoms (alive or 

killed, and if killed, which year it was killed), we diagnosed the damage of each tree and classified the

different levels of damage into seven categories (Cat.) (table 1). 

Spatial Relationships Between Two Categories of Trees

To examine if the trees attacked by P. quercivorus in a given year influenced the occurrence of newly

attacked trees in the following year, we analyzed the overlap between the trees attacked in 2009 and

those newly attacked in 2010 using Iwao’s m-m* method (Iwao 1977). When the individuals

 1 Laboratory of terrestrial microbial ecology, Graduate school of Agriculture, Kyoto University, Sakyo-ku, 606-8502, Kyoto, 

Japan.

2 Laboratory of forest biology, Graduate school of Agriculture, Kyoto University, Sakyo-ku, 606-8502, Kyoto, Japan.

Corresponding author: futai@kais.kyoto-u.ac.jp.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

127

Table 1—Categorization of host trees based on their damage history

Boring attack in Condition in Category

2009 2010 October 2010 (Cat.)

+ + Alive I

+ + Dead II

+ - Alive III

+ - Dead IV

- + Alive V

- + Dead VI

- - Alive VII

belonging to species X and Y are distributed over the same space, the mean crowding on sp. X by sp. 

Y can be given by 

 

and mean crowding on sp. Y by sp. X by

 

where xXj and xYj are the numbers of individuals of sp. X and sp. Y in the j quadrat respectively, and 

Q is the total number of quadrats contained in the whole area. The mean crowding within each 

species is denoted by 

and .

Then, as a measure of the degree of spatial correlation or the degree of overlapping relative to the 

independent distributions, we have

 or 

where the value ω changes from its maximum of +1 for complete overlapping, to the minimum of -1 

for complete exclusion. In this method, we calculated the ω value as an index of spatial overlapping 

of two categories of trees while changing the unit size from 25 m2 to 625 m2 (Iwao 1972).

The Densities of Beetle Holes on the Stems of Host Trees

To estimate the number of beetles visiting each host tree, the densities of beetle holes on the host 

stems were compared among five categories of trees. We also examined the density of beetle holes in 

relation to the distance from the trees of Category III. 
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Results and Discussion

Disease Spreading in the Stand 

There were 288 trees in the spring of 2009. Then in June and July 2009, 177 trees were attacked; 111 

trees remained without attack. Among the 177 trees attacked, only 10 trees were killed and the 

remaining 167 trees survived. In the following year, 2010, 113 (67.7 percent) of 167 trees that 

survived the 2009 attack were re-attacked, while the other 54 trees avoided the attack.

Among 111 trees that avoided the attack in 2009, 103 trees (92.8 percent) were newly attacked in 

2010. So, only 8 trees remained free from Platypus attack over 2 successive years. Thus, the 

probability of being attacked in 2010 was markedly lower in the trees that had survived the attack in 

2009 (67.7 percent) than those not attacked in 2009 (92.8 percent).

Spatial Relationships Between Two Categories of Trees

As shown in fig. 1, the distribution of a group of trees (Category III) that had survived beetle attack in 

2009 and avoided the attack of 2010 did not overlap the distribution of the trees newly attacked in 

2010 (Category V, VI). On the other hand, the distribution of Category I trees that had survived the 

attack in 2009 and were reattacked in 2010 overlapped the trees that were newly attacked in 2010. 

Iwao’s m-m* analysis revealed that the trees newly attacked in 2010 (Category V, VI) were spatially 

segregated from the trees of Category III at the quadrat sizes of 100 m2 and 225 m2 (fig. 2). Thus, 

beetles of P. quercivorus seem to be repelled from the trees of Category III, and are thereby 

segregated from the trees newly attacked in 2010. 

Figure 1—Distribution of the trees newly attacked in 2010 (Category V and VI) and those that had 

survived beetle attack in 2009 and were re-attacked (Category I) or avoided the attack (Category III) 

in 2010.

The Densities of Beetle Holes 

As shown in fig. 3, the density of beetle holes on the surviving trees, such as those in Cat. I and III, 

were significantly lower than those on the stems of dead trees (Cat. II and VI). This suggests that the 

density of boring attack affects host survivability. More importantly, Platypus beetles seem to select 

host trees, preferring the trees of Cat.II and VI, while seeming to be repelled by the trees of Cat. I and 

III. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

129

Figure 2—Spatial overlapping between the trees newly attacked in 2010 (Category V, VI) and the 

trees that had survived beetle attack in 2009 and were re-attacked (Category I) or avoided the attack 

(Category III) in 2010.

Figure 3—The density of beetle holes made on the stems of five categories of trees. 

For categories see table 1.

We also examined the density of beetle attack in relation to the distance from the trees of Category 

III, and found that the density of beetle attack was lower within a 5 m radius around the trees of 

Category III than that of the outer area, suggesting the lower flight frequency in the vicinity of the 

Category III trees. The trees that exuded sap were mainly found among the trees surviving the beetle 

attack in 2009, such as those in Category I and III. 

From these findings we hypothesize that there are some differences in resistance against the attack 

of P. quercivorus beetle among oak trees in a stand. Resistant trees may survive the first attack of the 

beetle and then release some repellent from the trunk and thereby escape the following attack.

Acknowledgments

We thank Dr. H. Barclay, a visiting professor from Pacific Forestry Centre, Victoria, BC, Canada, for critical 

reading of the manuscript and linguistic correction.
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Literature Cited

Iwao, S. 1972. Application of m-m* method to the analysis of spatial patterns by changing the quadrat size. 

Researches on Population Ecology. 14: 97–128.

Iwao, S. 1977. Analysis of spatial association between two species based on the interspecies mean crowding. 

Researches on Population Ecology. 18: 243–260.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

131

The Effect of Hemlock Woolly Adelgid Infestation on 

Water Relations of Carolina and Eastern Hemlock

Laura Rivera,

1 JC Domec,1 John Frampton,1 Fred Hain,1 John King,1 and

Ben Smith1

Abstract

In North America, hemlock woolly adelgid (HWA; Adelges tsugae) is an exotic insect pest from Asia that is 

causing rapid decline of native eastern hemlock (Tsuga canadensis (L.) Carr.) and Carolina hemlock (Tsuga 

caroliniana Engelm.) populations. The exact physiological mechanisms that cause tree decline and mortality are 

not known, despite substantial research efforts on ecological impacts and potential control measures of HWA. 

Eastern and Carolina hemlock may be reacting to infestation in a manner similar to the response of Fraser fir 

(Abies fraseri (Pursh.) Poir.) to infestation by balsam woolly adelgid (BWA; Adelges picea). It is known that 

Fraser fir produces abnormal xylem in response to BWA feeding. This abnormal xylem obstructs water 

movement within the trees, causing Fraser fir to die of water stress.

In this study, water relations within 15 eastern and Carolina hemlock were evaluated to determine if infestation 

by HWA was causing water stress. Water potential, carbon-13 isotope ratio, stem conductivity, and stomatal 

conductance measurements were conducted on samples derived from those trees. In addition, branch samples 

were analyzed for possible wood anatomy alterations as a result of infestation. Pre-dawn branch water potential 

measurements were more negative in infested hemlock than in non-infested trees. Carbon isotope ratios of the 

branches were more positive for infested trees, while stomatal conductance was lower in infested trees. These 

results indicate that infested eastern and Carolina hemlock are experiencing drought-like symptoms. Wood 

anatomy of the branches provided evidence that infested hemlocks are experiencing abnormal wood production 

in the xylem, including lower earlywood to latewood ratios and increased frequency of false rings. The 

significant reduction in conducting sapwood area, terminal branch growth, and leaf area in infested trees were 

sufficient to influence sap flux and whole-tree water use.

 1 North Carolina State University, Raleigh, NC.

Corresponding author: bcsmith6@ncsu.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Genotype x Environment Interaction and Growth 

Stability of Several Elm Clones Resistant to 

Dutch Elm Disease

Alberto Santini,1 Francesco Pecori,1 Alessia L. Pepori,1

and Luisa Ghelardini1

Abstract

The elm breeding program carried out in Italy at the Institute of Plant Protection - Consiglio

Nazionale delle Ricercje (CNR) during the last 40 years aimed to develop Dutch elm disease (DED)-

resistant elm selections specific to the Mediterranean environment. The need for genotypes adapted to 

Mediterranean conditions was evident from the poor performance of the Dutch elm clones in the hot and 

dry areas of central Italy, and the opportunity to breed better adapted hybrids was offered by the favorable 

adaptation in Italy of the Siberian elm (Ulmus pumila L.), a species that does not thrive in central Europe 

and the Netherlands. For this reason, a base of valuable individuals of native elms was bred with several 

accessions of Asian elm species that proved to easily acclimate to different conditions in the Mediterranean 

region. As a result, four resistant hybrid elms have been patented and released to the market, and many 

experimentally tested DED-resistant elm hybrids of different parentage are under evaluation to be released 

in the near future.

Conventionally, hybrid elm clones obtained within DED resistance breeding programs were selected to 

meet requirements for use as ornamentals. However, it has been long and commonly observed that these 

clones may show hybrid vigour and enhanced growth. Nowadays, DED-resistant hybrid elm clones, which 

have been released to the market or are under evaluation for an upcoming release, are numerous enough to 

be considered for timber production or short rotation coppice (SRC). But experimental testing of the growth 

performances of these clones in different environments is still lacking. Here, growth and stability of 

performance of several DED-resistant hybrid elm clones planted at three experimental sites with contrasting 

environmental conditions in Italy were studied. Height and diameter were measured yearly from 2001 to 

2009, and the mean yearly increments after plant establishment were calculated. The study revealed a 

general good growth performance of the majority of the clones with mean height increments above 1 m per 

year, and an excellent growth performance of some genotypes. Analysis of variance showed significant 

effects of clone, site, and clone x site interaction, for both height and diameter increments. Stability analysis 

of diameter and height increments was performed using two parametric (Coefficient of variation, CV 

percent and Wricke's Ecovalence, W2

) and two non-parametric (Hühn’s Si(1) and Si(2)) indexes. According 

to all indexes, two clones showed superior and stable growth. These clones may be suitable for planting in a 

range of environments. In addition, several other clones had high growth in general or at a particular site. 

The results support our belief that these elm clones could be successfully used for timber and biomass 

production, and provide new knowledge for an informed choice of the most suitable genotypes.

 1 Istituto per la Protezione delle Piante – C.N.R Via Madonna del Piano, 10 50019 Sesto fiorentino, Italy. 

Corresponding author:a.santini@ipp.cnr.it.
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133

Rapid Evolution of Introduced Tree Pathogens Via 

Episodic Selection and Horizontal Gene Transfer

Clive Brasier1

Episodic and Routine Selection

Routine selection is simply defined as “the ecological constraints experienced by an endemic

organism that favor a relatively stable but fluctuating population structure over time.” Its antithesis is 

episodic selection, defined as “any sudden ecological disturbance likely to lead to a significant 

alteration in a species’ population structure” (Brasier 1986, 1995). In plant pathogens, examples of 

episodic selection are common in the context of modern anthropogenically driven disturbance 

conditions such as sudden exposure to crop monoculture or to a new fungicide, host, or vector; or 

sudden introduction into a new biogeographic zone; an increasingly frequent event because of 

increasing international trade in plants.

The introduction of pathogens into new biogeographic zones is the concern of the present paper. 

Such an introduction is a major episodic selection event for any organism, let alone a plant pathogen. 

It can result in sudden release from many of the routine selection constraints experienced in the 

endemic environment; and sudden exposure to new constraints such as new hosts, substrates, 

competitors, parasites, phylogenetic relatives, vectors, and microclimates (Brasier 1995). For this 

reason, every introduced pathogen is an uncontrolled, open-ended experiment in evolution (Brasier 

2008).

In populations of outcrossing (heterothallic) pathogens, two significant evolutionary processes can 

occur as a result of introduction-mediated episodic selection. One is the emergence of highly fitted 

clones, examples being the clones of the Dutch elm disease pathogens Ophiostoma ulmi and O. novoulmi that spread across Eurasia and North America between 1910 to1930 and 1940 to 1990 

respectively (Brasier 1996, Brasier and Kirk 2000, Brasier et al. 2004a, Mitchell and Brasier 1994). If 

the episodic selection conditions are maintained over the longer term, this process could ultimately 

lead to clonal speciation (Brasier 1995). The second process, and again the particular concern of this 

paper, is rapid evolution via interspecific hybridization.

Interspecific Hybridization Between Introduced and Resident 

Pathogens

Novel contact between phylogenetically related introduced and resident pathogens will often result in 

their direct ecological competition for resources. It also provides an opportunity for rapid evolution 

via the transfer of genes between the residents and the immigrants (Brasier 2000b). Sympatric fungi, 

like animals, are likely to exhibit strong genetic or behavioral barriers to inter-specific gene flow as a 

result of prior co-evolution. Allopatric fungi, arising in geographical isolation, may not have evolved 

such barriers. Today, many previously allopatric fungal pathogens are being brought into contact by 

man, especially through inappropriate plant imports (Brasier 2008). Figure 1 shows an increase in 

major tree disease events in the United Kingdom since the mid-1990s, some 25 years after the arrival 

of O. novo-ulmi. Many of these new events are caused by introduced pathogens.

 1

Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, U.K.

Corresponding author: clive.brasier@forestry.gsi.gov.uk. 
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Figure 1—Cumulative major tree disease outbreaks in the United Kingdom since 1970. (Courtesy of 

Sandra Denman, Forest Research, UK) 

A variety of factors will influence whether or not contact between two previously geographically 

isolated pathogens will result in hybridization (Brasier 1995, 2000b). These include their level of 

reproductive isolation (both pre-zygotic and post- zygotic); the degree of niche contact between them 

(fig. 2); and the genetic and ecological conditions that will determine whether any hybrids can survive

in the presence of the parent species. Often, hybrids will need to be fitter than one or both parents to 

become established. Modern conditions of anthropogenically driven environmental disturbance are 

more likely to favor the successful establishment of hybrids. In horticultural nurseries, which these 

days can be infested with non-native pathogens, unusual combinations of introduced and resident 

pathogens may be brought together. The plants themselves may be stressed by the watering and 

chemical regimes and therefore susceptible to pathogens and hybrids that would not normally 

colonize them under natural conditions (Brasier 2008). Plantations of non-native trees may also be 

more susceptible to infection because they are growing ‘off site,’ i.e., under sub-optimal conditions.
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Figure 2—Genetical and environmental constraints on emergence of species hybrids in fungal 

pathogens. Adapted from Brasier (1995).

There are many possible outcomes to fungal interspecific hybridization, as shown in table 1.These 

range from the acquisition of a few nuclear genes or cytoplasmic elements to the emergence of a new 

organism that contains the genomes of both parents. Either way, with fungal pathogens there is a risk 

that the resulting genetic modification will be damaging to the natural environment. Before 1990, 

examples of hybridization between fungal pathogens were very rare. Recently, striking examples 

have occurred in all the major fungal groups, and most especially among tree pathogens. Several of 

the more prominent examples will now be discussed. A basic knowledge of the biology of the 

organisms will be assumed. 

Table 1—Possible outcomes of contact between two previously 

geographically isolated pathogens (see fig. 2)a.

• No change: no ecological displacement and no interspecific gene transfer.

• Introgression: acquisition and fixation of genes (also viruses, plasmids) by one or 

both parent species. Example: Ophiostoma ulmi x O. novo-ulmi. 

• Extinction of the ecologically ‘weaker’ species via competition.

• Extinction of the ‘weaker’ species and genetic modification of the other species via 

introgression. Example: Ophiostoma ulmi x O. novo-ulmi.

• Emergence of a hybrid swarm: free recombination between two species resulting in 

many genotypes. Optimal phenotypes may emerge via natural selection. Examples: 

the Ophiostoma novo-ulmi subspecies hybrids; Heterobasidion annosum x H.

irregulare; Melampsora occidentalis x M. medusae.

• Emergence of an allodiploid or allotetreploid hybrid species: a hybrid with the 

chromosome compliments of both parents. May be stable or give rise to further new 

heteroploid forms via haploidisation or diploidisation. Example: Phytophthora alni, 

subspecies alni.

a

Adapted from Brasier (1995). The examples given are described in the main text.
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Emerging Hybrids in the Dutch Elm Disease Pathogens 

(Ascomycetes)

The intercontinental spread of Dutch elm disease in the last century resulted in two massive 

pandemics on elms across the Northern Hemisphere (fig. 3). It has also provided classic examples of

hybridization operating at two different taxonomic levels with two very different outcomes: 

introgression and full genetic recombination. 

Figure 3—Spread of the pathogens Ophiostoma ulmi and O. novo-ulmi in the two pandemics of Dutch 

elm disease. Adapted from Brasier (2001).

At the species level, Ophiostoma novo-ulmi, responsible for the current second pandemic of Dutch 

elm disease beginning in the 1940s, has been displacing O. ulmi, responsible for the first pandemic in 

the 1920s to 40s (fig. 3). The two species are rather differently adapted, O. novo ulmi being an 

aggressive pathogen that grows fastest at ~ 22 °C; O. ulmi being a weaker pathogen that grows fastest 

at ~28 °C. Their molecular profiles indicate they are anciently divergent. The geographic origin of O. 

novo-ulmi remains unknown, but O. ulmi may have come from Japan. During the replacement of O. 

ulmi by O. novo-ulmi across Eurasia, a series of remarkable evolutionary events has occurred. These

can be summarized as follows:

1. When O. novo-ulmi arrived in Europe (coming from east and west, fig. 3) the resident O. 

ulmi population was highly variable both phenotypically and genetically. Whereas O. novoulmi initially spread as frontal clones of a single vegetative compatibility (vic) type and only 

a single sexual mating type: Mat-2. Hence these frontal clones could not undergo sexual 

reproduction.

2. O. novo-ulmi and O. ulmi occupied the same ecological niche, coming into close physical 

contact in the breeding galleries of the vector beetles in diseased elm bark.

3. O. novo-ulmi and O. ulmi are only partially reproductively isolated (at both pre-zygotic and 

post-zygotic levels), probably due to their having evolved allopatrically. Hybrids formed 

between them via sexual crosses. These hybrids were highly unfit and transient, but they 

provided a genetic bridge between O. ulmi and O. novo-ulmi, probably via backcrosses.

4. Debilitating dsRNA viruses also spread from O. ulmi into the O. novo-ulmi frontal vic

clones, again as a result of their close contact in the bark phase. The build-up of these viruses 

considerably lowered the fitness of O. novo-ulmi and therefore ‘threatened’ its survival.
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5. Simultaneously, O. novo-ulmi populations acquired the ‘missing’ Mat-1 mating type allele

and new vic alleles from O. ulmi via introgression from the hybrids. 

6. As a result the O. novo-ulmi population rapidly diversified into many new vic types via 

sexual recombination (crosses between Mat-1 and Mat-2 types).O. novo-ulmi also acquired 

other O. ulmi genes such as pathogenicity and cell surface hydrophobin genes at this time. 

7. Virus spread in the O. novo-ulmi population was suppressed by a sudden increase in the 

diversity of its vic types.

8. O. ulmi rapidly became extinct in any one location, declining at circa 10 percent per annum. 

It could not compete with the more aggressive, faster growing O. novo-ulmi.

9. O. novo-ulmi ‘retained’ the novel Mat-1 and vic genes by fixation but ‘discarded’ other O. 

ulmi DNA that rendered it less fit, such as O. ulmi pathogenicity genes. Figure 4 shows the

strong negative impact of small amounts of O. ulmi DNA on the fitness of emerging O. 

novo-ulmi introgressants. 

10. Some key references: Brasier 1986, 1988, 1996, 2000a; Brasier and Kirk 2000; Brasieret al. 

1998, 2004a; Buck et al.2002; Et Touil et al. 1999; Masuya et al. 2009; Paoletti et al. 2006; 

Pipe et al. 2000, 2001. For further references see www.forestry.gov.uk/fr/infd-8mraqc.

Figure 4—Influence of introgressed Ophiostoma ulmi DNA on fitness (pathogenicity) of O. novo-ulmi

isolates. Note that small amounts (2 to 4 percent) of introgressed DNA have a major effect on fitness. 

(M.Paolettii, C.M. Brasier, and K.W. Buck, unpublished data)

The O. novo-ulmi x O. ulmi interaction illustrates most of the main variables that influence the 

outcome of interspecific hybridization (see fig. 2). It also shows how complex, rapid, and dynamic the 

underlying evolutionary events can be. Ophiostoma ulmi was replaced because it could not compete 

either with the invading O. novo-ulmi or with the ensuing introgressants. Strong selection pressure 

exerted by the deleterious viruses probably favored the survival of the introgressants over the original 

O. novo-ulmi genotypes. It is somewhat ironic that the viruses as well as the Mat-1 and vic genes 

were probably acquired by O. novo-ulmi from O. ulmi via somatic or sexual fusion—rather like a 

sexually transmitted disease. Also, that in Eurasia and North America today O. novo-ulmi now carries 

O. ulmi genes almost by definition, much as Homo sapiens now carries the DNA of H. 

neanderthalensis. Ophiostoma novo-ulmi is no longer the same organism that it was when it was first 

introduced into Eurasia and North America around 70 years ago (fig. 3).

But that is far from the end of the matter. Another remarkable hybridization event is now 

occurring. Ophiostoma novo-ulmi actually spread across the northern hemisphere as two subspecies 

named americana and novo-ulmi (fig. 5). These differ in colony development, perithecial (sexual fruit 

body) morphology, mean pathogenicity, and molecular profiles (Brasier and Kirk 2001). During the 
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1980s, the distributions of the two subspecies began to overlap along a zone running approximately 

from southern Sweden to southern Italy, with an outlier in southern Ireland (fig. 6). In this zone they 

have come into strong niche contact in the galleries of the vector beetles. 

There is also only a weak pre-zygotic mating barrier between the two subspecies and their sexual 

progeny are highly fit. Fully recombinant hybrid swarms have begun to appear, probably involving

many further backcrosses and intercrosses (Brasier and Kirk 2010). A recent study has shown that 30 

years on the surviving hybrids in the overlap zones are still very genetically heterogeneous, but now 

exhibit the colony and pathogenicity characteristics of subspecies americana and the perithecial

characteristics of O. novo-ulmi (C.M. Brasier, unpublished). Evidently a new phenotype of O. novoulmi is emerging from the hybrid swarms under natural selection. Ophiostoma novo-ulmi is 

undergoing yet another evolutionary transformation. If this new phenotype stabilizes and becomes 

widespread it will need to be formally taxonomically recognized.

Figure 5—Spread of the two Ophiostoma novo-ulmi subspecies. Note their recent overlap in western 

Europe. Adapted from Brasier (2001).

Figure 6—Known distribution of the two Ophiostoma novo-ulmi subspecies in 1995 (omitting central 

Asia), showing potential hybrid zones. Adapted from Brasier and Kirk (2010) and based on >2,500 

field samples collected by the author.
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Emerging Hybrids in Melampsora, Heterobasidion, and 

Phytophthora (Basidiomycetes and Oomycetes)

In the Pacific Northwest of North America, hybrids have recently appeared between the resident 

Basidiomycete rust pathogen Melampsora occidentalis and the introduced M. medusae. Melampsora

occidentalis occurs naturally on the leaves of the local Populus trichocarpa in the Columbia Valley. 

Melampsora medusae is endemic on P. deltoides in the southeastern United States. A swarm of 

hybrid rust genotypes and morphotypes (‘M. x columbiana’) has emerged as a result of backcrosses 

and intercrosses between the two species on P. trichocarpa (Newcombe et al.2000, Newcombe et al. 

2001). The hybrids show abundant pathogenic variation and commercially produced P. trichocarpa x 

P. deltoids clones are vulnerable to this variation. It is noteworthy that screening using host 

differentials carrying known resistance genes using M. x columbiana hybrids has led to the discovery 

of three new genes for rust resistance in poplar. The long-term evolutionary potential of these hybrids 

is presently unclear. 

In the economically important conifer root rot Basidiomycete Heterobasidion, a wide range of 

hybrids and introgressants between the resident H. annosum and the introduced North American H. 

irregulare have recently appeared on native pines in the Castel Porziano region of southwest Italy

(Gonthier and Garbelotto 2011, Gonthier et al. 2007). Both species specialize in attacking pines. 

Heterobasidion irregulare may have been imported into the area on pine packing material during the 

Second World War (M. Garbelotto, professor, University of California, Berkeley, personal 

communication). In this case it is especially noteworthy that the hybridization process has included 

the evolution of novel alleles via intralocus recombination. Introgression is mostly occurring 

unilaterally from the resident to the introduced species. Currently it also appears to be occurring 

randomly—by population expansion rather than natural selection—but again, the long-term 

evolutionary potential of this development is unclear.

In the well-known Oomycete genus Phytophthora, a very aggressive new hybrid species, P. alni, 

has emerged on alder in Europe, possibly as a result of a recent hybridization event in a European 

nursery. Phytophthoras are diploid in the vegetative state but the common hybrid form, P. alni

subspecies alni, is a near allotetreploid (i.e., with two sets of chromosomes, one from each parent). It 

appears to be sexually sterile (probably due to pairing failures at first meiotic division) and its 

genome, such as its rDNA ITS region profile, is apparently still evolving (Brasier 2003, Brasier and 

Kirk 2001, Brasier et al. 1999, Brasier et al. 2004b, Delcan and Brasier 2001). Phytophthora alni

subspecies alni is causing serious damage to native riparian alders, especially A. glutinosa, along 

rivers in France, Germany, and the United Kingdom (Gibbs et al. 2003, Jung and Blashke 2004) and 

has now spread to at least 15 neighboring countries. 

The ‘parents’ of P. alni subspecies alni were originally suggested to be taxa close to P. cambivora

and P. fragariae (Brasier et al. 1999). New evidence suggests otherwise. Two additional

phenotypically and genetically unique forms of P. alni also occur in Europe, but at low frequency: P. 

alni subspecies uniformis and P. alni subspecies multiformis (Brasier et al. 2004b). Both are only 

weak pathogens on alder (Brasier and Kirk 2000). Since these forms had chromosome compliments 

closer to diploid they were originally suggested to be genetic breakdown products of the allotetreploid

P. alni subspecies alni. The new evidence, from additional molecular profiling, indicates that these 

two forms are most probably the parents of P. alni subspecies alni (Ioos et al. 2006, 2007a, 2007b). 

This event has therefore resulted in a hybrid that is much more aggressive than either parent. 

Phytophthora alni subspecies multiformis may itself be a product of a reticulation event.

Conclusions

The five examples provided illustrate most of the theoretical outcomes of interspecific hybridization 

between fungal pathogens, ranging from genetically modified pathogens to new pathogen species 

(table 1). Similar examples are being reported increasingly in the literature. An increase in the 
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introduction of plant pathogens into new biogeographic areas (by the plant trade and other 

agencies)—episodic selection events—coupled with increasing environmental disturbance, is leading 

to an increase in rapid pathogen evolution via interspecific hybridization. 

At present we may be recording only the ‘tip of the iceberg’: hybridization events associated with 

more overt disease episodes such as epidemics. More subtle events—such as the introgression of one 

or two loci into organisms currently viewed only as a minor threat—are perhaps less likely to be 

recorded unless they lead to a significant change in pathogen behavior. 

Interspecific hybridization between pathogens, whether overt or subtle, must now be considered a

major genetic risk issue, not only for the biosecurity of our forests and natural ecosystems, but also 

for tree disease resistance breeding programs. It adds another element of uncertainty to disease 

control policy and to practice. How do you breed against an emerging hybrid swarm or against the 

risk of future horizontal gene transfer? 

Unfortunately present international plant health protocol, formulated by the Food and Agriculture 

Organization of the United Nations (FAO) and the World Trade Organization, and operated by

organizations such as the Animal and Plant Health Inspection System (APHIS) in the United States

and the Standing Committee on Plant Health in the European Union, has failed to provide an 

acceptable level of protection against the export and import of exotic pathogens; and failed to educate 

the public on the issue (Brasier 2008, Stenlid et al. 2011, Webber 2010). In present circumstances, 

none of the world’s tree populations, whether native or introduced, whether natural or the products of 

sophisticated tree breeding, can be considered secure. 

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Dynamics of Surviving Ash (Fraxinus spp.) 

Populations in Areas Long Infested by

Emerald Ash Borer (Agrilus planipennis)

Kathleen S. Knight,1 Daniel Herms,2 Reid Plumb,3 Eileen Sawyer,3 Daniel 

Spalink,3 Elizabeth Pisarczyk,3 Bernadette Wiggin,3 Rachel Kappler,3 Emily 

Ziegler,3 and Karen Menard3

Abstract

Emerald ash borer (EAB) (Agrilus planipennis), an introduced wood-boring insect, has killed millions of ash 

(Fraxinus spp.) trees in the Midwest region of the United States and Canada. However, in some areas where 

EAB has caused almost complete mortality of mature ash trees, a small number of healthy ash trees 

intermingled with the dead ash trees have been discovered, sparking interest in these “lingering” ash trees. Here 

we present the results of surveys in 2010 and 2011 of two populations of surviving ash trees in southeast 

Michigan and northwest Ohio, where the vast majority of ash trees had died by 2008. A 2010 survey of ash 

trees ≥10 cm DBH (diameter at breast height, 1.4 m) along 10 km of floodplain forest in northwest Ohio found 

2.6 percent of the ash trees were alive and 1 percent of the ash trees were healthy in an area where most of the 

ash trees had died by 2008. The canopy condition and EAB symptoms of these surviving trees were recorded. A 

repeated survey in 2011 found that most of the surviving trees that had healthy canopies in 2010 remained 

healthy in 2011, while trees with unhealthy canopies declined or died by 2011. In southeast Michigan, a 

population of living ash trees was discovered in 2010, at which time 39 trees were tagged and their canopy 

condition and EAB symptoms were recorded. When the trees were re-surveyed in 2011, the same pattern 

observed in northwest Ohio was evident: most of the trees that had healthy canopies in 2010 remained healthy 

in 2011, while trees with unhealthy canopies declined or had died by 2011. At both sites, some of the trees with 

healthy canopies show evidence of past EAB infestation, while others had no symptoms. Research is ongoing to 

determine whether these “lingering” ash trees express resistance or tolerance to EAB, or are simply the 

last to die.

Key words: emerald ash borer, Agrilus planipennis, Fraxinus spp., lingering ash

Introduction

Emerald ash borer (EAB) (Agrilus planipennis) has killed millions of ash trees (Fraxinus spp.) in 

North America since its accidental introduction from Asia (Poland and McCullough 2006). All 

eastern North American ash tree species are susceptible, including green (F. pennsylvanica Marsh.), 

white (F. Americana L.), black (F. nigra Marsh.), blue (F. quadrangulata Michx.), and pumpkin ash 

(F. profunda (Bush) Bush) (Anulewicz et al. 2007, Knight et al. 2010, Smith 2006). Ash stands can 

progress from healthy to nearly complete mortality within 5 to 7 years (Knight et al. 2010), and ash 

mortality now exceeds 99 percent in forested areas of southeast Michigan (Herms et al. 2009) and 

northwest Ohio (Knight et al. 2010). As EAB continues to spread, it threatens to devastate ash 

throughout its range.

The initial introduction of EAB to North America occurred in an urban area near Detroit, where 

planted ash trees were primarily grafted horticultural selections representing only a few genotypes.

 1 USDA Forest Service, Northern Research Station, 359 Main Rd, Delaware, OH 43015. 

2 Department of Entomology, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 

Madison Ave., Wooster, OH 44691. 

3 Metroparks of the Toledo Area, Toledo, OH.

Corresponding author: ksknight@fs.fed.us.
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No resistance to EAB was observed in these populations. Later, EAB spread into forests with greater 

genetic variation among trees. Healthy, living ash trees have been observed in areas infested by EAB 

for several years where the vast majority of ash trees have been dead for a few years; these healthy 

trees have been termed “lingering ash.” Lingering ash trees are intermingled with dead ash trees with 

EAB symptoms, so it is unlikely that they are escapes. It is possible that these are simply the last trees 

to die, as EAB remains in these areas, but at low densities (Knight et al. 2010). It is also possible that 

these trees are resistant or tolerant to EAB and have either remained healthy or recovered from EAB 

damage.

Objectives:

1. Determine the incidence of ash survival at a site in northwest Ohio where “lingering ash” 

have been observed.

2. Study the population dynamics and health of surviving ash populations over time at a site in 

northwest Ohio and a site in southeast Michigan 

Methods

Two populations of surviving ash trees, one in northwest Ohio and one in southeast Michigan (fig. 1), 

were surveyed in 2010 and 2011 using the method described by Smith (2006) for rating canopy 

condition and recording symptoms of EAB. Canopy condition was rated on a 1 to 5 scale, where 1 is 

a healthy tree, 5 is a dead tree, and 2 to 4 are progressive stages of thinning and dieback (fig. 2). Tree 

stress symptoms associated with EAB activity were recorded as present or absent. These symptoms 

include woodpecker holes and EAB exit holes visible on the tree trunk, splitting bark, new epicormic 

shoots, and basal sprouts. Each tree’s trunk diameter at 1.38 m height (DBH) and the global position

coordinates were also recorded. Living ash trees surveyed in 2010 were re-surveyed in 2011 using the 

same methods.

Figure 1—Study sites: Oak Openings Preserve Metropark in northwest Ohio and Indian Springs 

Metropark in southeast Michigan.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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 1 2 3 4 5

Figure 2—Photos showing ash canopy condition rating scale (Smith 2006). (Photos: D.A. Herms)

Northwest Ohio

Oak Openings Preserve Metropark, located in western Lucas County, Ohio, is a nearly 2,024 ha

nature preserve in the heart of the Oak Openings region. This biologically rich region, formed at the 

western shore of the ancient glacial lakes Warren, Wayne, and Lundy, has extensive deposits of sand

underlain by a glacial till consisting of drainage-impeding clay minerals (Brewer and Vankat 2004). 

Variations in sand and soil depth, pH, moisture, and elevation create a multitude of habitats ranging 

from sand blowouts, sand barrens, black oak savannas, wet prairies, and floodplain forests (Easterly 

1976). The original floodplain forests of the early 1800s were dominated by American elm (Ulmus 

americana L.), black willow (Salix nigra Marsh.), green ash, sycamore (Platanus occidentalis L.), 

basswood (Tilia americana L.) and quaking aspen (Populus tremuloides Michx.), with less dominant 

species of silver maple (Acer saccharinum L.), red oak (Quercus rubra L.), Ohio buckeye (Aesculus 

glabra Willd.), honey locust (Gleditsia triacanthos L.), and hackberry (Celtis occidentalis L.) 

(Brewer and Vankat 2004). Although successive waves of drainage, urbanization, fire-suppression, 

farming, and lumbering have dramatically fragmented the landscape, this region nevertheless remains 

home to over 140 state and federally listed species (Brewer and Vankat 2004). The modern floodplain 

is a mix of closed-crown floodplain forest and areas of herbaceous vegetation with scattered trees.

The study area was located along the 10 km section of the Swan Creek floodplain that resides 

within the Oak Openings Preserve Metropark boundaries (fig. 3). The study area encompassed 

approximately 1 km2 of the 1.23 km2 100-year floodplain. We surveyed living ash trees (foliage 

present) ≥10 cm DBH within 50 m of the creek, and, in addition, dead ash and other live or dead tree 

species in 20 plots systematically placed every 1,000 m along the length of the river. In areas where 

the floodplain was wider than 50 m, trees beyond 50 m were not surveyed, and in areas where the 

floodplain was less than 50 m wide, we surveyed to the edge of the floodplain. Additional living ash 

trees that were discovered in 2011 were measured and recorded as well.

The 20 plots for surveying standing dead ash trees, and other tree species, were 100 m long and 50 

m wide. Plots were located within the 100-year floodplain using geographical information system in 

areas where the floodplain was 50 m wide and approximately 1,000 m intervals along the contour of 

Swan Creek. Approximately 10 percent of the length of Swan Creek was included in the plots.

Information recorded on trees in these plots included DBH and global position. By 2010, many ash 

trees had already fallen and were difficult to tally as they washed downstream into piles during 

flooding. Therefore, we used the mean percentage of ash trees fallen in three sites in the Swan Creek 

floodplain that each had three 0.04 ha monitoring plots that were monitored yearly since 2005 to 

estimate the number of ash trees fallen in the floodplain.
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Figure 3—Swan Creek floodplain at Oak Openings Preserve Metropark, Ohio.

Southeast Michigan

Indian Springs Metropark, in Oakland County in southeast Michigan, encompasses 896 ha of wooded 

wetlands, upland forest, and rolling meadows. Forested areas are dominated by a diversity of mixed

hardwoods. A 3-ha area with lingering ash, discovered in 2009 by Kevin Rice (Dept. of Entomology, 

The Ohio State University), is an upland successional area with a mix of closed canopy young forest 

and old fields with scattered trees dominated by herbaceous vegetation. Thirty-nine living ash trees 

were surveyed in 2010 and re-surveyed in 2011. The trees were selected to represent some healthy 

ash trees and some declining ash trees within the larger population of living ash trees at the site.

Results

Mortality Estimate

Until recently, ash was the most abundant tree species in the Swan Creek floodplain. Now, nearly all 

ash trees have died and the most abundant tree species is American elm (table 1). The 20 surveyed 

plots, representing approximately 10 percent of the floodplain within 50 m of the river, and 

representing 8 percent of the 100-year floodplain area, contained 858 standing dead ash trees.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Table 1—Standing trees in survey plots of Swan Creek floodplain

Species Health Number of trees in plots Average DBH (cm)

American elm Alive 864 19

Green ash Dead 858 29

Boxelder Alive 615 22

Silver maple Alive 513 29

American elm Dead 287 21

Cottonwood Alive 237 41

Hawthorne Alive 145 15

Boxelder Dead 118 18

Black walnut Alive 73 34

Green ash Alive 53 17

White oak Alive 43 34

Black willow Alive 36 36

Silver maple Dead 32 22

Red maple Alive 30 30

Red oak Alive 30 32

Hackberry Alive 24 21

Black willow Dead 18 38

Cottonwood Dead 17 37

Mulberry Alive 11 21

Sycamore Alive 9 39

The three sites monitored since 2005 had a mean of 74 percent of the ash trees standing dead 

(including trees that were leaning and trees that were snapped above DBH) and 26 percent fallen by 

2010. There was considerable variation among the three sites, ranging from 5 percent to 56 percent 

fallen trees, which creates considerable uncertainty in our estimates. Based on these data, we estimate 

that 8,580 standing dead and 3,015 fallen ash trees are within 50 m of the river, for a total of 11,595

dead ash trees. Depending on the estimate used for fallen trees, the total could range from 9,032 to 

19,500 dead ash trees. These estimates are based on the following formulas: 

Standing dead = 858×10 = 8,580

Fallen = (0.26×(858/0.74))×10 = 3,015

Total = 8,580 + 3,015 = 11,595.

If the rest of the floodplain is similar in tree species composition to our plots that were within 

50 m of the river, we estimate that the 100-year floodplain contains 14,494 dead ash trees, of which 

10,725 were standing dead and 3,769 were fallen in 2010. Depending on the estimate used for fallen 

trees, the total number of dead trees could range from 11,289 to 24,375. These estimates are based on 

the following formulas:

Standing dead = 858×(100/8) = 10,725

Fallen = (0.26×(858/0.74))×(100/8) = 3,769
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Total = 10,725+3,769 = 14,494.

The 2010 survey found 299 live ash trees within 50 m of the river, representing 2.5 percent of the 

estimated total number of ash trees present within 50 m of the river when EAB arrived. Depending on 

the estimate used for fallen trees, the percent of live trees could range from 1.5 percent to 3.2 percent. 

Spatially, these trees were mixed with dead ash trees that had clearly died from EAB several years 

ago.

Surviving Ash Health in 2010 and 2011 

The 2010 and 2011 data for the surviving trees in Ohio and Michigan show remarkably similar 

patterns. At Oak Openings in 2010, 111 ash trees were healthy (rated a 1) and 188 were in stages of 

decline (fig. 4). The healthy trees represented 0.9 percent of the original population, ranging from 0.6 

percent to 1.2 percent depending on the estimate used for fallen trees. Of the healthy trees, 48 percent

exhibited symptoms of EAB and 52 percent had no symptoms of EAB. Of the declining trees, 93

percent exhibited symptoms of EAB. At Indian Springs in 2010, 20 healthy ash trees and 19 trees in 

various stages of decline were selected (fig. 5). Of the healthy trees, 85 percent exhibited symptoms 

of EAB and 15 percent expressed no symptoms, while 100 percent of the declining trees had EAB 

symptoms. In 2011 at both sites, the majority of the trees that were healthy in 2010 remained healthy, 

while trees that were declining in 2010 continued to decline or died in 2011. The less healthy the tree 

was in 2010, the more likely it was to die in 2011 (table 2a and 2b).

Figure 4—Ash tree health at Oak Openings.

0

10

20

30

40

50

1 2 3 4 5

Percent of ash trees alive in 2010

Crown health condition

2010

2011
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Figure 5—Ash tree health at Indian Springs.

Table 2a—Ash health transitions at Oak Openings 

Health condition

2010 Health condition 2011

1 2 3 4 5

1 74a 18 3 5 0

2 11 40 26 14 9

3 0 0 11 41 48

4 0 0 0 8 92

a For each health class in 2010, the percent of trees that transitioned to each health class in 2011 is reported.

Table 2b—Ash health transitions at Indian Springs 

Health condition 

2010 Health condition 2011

1 2 3 4 5

 1 78a 11 6 6 0

 2 33 67 0 0 0

 3 0 20 40 40 0

 4 0 0 0 11 89

a For each health class in 2010, the percent of trees that transitioned to each health class in 2011 is reported.

Discussion

The study suggests that at these sites, a small proportion (approximately 1 percent at Oak Openings) 

of the ash trees remained healthy after EAB killed the vast majority of the trees. The living ash tree 

populations at the two sites exhibited remarkably similar population dynamics over time. At Oak 

Openings, 74 percent of the healthy trees remained healthy, while at Indian Springs, 78 percent of the 

0

10

20

30

40

50

1 2 3 4 5

Percent of tagged ash trees

Crown health condition

2010

2011
GENERAL TECHNICAL REPORT PSW-GTR-240

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healthy trees remained healthy. Declining trees were likely to continue declining or die. This suggests 

that the healthy trees observed at these sites have not recovered after initially declining, but rather 

have always been healthy. It is too early to determine the ultimate fate of these lingering ash as EAB 

populations continue to persist at low densities at both sites. However, it is possible that the surviving 

trees are tolerant to EAB. We will continue to monitor the surviving trees to understand the dynamics 

of potential EAB tolerance in this natural system.

Rare allopatric resistance or tolerance to EAB by North American ash species would not be 

surprising, as Asian ash species exhibit resistance and North American ash species have been shown 

to vary in attractiveness to EAB adults. Asian ash species have been shown to have greater resistance 

to EAB than North American ash cultivars in common garden studies in North America (Rebek et al.

2008) and plantations in China (Liu et al. 2007). Outbreaks of EAB in Asia appear to be infrequent 

and associated with stress in Asian ash trees (Gould et al. 2005, Liu et al. 2007). In studies of North 

American ash trees, blue ash is less preferred than other species in field (Anulewicz et al 2007) and 

laboratory (Pureswaran and Poland 2009) studies. Mechanisms underlying the differences in host 

selection may lie in volatile compounds that allow ovipositing females to locate hosts (Pureswaran et 

al. 2007, Rodriguez-Saona et al. 2006), while defensive secondary compounds in phloem tissue may 

play a role in host resistance in Asian ash species by decreasing larval growth and survival (Cipollini 

et al. 2011). Thus, nonpreference, antibiosis, compensatory growth (McNaughton 1983), or a 

combination of these mechanisms may play a role in the existence of lingering ash. We speculate that 

different mechanisms may be responsible for the healthy ash trees in our study that exhibited 

symptoms of EAB and the healthy ash trees that exhibited no symptoms of EAB. Alternatively, it is 

possible that previously infested trees are subsequently preferentially attacked, which could generate 

a similar pattern when EAB densities are low, at least in the short term.

Rare resistance or tolerance to nonnative pests and pathogens (allopatric resistance) has been 

commonly observed in many other tree species worldwide, and screening and breeding programs are 

underway for many of these species. Examples from eastern North America include American elm 

tolerant to Dutch elm disease (caused by Ophiostoma novo-ulmi) (Townsend et al. 2005) and beech 

(Fagus grandifolia Ehrh.) tolerant to beech bark disease (Houston 1983, Koch et al. 2010). The 

development of EAB-resistant ash trees is of interest for long-term reforestation and preservation of 

ash in natural and urban forests. Our observations that lingering ash can remain healthy over time 

suggest that they may be a source of resistance or tolerance to EAB. Alternatively, these surviving 

ash trees may represent the residual tail of the survivorship curve after EAB populations have 

declined to low density as their carrying capacity has crashed. Further study is needed including 

exposure of these genotypes to EAB infestations under controlled conditions. We are collaborating 

with Jennifer Koch (USDA Forest Service) to propagate lingering ash trees through grafting and seed 

collection. Through a larger collaborative program, the trees are then tested for resistance or tolerance 

to EAB and the potential mechanisms are studied (Koch et al., Breeding strategies for the 

development of Emerald Ash Borer resistant North American ash, this proceedings).

Acknowledgments

Helpful comments from Jennifer Koch and Mary Mason improved this manuscript. We thank the USDA Forest 

Service, and staff and students of The Ohio State University, for assistance with field data collection. We thank 

the Metroparks of the Toledo Area and the Huron-Clinton Metroparks for access to study sites. This research 

was supported by funding from the American Recovery and Reinvestment Act, USDA National Research 

Initiative, USDA Animal and Plant Health Inspection Service, USDA National Institute of Food and 

Agriculture, USDA Forest Service Northern Research Station, and state and federal appropriations to the Ohio 

Agricultural Research and Development Center and The Ohio State University. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

151

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Knight, K.S.; Herms, D.A.; Cardina, J.; Long, R.P.; Rebbeck, J.; Gandhi, K.J.K.; Smith, A.; Klooster, 

W.S.; Herms, C.P. 2010. Emerald ash borer aftermath forests: the dynamics of ash mortality and the 

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half-sibling American beech families. Canadian Journal of Forest Research. 40: 265–272.

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abundance of Agrilus planipennis (Coleoptera: Buprestidae) and its natural enemies Oobius agrili

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McNaughton, S.J. 1983. Compensatory plant growth as a response to herbivory. Oikos. 42: 329–336.

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Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

153

Coast Live Oak Resistance to

Phytophthora ramorum

B.A. McPherson,1 David L. Wood,1 Sylvia R. Mori,2 and Pierluigi Bonello3

Abstract

The oomycete Phytophthora ramorum is a plant pathogen with an unusually broad host range. Recognized in 

2000 as a previously unknown and likely introduced species, this pathogen has become established in central 

and northern coastal California, southwestern Oregon, and Western Europe. Tree species that may be killed by 

stem cankers include true oaks (Quercus agrifolia Née, Q. kelloggii Newb., Q. parvula var. shrevei (C.H. 

Muller) Nixon, and Q. chrysolepis Liebm.), and tanoaks (Notholithocarpus densiflorus Hook. and Arn.), in 

North America; and Japanese larch (Larix kaempferi (Lam.) Carrière) in the United Kingdom. The disease, 

referred to as sudden oak death (SOD), is changing the composition of forests in California and potentially 

threatens forests worldwide. Coast live oak has been heavily-impacted by the epidemic, with infection and 

mortality rates in infested areas averaging 5 percent and 3 percent, per year, respectively, in forests of Marin 

County, California. Since this species is one of the dominant mast-producing trees in much of coastal

California, the species composition and ecological integrity of many of these forests are threatened. Median 

survival of naturally infected coast live oaks in long-term disease progression plots established in 2000 was 

estimated as 9.7 years, using Weibull survival models. Beetle attacks in cankers reduced survival times by 65 to 

80 percent. While coast live oak mortality has steadily increased since 2000, the infection rate in these sites has 

been declining. In heavily-affected areas, asymptomatic coast live oaks persist that have never exhibited the 

bleeding symptom of infection or that are in apparent remission from previously recorded bleeding.

Resistance to canker pathogens in trees can be assessed by measuring canker sizes induced by inoculation of the 

pathogen into mechanical wounds, which in resistant trees are not significantly different from wounds alone or 

are below a critical threshold. We inoculated a total of 80 asymptomatic mature coast live oaks with P. 

ramorum and mock-inoculated (controls; wounded without inoculation) 40 trees in two natural populations in 

Marin County in July 2002 and followed disease development to July 2009. By April 2003, 70 percent of the

inoculated trees, and none of the controls, exhibited bleeding. We estimated the probability of survival to 2009 

using a Logit model to regress on maximum external canker length measured 9 months after inoculation. Trees 

with external cankers < 21.2 cm (50 percent of the trees) had ≥ 80 percent probability of survival, while cankers 

< 6.6 cm (35 percent of the trees) predicted ≥ 90 percent probability of survival. The trees that died by 2009 (20 

percent of the total inoculated with the pathogen) all had external cankers ≥ 40 cm in 2003. Because the study 

started in 2002, after the most susceptible individuals in the stands likely had already been infected or killed by 

P. ramorum infection, our data may overestimate the proportion of resistant trees in the pre-epidemic stands. 

Even so, our study shows that coast live oaks in northern California exhibit substantial resistance to P. 

ramorum. The distribution of canker lengths is consistent with quantitative multi-gene resistance to the 

pathogen.

 1 University of California, Department of Environmental Sciences, Policy and Management, Berkeley, CA 94720. 2 USDA Forest Service, Pacific Southwest Research Station, Albany, CA 94710. 3 Ohio State University, Columbus, OH 43210.

Corresponding author: bmcpherson@berkeley.edu.
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Relationship Between Field Resistance to 

Phytophthora ramorum and Constitutive Phenolic 

Chemistry of Coast Live Oak

A.M. Nagle,

1 B.A. McPherson,2 D.L. Wood,2 M. Garbelotto,2 A.O. Conrad,1

S. Opiyo,1 and P. Bonello1

Abstract

Sudden oak death, caused by Phytophthora ramorum, has resulted in high levels of coast live oak (Quercus

agrifolia Nee (CLO) mortality. However, some CLO survive in areas with high disease pressure and may thus 

be resistant. We tested the hypothesis that such field resistant trees contain constitutively higher levels of 

phenolics than susceptible trees. Phloem was sampled from the trunks of two groups of trees (one previously 

inoculated, one naturally infected with P. ramorum) categorized over the course of several years as putatively

resistant (PR, no symptoms), in remission (IR, showed symptoms but then recovered), and symptomatic (S).

Individual and total soluble phenolics from these trees were quantified. There were no significant differences in 

individual or total soluble phenolics between groups of naturally infected trees. However, inoculated PR and IR 

trees were characterized by higher constitutive levels of total phenolics, as well as ellagic acid and tyrosol 

hexoside pentoside, than S trees. Threshold concentrations that predicted an individual tree’s response to 

inoculation with P. ramorum were determined using logistic regression analysis for ellagic acid, tyrosol 

hexoside pentoside, and total phenolics. The identification of low molecular weight compounds (biomarkers) 

associated with resistance may lead to minimally invasive assays for assessing the response of individual coast 

live oaks or populations to P. ramorum. The ability to identify resistant trees prior to the arrival of the pathogen 

increases the options for managing threatened forests, e.g., by protecting highly resistant stands from logging, 

fire, and development.

 1 The Ohio State University, Columbus, OH 43210. 2 University of California, Berkeley, CA 94720.

Corresponding author: bonello.2@osu.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Modeling Elm Growth and

Dutch Elm Disease Susceptibility

Alberto Santini1 and Luisa Ghelardini1

Abstract

Elm susceptibility to Dutch elm disease (DED) displays strong seasonal variation. The period during which 

elms can become infected and express DED symptoms is generally restricted to several weeks after growth 

resumption in spring, although it can vary among species, provenances, and environmental conditions. The 

reason for this phenomenon is not understood, but the few studies correlating DED susceptibility with the 

host’s rhythm of seasonal morphogenesis suggest that the seasonal variation in disease development 

depends on the pattern of growth. Susceptibility to DED is correlated to the date of bud burst in the 

European field elm, suggesting that a differentiation in spring phenology might cause an asynchrony 

between the period of susceptibility of the host and the phase of disease transmission by the insect vector 

allowing the tree to escape the disease.

In order to verify which conditions are determinant to express a susceptible response in elm, a thorough 

study of the time course of longitudinal growth, including reactivation of cambial activity and timing of bud 

burst, leaf area development, radial growth, wood ring anatomy, and their relations with DED 

susceptibility, assessed through repeated artificial inoculations from late winter to early summer, was 

conducted in an intermediately susceptible elm clone. Growth and seasonal variation in the response to 

DED infection were modeled in 2 consecutive years.

Three different patterns of shoot growth were observed at the same time in elm: predetermined growth, free 

growth, and growth for successive flushes. Each of the observed types of elongation growth had a specific 

spatial and temporal distribution, which corresponded to different anatomical patterns in the wood and 

different responses to DED inoculation. 

The period of maximum susceptibility to DED coincided with a phase of initial and slow growth, during 

which reserves were rapidly exhausted, expanding leaves were behaving as energy sinks, and early wood 

was formed. The first decrement in DED susceptibility coincided with the transition to a subsequent phase 

of fast growth, when hefty energy was supplied by an increasing number of mature leaves able to efficiently 

photosynthesize, and a more compact late wood was formed. 

The results fit the “Growth-Differentiation Balance Hypothesis” (Herms, D.A.; Mattson, W.J. 1992. The 

dilemma of plants - to grow or defend. Quarterly Review of Biology. 67 (4): 478.), which provides a 

framework for predicting possible allocation trade-offs in between differentiation-related and growthrelated processes over a range of environmental conditions (Lorio, P.L. 1986. Growth-differentiation 

balance: a basis for understanding southern pine beetle tree interactions. Forest Ecology and Management. 

14: 259–273.). Similarly to all other plants, elms are challenged by the highly unpleasant dilemma: to grow 

or to defend.

 1 Institute of Plant Protection, CNR, Via Madonna del Piano, 10, 50019 Sesto fiorentino, Firenze, Italy.

Corresponding author: a.santini@ipp.cnr.it.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Altered Distribution of Susceptibility Phenotypes 

Implies Environmental Modulation of Genetic 

Resistance

Thomas R. Gordon1 and Neil McRoberts1

Resistance to disease is determined by the genetic capacity of a plant to recognize and respond to a 

pathogen, as modified to varying degrees by the environment in which the interaction occurs. 

Physical factors such as temperature and moisture can limit the ability of a pathogen to infect and 

cause disease, and may also influence the response of the host through effects on gene expression 

and/or by imposing constraints on physiological activities required to deliver an effective defense. 

Recent research has also drawn attention to the potential for the biotic environment to modulate 

susceptibility to disease. Thus, resistance may be enhanced by endophytic microbes and also by sublethal exposure to a plant pathogen. For example, studies under controlled conditions document that 

systemic-induced resistance (SIR) to pitch canker, caused by Fusarium circinatum, is operative in 

Pinus radiata D. Don (Monterey pine) (Bonello et al. 2001). Evidence for SIR in natural populations 

of P. radiata derives from studies showing that trees are more resistant to pitch canker in areas where 

the disease is of long residence than trees in areas where the disease is only recently established. 

Likewise, in a given stand, trees tend to become more resistant with time after establishment of pitch 

canker (Gordon et al. 2011). These observations suggest that susceptibility to a disease may be 

influenced as much by the history of exposure to a pathogen as by inherent genetic resistance. 

To explore this possibility, we compared the distribution of virulence phenotypes in a 

native stand of P. radiata to a population of seedlings reared from seed collected from the 

same stand. Seedlings were maintained in a greenhouse and were not exposed to F.

circinatum. At 1.5-years-of-age, each tree was inoculated once on the main stem, as described 

by Gordon et al. (1998). Three weeks later, the length of the lesion at the site of inoculation 

was measured. Lesion length was used as a proxy for susceptibility, with the more resistant 

trees sustaining shorter lesions than trees that are more susceptible. Lesion lengths on 614 

trees were approximately normally distributed around a mean of 29.5 mm (fig. 1), which is 

consistent with resistance to pitch canker being a quantitatively inherited trait (Matheson et 

al. 2006). In contrast, lesion lengths in parent trees were not normally distributed, but instead 

were arrayed in a right skewed distribution (fig. 2). 

The comparatively high proportion of short lesion lengths among parent trees cannot be 

attributed to a loss of susceptible trees due to pitch canker because the disease became 

established only after the sampled trees were mature, and no pitch canker caused mortality 

had occurred within that population. On the other hand, we cannot exclude the possibility that 

ontogenetic resistance contributed to the observed difference between progeny and parent 

trees. However, there is no evidence that susceptibility to pitch canker changes with the age 

of a tree. This is illustrated by results of an experiment in which 148 trees ranging in size 

from 3.5 to 77.5 cm were inoculated on each of three branches (fig. 3). Regression of lesion 

length on tree diameter (DBH) yields an R2 of 0.006, indicating that DBH (as a surrogate for 

age) accounts for a negligible proportion of the observed variation.

 1 Department of Plant Pathology, University of California, Davis, CA 95616.

Corresponding author: trgordon@ucdavis.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Figure 1—The number of trees corresponding to each lesion length category. Each category 

encompasses a range of 5mm, with the number on the X-axis representing the upper end of the 

range in mm.

Figure 2—The number of trees corresponding to each lesion length category. Each category 

encompasses a range of 5mm, with the number on the X-axis representing the upper end of the 

range in mm.

An alternative explanation for a different distribution of lesion lengths between parent trees and 

their progeny is that environmental factors are influencing susceptibility, and more specifically, that 

exposure to parasitic microbes leads to a shift in the distribution of susceptibility phenotypes toward 

greater resistance. To test the merits of this proposition, we developed a simple model to simulate the 

transition from a normal distribution of lesion lengths (as occurs in unexposed seedlings) to the rightskewed distribution observed in the parent population. The model includes a stochastic contagious 

contact distribution between trees and the pathogen, which models exposure, and a host phenotypic 

response function that determines the impact of exposure events on lesion length. An outline of the 

model is shown in the schematic in fig. 4.

0

20

40

60

80

100

120

140

160

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Lesion length category

Number of trees

0

2

4

6

8

10

12

14

16

18

20

4 8 12 16 20 24 28 32

Lesion length category

Number of trees
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Figure 3—Each point corresponds to the length of a lesion on an inoculated branch (Y-axis) and the 

DBH (X-axis) of the tree. Each tree was inoculated on three branches and so is represented by three 

data points in this graph. 

Figure 4—A schematic of the model used to simulate the transforming effect of induced resistance on 

the distribution of susceptibility phenotypes. (SAR = systemic acquired resistance).

Using parameters that are consistent with what we know about the epidemiology of pitch canker, 

the model effectively converts a normal distribution to one that resembles the pattern observed in 

nature. Experimental support for the operation of this process will be sought by establishing a 

population from seed and documenting that changes over time are associated with exposure to the 

pitch canker pathogen.

Literature Cited

Bonello, P.; Gordon, T.R.; Storer, A.J. 2001. Systemic induced resistance in Monterey pine. Forest 

Pathology. 31: 1–8.

Gordon, T.R.; Kirkpatrick, S.C.; Aegerter, B.J.; Fisher, A.J.; Storer, A.J.; Wood, D.L. 2011. 

Evidence for the natural occurrence of induced resistance to pitch canker, caused by Gibberella 

circinata, in populations of Pinus radiata. Forest Pathology. 41: 227–232.

Gordon, T.R.; Wikler, K.R.; Clark, S.L.; Okamoto, D.; Storer, A.J.; Bonello, P. 1998. Resistance 

to pitch canker disease, caused by Fusarium subglutinans f. sp. pini, in Monterey pine (Pinus 

radiata). Plant Pathology. 47: 706–711.

Matheson, A.C.; Devey, M.E.; Gordon, T.R.; Werner, W.; Vogler, D.R.; Balocchi, C.; Carson, 

M.J. 2006. Heritability of response to inoculation by pine pitch canker of seedlings of radiata 

pine. Australian Forestry Journal. 70: 101–106.

0

50

100

150

200

250

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

DBH

Lesion length
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Latent Infection by Fusarium circinatum Influences 

Susceptibility of Monterey Pine Seedlings to 

Pitch Canker

Cassandra L. Swett1 and Thomas R. Gordon1

Pitch canker, caused by Fusarium circinatum, is a serious disease affecting Pinus radiata D. Don 

(Monterey pine) in nurseries, landscapes, and native forests. A typical symptom of pitch canker is 

canopy dieback resulting from girdling lesions on terminal branches (Gordon et al. 2001). More 

extensive dieback can result from coalescing lesions on large branches or on the main stem of the tree. 

The severity of disease depends, in part, on susceptibility of the individual tree. Some will suffer no 

more than a few infected branch tips, whereas others sustain extensive damage and may ultimately 

die from the disease, often in conjunction with other forms of stress. However, some trees that 

become severely diseased eventually recover, with the absence of new infections attributed to 

systemic induced resistance (Gordon et al. 2011). To date, induced resistance in Monterey pine has 

been examined only in mature trees, but the disease can also affect seedlings, with potentially 

significant impacts on regeneration. Although the pitch canker pathogen can be a cause of mortality 

in seedlings, those that are not killed may remain infected without showing symptoms (Gordon et al.

2001, Storer et al. 2001). The present study was undertaken to determine if seedlings with 

symptomless infections manifest systemic-induced resistance to pitch canker.

To establish symptomless infected seedlings, seed was sown in sand infested with either 100 or 

1,000 propagules per gram, referred to as the low and high inoculum treatments, respectively. Control 

seedlings were grown in non-infested sand. Six months after sowing, symptomless seedlings 

representative of each treatment were challenge inoculated by depositing a suspension of 1.25 x 104

spores per ml into a 1.0 mm diameter wound on the main stem. Susceptibility to pitch canker was 

quantified as the length of the lesion developing at the site of inoculation. 

The results showed that resistance was significantly increased in seedlings previously exposed to 

the pathogen (P < 0.001). Stem lesions were 32 to 54 percent shorter than controls in the low 

inoculum induction treatment and 63 percent shorter in the high inoculum treatment (fig. 1). In 

addition, a greater proportion of plants appeared healthy in the high inoculum treatment, compared to 

untreated plants (P = 0.033) (fig. 2). 

Figure 1—Lesion sizes on inoculated trees (n=60) 19 days after inoculations. 

 1 Department of Plant Pathology, University of California, Davis, CA 95616.

Corresponding author: trgordon@ucdavis.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Figure 2—Percent of plants (n = 60) appearing healthy 19 days after inoculation. 

Similar results were obtained in experiments using 18-month-old seedlings, suggesting that 

systemic-induced resistance can persist as seedlings mature. Together, these results indicate that 

symptomless root infections can induce systemic resistance in seedlings, potentially enhancing 

survival rates. 

The growth-defense balance hypothesis predicts that increased expression of secondary metabolic 

pathways associated with disease resistance will decrease allocation of resources to growth. Contrary 

to this prediction, plant growth was not reduced in induced plants (fig. 3). 

Figure 3—Effect of induced resistance on plant growth.

This is the first study to describe systemic-induced resistance in tree seedlings, and offers insight 

into the ecological role of Fusarium circinatum as an endophyte. If subsequent studies confirm these 

findings, we aim to determine if similar effects can be documented to occur under natural conditions. 

If so, it will be of interest to know what factors determine whether infections at the seedling stage 

result in death or a longer lasting association that may enhance resistance to subsequent challenge by 

the pitch canker pathogen. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Literature Cited

Gordon, T.R.; Kirkpatrick, S.C.; Aegerter, B.J.; Fisher, A.J.; Storer, A.J.; Wood, D.L. 2011. Evidence for 

the natural occurrence of induced resistance to pitch canker, caused by Gibberella circinata, in populations 

of Pinus radiata. Forest Pathology. 41: 227–232.

Gordon, T.R.; Storer, A.J.; Wood, D.L. 2001. The pitch canker epidemic in California. Plant Disease. 85: 

1128–1139.

Storer, A.J.; Wood, D.L.; Gordon, T.R.; Libby, W.J. 2001. Restoring native Monterey pine forests in the 

presence of an exotic pathogen. Journal of Forestry. 99: 14–18.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Breeding for Resistance in Norway Spruce to the 

Root and Butt Rot Fungi Heterobasidion spp.

G. Swedjemark,1 A.K. Borg-Karlson,2 and B. Karlsson1

Abstract

Results from previous studies of resistance in Norway spruce (Picea abies (L.) Karst.) to the pathogens 

Heterobasidion spp. show significant genotypic variation in fungal growth and spore susceptibility among 

Norway spruce clones. The genetic variation and the heritability are large enough for practical breeding 

purposes and there is no adverse correlation between Heterobasidion infection and growth traits.There is an 

economic incentive to use plant materials which are more resistant than those used in reforestation today. 

Recent research on developing the methods for testing resistance in plant material have demonstrated that 

we need to identify new markers and early traits for screening young material before we can include 

resistance to the pathogen in existing breeding programs. In an ongoing project we are analyzing the 

differences in the chemical defense and the transcriptome of Heterobasidion spp.-infected P. abies clones 

with different susceptibility to Heterobasidion spp. infection. Preliminary results show that the quantity of 

certain substances involved in defense mechanisms is specific for resistant clones. For example, astringin 

may be a constitutive chemical marker for resistance. 

Key words: Heterobasidion spp., Norway spruce, Picea abies, resistance breeding

Introduction

The root pathogens Heterobasidion spp. are able to infect living conifer roots of all ages (Asiegbu et 

al. 2005). The Heterobasdion species complex consists of five different species, of which two are 

common in Scandinavia: Heterobasidion annosum and Heterobasidion parviporum (Dahlman 

2010). The complex causes serious damage to coniferous forests throughout the Northern 

Hemisphere and, under optimal conditions, can rapidly destroy a stand. The fungus spreads both by 

spores and by vegetative growth: spores land on recently exposed wood, such as freshly cut stumps, 

germinate, and form a mycelium, which colonizes the stump and its roots. It then infects healthy 

trees by growing via root contacts and grafts between stumps and trees and then extends through the 

roots and up into the stem. 

Results from our previous studies (inoculation of differently aged clonal material and screening 

of naturally infected clone stands) of resistance in Norway spruce (Picea abies (L.) Karst.) to 

Heterobasidion spp. are promising. There is significant genetic variation in resistance to fungal 

growth and in spore susceptibility in Norway spruce. The results show the same pattern in 

inoculation experiments and in naturally-infected Norway spruce clone stands (Karlsson and

Swedjemark 2006, Swedjemark 1995). The genetic variation and the heritability are large enough 

for practical breeding purposes and there is no adverse correlation between Heterobasidion infection

and growth traits. The economic calculations show that losses due to root rot are significant; there is 

an economic incentive to use plant materials which are more resistant than those used today

(Karlsson and Swedjemark 2006). 

Genetic variation in Norway spruce has been detected for several traits such as growth capacity 

and wood density (Karlsson and Högberg 1998). Economic gain from using improved plant material 

could be even greater if resistance factors were included in selection programs. Previous results 

from Heterobasidion spp. inoculation experiments and inventories in field trials have shown that 

genetic variation in fungal extension and infection frequency is of the same magnitude as in other 

traits used in applied tree breeding. The genetic improvement was 15 percent less than the average 

 1 Skogforsk, Ekebo 2250, SE-26890 Svalöv, Sweden. 2 Ecological Chemistry Group, Department of Chemistry, KTH, Sweden.

Corresponding author: gunilla.swedjmark@skogforsk.se. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

163

infection frequency when selecting the 10 percent top clones (Karlsson and Swedjemark 2006,

Swedjemark 1995).

Inoculation experiments on twigs showed that the infection pattern is the same in twigs as in 

cuttings, which makes it possible to perform non-destructive inoculation experiments in field 

trials. In inoculation experiments on cuttings, a systemic induced resistance reaction was found up 

to 18 months after the first treatment with the pathogen (Swedjemark and Karlsson 2006, 

Swedjemark et al. 2007).

Being long-lived organisms, Norway spruce trees rely on both induced and constitutive 

defenses to restrict the spread of invading fungi and insects. The first line of defense in Norway 

spruce trees is the bark. The combination of the physical properties of lignified and suberized 

walls and the chemical properties of phenolics and terpenes makes bark a very efficient barrier 

against infection. Once an injury or an infection is recognized by the tree, induced defenses are 

activated, including cell wall re-enforcements, production of lytic enzymes, and secondary 

metabolites such as phenols, stilbenes, lignans, flavonoids, and terpenes (Keeling and Bohlmann 

2006, Witzell and Martin 2008)

One generally debated hypothesis claims that the production of defense compounds depend on 

abiotic factors, e.g., the amount of nutrition available (Herms and Mattson 1992). In one of our 

studies on spruce, we have partly confirmed this hypothesis, as the induced amounts of 

monoterpenes were less in trees that obtained an excess nutrition. We also found that the absolute 

amounts of terpenes were positively correlated to the inoculated fungal performance (Danielsson et 

al. 2011).

The terpene chemistry in healthy P. abies is well documented, both within and between grafts,

and in natural stands (Borg-Karlson et al. 1993, Persson et al.1996). Large variations between and 

within trees are found for the monoterpenes, and there also exists “chemo types” characterized by 

the occurrence of 3-carene. We have shown that only a few monoterpene enantiomers, including 

(+)-3-carene, are induced by fungi (Zhao et al. 2010) representing specific terpene synthases . With 

our technique, we can analyze the effects of stress elicitors in more detail, and we aim to show that 

individual changes in terpenes are highly dynamic and inducible. 

The phenols stilbene, (E)-astringin and corresponding astringin dimers (piceasides), and a 

number of unknown phenolics, seem to be of major importance as resistant markers (Danielsson et 

al. 2011) and will be the focus of our further investigations. We have recently identified clones that 

produce enhanced amounts of astringin which is of interest in our search of resistant markers. 

The aim of our research is to find the best constitutive or induced markers for resistance to be 

able to select parent clones to be included in existing breeding programs for Norway spruce. 

Knowledge of reliable resistance markers is essential for the production of future plant materials 

intended for selection of parent clones for the breeding population, and thus for commercial 

reforestation material. 

Material and Methods

Plant Material

Since 1994, we have had access to Norway spruce plant and tree material included in the Swedish 

regional clonal forestry program at The Forest Research Institute in Sweden (Karlsson and

Högberg 1998). During the years, we have tested ca 1,000 spruce clones for resistance to 

Heterobasidion spp., using artificial inoculation of plants, trees, twigs, stumps, and somatic 

embryos. In addition we have screened naturally infested clonal stands by felling trees, using a 

borer, or a root rot detector (Rotfinder©). Grafts of selected clones have been prepared and we 

have made controlled crossings between parents with known levels of resistance.

Methods for Artificial Inoculation

• Cuttings, roots, and twigs. Wounds are made aseptically in the bark to reach the xylem 

surface. Mycelium is attached to the wound. Incubation period is ca 3 weeks. At 

sampling, the stem is consecutively cut into 5 mm thick disks. The disks from each 
GENERAL TECHNICAL REPORT PSW-GTR-240

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sample are incubated under humid conditions and are then checked for conidia of the 

fungi. The fungal extension is estimated for each sample.

• Artificial inoculation on stumps. The trees are felled, leaving stumps ca 50 cm high. 

Immediately after felling, the stumps are covered with a suspension of Heterobasidion

spp. conidiospores (suspended in sterile water). After incubation (ca 3 months), the 

stumps are sliced into 1 cm thick discs, numbered, and stored in a plastic bag under 

humid conditions. After ca 10 days, the discs are checked for conidia development on the 

mycelia and the extension of fungal growth is measured.

Quantitative RT-PCR Analysis of PAL-Transcripts

The enzyme PAL (Phenylalanin Ammonia-Lyase) is central in the phenylpropanoid pathway and 

lignin biosynthesis, but also in production of stilbenes and flavonoids. Using quantitative real-time

polymerase chain reaction (Quantitative RT-PCR) analysis of PAL transcript levels, the level of 

activation is measured in the tissues around the fungal growth. Small samples close to the fungi and 

at certain distances from the fungi are collected and frozen in -80 o

C and processed according to 

Hietala et al. (2003).

Methods for Terpene and Phenol Analysis

Samples extracted in hexane, methanol, or by solid phase micro extraction (SPME) or thermal 

adsorption/desorption methods, are taken in order to measure the “metabolomics.” We have wellestablished protocols for the extraction procedures and the separation of constituents in the complex 

mixtures is reliable. Sampling by solvent extraction gives information about the constitutive and 

induced resins, and the hexane extracts is analyzed by the two-dimensional GC-MS to separate the 

enantiomers of the chiral terpenes (Persson et al 1993). The dynamics of selected chiral

monoterpenes α-pinene, β-pinene, and limonene and sesquiterpenes is analyzed in detail by SPMEGC-MS technique adsorption solvent or thermal desorption in 2D-GC-MS. Volatiles released by 

healthy and infested trees are collected and we localize the front of the mycelium by measuring the 

emission of specific de novo-produced stress compounds (methyl salicylate and α-and βfarnesenes).

The methanol/water extract rich in phenolic compounds is analyzed by using reversed phaseHPLC-MSn

. Efforts to identify the unknown compounds is made by HPLC-prep-microprobe-NMR 

in cooperation with the NMR group at the Max Planck Institute of Chemical Ecology in Jena,

Germany, a group already focusing in this area.

Recent Results and Discussion

A significant problem when studying resistance in forest ecosystems is the relevance of results from 

artificial trials in young material versus the natural situation. Are we really measuring actual 

resistance to the pathogen, and what will happen during a rotation period? The correlation between 

inoculation results is poor when comparing inoculation in the same clones from different 

propagation batches (Karlsson et al. 2008). Recent results indicate that it may be an environmental 

disturbance (stress) of the cuttings which triggers the induced resistance system, which may 

influence results from inoculation experiments. Induced resistance has showed to be a clonedependent trait. A systemic-induced resistance reaction was found 18 months after the first 

treatment with Heterobasidion (Swedjemark et al. 2007). Inoculated twigs heavily attacked by 

Sacchiphantes abietis were more efficient in excluding the pathogen from the wood, indicating an 

induced resistance reaction (Swedjemark and Karlsson 2006). Plants infested with mites were tested 

for levels of α-and β-farnesenes (indicating stress); clones exposed to mite infection had high levels 

of volatiles and had a shorter extension of fungal growth when inoculated compared to unaffected 

control plants, indicating that the induced resistance reaction influenced the result. (Borg-Karlson 

and Swedjemark, unpublished data). Recent research has shown that induced stress affects tree 

resistance, where the least visually stressed trees were the most infested by blue stain fungi (Zhao et

al. 2010). However, recent results show that the transcriptional regulation always shows the same 

pattern in the same spruce clones of different ages when inoculated with Heterobasidion. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

165

(Danielsson et al. 2011) These findings make it possible to rely on inoculation studies if 

supplemented with analysis of transcriptional regulation.

Inoculations of H. annosum in roots of clones with high and low susceptibility show that the 

initial phenol composition differed between highly and less susceptible clones. Less susceptible 

clones were characterized by higher constitutive concentrations of the sesquiterpene αlongipinene, the stilbene (E)-astringin and astringin dimers (piceasides), and a number of 

unknown phenolics, possibly indicating further biochemical reactions downstream.

The levels of the flavonoid (+)-catechin showed a temporal variation; it accumulated between 

5 and 15 days after inoculation (dpi) in response to H. annosum infection in the less susceptible 

genotypes. The transcriptome data suggested that the accumulation of free (+)-catechin was 

preceded by an induction of genes in the flavonoid and proanthocyanidin biosynthesis pathway 

such as leucoanthocyanidin reductase. Quantitative PCR analyses verified the induction of genes 

in the phenylpropanoid and flavonoid pathway. The qPCR data also highlighted genotypedependent differences in the transcriptional regulation of these pathways. 

It is obvious from our results that the transcriptional changes in responses to H. annosum or 

wounding treatments are substantially different between the two less susceptible genotypes. The 

pattern of extractable (+)-catechin in bark differ between the clones. The qPCR data also indicate 

differences between the clones; the PAL genes are down-regulated already at 15 dpi in wounded 

susceptible clones while they remain slightly up-regulated at 28 dpi irrespective of treatment in 

more resistant clones. The result indicates that the genotypes may perhaps depend on different 

successful defense strategies in the interaction with Heterobasidion. Clearly, an extended

comparison between the chemistry and the transcriptional responses in the interaction with 

Heterobasidion between several independent genotypes is needed to catalogue the mechanisms of 

successful host defense strategies (Danielsson et al. 2011).

Conclusions

• There is significant genetic variation in resistance to fungal growth and in spore 

susceptibility in Norway spruce. Genetic variation and the heritability are large enough 

for practical breeding purposes and there is no adverse correlation between 

Heterobasidion infection and growth traits.

• Concentration of secondary metabolites in the tree differs between trees more or less

susceptible to the pathogen: astringin could be a candidate as a constitutive marker for 

resistance.

• Concentration of specific phenols differs between trees more or less susceptible to the 

pathogen.

• Constitutive concentrations of terpenes do not seem to be specific for trees more or less 

resistant to Heterobasidion.

• The transcriptome always shows the same pattern in the same spruce clones when 

inoculated with Heterobasidion.

• Transcriptome data correlates with the chemical profile.

Literature Cited

Asiegbu, F.; Adomas, A.; Stenlid, J. 2005. Conifer root and butt rot caused by Heterobasidion annosum

(Fr.) Bref. s.l. Molecular Plant Pathology. 6: 395–409.

Borg-Karlson, A.K.; Lindström, M.; Persson, M.; Norin, T.; Valterová, I. 1993. Enantiomeric 

composition of monoterpene hydrocarbons in different tissues of Norway spruce Picea abies (L.) Karst. 

A multidimensional gas chromatography study. Acta Chemica Scandinavica. 47: 138–144.

Dahlman, K. 2010. Heterobasidion root rot. Genetic mapping of virulence and evolutionary history.

Uppsala: Swedish University of Agricultural Sciences. 67 p. Ph.D thesis Nr 2010:81.

Danielsson, M.; Lundén, K.; Arnerup, J.; Hu, J.; Zhao, T.; Swedjemark, G.; Elfstrand, M.; BorgKarlson, A.K.; Stenlid, J. 2011. Chemical and transcriptional responses of Norway spruce clones with 
GENERAL TECHNICAL REPORT PSW-GTR-240

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varying susceptibility to Heterobasidion spp. infection. BMC Plant Biology. doi:10.1186/1471-2229-11-

154.

Herms, D.A.; Mattson, W. J. 1992. The dilemma of plants: to grow or defend. Quarterly Review of 

Biology. 67: 283–335

Hietala, A.M.; Eikenes, M.; Kvaalen, H.; Solheim, H.; Fossdal, C.M. 2003. Multiplex real-time PCR for 

monotoring Heterobasidion annosum colonization in Norway spruce clones that differ in disease 

resistance. Applied and Environmental Microbiology. 69: 4413–4420.

Karlsson, B.; Swedjemark, G. 2006. Genotypic variation in natural infection frequency of

Heterobasidion sp. in a 20-year old clone trial of Picea abies (L.) Karst. in south Sweden. Scandinavian 

Journal of Forest Research. 21: 108–114.

Karlsson, B.; Högberg, K.A. 1998. Genotypic parameters and clone x site interaction in clone tests of 

Norway spruce (Picea abies (L.) Karst.). Forest Genetics. 5: 21–30.

Karlsson, B.; Tsopelas, P.; Zamponi, L.; Capretti, P.; Soulioti, N.; Swedjemark, G. 2008.

Susceptibility to Heterobasidion parviporum in Picea abies clones grown in different environments.

Forest Pathology. 38: 83–89.

Keeling, C.I.; Bohlmann, J. 2006. Genes, enzymes and chemicals of terpenoid diversity in the constitutive 

and induced defence of conifers against insects and pathogens. New Phytologist. 170: 657–675.

Persson, M.; Borg-Karlson, A.K.; Norin, T. 1993. Enantiomeric composition of the six main 

monoterpene hydrocarbons in different tissues of Picea abies (L.) Karst. Phytochemistry. 33: 303–307.

Persson, M.; Sjödin, K.; Borg-Karlson, A.K.; Norin, T.; Ekberg, I. 1996. Relative amounts and 

enantiomeric compositions of monoterpene hydrocarbons in xylem and needles of Picea abies

(Pinaceae). Phytochemistry. 42: 1289–1297.

Swedjemark, G. 1995. Heterobasidion annosum root rot in Picea abies: variability and resistance. SLU 

Info/Repro, Uppsala: 1995 ISBN 91-576-5053-5. Ph.D. thesis.

Swedjemark, G.; Karlsson, B. 2006. Mycelial growth and exclusion of Heterobasidion parviporum

inoculated in branches of 15-year old Picea abies clones. Forest Pathology. 36: 1–6.

Swedjemark, G.; Karlsson, B.; Stenlid, J. 2007. Exclusion of Heterobasidion parviporum from 

inoculated clones of Picea abies and evidence of systemic induced resistance. Scandinavian Journal of 

Forest Research. 22: 110–117. 

Witzell, J.; Martin, J.A. 2008. Phenolic metabolites in the resistance of northern forest trees to pathogens -

past experiences and future prospects. Canadian Journal of Forest Research-Revue Canadienne De 

Recherche Forestiere. 38: 2711–2727.

Zhao, T.; Krokene, P.; Björklund, N.; Långström, B.; Solheim, H.; Christiansen, E.; Borg-Karlson, 

A.K. 2010. The influence of Ceratocystis polonica inoculation and methyl jasmonate application on 

terpene chemistry and host resistance in Norway spruce, Picea abies. Phytochemistry. 71: 1332–1341.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Using Survival Analysis for Assessing Resistance to 

Phytophthora lateralis in Port-Orford-Cedar Families

Sylvia R. Mori,

1 Richard A. Sniezko,2 Angelia Kegley,2 and Jim Hamlin3

Abstract

In a greenhouse trial to examine genetic resistance among seedling families (half-sib, full-sib, and selfed) of PortOrford-cedar (Chamaecyparis lawsoniana (A. Murr.) Parl.) to the root pathogen Phytophthora lateralis, the root tips 

of seedlings were inoculated, and the subsequent mortality was followed over a 3 year period. Mortality among 

families at the end of the trial varied from 0 to 100 percent, with continuing mortality for some families occurring 

throughout the trial period. A preliminary examination of mortality curves for the families indicated that some 

families were highly susceptible (100 percent mortality in less than 290 days), some appeared to have high survival 

putatively from a single major gene (0 to 50 percent mortality with little or no additional mortality after 180 days), 

and the majority of families appeared to show varying degrees of partial resistance, with different final levels of 

mortality and with the within-family mortality occurring at different rates. Here, we explore the use of survival 

analysis to examine the different patterns of mortality among the 125 families and place them into different 

resistance groups. To illustrate the usage of survival analysis, we applied some of these techniques to all families 

and others to a subsample of eight of the 125 seedling families. We used the Kaplan-Meier estimator to plot their

survival function and compare them visually. One main objective was to discern how many of the 124 families were 

significantly different than the high susceptible control family (family 73), which in previous trials had reached 100 

percent mortality in a relatively short time. We used the log-rank test (non-parametric approach) to compare groups 

of families with similar patterns. Using separate pairwise comparisons, this approach identified five (of the 124) 

families that were not significantly different from the susceptible control (family 73). In some other comparisons, 

the log-rank test did not separate them as well as anticipated based on the visual plots. This is mainly due to the 

small sample size and large variability. Only when survival curves looked extremely different from each other were 

there statistically significant differences. Therefore, the log-rank test should be used with caution, and as an 

exploratory tool. When possible, other techniques should be used such as Cox Proportional Hazard (PH, semiparametric approach), and Weibull or Gamma regression models (parametric approach). The Cox PH model was 

used to compare a family’s mortality risk with respect to another family; for example, high mortality family 73 had

4.4 times the risk of mortality than high mortality family 101. When possible, we also compared families and groups

of families’ survival patterns using the Gamma survival regression model (the 3-parameter Gamma survival curve 

fitted the data best) by comparing their estimated parameters or analyzing their quantile times (Q25, Q50 and Q75).

In conclusion: (1) survival analysis is a useful tool for understanding the distribution of mortality responses, (2) 

small sample sizes and lack of homogeneity of the data can produce inconclusive results, (3) parametric models 

don’t fit the data with long plateaus well, and (4) if parametric models fit well, they can be used to compare the 

mortality statistical distributions by means of their estimated parameters or estimated key quantile times.

 1 USDA Forest Service, Pacific Southwest Research Station, 800 Buchanan, Albany, CA 94710. 2 USDA Forest Service, Dorena Genetic Resource Center, 34963 Shoreview Road, Cottage Grove, OR 97424. 3 USDA Forest Service, USDA, Umpqua National Forest, Roseburg, OR 97471.

Corresponding author: smori@fs.fed.us.

169

Molecular and Genomics Tools in 

Resistance Programs

Port-Orford-cedar (Chamaecyparis lawsoniana) (a) Phytophthora lateralis

mortality in Redwood National Park, (b) resistant parent 510015 surrounded by 

dead Port-Orford-cedar trees, (c) dead and dying Port-Orford-cedar along 

Highway 101 in southern Oregon, and (d) breeding Port-Orford-cedar for 

resistance in containerized orchard (Photos: R.Sniezko (a,c,d); C.Frank (b)).

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Integrating Molecular Tools and Conventional 

Approaches in the Oregon State University Hazelnut 

Breeding Program

Shawn A. Mehlenbacher1

The Oregon State University (OSU) hazelnut breeding program, initiated in 1969, continues to 

develop new cultivars for the hazelnut industry that combine suitability to the blanched kernel market 

with resistance to eastern filbert blight (EFB) caused by Anisogramma anomala. Oregon's hazelnut 

growers support the program through the Oregon Hazelnut Commission. The need for improved 

disease-resistant cultivars and pollinizers is the program's driving force. About 5000 seedlings have 

been planted each year during the past 2 decades, and 10 cultivars and 10 pollinizers have been 

released. Oregon produces 99 percent of the United States crop, but only 3 to 5 percent of the of the 

world crop. Beginning in 2009, plantings have expanded from 12,000 ha by 1200 ha per year. Most 

new orchards are being planted to 'Jefferson,' increased by micropropagation.

Cultivars of European hazelnut (Corylus avellana L.) are highly heterozygous and clonally 

propagated. Hazelnuts were already being cultivated in Italy and Spain at the time of the Roman 

Empire as well as at an earlier date on the Black Sea coast of Turkey and in the Caucasus Mountains.

All cultivars in these countries were selected from the local vegetation. Corylus avellana is native to 

most of Europe, Turkey, and the Caucasus republics were it grows as a small-stature, multi-stemmed 

shrub. Diverse germplasm has been imported as scions or seeds. The Corvallis collections contain 

~900 accessions and include representatives of all 11 Corylus species and ~500 clones of Corylus 

avellana. Hazelnut is a model for the order Fagales and family Betulaceae. All Corylus species are 

diploid, have a small genome (0.48 pg/1C, 380 MB), and an estimated 35,000 genes. Morphology, 

phenology, and all marker types investigated to date have shown C. avellana to be highly 

polymorphic.

Objectives of the OSU hazelnut breeding program fall into two categories: suitability to the 

blanched kernel market and resistance to EFB. Some 93 percent of the world's hazelnuts are sold as 

kernels to chocolate makers and bakers; only 7 percent are sold in-shell for the table market. Kernel 

varieties should be resistant to bud mites (primarily Phytoptus avellanae), start to bear nuts at an early 

age, have consistent high nut yields, round nut shape, thin shells, easy pellicle removal, few nut and 

kernel defects, early nut maturity, and free-falling nuts. The process to develop varieties begins with 

40 to 50 controlled pollinations each year. The hybrid seeds are harvested in late August and are 

soaked and placed in stratification in early November. The seedlings are grown in 4 L pots in the 

greenhouse in the first year and then transplanted 1 m apart in the field in October in rows that are 3 

m apart. Most seedlings bear their first nuts in the fifth leaf. Evaluation of the seedlings in years 5 to 8 

reduces their number to 25 to 40 selections, which are propagated by tie-off layerage. The layers are 

lined out in a nursery row for one season and then used to plant replicated yield trials. Data is 

collected from the trials in years 3 to 7. The breeding cycle from seed to seed is 8 years. New 

cultivars are released 16 to 17 years after the cross is made.

A few traits are controlled by single loci, including pollen-stigma incompatibility and some EFB 

resistance sources. Most of the program's breeding objectives involve quantitative traits for which 

heritability was estimated by regression (Mehlenbacher et al. 1993, Yao and Mehlenbacher 2000).

Heritability is >50 percent for most traits we have investigated, so it is unlikely that marker-assisted 

selection will be more effective than phenotypic selection.

 1 Dept. of Horticulture, 4017 ALS Building, Oregon State University, Corvallis, OR 97331.

Corresponding author: mehlenbs@hort.oregonstate.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Greenhouse inoculation with EFB is used to classify accessions as resistant or susceptible. Three 

trees per accession are grown in the greenhouse, inoculated with a spore suspension, and maintained 

under high humidity. The trees are placed outside where they go dormant, receive chilling, and then 

develop cankers 15 to 20 months after inoculation. Very high resistance has been identified in 23 

accessions, which represent about 2 percent of those tested (Chen et al. 2007, Lunde et al. 2000,

Sathuvalli et al. 2010b). Several resistant accessions have been used as parents in breeding. The 

resistance in 'Gasaway' is conferred by a dominant allele at a single locus which is carried by the 

recently released 'Yamhill' and 'Jefferson.' A very high level of resistance was detected in accessions 

imported as scions: the Spanish 'Ratoli' and 'Culpla,' Georgian OSU 759.010, Serbian 'Crvenje' and 

'Uebov,' and eight selections from a forestry institute near Moscow, Russia. Seeds have been 

imported from several sources, and selected trees in these populations have been tested for EFB 

response. Highly resistant selections include OSU 408.040 from Minnesota; OSU 495.072 from 

Russia; and selections from southern Russia, Crimea (Ukraine), and the Republic of Georgia. DNA 

marker-assisted selection (MAS) has been practiced for 'Gasaway' resistance for more than 12 years.

The MAS was initiated because of the pathogen’s 2-year life cycle, a desire to select for resistance in 

the absence of the pathogen, and difficulties in determining disease response. Random Amplified 

Polymorphic DNA (RAPD) markers 152-800 and 268-580 flank the 'Gasaway' resistance locus, are 

robust and easy to score, and are useful in multiple populations (Mehlenbacher et al. 2004). Seedlings 

that carry one or both RAPD markers are planted in the field. Resistance from 'Ratoli,' OSU 408.040,

and Georgian OSU 759.010 is also simply inherited, and linked DNA markers have been identified 

(Chen et al. 2005; Sathuvalli et al. 2010a, 2011).

A linkage map for hazelnut was constructed using RAPD markers (Mehlenbacher et al. 2006), and 

microsatellite markers were added to serve as anchors. EFB resistance from 'Gasaway' was assigned 

to linkage group 6 (LG6), from 'Ratoli' to LG 7, and from Georgian OSU 759.010 to LG 2. We will 

use DNA markers to pyramid different resistance genes. Microsatellite markers show transferability 

across genera in the Betulaceae (Gürcan and Mehlenbacher 2010), and have been used to fingerprint 

accessions and study genetic diversity.

Susceptible cultivars vary widely in level of susceptibility. To quantify susceptibility, cultivars 

and selections are propagated by tie-off layerage. The rooted layers are grown for 1 season in pots in 

the lathhouse. In the spring, 12 potted trees of each cultivar (including checks) are placed under a 

structure topped with diseased wood. Sprinklers keep the cankered branches wet, and spores rain 

down on the young shoot tips. After several weeks of exposure, the trees are planted in a nursery row 

where cankers are allowed to develop. Cankers are counted and measured 18 to 20 months after 

exposure. The total length of cankers is calculated for each tree, and a square root transformation used 

to reduce the relationship between mean and variance. Quantitative resistance, expressed as fewer and 

smaller cankers, is present in OSU releases 'Lewis,' 'Clark,' and 'Sacajawea'. In a set of nine 

progenies, the heritability estimate for proportion of wood diseased was 47 percent (Osterbauer et al. 

1997).

Oregon State University has partnered with Rutgers University, the University of NebraskaLincoln, and the Arbor Day Foundation to form the Hybrid Hazelnut Research Consortium. The 

Consortium’s goal is to develop C. americana × C. avellana hybrids with EFB resistance and 

adaptation to the climate of the eastern and Great Plains states. The high disease pressure in field 

plots at Rutgers University provides useful information.

With Todd Mockler and Illumina, the genome and transcriptome of ‘Jefferson’ hazelnut was 

sequenced, and seven accessions are being resequenced. The assembled sequences are being mined

for microsatellites and single nucleotide polymorphisms (SNPs), with a special interest in markers for 

newly identified EFB resistance genes. Map-based cloning of the 'Gasaway' EFB resistance gene 

identified five candidate genes. A similar approach is in progress for the incompatibility locus. For 

use in marker-assisted selection with 4000 seedlings per year, markers must be high throughput, 

robust, and valid in different populations. For use with 400 selections per year, other markers types 

such as microsatellites are feasible. SNPs are abundant in the hazelnut genome (~1 in 50 bp). High 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

173

Resolution Melting (HRM) analysis using a LightScanner (Idaho Technology) is promising for costeffective scoring of SNP markers. However, MAS is less useful for traits with high heritability, and 

most traits in hazelnut are highly heritable.

Literature Cited

Chen, H.; Mehlenbacher, S.A.; Smith, D.C. 2005. AFLP markers linked to eastern filbert blight resistance 

from OSU 408.040 hazelnut. Journal of the American Society for Horticultural Science. 130: 412–417.

Chen, H.; Mehlenbacher, S.A.; Smith, D.C. 2007. Hazelnut accessions provide new sources of resistance to 

eastern filbert blight. HortScience. 42: 466–469.

Gürcan, K.; Mehlenbacher, S.A. 2010. Transferability of microsatellite markers in the Betulaceae. Journal of 

the American Society for Horticultural Science. 135: 159–173. 

Lunde, C.F.; Mehlenbacher, S.A.; Smith, D.C. 2000. Survey of hazelnut cultivars for response to eastern 

filbert blight inoculation. HortScience. 35: 729–731.

Mehlenbacher, S.A.; Brown, R.N.; Davis., J.W.; Chen, H.; Bassil, N.V.; Smith, D.C.; Kubisiak, T.L. 2004.

RAPD markers linked to eastern filbert blight resistance in Corylus avellana. Theoretical and Applied 

Genetics. 108: 651–656. 

Mehlenbacher, S.A.; Brown, R.N.; Nouhra, E.R.; Gokirmak, T.; Bassil, N.V.; Kubisiak, T.L. 2006. A 

genetic linkage map for hazelnut (Corylus avellana L.) based on RAPD and SSR markers. Genome. 49:

122–133. 

Mehlenbacher, S.A.; Smith, D.C.; Brenner, L.K. 1993. Variance components and heritability of nut and 

kernel defects in hazelnut. Plant Breeding. 110: 144–152.

Osterbauer, N.K.; Johnson, K.B.; Mehlenbacher, S.A.; Sawyer, T.L. 1997. Analysis of resistance to eastern 

filbert blight in Corylus avellana. Plant Disease. 81: 388–394.

Sathuvalli, V.R.; Chen, H.L.; Mehlenbacher, S.A.; Smith, D.C. 2010a. DNA markers linked to eastern 

filbert blight resistance in 'Ratoli' hazelnut. Tree Genetics and Genomes. 7: 337–345.

Sathuvalli, V.R.; Mehlenbacher, S.A.; Smith, D.C. 2010b. Response of hazelnut accessions to greenhouse 

inoculation with Anisogramma anomala. HortScience. 45:1116–1119.

Sathuvalli, V.R.; Mehlenbacher, S.A.; Smith, D.C. 2011. DNA markers linked to eastern filbert blight 

resistance from a Georgian hazelnut accession. Journal of the American Society for Horticultural Science.

136: 350–357.

Yao, Q.; Mehlenbacher, S.A. 2000. Heritability, variance components and correlation of morphological and 

phenological traits in hazelnut. Plant Breeding. 119: 369–381.
GENERAL TECHNICAL REPORT PSW—GTR-240

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High-Resolution Genetic and Physical Mapping of 

Eastern Filbert Blight Resistance in Hazelnut

Vidyasagar Sathuvalli1 and Shawn A. Mehlenbacher1

Eastern filbert blight (EFB), caused by the pyrenomycete Anisogramma anomala, is a serious threat 

to the hazelnut (Corylus avellana L.) industry in the Pacific Northwest. A dominant allele at a single 

locus from the obsolete pollenizer ‘Gasaway’ confers a very high level of resistance, and has been 

extensively used in the hazelnut breeding program at Oregon State University. Several linked random

amplified polymorphic DNA (RAPD) markers have been identified. Map-based cloning of the EFB 

resistance gene from ‘Gasaway’ was initiated by constructing a bacterial artificial chromosome

(BAC) library for ‘Jefferson’ which is heterozygous for resistance (Sathuvalli and Mehlenbacher

2011). The BAC library was constructed using the cloning enzyme MboI and the vector pECBAC1 

(BamHI site). The library consists of 39,936 clones arrayed in microtiter plates with an average insert 

size of 117 kb and estimated coverage of 12 genome equivalents. 

A mapping population of 1,488 seedlings, which segregate for resistance, was developed and 

scored for flanking RAPD markers 152-800 and 268-580 to identify potential recombinants.

Chromosome walking, initiated using primers designed from eight RAPD markers (Mehlenbacher et 

al. 2004, 2006) closely linked to resistance and extended with two further rounds of walking, 

identified a total of 93 BACs in the resistance region. The BAC library was screened using a PCRbased pooling and subpooling strategy (Sathuvalli and Mehlenbacher 2011). From the BACs, a total 

of 629 primers were designed, of which 63 pairs were polymorphic. Of these, we mapped 41 new 

markers to the resistance region. The new markers included 23 sequence-characterized amplified 

regions (SCARs), seven single-stranded conformational polymorphism (SSCP), seven high-resolution 

melting (HRM), and four simple-sequence repeat (SSR) markers. A high-resolution genetic map of 

the resistance region was created with 51 markers (41 from BACs, eight RAPD markers, and two

SCAR markers derived from RAPDs) and the resistance phenotype in the mapping population using 

the program JoinMap v 4.0 (Van Ooijen 2006).

High-information content fingerprinting (HICF) was carried out on 93 BACs which were then 

assembled using the program FPC v 9.3 (Nelson and Soderlund 2009) into 22 contigs and 23 

singletons. The combined maps (genetic and physical) identified a single contig that spans the EFB 

resistance locus. The 1,488 seedlings showed two recombination breakpoints within a single contig of 

three BACs (43F13, 66C22 and 85B7) and a physical distance of 135 kb. This region is the target for 

whole BAC sequencing and a search for disease resistance genes.

Whole BACs in the resistance region (< 1cM) were sequenced using an Illumina IIx genome 

analyzer, with multiplexing and barcoded adapters to reduce the cost, and paired-end reads to 

facilitate de novo sequence assembly. De novo assembly was carried out using the programs Velvet

(Zerbino and Birney 2008) and SOPRA (Dayarian et al. 2010), and the resulting contigs were further 

aligned using CodonCode aligner software. The BAC sequencing and assembly generated contigs

whose length ranged from 393 bp to 108,194 bp. Estimated coverage of the BACs ranged from 64 to 

100 percent. The gene prediction program AUGUSTUS (Stanke et al. 2008) identified 233 genes 

from these sequences using Arabidopsis as the model. Of these, RNA-Seq data supported 32 genes at 

100 percent support, of which five were in the contig that contains the resistance gene. The predicted 

gene sequences were compared with sequences in GenBank using a BLASTP search and identified 

two putative genes encoding a p-loop NTPase and F-box super family in the resistance region. Genes 

 1 Department of Horticulture, 4017 ALS Building, Oregon State University, Corvallis, OR 97331.

Corresponding author: vidyasas@hort.oregonstate.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

175

in these two superfamilies have defense response properties. Future expression, complementation, 

and mapping studies are essential to confirm which gene confers resistance.

Literature Cited

Dayarian, A.; Michael, T.P.; Sengupta, A.M. 2010. SOPRA: scaffolding algorithm for paired reads via 

statistical optimization. BMC Bioinformatics. 11: 345.

Mehlenbacher, S.A.; Brown, R.N.; Davis, J.W.; Chen, H.; Bassil, N.; Smith, D.C. 2004. RAPD markers 

linked to eastern filbert blight resistance in Corylus avellana. Theoretical and Applied Genetics. 108: 651–

656.

Mehlenbacher, S.A.; Brown, R.N.; Nouhra, E.R.; Gökirmak, T.; Bassil, N.V.; Kubisiak, T.L. 2006. A 

genetic linkage map for hazelnut (Corylus avellana L.) based on RAPD and SSR markers. Genome. 49: 

122–133.

Nelson, W.; Soderlund, C. 2009. Integrating sequence with FPC fingerprint maps. Nucleic Acids Research. 

37(5): e36.

Sathuvalli, V.R.; Mehlenbacher, S.A. 2011. A bacterial artificial chromosome library for ‘Jefferson’ hazelnut: 

a resource for map-based cloning of eastern filbert blight resistance and pollen-stigma incompatibility genes.

Genome. 54: 862–867.

Stanke, M.; Diekhans, M.; Baertsch, R.; Haussler, D. 2008. Using native and synthetically mapped cDNA 

alignments to improve de novo gene finding. Bioinformatics. 24: 637–644.

Van Ooijen, J.W. 2006. JoinMap 4.0, software for the calculation of genetic linkage maps. Wageningen, The 

Netherlands: Kyazama B.V.

Zerbino, D.R.; Birney, E. 2008. Velvet: algorithms for de novo short read assembly using de Bruijn graphs.

Genome Research. 18: 821–829. 
GENERAL TECHNICAL REPORT PSW-GTR-240

176

Molecular and Genetic Basis for Partial Resistance 

of Western White Pine against Cronartium ribicola

Jun-Jun Liu,1 Arezoo Zamany,

1 and Richard Sniezko2

Abstract

Western white pine (Pinus monticola Douglas ex D. Don) is an important forest species in North America. 

Forest genetics programs have been breeding for durable genetic resistance against white pine blister rust 

(WPBR) caused by Cronartium ribicola in the past few decades. As various genetic resistance resources are 

screened and available from breeding programs, we are interested in understanding the genetic mechanisms at 

the molecular level. This will facilitate a breeding strategy of pyramiding several resistance mechanisms in 

white pine elite stocks, which would make durable resistance possible for effective and long-term WPBR 

management. 

Both major gene resistance and quantitative partial resistance have been identified in P. monticola populations. 

In contrast to the Cr2-mediated major gene resistance that occurs on pine needles by the hypersensitive 

response (HR), quantitative partial resistance was observed in stems after C. ribicola infection as slow canker 

growth, or bark reaction. Quantitative stem resistance was displayed with various phenotypes in different seed 

families, ranging from complete susceptibility (rust dead) to complete stem resistance (stem symptom-free), 

including normal canker development, partial bark reaction with incomplete inhibition of fungal growth, 

complete bark reaction without fungal activity in the bark, delayed stem infection, fewer stem infections, or 

longer time to mortality as compared to susceptible controls. Partial resistance is believed to be a heritable and 

polygenic resistance trait that allows greater survivability of infected white pine trees. Based on phenotypic data 

collected yearly from western white pine seed families sowed in 2003 and inoculated with C. ribicola in 2004, 

partial resistance levels were analyzed and ranked for each seedling.

Using these plant materials, we used a candidate gene-based association study to reveal what genomic DNA 

variants underlie phenotypic variation of white pine partial resistance. A set of pathogenesis-related (PR) genes 

was selected as candidates, including defense-related genes encoding for ß-1,3-glucanases (PR2), chitinases 

(PR3), thaumatin-like proteins (PR5), cytosolic nuclease-like proteins (PR10), and anti-microbial proteins 

(AMPs). Genomic DNA sequences of these genes were determined in white pine trees. DNA sequence data of 

selected candidate genes showed rich single nucleotide polymorphisms (SNPs) in white pine seed families. We 

will report on the genetic variations of these genes. Development of DNA markers will be discussed for markerassisted selection of partial resistance to speed up of white pine breeding programs.

Defense responses of white pine trees with partial resistance were also investigated by measuring PR protein 

accumulation post C. ribicola infection. Using Pm-AMP1 as a biochemical indicator, a few patterns were 

documented for protein regulation in white pine seed families. Our results revealed that different molecular 

mechanisms may be responsible for various phenotypes of partial resistance in white pine populations. Crosspollination between seed families with different resistance mechanisms would produce progeny with more 

durable resistance against C. ribicola.

 1 Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, 506 West Burnside Road, Victoria, British 

Columbia, V8Z 1M5, Canada.

2 USDA Forest Service, Dorena Genetic Resource Center, 34963 Shoreview Road, Cottage Grove, OR 97424.

Corresponding author: Jun-Jun.Liu@NRCan-RNCan.gc.ca.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

177

Mapping Resistance to Phytophthora cinnamomi in 

Chestnut (Castanea sp.)

Bode A. Olukolu,1 C. Dana Nelson,2 and Albert G. Abbott1

Abstract

Phytophthora cinnamomi (Phytophthora crown and root rot, or ink disease) is now known to infect several 

hundred plant species in the world and is especially linked to the widespread death of mature chestnut 

(Castanea) and evergreen oak (Quercus ilex L.) trees in southeast United States. With an expanding 

geographical distribution of P. cinnamomi in Northern America, and coupled with the chestnut blight disease 

(caused by Cryphonectria parasitica) that initially decimated the 4 billion-strong American chestnut population 

(about 30 percent of trees in the Appalachian mountains), P. cinnamomi is becoming a crucial limiting factor in 

natural regeneration and reforestation due to the high susceptibility of both young seedlings and mature trees. In 

this preliminary study, we report the use of various genomic resources for identifying quantitative trait loci

(QTLs) and candidate genes (CGs) underlying resistance to root rot disease. The strategy involved the use of 

single nucleotide polymorphism (SNP) markers and a small segregating population (48 progenies) for the 

construction of a transcriptome-based map and identification of root rot disease resistance QTLs. Using a 

resistant Chinese chestnut and susceptible American chestnut parents, two major QTLs were detected on the 

chestnut linkage group E (64.8 cM map length) at a logarithm of the odds (LOD) of 4.42 and 5.39. These QTLs 

spanned 3 cM (12-15 cM) and 16 cM (42-62 cM), respectively and explained 34.6 ± 11 percent and 40.4 ± 10.9

percent of the total phenotypic variance, respectively. Following the alignment of this low resolution map (211 

mapped SNP markers) against the high density consensus chestnut map, additional expressed sequence tag 

(EST)-based markers provided better marker saturation of the QTLs. Two of these EST-based markers within 

the QTLs reveal two plausible CGs that include CCR1 (Cinnamoyl CoA Reductase 1) and BAG1 (BCL-2-

Associated Athanogene 1). Additionally, comparative analysis with the peach genome using the chestnut 

physical map revealed that the chestnut QTL regions correspond to homologous segments of the peach genome 

on chromosome 3 and 4. The homologous regions in peach identified three plausible CGs including RPH1 

(resistance to Phytophthora), NPR3/NPR4 (non-expresser of pathogenesis-related genes 3/4) and BAG4 (BCL2-Associated Athanogene 4). Following Southern hybridization on chestnut bacterial artificial chromosome 

(BAC) filters using overgo probes, the co-localization of the CGs with the QTLs was confirmed. For further 

validation based on gene expression, a larger mapping population, or an association mapping panel, is required.

These results can provide functional markers for precise and accurate marker-assisted breeding for introgression 

of resistance genes into the American chestnut. Transgenic trees are currently also been developed in parallel 

based on cloned CGs.

 1 Genetics and Biochemistry Department, 100 Jordan Hall, Clemson University, Clemson, SC 29634. 2 USDA Forest Service, Southern Research Station, Southern Institute of Forest Genetics, 23332 Success Road, Saucier, MS 

39574.

Corresponding author: baolukol@ncsu.edu.

179

Evaluating Resistance

Testing six species of white pines (Pinus albicaulis, P. aristata, P. flexilis, P. 

lambertiana, P. monticola, P. strobiformis) for white pine blister rust resistance at 

Dorena Genetic Resource Center, Cottage Grove, OR.

(Photo: R. Sniezko)

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

181

Methods for Screening Port-Orford-Cedar for 

Resistance to Phytophthora lateralis

Everett M. Hansen,

1 Paul Reeser,1 Wendy Sutton,1 and Richard A. Sniezko2

Abstract

Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murray) Parl.) (POC) is an economically and ecologically 

valuable tree in the forests of southwest Oregon and northern California and in the horticultural trade 

worldwide. Phytophthora lateralis, the aggressive, invasive cause of POC root disease, was introduced to the 

native range of POC about 1950, and has since killed trees along roads and streams throughout the forest area 

where the tree grows. The federal forest management agencies responded with an integrated disease 

management program designed to slow further spread of the pathogen in order to protect remaining POC and 

restore the species with planted resistant stock in impacted areas. Various short-term resistance screening 

methods have been used since the 1980s. Ultimately, in any resistance program, the screening assays have to be 

ground truthed using field trials and the screening methods updated and refined to assist in resistance 

evaluations. In this paper we describe and compare the various inoculation methods and screening tests we use 

to evaluate POC parents and their progenies for resistance and offer evolving insights on their utility. This 

should provide food for thought for other newer resistance programs working with other pathogens or insects.

Introduction

Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murray) Parl.) (POC) is an economically and 

ecologically valuable tree in the forests of southwest Oregon and northern California and in the 

horticultural trade worldwide. Phytophthora lateralis, an exotic, invasive root pathogen, is the 

primary cause of POC mortality in southwest Oregon and northwest California (Betlejewski et al. 

2011). The origin of P. lateralis is not known, although it was first described killing POC in 

horticultural nurseries in the 1920s (Hansen et al. 2000). Recently P. lateralis or a very close relative 

was reported from native Chamaecyparis obtusa (Siebold & Zucc.) Siebold & Zucc. ex Endl. forests

in Taiwan (Brasier et al. 2010), suggesting that the pathogen originated in eastern Asia. In North 

America it spreads as sporangia and zoospores in streams and is transported by vehicles along roads 

in the forest (Hansen et al. 2000). Even in infested areas, however, occasional Port-Orford-cedars 

escape mortality by P. lateralis. 

Phytophthora lateralis was introduced to the forest range of POC about 1950. It has since killed 

trees along roads and streams throughout the tree’s range. The federal forest management agencies—

U.S. Department of Agriculture, Forest Service (USFS) and U.S. Department of the Interior, Bureau 

of Land Management (BLM)—have responded with a disease management program integrated into 

forest plans (Betlejewski et al. 2003, USDA FS and USDI BLM 2004). Mandated actions are 

designed to slow further spread of the pathogen in order to protect remaining POC, and to restore the 

species with planted resistant stock in areas where the pathogen has eliminated POC. Since the late 

1980s, the USFS and the BLM, in cooperation with Oregon State University (OSU), have conducted 

an intensive program to identify and test resistant trees from the field and propagate them in a seed 

orchard with the goal of providing resistant seedlings for regeneration 

(http://www.fs.fed.us/r6/dorena/poc/; Sniezko 2006, Sniezko et al. 2006; Sniezko et al., Operational 

program to develop Phytophthora lateralis resistant populations of Port-Orford-cedar 

(Chamaecyparis lawsoniana), these proceedings). In this paper we describe the various inoculation 

methods and screening tests we use to evaluate POC parents for resistance.

 1 Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis, OR 97331.

2 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, OR 97424.

Corresponding author: hansene@science.oregonstate.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Most POC trees in the field are very susceptible, but genetic resistance to this pathogen has been 

demonstrated (Hansen et al. 1989, Sniezko 2006, Sniezko et al. 2006). Susceptible seedling families 

showed only 0 to 10 percent survival using a variety of inoculation techniques. Rooted cuttings of the 

most resistant parents were seldom killed, and seedling families of these parents exhibited 25 to 100

percent survival, depending on family and inoculation technique (Oh et al. 2006; Sniezko et al., Nine 

year survival of 16 Phytophthora lateralis resistant and susceptible Port-Orford-cedar families in a 

southern Oregon field trial, these proceedings). Symptom development on resistant trees, including 

reduced colonization and reisolation success, sunken lesions, and resinosis, was consistent with a

hypersensitive reaction (Oh and Hansen 2007). 

Methods

Screening Protocol 

The USFS and BLM funded and soon led the effort to locate candidate resistant trees in the forest and 

to collect vegetative material and seeds for testing. Inoculations and resistance screening carried out 

at OSU have evolved through time to take advantage of the unique features of POC and to meet the 

changing needs of the program. Candidate parents from the field are initially challenged with a rapid 

stem dip zoospore inoculation test. “Winners” are then propagated as rooted cuttings, which are in 

turn exposed to zoospore inoculum in a root dip test. The best individuals are incorporated into 

containerized seed orchards at Dorena Genetic Resource Center (DGRC) in Cottage Grove, Oregon. 

Seedlings from cross or self pollination within the orchards are further challenged with the root dip 

test and by planting into infested soil for a 2-year exposure in outdoor raised beds. Finally, the most 

promising materials are included in long-term field tests on infested sites and maintained in the seed 

orchards by breeding zone. Over 10,000 field selections have been screened and over 1,100 trees have 

been chosen for further evaluation (Sniezko et al., Operational program to develop Phytophthora 

lateralis resistance populations of Port-Orford-cedar (Chamaecyparis lawsoniana), these

proceedings). 

Inoculum Production

Port-Orford-cedar cuttings, rooted cuttings, and seedlings are inoculated with zoospores. Two isolates 

of P. lateralis, collected from diseased POC trees in southern Oregon or northern California, were 

used. Isolates were grown on homemade corn meal agar amended with B-sitosterol (CMAB) (15 g 

Bacto agar, 50 ml corn steep, 20 mg B -sitosterol). Corn steep was made by mixing 100 g yellow corn 

meal in 650 ml de-ionized water at 60 °C for 1 hr. Solids were removed by pouring through eight

layers of cheesecloth then filtering through Celite 545. Cultures were maintained in water storage in 

the culture collection at Oregon State University. 

To produce zoospores, isolates were grown for one week on CMAB then three, 3 mm dia. plugs 

cut from the advancing edge of colonies were transferred to pea broth (150 g dry split peas per liter, 

autoclaved for 3 min and strained through a double layer of cheesecloth, then autoclaved for 60 min 

at 121 °C) and grown for 7 days. The culture medium was removed, plates were rinsed with deionized water, then filled with 10 ml filtered water from a local stream. After 24 hours, zoospore 

release was triggered, if needed, by chilling plates 1 hr at 5 °C then returning to room temperature.

Stem Dip Test 

Initial screening of POC trees for resistance to P. lateralis is done on fresh cuttings from candidate 

trees. Six to 10 branch tips, each about 30 cm long, were clipped from each candidate tree, labeled, 

and transported cool and moist to the OSU lab for testing.

For zoospore inoculations, zoospores were produced as described above, and the water containing 

the colonies and zoospores was poured from plates into 1 L glass jars. Water and zoospores from one 

plate of each isolate, along with additional water to total 40 ml, were combined in each jar to an 

approximate depth of 1 cm.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Branches were freshly trimmed to 20 to 30 cm length, mixed among the candidate and control 

trees in two replications (three to five branches per parent per replication), and clustered in bundles of 

10 then placed cut end down in each glass jar with zoospores, care being taken to insure that each cut 

end was immersed (fig. 1).

Figure 1—Stem dip test. Left: Branch tips with cut ends immersed in zoospore suspension. Right: 

Lesions on susceptible control (left) and resistant control (right) branches after three weeks.

Branches remain with cut ends in the zoospore suspension for 24 hours and then are transferred 

individually to plastic growth tubes filled with wet vermiculite. Racks of test cuttings are held in the 

greenhouse and watered regularly. After 3 weeks, they are scored. Each branch tip was removed from 

the vermiculite, rinsed, and scraped at the cut end to reveal the discolored lesion in the inner bark. 

Lesion length was recorded. The best 10 percent of all parent trees in each test, including all trees 

with average lesion lengths less than or equal to the resistant control, were declared “winners” and 

advanced to further testing.

Root Dip Test

Port-Orford-cedar for root dip inoculation trials was grown from cuttings from “winner” parent trees. 

Cuttings were rooted under mist and grown about 12 months. Rooted cuttings were grown in 

individual 3.7 cm top diameter, 20.6 cm length, ‘supercell’ plastic conical tubes (Cone-tainerTM).

Seedlings, mostly from controlled crosses at DGRC, were grown for about 9 months in supercells. 

Seedlings or rooted cuttings were grown until roots emerged from the bottom of the containers. 

Emergent roots were trimmed immediately prior to inoculation. 

Seedlings or rooted cuttings were randomized and groups of six were clustered in each inoculation 

container. The lower 1 cm of the root system (in the growth tube) was immersed in a P. lateralis

zoospore suspension (prepared as above) for 24 hours (fig. 2). The inoculated trees were then 

incubated in the greenhouse for up to 36 months, and symptom development was periodically 

recorded. At the first sign of yellowing or wilting (usually after about 6 months), a small slice in the 

bark at soil level was made and the stem examined for the characteristic necrotic lesion caused by P. 

lateralis. Mortality was recorded when the lesion reached the top of the growth tube. Mortality, 

evidenced by diagnostic lesion at the ground line, was noted regularly and the days from planting to 

mortality recorded.
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Figure 2—Root dip test. Container grown seedlings and rooted cuttings, inoculated by immersing the 

bottom 1 cm of the container in zoospore suspension. 

Port-Orford-cedar seedlings and rooted cuttings were also planted in infested soil in an outdoor 

raised bed (fig. 3). Two raised beds were established in 2000, each 1.5 x 10 m and 30 cm deep, and 

contained a soil mix comprised of 1/4 loam, 1/4 peat, 1/4 pumice, 1/4 fine bark, plus 1/2 kg 

gypsum/m3

, and 1 kg lime/m3. The soil mix was infested with P. lateralis by mixing sporulating 

cultures and chopped roots of infected POC seedlings with the soil. Port-Orford-cedar trees were 

planted in family row plots in a replicated design and received regular summer irrigation. Mortality, 

evidenced by diagnostic lesion at the ground line, was noted regularly and the days from planting to 

mortality recorded.

Figure 3—Raised bed test. Seedlings and rooted cuttings planted in infested soil. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Outplanting Tests 

Port-Orford-cedar seedlings and rooted cuttings were also planted in artificially or naturally infested 

soil at field sites. Some of the first selected families and clones were planted in 1989 and 1990 in a 

replicated block design at the OSU Botany and Plant Pathology Field Lab near Corvallis, Oregon 

(table 1). Parent trees other than parent id numbers CF1 and ESOC were initially collected by 

Professor L. F. Roth from forested areas in Coos County, Oregon, either because they were survivors 

in areas of widespread mortality or as unselected control trees from areas with no previous mortality. 

The 1/10 ha plot had been artificially infested with P. lateralis some years previously, and was 

initially used for screening different species of Chamaecyparis for resistance to P. lateralis. Mortality 

and cause of death were recorded annually to the present. Surviving trees are now 10 to 20 cm dbh. 

Additional long-term outplanting test sites were selected at forest sites where local concentrations of 

POC had been recently killed by P. lateralis (fig. 4). 

Table 1—Survival of POC trees selected for resistance or as susceptible controls at the 

Oregon State University Botany Farm outplanting site 

Parent tree Year planted Trees planted (n) Trees alive July 2011 (%)

CF1-cuttings 1989 25 48.0

CF1-seedlings 1989 25 24.0

CF2 1989 20 80.0

CF3 1989 4 50.0

CF4 1989 20 15.0

ESOC 1990 30 3.3

CHECK10 1990 20 0.0

CB1 1989 20 40.0

CB2 1989 20 0.0

CB4 1989 18 5.6

CB5 1989 20 0.0

CHECK13 1989 20 0.0

CHECK14 1989 20 0.0

Figure 4—Forest outplanting site established on naturally infested ground within a mixed Douglas-firPOC forest in Douglas County, Oregon. 
GENERAL TECHNICAL REPORT PSW-GTR-240

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Results

Sniezko et al. present extensive validation data for the screening tests, comparing greenhouse and 

raised bed rankings with outplanting test results elsewhere in this proceedings. Here we present two 

additional examples. The stem dip test reliably separated a small number of POC families with good 

survival at the Botany Farm planting from susceptible families with poor or no survival (table 2).

Similarly, root dip test scores reliably predicted survival after 2 1⁄2 years at an infested outplanting site 

(fig. 5).

Table 2—Stem dip lesion length scores for branch tips collected from susceptible and 

resistant trees from the Botany Farm outplanting site 

Parent tree Alive 2011 (%) Stem dip score (mm)

Resistant control 9.2

CF1 (cuttings) 48 9.4

CF2 80 12.5

CB1 45 10.7

CB2 0 28.2

CB4 6 28.9

CB5 0 28.9

Susceptible control 32.7

Figure 5—Root dip vs outplanting survival, Bill Creek test site. See Sniezko et al. 2006 for trial 

background.

Discussion

Short-term assays of resistance are a vital part of operational resistance programs. The four methods 

of short-term screening currently used for POC to discern P. lateralis resistance—(1) stem dip; (2) 

root dip using rooted cuttings; (3) root dip using seedlings; and (4) raised bed—all have potential 

advantages. The stem dip test can be done fastest and cheapest. The raised bed trial may mimic field 

conditions or could be monitored for three years or more for continued mortality or evolution of 

virulence. The root dip tests may allow evaluation of hundreds of families or clones at a time and 

provide very controlled conditions. However, ultimately the utility of these tests depends on how their 

results relate to field trials. At this point in time, the seedling root dip assay, and probably the raised 

bed trials, provide the best overall indication that there might be several types of resistance and 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

187

information on the upper level of survival to expect under field conditions (Sniezko et al., Nine year 

survival of 16 Phytophthora lateralis resistant and susceptible Port-Orford-cedar families in a 

southern Oregon field trial, these proceedings; Sniezko and Savin, unpublished). 

The POC resistance screening program is an evolving success. Resistant seed with demonstrated 

survival potential is available for regeneration programs in the most important POC seed zones. The 

program continues, screening additional forest trees as candidates are identified, and root dip testing a 

backlog of stem dip “winners” to add additional parents to the orchards. In addition, orchards are 

continuously rogued and upgraded as field testing results are available and through selection of 

resistant progeny from controlled crosses.

Each of the screening tests has advantages as well as limitations. The original intention of the stem 

dip test was to provide a quick first screen of candidate trees assuming most would prove susceptible 

and could be eliminated before costly propagation efforts are made. It is likely that such an indirect 

test can only select for one type of resistance. However, with the compilation of data from a large 

number of selections, a re-evaluation of the utility of the stem dip test is underway. The test identifies 

some of the parents with the highest and lowest levels of resistance, but has a low correlation with the 

root dip tests (Sniezko and Savin, unpublished). It is not sensitive enough to rank trees with 

intermediate levels of disease tolerance. There appear to be several types of resistance in POC 

(Sniezko 2006; Sniezko, unpublished), and more understanding of these mechanisms is needed to 

fully evaluate the limitations of the stem dip method. We hope to use other current existing data or

selections to more fully investigate the utility of this method. Other ‘indirect’ methods (e.g.,

biochemical markers, or genetic markers) of selecting for different types of resistance could be useful 

in short-cutting the initial testing step.

Rankings from root dip testing correlate well with outplanting test results (Sniezko et al., Nine 

year survival of 16 Phytophthora lateralis resistant and susceptible Port-Orford-cedar families in a 

southern Oregon field trial, these proceedings). The infection pathway is more realistic and the longer 

test period allows at least preliminary identification of “slow to die” families. Seedlings outgrow their 

containers after 2 or 3 years, however, and rooted cuttings are often difficult to evaluate because they 

may be slow to develop a robust root system. In a recent comparison of rooting cuttings and seedling 

families, there was good agreement, but the seedling assay appeared more consistent (Sniezko and 

Savin, unpublished). The current seedling assay takes several years to complete; the seedlings are 

grown for 9 months and then followed for 1 to 3 years after inoculation. Now that we have well 

defined resistant and susceptible families, there may be merit to see if we can short-cut the process by 

utilizing much younger, smaller seedlings in smaller containers. If so, this could reduce some costs as 

well as speed evaluation.

The various outplanting trials, especially on high-hazard forest sites, provide the ultimate test of 

the program, but are also most difficult to establish and maintain. It is challenging to find sites of 

sufficient area with uniform inoculum and topography. Regular recording of mortality is often 

precluded by limited travel budgets, and other mortality agents confuse the analysis.

Lessons learned in the POC resistance program should be valuable in providing guidance to other 

programs. Blind faith in a single short-term seedling assay may lead to over- or under-estimation of 

the level and types of resistance or may preclude utilizing some types of resistance. Establishing some 

field trials early to help validate short-term screening methods is essential to helping maximize 

progress in the development of resistant populations. The development of new screening assays such 

as biomarkers or genetic markers could be a useful addition to any resistance program, provided they 

are validated with field trials.

Acknowledgments

We thank the USDA Forest Service and USDI Bureau of Land Management for their continuing support of the 

program. We also thank Dave Shaw and Douglas Savin for their reviews of an earlier version of this paper.
GENERAL TECHNICAL REPORT PSW-GTR-240

188

Literature Cited

Betlejewski, F.; Casavan, K.; Dawson, A.; Goheen, D.; Mastrofini, K.; Rose, D.; White, D. 2003. A Rangewide assessment of Port-Orford-cedar (Chamaecyparis lawsoniana) on federal lands. Oregon/Washington 

State Office, Portland, OR: U.S. Department of Agriculture, Forest Service, and U.S. Department of the 

Interior, Bureau of Land Management. 

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5316517.pdf 182 p. (13 March 2012). 

Betlejewski, F.; Goheen, D.; Angwin, P.; Sniezko, R. 2011. Port-Orford-cedar root disease. Forest Insect and 

Disease Leaflet 131. Portland, Oregon: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Region. 12 p.

Brasier, C.M.; Vettraino, A.M.; Chang, T.T.; Vannini, A. 2010. Phytophthora lateralis discovered in an old 

growth Chamaecyparis forest in Taiwan. Plant Pathology. 59: 595–603. 

Hansen, E.M.; Hamm, P.B.; Roth. L.F. 1989. Testing Port Orford cedar for resistance to Phytophthora. Plant 

Disease. 73:791–794.

Hansen, E.M.; Goheen, D.J.; Jules, E.; Ullian, B. 2000. Managing Port-Orford-cedar and the introduced 

pathogen Phytophthora lateralis. Plant Disease. 84: 4–14.

Oh, E.; Hansen, E.M.; Sniezko, R.A. 2006. Port-Orford-cedar resistant to Phytophthora lateralis. Forest

Pathology. 36:385–394.

Oh, E.; Hansen E.M. 2007. Histopathology of infection and colonization of Port-Orford-cedar by 

Phytophthora lateralis. Phytopathology. 97: 684–693.

Sniezko, R.A. 2006. Resistance breeding against nonnative pathogens in forest trees — current successes in 

North America. Canadian Journal of Plant Pathology. 28: S270–S279.

Sniezko, R.A.; Kolpak, S.E.; Hansen, E.M.; Goheen, D.J.; Elliott, L.J.; Angwin, P.A. 2006. Field survival 

of Phytophthora lateralis resistant and susceptible Port-Orford-cedar families. In: Brasier, C.; Jung, T.; 

Osswald, W., eds. Progress in research on Phytophthora diseases of forest trees: Proceedings of the Third 

International IUFRO Working Party S07.02.09. Farnham, UK: Forest Research: 104–108.

USDA FS and USDI BLM 2004. Final supplemental environmental impact statement – management of PortOrford-cedar in southwest Oregon, Portland OR: U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Region. http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5316520.pdf . (06 June 

2012).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

189

Resistance to Phytophthora cinnamomi in the 

Genus Abies

John Frampton,1 Fikret Isik,

1 Mike Benson,2 Jaroslav Kobliha,3 and

Jan Stejskal3

Abstract

A major limiting factor for the culture of true firs as Christmas trees is their susceptibility to Oomycete species 

belonging to the genus Phytophthora. In North Carolina alone, the Fraser fir (Abies fraseri [Pursh] Poir.) 

Christmas tree industry loses 6 to 7 million dollars annually to root rot primarily caused by Phytophthora 

cinnamomi. Because no resistance has been found in Fraser fir, in 2003, the North Carolina State University 

(NCSU) Christmas Tree Genetics (CTG) Program conducted an extensive resistance screening of 32 Abies

species (50 unique taxa) from around the world.

Overall mortality was high (88 percent), but was less frequent in two of the eight taxonomic sections of the 

genus evaluated: Momi (65.5 percent) and Abies (79.3 percent). Species mortality rates in all other sections 

exceeded 93 percent. Final species mortality ranged from 10.6 percent (A. firma Sieb. et Zucc.) to 100.0 percent

(several species). Hierarchical cluster analysis was used to classify species into groups representing six levels of 

resistance, designated as very resistant (A. firma), resistant (A. pindrow [D.Don] Royle), moderately resistant 

(four species), intermediate (five species), susceptible (seven species), and very susceptible (17 species).

In the species screening trial, Turkish fir (A. bornmuelleriana Mattf.) and closely related Trojan fir (A. equitrojani Coode et Cullen) ranked third and tenth for resistance, but mortality in these species was relatively high. 

A systematic approach to understand and better use Phytophthora resistance within Turkish and Trojan fir was 

undertaken. Using seeds from a 2005 cone collection expedition to Turkey, greenhouse-grown seedlings from 

105 open-pollinated families were inoculated with P. cinnamomi. Sixteen weeks after inoculation, overall 

seedling mortality was 56 percent for Trojan fir and 35 percent for Turkish fir. As a comparison, 97 percent of 

inoculated Fraser fir seedlings, but only 3 percent of inoculated momi fir seedlings, died. For Turkish and 

Trojan fir, there was a distinct relationship between mortality and geographic origin; mortality percentage 

decreased from west to east. Additionally, estimates of family mean heritabilities were extremely high for both 

Turkish (0.96+0.010) and Trojan (0.97+0.011) fir.

In another approach, hybrid firs are being developed through a collaborative effort between the NCSU CTG 

Program and researchers at the Czech University of Life Sciences (CULS) Prague. Researchers at CULS have

utilized Toros fir (A. cilicica Carr.) from southern Turkey and Greek fir (A. cephalonica Loud.) in a long-term 

hybrid breeding effort aimed at developing a faster growing fir that is hardier to changing ecological conditions 

than the native European silver fir (A. alba Mill.). Toros and Greek fir were ranked fourth and eighth, 

respectively, in the species screening trial. Seedlings of F1, F2, and complex hybrids with Fraser fir are being 

assessed for Phytophthora resistance.

 1 Dept. of Forestry and Environmental Resources, N.C. State University, Raleigh, NC 27695. 2 Dept. of Plant Pathology, N.C. State University, Raleigh, NC 27695. 3 Dept. of Dendrology and Forest Tree Breeding, Faculty of Forestry and Wood Sciences, Czech University of Life 

Sciences, Prague, Kamýcká 1176, 165 21 Praha 6 – Suchdol, Czech Republic.

Corresponding author: frampton@ncsu.edu.
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Host Resistance Screening for Balsam Woolly 

Adelgid: A Comparison of Seedlings from

12 Fir Species

Leslie Newton,

1 John Frampton,2 and Fred Hain1

Introduction

The balsam woolly adelgid, Adelges piceae (Hemiptera: Adelgidae) (BWA), first reported on Fraser 

fir, Abies fraseri (Pursh) Poiret, on Mount Mitchell in 1955 (Amman 1966, Boyce 1955), is a major 

pest in Christmas tree plantations and in native stands. Nearly all Fraser fir Christmas trees produced 

in North Carolina need to be treated one or more times during their 5- to 10-year rotation to prevent 

or lessen damage caused by this adelgid. These chemical treatments cost the industry over 1.5 million

dollars per year (Potter et al. 2005) and may compromise the effectiveness of integrated pest 

management systems. The development of BWA-resistant Fraser fir trees would be a relatively 

inexpensive solution to a difficult pest problem and would minimize adverse effects from 

management strategies.

The balsam woolly adelgid, specific to the genus Abies, reproduces through parthenogenesis and 

completes two or more generations per year (Arthur and Hain 1984, Balch 1952). The early phase of 

the first instar (crawler) is the only motile stage. Feeding sites are chosen for accessibility to 

parenchyma cells (Balch 1952) and, once settled, the adelgid remains fairly sessile for the remainder 

of its life. Susceptibility to BWA varies among Abies species. Host responses include gouting 

(abnormal cell growth resembling a gall) at the feeding site, loss of apical dominance, and the 

production of abnormal xylem (‘redwood’ or ‘rotholtz’). 

In the 100+ years that BWA has been in North America, studies on its biology and interaction 

with host trees have been conducted in the Pacific Northwest, Canada, and the Southern Appalachians 

on multiple fir species of different ages, utilizing various sources of the adelgid. Three subspecies of 

BWA have been identified in North America (Foottit and Mackauer 1983). Our long-term objective is 

to develop BWA-resistant Fraser fir trees for the Christmas tree industry and native stand restoration. 

Our objective for this study was to screen for resistance across multiple fir species utilizing trees of 

equal age, grown under the same conditions and infested with BWA from the same source, and to 

observe the reactions of both host and insect. 

Methods

A BWA resistance screening trial was established in a greenhouse at the Upper Mountain Research 

Station in Ashe County, North Carolina in August 2007, utilizing 4-year-old seedlings. The study 

included 12 fir species, 9 representing the range of known susceptibility and 3 representing unknown 

susceptibilities. Susceptible species included Fraser fir (three seed sources: Roan Mountain, Richland 

Balsam, Mount Mitchell), balsam fir (A. balsamea), West Virginia Canaan fir (A. balsamea var. 

phanerolepis), corkbark fir (A. lasiocarpa var. arizonica), and Korean fir (A. koreana). Moderately 

resistant or tolerant species included European silver fir (A. alba) and white fir (A. concolor). 

Resistant species included Veitch fir (A. veitchii) and Momi fir (A. firma). Species representing 

unknown susceptibility included Turkish fir (A. bornmuellariana), Trojan fir (A. equi-trojani), and 

West Himalayan or Pindrow fir (A. pindrow). The greenhouse was divided into four blocks, each 

 1 North Carolina State University, Department of Entomology, Raleigh, NC 27695-7626. 2 North Carolina State University, Department of Forestry and Environmental Resources, Raleigh, NC 27695.

Corresponding author: fred_hain@ncsu.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

191

containing two treatment plots representing seasonal effect (August/September). There were 48 trees 

per species in each of the 8 treatment plots, for a total of 672 trees. Trees were exposed to BWA by 

suspending logs of BWA-infested Fraser fir over each treatment plot. This technique (Newton et al., 

2011) mimics natural dispersal, allowing crawlers to drop onto the trees. 

In December 2007 one-half of the study was dismantled and in May 2008 the remaining half was 

dismantled. Data collected from the first group of trees included the number and location of settled 

first instars (neosistentes). Data from the second group included presence/absence of settled instars, 

an assessment of infestation (yes/no), the number of BWA adults with eggs, the number of eggs, 

fecundity (mean number of eggs per adult), evidence of early BWA development (yes/no), and 

presence/absence of gouting. A subsampling technique was developed; the most predictive scheme 

was to take the first three branches from the top of the tree with secondary or tertiary branching (i.e., 

the bushiest branches from the second and third whorls). Data were collected from branches and 

boles, but we report here only branch responses. Because there was a significant difference in height 

between the species, infestation levels are expressed as counts per cm of branch. 

Results

From the first group of trees (December 2007), there were no significant differences among species in 

the number of settled neosistentes per branch. Least squares means ranged from 0.03 (± 0.51) for A. 

concolor (white fir) to 2.09 (± 0.59) for A. lasiocarpa ‘arizonica’ (corkbark fir). The susceptible 

species consistently ranked higher than the resistant and tolerant species. Crawlers settled at the base 

of the branches, at the nodes under old bud scales, at the base of leaves along the branch stem, and at 

the base of buds (both lateral and apical). They appeared to show a preference for the base of buds 

regardless of species. 

From the second group of trees (May 2008), there were no differences among species in 

presence/absence of settled instars, with 81 to 100 percent of trees showing settled instars. Although 

one can consider trees with settled instars to be infested, here we consider ‘infested’ to mean that 

adults with egg clutches have developed on the tree. Differences in the proportion of infested trees 

(supporting adults with eggs) were highly significant (p < .0001) and, with few exceptions, the results 

were consistent with known susceptibility levels. Susceptible species generally showed higher 

proportions of infested trees than the tolerant or resistant species. Two of the unknowns, A. equitrojani (Trojan fir) and A. bornmuelleriana (Turkish fir), were at opposite ends of this range, with 

almost all (0.99) of the Trojan fir trees becoming infested and only about one-third (0.34) of the 

Turkish fir trees. In the number of adults and eggs per cm branch, Trojan fir ranked significantly 

higher than Korean, Turkish, and white firs.

Mean fecundity levels reflected significant (p = .0143) differences among species, ranging from 

4.4 eggs per adult in A. pindrow to 16.8 in A. alba. Trojan fir ranked high, along with corkbark fir, 

Fraser fir, and balsam fir, species known to be very susceptible to BWA. Early BWA development 

was evident on approximately half of the trees from most species. Turkish fir appeared least likely to 

exhibit early development (0.16) and Trojan fir the most likely (0.63). Fraser fir (Roan Mountain) 

exhibited the highest proportion of trees with gouting. Gouting responses were highly significant (p < 

.0001) with susceptible species ranking highest, tolerant and resistant species lowest, and the 

unknowns in the middle – one notable exception is Korean fir, which showed the least amount of 

gouting (0.18). 

Discussion

Data collected from the first group of trees, harvested before winter dormancy, provide evidence that 

the trees were exposed to a sufficient number of crawlers to complete development to the next 

generation. Although the differences among species were not statistically significant, the rankings 

were consistent with a priori resistance classifications – susceptible species ranked higher than 
GENERAL TECHNICAL REPORT PSW-GTR-240

192

resistant or tolerant species. This may reflect some constitutive defense that inhibits settling by the 

crawler. The preference for buds exhibited by the crawlers may indicate the presence of higher 

nutritive values in that region. 

The second group of trees, when harvested, had gone through winter dormancy and entered into 

the growing season (although budbreak had not yet begun). Most trees from all species exhibited the 

presence of settled instars. At least some trees from each species were able to support the 

development of adults with eggs, but the differences in the proportion of trees that became fully 

infested suggest that this parameter alone (adults with eggs) may not be adequate for assessing 

resistance. Complete development took place on 89 percent of the A. firma fir trees, a species that 

exhibits resistance to BWA when mature (Mitchell 1966). One of the most interesting results revolves 

around the differences between two of the ‘unknown’ species, A. equi-trojani (Trojan fir) and A. 

bornmuelleriana (Turkish fir). Both are from the same global region, but here Trojan fir consistently 

suggests high susceptibility to BWA and Turkish fir consistently suggests resistance. This is observed 

in the proportion of trees that became fully infested, the numbers of adults with eggs, the number of 

eggs in general, the ability of BWA to complete development very early in the season, and, to a 

certain extent, fecundity. One important factor in the ‘susceptibility status’ of Trojan fir relates to the 

gouting response, generally considered detrimental to the health of the tree. While European silver fir 

can become infested with BWA, it suffers very little from BWA attack and here exhibited the highest 

fecundity levels (16.8 eggs per adult), but comparatively little gouting response (27 percent of the 

trees). Trojan fir appears to be highly susceptible, but it ranked with European silver fir in both 

fecundity levels and gouting responses. The other ‘unknown,’ Pindrow fir, generally ranked near the 

resistant and tolerant species. Abies veitchii (Veitch fir) consistently exhibited resistance and Fraser 

fir and the balsam firs consistently exhibited susceptibility. Abies koreana (Korean fir) is considered 

to be susceptible to BWA (Mitchell 1966); however, here its responses suggest resistance in the 

seedling class.

Acknowledgments

We gratefully acknowledge John Strider and the NC State Forest Entomology Lab, Anne Margaret Braham and 

the NC State Christmas Tree Genetics Program, Les Miller and the Upper Mountain Research Station for 

technical assistance, and the USDA Cooperative State Research, Education, and Extension Service (CSREES) 

and the NC State Department of Entomology for funding assistance.

Literature Cited

Amman, G.D. 1966. Aphidecta obliterata (Coleoptera: Coccinellidae) an introduced predator of the balsam 

woolly aphid, Chermes piceae (Homoptera: Chermidae), established in North Carolina. Journal of 

Economic Entomology. 59: 506–511.

Arthur, F.H.; Hain, F.P. 1984. Seasonal history of the balsam woolly adelgid (Homoptera: Adelgidae) in 

natural stands and plantations of Fraser fir. Journal of Economic Entomology. 77: 1154–1158.

Balch, R.E. 1952. Studies of the balsam woolly aphid, Adelges piceae (Ratz.) and its effect on balsam fir, Abies 

balsamea (L.) Mill. Publication 867. (place of publication unknown): Canadian Department of Agriculture.

76 p.

Boyce, J.S. 1955. Memorandum of October 7, 1955 to North Carolina National Forests, Toecane Ranger 

District, Burnsville, North Carolina. Asheville, NC: U.S. Department of Agriculture, Forest Service, 

Southeastern Forest Experiment Station. (As reported in Amman 1966.)

Foottit, R.G.; Mackauer, M. 1983. Subspecies of the balsam woolly aphid, Adelges piceae (Homoptera: 

Adelgidae), in North America. Annals of the Entomological Society of America. 7: 299–304.

Mitchell, R.G. 1966. Infestation characteristics of the balsam woolly aphid in the Pacific Northwest. Research 

Paper PNW-35. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and 

Range Experiment Station. 18 p.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

193

Newton, L.; Frampton, J.; Monahan, J.; Goldfarb, B.; Hain, F. 2011. Two novel techniques to screen Abies

seedlings for resistance to the balsam woolly adelgid, Adelges piceae. Journal of Insect Science. 11:158.

Potter, K.M.; Frampton, J.; Sidebottom, J. 2005. Impacts of balsam woolly adelgid on the southern 

Appalachian spruce-fir ecosystem and the North Carolina Christmas tree industry. In: Onken, B.; Reardon, 

R., eds. Proceedings, 3rd symposium on hemlock woolly adelgid in the eastern United States. FHTET-2005-

01. Morgantown, WV: U.S. Department of Agriculture, Forest Service, Forest Health Technology 

Enterprise Team: 25–41.
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Resistance to Phytophthora cinnamomi Among 

Seedlings From Backcross Families of Hybrid 

American Chestnut

Steven N. Jeffers,1 Inga M. Meadows,1 Joseph B. James,2 and Paul H. Sisco3

American chestnut (Castanea dentata (Marsh.) Borkh.) once was a primary hardwood species in 

forests of the eastern United States. Sometime during the late 18th century, it is speculated that 

Phytophthora cinnamomi, which causes Phytophthora root rot (PRR) on many woody plant species, 

was introduced to the southeast region of the United States, and this pathogen spread as people moved 

inland from the coast (Zentmyer 1980). In the 1800s, PRR (also known as ink disease) caused 

extensive mortality to American chestnut trees in the southern portion of its range (Freinkel 2007, 

Zentmyer 1980). Then, in the early 1900s, chestnut blight, caused by Cryphonectria parasitica, 

almost eliminated American chestnut trees from eastern forests (Freinkel 2007). Since 1989, The 

American Chestnut Foundation (TACF) has been producing hybrid chestnut seedlings by crossing 

Chinese chestnut (C. mollissima), which is resistant to C. parasitica, with American chestnut and then 

backcrossing progeny to C. dentata to produce resistant American-type chestnut trees. Since 2000, 

hybrid seedlings planted in some locations in southeastern states have died from PRR before they 

could be challenged by naturally-occurring populations of C. parasitica. Therefore, we wanted to 

determine if any of the backcross trees selected for resistance to C. parasitica were resistant to P. 

cinnamomi as well because Chinese chestnut also is resistant to this pathogen.

Each year from 2004 to 2010, hybrid seeds from trees of known parentage were obtained from 

TACF cooperators, and seeds from C. dentata and C. mollissima were collected in the field. All seeds 

from a single cross were considered to be a family. Seeds were stratified and then planted outside in 

April in six or seven replicate 570 L plastic tubs filled with a soilless container mix at a field site in 

Oconee County, South Carolina. Each tub contained a sub-sample of seeds from each family. 

Inoculum consisted of two isolates of P. cinnamomi originally recovered from diseased chestnut 

seedlings growing at the study site. Isolates were grown on autoclaved rice grains or sterilized 

vermiculite moistened with V8 juice broth (Meadows et al. 2011, Roiger and Jeffers 1991). Seedlings 

were inoculated 12 to 14 weeks after planting. Inocula from the two isolates were combined, mixed 

thoroughly, and then evenly distributed in 1- to 3-cm-deep furrows between rows of seedlings. 

Seedlings were watered as needed throughout the study period, and the container mix in each tub was 

brought to saturation at least once while plants were actively growing to encourage disease 

development. Plants were evaluated in December when fully dormant. Each seedling was scored for 

mortality and PRR severity by examining the roots. A complete description of our methods was 

reported previously (Jeffers et al. 2009).

Each year, seedlings began to die approximately 3 weeks after inoculation and continued to die 

until the end of the growing season. Castanea dentata seedlings consistently were susceptible and 

died, and C. mollissima seedlings consistently were resistant and survived. In 7 years, we have tested 

197 hybrid families from generations that ranged from F1 to BC4 (table 1). Families with seedlings 

resistant to P. cinnamomi occurred each year, but the number of resistant seedlings and PRR severity 

ratings varied considerably among families. Mean annual mortality for all hybrid seedling families 

combined was 83 percent over the 7-year period. The genes for resistance to P. cinnamomi and C. 

parasitica do not appear to be linked because most families were derived from backcross trees 

selected for blight resistance, but PRR resistance was infrequent. The pattern of inheritance of 

 1

School of Agricultural, Forest, and Environmental Sciences, Clemson University; Clemson, SC 29634. 2

Chestnut Return Farm; Seneca, SC 29678. 3 The American Chestnut Foundation, Asheville, NC 28801.

Corresponding author: sjffrs@clemson.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

195

resistance to PRR was consistent with it being controlled by a single gene. This may facilitate 

introgression of PRR resistance into breeding populations of backcrossed American chestnut. The 

paper by Olukolu et al. in these proceedings (Genomics assisted breeding for resistance to 

Phytophthora cinnamomi in chestnut [Castanea sp.]) provides more detailed information on the genes 

controlling resistance to P. cinnamomi in hybrid chestnut seedlings.

Table 1—Numbers of hybrid American chestnut families screened for resistance to

Phytophthora cinnamomi over 7 years: 2004 to 2010

Year No. families No. with resistance Mortality (%)

2004 5 3 94

2005 20 9 90

2006 23 5 99

2007 41 5 80

2008 22 4 75

2009 32 8 68

2010 54 6 72

Screening of hybrid American chestnut seedlings continued in 2011 (48 families screened) and 

will be conducted each year for the foreseeable future as we search for resistance to PRR. In addition 

to hybrid seedlings, we also began to evaluate transgenic plants in 2011. These plants were developed 

for resistance to chestnut blight by colleagues at the State University of New York at Syracuse 

(College of Environmental Science and Forestry) and at the University of Georgia (Warnell School of 

Forestry and Natural Resources). It will be interesting to see if these plants have any resistance to P. 

cinnamomi.

Literature Cited

Freinkel, S. 2007. American chestnut – The life, death and rebirth of a perfect tree. Berkeley, CA: University of 

California Press. 284 p.

Jeffers, S.N.; James, J.B.; Sisco, P.H. 2009. Screening for resistance to Phytophthora cinnamomi in hybrid 

seedlings of American chestnut. In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth 

meeting of the International Union of Forest Research Organizations (IUFRO) working party S07.02.09: 

Phytophthoras in Forests & Natural Ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station: 188–194.

Meadows, I.M.; Zwart, D.C.; Jeffers, S.N.; Waldrop, T.A.; Bridges, W.C., Jr. 2011. Effects of fuel 

reduction treatments on incidence of Phytophthora species in soil of a southern Appalachian Mountain 

forest. Plant Disease. 95: 811–820. 

Roiger, D.J.; Jeffers, S.N. 1991. Evaluation of Trichoderma species for biological control of Phytophthora 

crown and root rot of apple seedlings. Phytopathology. 81: 910-917.

Zentmyer, G.A. 1980. Phytophthora cinnamomi and the diseases it causes. St. Paul, MN: APS Press. The 

American Phytopathological Society. 96 p.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Screening for Resistance to Beech Bark Disease: 

Improvements and Results From Seedlings and 

Grafted Field Selections 

Jennifer L. Koch,

1 Mary E. Mason,2 and David W. Carey1

Abstract

Beech bark disease (BBD) is an insect-disease complex that has been killing American beech (Fagus 

grandifolia Ehrh.) trees since the accidental introduction of the beech scale insect (Cryptococcus fagisuga) to 

Canada around 1890. Insect infestation is followed by infection with Neonectria ditissima or N. faginata.

Mortality levels in the first wave of the disease can be as high as 50 percent, with consequent loss to stand 

health, merchantable timber, and many wildlife and ecosystem services. It is currently estimated that between 1 

and 5 percent of the native American beech are resistant to beech bark disease, and resistance has been shown to 

be to the insect part of the complex.

Recent work has shown that artificial infestation techniques can be used to screen seedlings for scale resistance.

Here we present results from additional beech insect resistance screening experiments, including additional 

seedling families, grafted parental ramets of seedlings, and nonnative beech species. Results further confirm the 

utility of the screen to allow selection of better performing individuals, even within families that perform poorly 

overall, and to rank families for overall performance. When full-sibling families using parents of known scale 

phenotype were screened, an enriched proportion of resistant progeny were observed only in families with two 

resistant parents.

Trees selected in the field can be grafted and the assay is useful to confirm the field-assessed scale resistant 

phenotype. We are currently identifying, grafting, and testing scale-resistant beech trees as part of a multi-state, 

multi-agency cooperative effort. Confirmed resistant genotypes will be used to establish seed orchards to supply 

regionally adapted disease-resistant beechnuts for use in restoration plantings and BBD management. The use 

of resistant parents is necessary to produce significant improvement over unselected seed lots.

Key words: beech bark disease, beech scale, Cryptococcus fagisuga, Neonectria, scale resistance

Introduction

Beech bark disease (BBD) was introduced into North America in Nova Scotia in the late 1890s and 

has been steadily spreading south and west over the last 120 years (Ehrlich 1934). Beech bark disease 

is caused by an insect-fungal complex. The beech scale insect (Cryptococcus fagisuga) is the insect 

component, and a canker-causing Neonectria (either Neonectria ditissima or N. faginata in North 

America) is the fungal component (Castlebury et al. 2006). Insect infestation appears first and is 

believed to predispose trees to fungal infection of insect-damaged bark (Ehrlich 1934). Many forests

where American beech is the dominant component of stands are already heavily impacted, with 

mortality in the ‘killing front’ as high as 50 percent followed by significant additional damage due to 

beech snap (Miller-Weeks 1983, Papaik et al. 2005). Residual and regenerating stands may be 

dominated by susceptible beech, with susceptible beech root sprouts capable of forming thickets that 

prevent regeneration of resistant beech or other species, and offer no economic and severely reduced 

ecological value as beech bark disease continues to kill susceptible beech over time (Morin et al. 

2007).

 1 USDA Forest Service, Northern Research Station, 359 Main Rd, Delaware, OH 43015.

2 Dept. Entomology, Ohio Agricultural Research and Development Center, The Ohio State University, 1680 Madison Ave., 

Wooster, OH 44691.

Corresponding author: jkoch@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

197

In native beech populations heavily infested with BBD, there are an estimated 1 to 5 percent of 

trees that remain healthy (Houston 1983). These apparently resistant beech trees are frequently found 

in clusters suggesting they may be clonal or related seedlings. Houston (Houston 1982) developed a

scale infestation technique and used it to demonstrate that these healthy trees are actually resistant to 

the scale insect component of BBD. To investigate the genetic control of resistance to beech scale in 

American beech, its inheritance, and its potential for improvement through breeding, the U.S. 

Department of Agriculture, Forest Service (USDA FS) has been conducting beech scale resistance 

studies. Koch and Carey (2005) selected and tested scale-resistant American beech trees in the field 

(to determine if they were resistant to scale (R) or susceptible to scale (S)) and performed controlled 

cross pollinations that produced RxR and RxS seedling families (Koch et al. 2006). Houston’s 

(Houston 1982) artificial infestation technique was adapted to screen the seedlings and significant

family effects were found for scale resistance (Koch et al. 2010). In this paper, we present results 

from several experiments conducted over several years, including:

 Screening cultivars and seedlings.

 Screening grafted trees and improving the screen.

 Screening families of seedlings.

 Development of regional seed orchards for breeding resistant beech.

Methods and Materials

Plant Materials and Growth Conditions

Beech plants were grown in containers, either in a greenhouse or shaded outside growing areas, as 

previously described (Koch et al. 2010), and repotted to larger containers as needed. In general,

seedlings were screened at 1 to 2 years of age and grafts were screened 1 to 2 years post-graft. The 

European beech seedlings and cultivars were purchased from Lawyer’s nursery (Plains, Montana).

Scion was grafted using either top (cleft) grafts or side-veneer grafts and utilizing a ‘hot-callous’ 

system to warm the graft union relative to the rootstock and scion (Koch et al. 2006). Rootstocks were 

grown from unselected seed collected in Ohio. Grafts were maintained in a greenhouse for the first 

year and received intensive aftercare (rewrapping grafting rubber, pruning rootstock, etc.). After the 

first year, plants were grown outside under shade (approximately 50 percent).

Grafted trees may produce flowers, either from mature scion in the year of grafting or after 

resuming normal cyclical flowering; these flowers were utilized in control pollinations. Flowering 

ramets used as female parents were isolated and emasculated to prevent rare self-pollination, then 

pollinated using camel hair brushes when receptive (Koch and Carey 2004). Flowering ramets used as 

male parents were isolated, and male flowers were collected when dehiscent. Pollen was collected 

and passed through fine mesh screen to remove any anthers or other debris, then stored at -80 o

C over 

desiccant until use. Ramets carrying nuts were maintained in the greenhouse and protected from 

insect infestation.

Beech nuts were collected before being shed from the burrs. The burrs were allowed to dry and 

open in the laboratory, and the nuts were then separated and stratified 90 days in Banrot® wetted peat 

moss at 4 to 10 o

C. When radicals began to emerge, nuts were transferred to soil-less media in the 

greenhouse to complete germination and begin growth.

Scale Screening

When grafted ramets and seedlings were of sufficient size to support the required number of scale 

screening pads, they were transferred to the screening facility at Holden Arboretum (Kirtland, Ohio).

Scale pads were applied as previously described (Koch et al. 2010) including the use of Tyvek®

covers to prevent excess moisture from reaching the foam pads. Field scale pads were established in a 

stand of susceptible beech trees at Holden Arboretum to provide a consistent supply of eggs for use in 

screening. One hundred eggs (through 2006) or 150 eggs (2007 and thereafter) were placed on each 

pad prior to the pad being affixed to the tree (seedling or grafted ramet). Viability assays were
GENERAL TECHNICAL REPORT PSW-GTR-240

198

conducted on each batch of eggs prior to use for quality control. Scale pads were scored 52 weeks 

after application to maximize the number of scale adults and egg clusters and to minimize the number 

of mobile juvenile nymphs at the time of scoring due to the difficulty in counting them (Koch et al. 

2010). Trees were overwintered with scale test pads affixed either in a 4 °C cold storage facility or a 

heated poly-house maintained just above freezing.

Scale pad data was examined for quality prior to further analysis and inclusion in statistical 

datasets. Multiple pads were reviewed for consistency and any obviously discordant pads were either 

discarded (for three or more pads total, leaving at least two), or the ramet or seedling was rescreened.

For seedlings or ramets with only two pads, an additional quality control metric was calculated to 

confirm the two pads were consistent. The quality control metric is adjusted difference:

Adjusted difference = (pad 2 value – pad 1 value )/(pad value mean + 0.0001).

An adjusted difference of zero corresponds to two identical pads, while an adjusted difference of 

one corresponds to the difference between the two pads being equal to the mean. Ramets or seedlings 

with an adjusted difference greater than 1.5 were considered to be inconsistent and were rescreened.

The cutoff value of 1.5 was originally chosen by graphing the adjusted difference values for the 2009 

dataset, and observing that there was a break in the distribution of adjusted difference around 1.5,

with approximately 90 percent of values below 1.5. Adjusted difference has been similarly distributed 

in later years.

Scale pads were scored resistant if they had zero (or occasional one small or unhealthy) scale 

adults and no egg clusters. Susceptible pads had both healthy scale adults and egg clusters, often in 

equal or greater number than the scale adults count indicating a robust and reproductive scale 

population. Pads were scored intermediate if they had only a few scale adults, or mainly unhealthy 

scale adults and no or only a few egg clusters (many less egg clusters than scale adults, e.g., a pad 

with 6 small adults and one egg cluster would be considered intermediate). Intermediate classification 

was less stringently defined, and when stringent classification was desired, these pads were classified 

as susceptible. Grafts or seedlings were scored resistant, intermediate, or susceptible based on the 

score of all the pads, with a resistant score being most conservative. A resistant genotype had only 

resistant pads. A genotype with a mix of resistant and intermediate pads was scored intermediate (or,

more stringently, susceptible). A susceptible genotype had susceptible pads or a mix of susceptible 

and intermediate pads. Additional exploratory data analysis including calculation of summary 

statistics and graphing were carried out using Minitab®.

Statistical Analysis

Repeatability (for fig. 3, section titled Screening Grafted Trees and Improving the Screen) was

estimated by r, the ratio of the variance attributed to genotype over the total variance. Minitab® was 

used to fit an analysis of variance (ANOVA) with model:

Scale adults = intercept + Genotype (Sample) + Error

Model fit was assessed by examination of residuals. The use of ANOVA was appropriate here 

because we were modeling the scale adults over the full experiment (essentially averaging), and the 

model fit was confirmed by examination of residuals. Variance components were reported for 

genotype, sample, error, and total, and used to compute r. Expected relative error variance was

calculated using the relationship:

E=(1-r)/n.

For family analysis (for fig. 5, table 4, and section titled Screening of Families of Seedlings), 

SAS® was used to fit a generalized linear model using the GENMOD procedure. Model fit was 

assessed by examination of residuals and alternate models were compared using deviance/degrees of 

freedom, AIC, AICC, and BIC scores, and graphing of residuals supplied by GENMOD. Model 

variation was guided by exploratory data analysis and included models with different distribution and 

link function options as well as models with and without interactions and main effects. The best-fit 

model was selected and the differences of least square means computed and interpreted only for the 

best fit model. Actual p-values are reported rather than an a priori cutoff or adjustment. The current 
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dataset contained many susceptible trees, so it was not as severely zero-inflated as data containing 

more highly resistant families. Therefore, the current data was sufficiently normalized by applying 

square root transformation prior to analysis. The best fit model was:

Square root (sum of adults) = intercept + family + scorer + year + error;

specifying tree (genotype within family) as the subject of repeat observations to correctly specify the

covariance structure and account for repeated measure on individual seedlings. Replication in the 

model comes from evaluating multiple pads per genotype.

Results and Discussion

Screening Cultivars and Seedlings

Koch et al. (2010) described an artificial scale infestation technique and reported data from several 

families screened using one pad per seedling per year and combining analysis over 2 years to obtain 

statistically analyzable results. In this paper, we report data from several smaller groups of seedlings 

that were also screened using one pad per seedling per year. Table 1 shows a summary of the results 

of these screenings. Results are consistent with those reported in Koch et al. (2010) in that only 

families produced from crosses where both parents were scored resistant had an enriched number of

resistant seedlings. Families with a beech scale susceptible parent and open-pollinated (all male 

parents presumed susceptible) families had only a few, if any, resistant progeny.

Table 1—Beech seedlings and families screened by USDA Forest Service

Entry Type Sourcea No. R 

trees

No. S 

trees

Avg 

adults

Avg 

eggs

Year(s) 

screened

DSP1973xOP seedling open-pollinated 

family, S mother

0 6 25.5 25.6 2004, 2005

1505xOP seedling open-pollinated 

family, R mother

2 14 32.21 33.24 2008

1505xSebois23 seedling control cross, RxR’ 17 7 14.8 15.86 2008

1505xSebois85 seedling control cross, RxR’ 2 9 3.22 15.33 2008

1520xOP seedling open-pollinated 

family, S mother

0 21 45.71 52.95 2008

DNxOP seedling open-pollinated

family, S mother

0 50 6.25 8.73 2005, 2006

a Source is either control-cross pollinated or open-pollinated family with parents field scored as R=resistant in field test, 

R’=field selected as healthy with no visible scale in post-aftermath forest, presumed scale resistant, S=susceptible in field 

test.

A number of exotic species and selections of beech are commercially available in the United 

States, and a sample of these were purchased and screened in 2005 and 2006 (table 2). Similar to 

observations with American beech (Fagus grandifolia Ehrh.), the additional species had both resistant 

and susceptible individuals. Several of the cultivars of European beech (Fagus sylvatica L.) appear to 

be resistant to beech scale. Also similar to the American beech data, susceptible trees of the different 

Fagus species showed a range in the size of the scale population they supported (fig. 1).
GENERAL TECHNICAL REPORT PSW-GTR-240

200

Table 2—Beech (Fagus) cultivars screened by USDA Forest Service

Entry Type Source No. R 

trees

No. S 

trees

Avg 

adultsa

Avg 

eggsa

Year(s) 

screened

F. orientalisb Asian 

species

commercial 

horticulture

5 0 - - 2006

F. sylatica v. 

cristata

European 

cultivar

commercial 

horticulture

6 0 - - 2006

F. sylatica v. 

rotundafolia

European 

cultivar

commercial 

horticulture

9 0 - - 2006

F. sylvatica v. 

spaethiana

European 

cultivar

commercial 

horticulture

6 4 0.7 3.0 2006

F.sylvatica v. 

asplendafolia

European 

cultivar

commercial 

horticulture

1 0 - - 2006

F. purpurea European 

sub-species

commercial 

horticulture

7 19 10.3 8.4 2008

F. sylvatica European 

sub-species

commercial 

horticulture

15 19 24.0 27.8 2008

a Average of the susceptible trees only: reflects the relative degree of susceptibility of the S population. The value cannot be 

calculated (is undefined) if all trees are resistant. b F. orientalis, purpurea, and sylvatica are species within Fagus. The other groups are horticultural selections or sub-species 

of Fagus sylvatica.

Figure 1—Boxplot of the distribution of beech scale adults count of several exotic species and 

cultivars of beech screened by the USDA Forest Service. Box: first to third quartile, line: median, 

‘whiskers:’ upper (or lower) quartile plus (minus) 1.5 times the inter-quartile range, asterisks: outliers. 

Screening Grafted Trees and Improving the Screen

Parent trees and other field selected trees were grafted, and ramets were screened to confirm the 

greenhouse screening with the field selection phenotypes. Early screens (2006) used one pad per 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

201

ramet and relied on screening multiple ramets to determine screening result (R or S). This replication 

of the genotypes allows the reproducibility of the screen to be assessed. An examination of the screen 

data showed generally consistent results, but problems were identified as well (discordant ramets;

table 3). Examination of the notes taken during scoring revealed several problems with early screens.

Often small side branches needed to be pruned in order to place the screening pads and Tyvek®

protectors on the tree. Scale insects appear to establish more readily on this freshly wounded tissue 

fig. 2A), sometimes even on normally resistant trees. To avoid the impact that pruning wounds have

on scale tests, pads are now installed either away from the wound entirely or with the foam facing the 

unwounded side of the branch only. An additional problem in scoring was a concomitant infestation 

of oyster shell scale (fig. 2B), including adults and juveniles present on and under beech scale test 

pads. While the adult stage is not difficult to distinguish, the egg and juvenile phases of the two scale 

insects are much more similar in appearance and can be confused. It is unknown what impact the two 

scale infestations may have on each other, but oyster shell scale infestation appeared to cause seedling 

mortality at the infestation levels present in the experiment, so it was likely having a significant 

impact on overall tree health during the beech scale screen.

Table 3—Performance of grafted ramets of trees in different years with different numbers of 

pads 

Genotype Source Year Pads 

per 

ramet

No. R 

ramets

No. S 

ramets

No. discordant 

pads/total pads

Avg. 

adults

Avg.

eggs

1503 selection 2006 1 6 1* n/a 1 1

1504 selection 2006 1 2 3* n/a 0.67 1

2179 unselected 2006 1 1 2 n/a 2.5 0

2189 unselected 2006 1 1 1 n/a 4 2

PA2691 selected 2006 1 3 2* n/a 2 0.5

PA2692 selected 2006 1 6 1* n/a 0 1

WV193 unselected 2006 1 0 1 n/a 7 2

WV963 unselected 2006 1 0 1 n/a 9 9

WV964 unselected 2006 1 1 2 n/a 7.5 5.5

WV966 unselected 2006 1 0 2 n/a 8 7.5

WV969 unselected 2006 1 0 2 n/a 8.5 2.5

WV970 unselected 2006 1 0 3 n/a 11.6 2

1504 selection 2009 1-2 3 0 1/5a 0 0

1505 selection 2009 2-3 5 0 1/13b 0 0

1506 selection 2009 1-3 0 4 1/8c 23.4 23.4

1520 selection 2009 2 0 1 0/2 15.5 23

DN00726 unselected 2009 2-3 0 5 1/13c 14.1 13.5

DN00740 unselected 2009 1-3 0 4 0/8 37.6 32.5

BEWL01 selected 2009 3 1 0 0/1 0 0

*discordant ramets likely mis-scored due to interfering conditions (insects, wounds, failed pad). a One pad with five adults and six egg clusters. b One pad with six adults and no egg clusters. c One pad with no adults and no egg clusters.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Figure 2—Beech scale infestation complications. Left (A): beech scale established around pruning 

wound (white arrow). Right (B): oyster shell scale adult and juvenile (white arrow) in area of beech 

scale juveniles and eggs (black arrow) on a tested tree. Note the very similar size and morphology of 

the juveniles.

The use of multiple pads per tree has made it possible to identify occasional discordant screening 

pads (pads inconsistent with the set of pads on the genotype). These discordant pads fall into two 

types: R pads on apparently S trees, and S or I pads on apparently R trees. The first conditions (R pad 

on S tree) can be considered a failed pad. A number of potential causes of pad failure have been 

identified including pads not receiving eggs and pads being placed backwards, or drying out before 

attachment to the tree during setup. Occasionally pads became loose over the duration of the 

experiment and/or the Tyvek® covers also became loose resulting in the pad being compromised. In 

the second case (S pad on R tree) the causes are potentially less clear, but a number of factors that 

impact tree health either locally or systemically can be supposed to compromise a score pad. Poor 

overall tree health or infection with other insects or fungi and damage or broken bark, especially fresh 

pruning wounds, are all noted on scoring sheets associated with discordant pads. Pads inadvertently 

placed below or straddling the graft union are an easily recognized case of S pads on R trees, and are 

discarded. With multiple pads per tree or ramet, discordant or failed pads can be easily detected and 

the data either carefully scrutinized for consistency or discarded.

Koch et al. (2010) used the relationship between expected relative error variance and number of 

pads (replicates) for a set of beech seedlings screened with only one pad from 2004 to 2006 to predict 

the number of pads that would improve control of error variance and improve the overall accuracy of 

the screen while maintaining a manageable workload. Increasing from one pad per tree to three to 

four pads per tree was estimated to give the best improvement of error variance. We calculated the 

same value for a set of grafted ramets screened in 2009 with multiple pads per ramet (fig. 3) and 

found a similar value of three to five pads per tree to give the optimal reduction in error variance 

(estimated by the inflection point of the curve, with pads above five giving only marginal 

improvement in error variance for each additional pad).

Based upon the 2006 results, and the predicted improvement based on increased replication, a new 

set of grafts was screened beginning in 2009 with a goal of screening three ramets with three to four

scale pads each for a total of nine reps per genotype. The improvement in the ability to identify and 

discard discordant pads greatly improves the accuracy and confidence of the screen based phenotype 

(table 3). Figure 4 shows our standardized scale egg score pad attached to a tree (fig. 4A), a grafted 

ramet with the improved replication of three scale pads with Tyvek® covers (fig. 4B), and our 

improved scale screening facility at Holden Arboretum (fig. 4C) that provides consistent, quality, 

dedicated beech growing space. The new growing space is optimized for growing beech trees and 

isolation from other Holden Arboretum growing stock reduces insect problems. This screening 

intensity allows consistent phenotyping of selected genotypes in one year, even if there are problems 

with a particular pad or ramet. Seedlings are also now screened at higher replication of test pads, 

generally with two applied per seedling. After scoring the pads, we compute a pad consistency metric, 

adjusted difference, and rescreen any seedlings with values over a cutoff value of 1.5. Trees with only 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

203

one pad scored due to a pad failure are also automatically rescreened. The improved screen 

replication, improved and consistent growing space, and adoption of quality control metrics combine 

to greatly improve the robustness and efficiency of the screening technique.

Figure 3—Expected relative error variance by number of pads screened. Expected relative error 

variance was calculated from the repeatability of grafts with two or more pads in 2009, based on the 

relationship E= (1-r)/n. Three to five pads is considered an optimal balance between the additional 

time and effort to add a pad and the reduction in expected relative error variance.

Screening Families of Seedlings

As field-selected trees are grafted for screening, grafted ramets occasionally flower. Any ramets that 

produce flowers are moved to the greenhouse and control pollinated to produce families of seedlings 

for screening. Ramets used as female parents are maintained in the greenhouse until nut collection, 

and this ‘containerized seed orchard’ approach is an efficient method to generate high quality 

controlled cross pollinated beechnuts (Koch et al. 2006). Ideally all trees grafted would flower and be 

used once or twice as a male parent and once or twice as a female parent to allow assessment of their 

suitability as R parents and to provide a source of improved seedlings for testing of planting 

requirements. A small group of five inter-related families were produced on flowering ramets in 2006 

and the resulting seedlings were screened at the scale screening facility from 2008 to 2011.

The distribution of total scale adults per scoring pad is shown in figure 5 for all the pads on all the 

trees in the experiment and has the features typical of these datasets in all experiments. The zero

‘spike’ is typical for scale screen datasets and is more or less pronounced depending on how many 

trees in the dataset are resistant (in this case it is less pronounced). Due to the number of zeroes, the 

data are sometimes zero inflated relative to the distribution that accurately models the non-zero 

portion of the data, so zero-inflated models are considered in the analysis in addition to several 

transformations (either directly or by varying the distribution and link function in the generalized 

linear model specifications). Only the best fit model based on typical goodness of fit measures (e.g.,

deviance/df=1, or smallest AIC) is used to determine significance of effects and evaluate family 

rankings.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Figure 4—Scale screening pads and current improved screening facility. A: Foam pad with scale

eggs is attached to the tree using wire ties. B: Several foam pads are placed on each tree and 

covered with Tyvek® to protect them from rain and irrigation water. C: Grafted ramets at the U.S. 

Department of Agriculture, Forest Service scale screening facility at Holden Arboretum.

A B

C
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Figure 5—Histogram of scale adults per tree with normal curve. The normal curve based on the mean 

and standard deviation for the sample is superimposed over the histogram. Both the high proportion 

of zero counts and the lack of normality expected for count data can be seen in the chart. Depending 

on the proportion of zeroes in a given dataset, simple transformation may be insufficient to normalize 

the data and zero-inflated models should also be investigated.

For this experiment (dataset), the best model for the total scale adults per tree analyzed the square 

root of the total adults with main effects for family, year, and person scoring (scorer), and using a 

generalized linear model specifying the normal distribution, identity link function, and tree as the 

subject of repeated observations. Effects were tested using the Wald statistics. Family and year were 

significant (both p<0.0001), but scorer was not (p=0.2049). Least squares means were computed for 

family and year and are shown in table 4. Consistent with the previous results, only the RxR family 

had operationally reasonable numbers of resistant progeny (over half), while the SxR and SxS 

families were dominated by susceptible individuals. The scale population carried by the three groups 

of families (RxR, SxR, and SxS) differed in the expected way with fewer scale insects on families 

with both resistant parents. We also found that the least squares mean for SxR families is significantly 

(p<0.0001 for difference of LSM=0) less than those for SxS families. These families also had a small 

number of resistant progeny, consistent with estimates of resistance in natural stands. The YEAR 

effect was also significant. Relative rankings will probably be most robust if completed within a year 

and control trees or families should be screened each year for multi-year experiments (to allow 

differences in years to be estimated statistically). Five different individuals scored trees over the 3 

years including the research scientist that developed the assay and summer interns. The lack of 

significance for scorer is reassuring that the screen and training is robust and should be transferable to 

other work groups or managers.

Development of Regional Seed Orchards for Resistant Beech

Early experiments (Koch et al. 2010) showed both the utility of the screen to identify beech scale 

resistant individuals and superior families and indicated that the degree of genetic determination of 

resistance to beech scale was sufficient to realize genetic gain through traditional tree breeding 

programs. However, these conclusions were based on the analysis of a small number of families. The

findings presented here further support the conclusions of Koch et al. (2010) and Koch (2010), and 
GENERAL TECHNICAL REPORT PSW-GTR-240

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Table 4—Least squares means for family (A) and year (B)

A. Family Field score of parents 

in cross

Proportion of 

resistant 

progeny

Least squares mean 

estimate for total 

adults

Significance

BEWL01x1504 R x R 0.607 1.4 Aa

DNx1504 S x R 0.08 23.9 B

DNx1505 S x R 0.21 27.40 B

DNx1506 S x S 0 59.2 C

DNxDN S x S 0 50.5 C

B. Year Least squares mean estimate Significance

2009 43.3 A

2010 15.0 B

2011 27.1 AB a Means that share a letter are not significantly different (all significant differences had an unadjusted p<0.0001).

taken together provide the framework to develop a strategy for managing beech bark disease. A 

multi-state, multi-agency workgroup has been assembled to select, screen, and propagate beech scaleresistant American beech for use in the installation of regional seed orchards. The initial efforts have 

focused on both the Lower and Upper Peninsulas of Michigan, and central Pennsylvania. A number 

of beech trees have been selected, grafted, and screened (table 5). Resistant beech trees appear easier 

to detect during the killing front phase, and harder to detect in aftermath forest. Based on these early 

results, selected trees are being field tested before grafting.

Table 5—Regional seed orchard project (American beech trees were selected by local 

cooperators, then grafted and screened for beech scale resistance by U.S. Forest Service 

personnel at the screening facility at the Holden Arboretum)

Region Beech 

bark 

disease 

status

Tree ID for screen result:

resistanta

Tree ID for 

screen result: 

intermediateb

Tree ID for

screen result:

susceptiblec

MichiganLower Peninsulakilling front 1503,1504,1505,1510 1506d

1520d

Michigan-Upper 

Peninsula

killing front 1201,1207,1208,1209,1210,1211,1213,

1214,1215,1217,1219,1220,1228,1229, 

1230, 1231, 1301

1202

1216

Pennsylvania aftermath PA2661, PA2663, PA2665, PA2691,

ANF4491304, ANF4491308,

ANF4491321, ANF4693608,

ANF4694223, ANF4693619,

ANF4694209, ANF4714904,

D-9-1, D-9-2

ANF4491304

ANF4694204

PA2692

PA2659

ANF4693615

ANF4693905

ANF4693622

West Virginia killing 

front/

aftermath

MNF3839335

MNF3839557

Ohio uninfested DN00726d

DN00740d

a No more than 3 adults and/or one egg cluster. b A few adult scale or mainly unhealthy adult scale, and only a few egg clusters. c Robust scale infestation present with both adults and eggs. d Originally selected as susceptible control.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Conclusions

We report results from screening several different sets of seedlings, including open-pollinated, 

controlled-pollinated, commercially available, and grafted ramets of American beech for resistance to 

the beech scale insect. Evaluation of these scale test results has guided improvement, quality control, 

and standardization (best practices) of the beech scale insect screening for both grafted trees and 

seedlings. These changes have also made the process of successfully scoring beech grafted trees and 

seedlings faster, more predictable, and more efficient. Analysis of family data continues to support 

the conclusions of Koch et al. (2010) that only families with two resistant parents have high numbers 

of resistant seedlings. Based on these finding, a pilot seed orchard project is being established where 

collaborators are identifying superior parents in their region for establishment in a seed orchard to 

supply resistant beech nuts for planting.

Acknowledgments

We would like to thank Kathleen Knight (NRS, USDA Forest Service), John Davis (University of Florida), 

Dana Nelson (SRS, USDA Forest Service), and John Stanovick (NRS, USDA Forest Service) for helpful 

comments on the manuscript. We thank the Holden Arboretum for partnership on the scale resistance screening 

facility. We thank our collaborators for the work and vision on the beech seed orchard project: Bob Heyd and 

Rich Mergener (Michigan Department of Natural Resources), Andrea Hille (Allegheny National Forest), Tom 

Hall (Pennsylvania Dept. of Conservation and Natural Resources), Dan Twardus (SPF, USDA Forest Service), 

Manfred Mielke (FHP, USDA Forest Service), Paul Berrang (Region 9, USDA Forest Service), and Glenn 

Juergans (Monangahela National Forest, retired). Funding for this work was supplied in part by the Evaluation 

Monitoring Program (NE-EM-B-11-03) and the Special Technology Development Program (NA-2009-01) of 

Forest Health Protection, USDA Forest Service.

Literature Cited

Castlebury, L.A.; Rossman, A.Y.; Hyten, A.S. 2006. Phylogentic relationships of Neonectria/Cylindrocarpon

on Fagus in North America. Canadian Journal of Botany. 84: 1417–1433.

Ehrlich, J. 1934. The beech bark disease: a Nectria disease of Fagus, following Cryptococcus fagi (Baer.). 

Canadian Journal of Research. 10(6): 593–692.

Houston, D.R. 1982. A technique to artificially infest beech bark with the beech scale, Cryptococcus fagisuga 

(Lindinger). Res. Pap. NE-507. Broomhall, PA: U.S. Department of Agriculture, Forest Service, 

Northeastern Research Station. 8 p.

Houston, D.R. 1983. American beech resistance to Cryptococcus fagisuga. In: [Editors unknown]. 

Proceedings, IUFRO beech bark disease working party conference. Gen. Tech. Rep. WO-37. Washington, 

DC: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station: 38–41. 

Koch, J.L. 2010. Beech bark disease: The oldest “new” threat to American beech in the United States. Outlooks 

on Pest Management. April 2010: 64–68.

Koch, J.L.; Carey, D.W. 2004. Controlled cross-pollinations with American beech trees that are resistant to 

beech bark disease. In: Yaussy, D.; Hix, D.M.; Goebel, P.C.; Long, R.P., eds. Proceedings, 14th Central 

Hardwood Forest Conference. Gen. Tech. Rep. NE-316. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northern Research Station: 358–364.

Koch, J.L.; Carey, D.W. 2005. The genetics of resistance of American beech to beech bark disease: 

knowledge through 2004. In: Evans, C.A.; Lucas, J.A.; Twery, M.J., eds. Proceedings of the beech bark 

disease symposium. Gen. Tech. Rep. NE-331. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Research Station: 98–105.

Koch, J.L.; Mason, M.E.; Carey, D.W. 2006. Advances in breeding American beech for resistance to beech 

bark disease. In: Proceedings of the 3rd Northern Forest Genetics Association meeting. Staff Paper Series 

No. 194. St. Paul, MN: Dept. of Forest Resources, University of Minnesota: 22–28.

Koch, J.L.; Mason, M.E.; Carey, D.W.; Nelson, C.D. 2010. Assessment of beech scale resistance in full and 

half-sibling American beech families. Canadian Journal of Forest Research. 40: 265–272.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Miller-Weeks, M. 1983. Current status of beech bark disease in New England and New York. In: Proceedings, 

IUFRO beech bark disease working party conference. Gen. Tech. Rep. WO-37. Washington, DC: U.S. 

Department of Agriculture, Forest Service, Northeastern Forest Experiment Station: 21–23.

Morin, R.S.; Liebhold, A.M.; Tobin, P.C.; Gottschalk, K.W.; Luzader, E. 2007. Spread of beech bark 

disease in the eastern United States and its relationship to regional forest composition. Canadian Journal of 

Forest Research. 37: 726–736.

Papaik, M.J.; Canham, C.D.; Latty, E.F.; Woods, K.D. 2005. Effects of an introduced pathogen on 

resistance to natural disturbance: beech bark disease and windthrow. Canadian Journal of Forest Research. 

35: 1832–1843.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Breeding Resistance to Butternut Canker Disease

James McKenna,1 Keith Woeste1and Michael Ostry2

Abstract

Butternut (Juglans cinerea L.) is being killed throughout its native range by an exotic fungus Ophiognomonia

clavigignenti-juglandacearum (Ocj). In recent years, many disease-free trees have been determined to be 

complex hybrids with an admixture of Japanese walnut (J. ailantifolia). Recently developed molecular and 

morphological characterizations allow us to accurately identify and separate hybrid and pure butternut progeny.

Disease-free-trees, from across butternut’s native range, are the basis of our breeding program in the Central 

Hardwood Region of the eastern United States. Our first clone banks and seed orchards were grafted and 

established in the 1990s and 2000s, and are now producing seed for resistance screening. In 2008, we 

challenged 5-year-old trees from our first two field progeny tests with Ocj.

The first test, planted in 2003, had 37 diverse families (n=319). Thirty-two of these seedling families were 

derived from a grafted orchard of putatively resistant selections. Five additional families were collected from 

healthy hybrid trees. In early fall of 2008, trees were inoculated with two isolates of Ocj obtained from branch 

cankers on trees in two locations in Indiana. The trees were scored 8, 12, 20, and 24 months after inoculation for 

canker incidence and severity. Native butternuts in the adjacent woods provided a source of inoculum whereby 

natural infections from Ocj began to occur in the third year. Cumulative natural canker incidence and severity 

were recorded at 5 and 7 years. 

The second test, planted in 2004, had 12 pure butternut half-sib families collected from a woodlot with: four 

resistant, four moderately resistant, four susceptible, and one resistant hybrid families (n=213). Resistance 

ratings were based on the disease status of the mother trees in the stand when the seed was harvested in the fall 

of 2002. In early fall of 2008, trees were inoculated with the same two isolates of Ocj used in the first test. The 

trees were scored 8, 12, 20, and 24 months after inoculation for canker incidence and severity. There was no 

natural infection in the second test.

Hybrid butternut families were more resistant to natural infection than the pure butternut families. Eight months 

after inoculation, canker incidence and severity varied significantly among butternut hybrid families and Ocj

isolate, but not among pure butternut families. After 12, 20, and 24 months, canker incidence and severity of 

pure butternut families changed. By 24 months, hybrid families in general have shown reduced canker 

expansion and a high level of resistance. Pure butternut families exhibit more variation from highly susceptible 

to resistant. Year-to-year variation in canker growth suggests that it may take several years to determine the 

resistance status of butternut with artificial stem inoculations.

 1 USDA Forest Service, Northern Research Station, Hardwood Tree Improvement and Regeneration Center at Purdue 

University, 715 West State St., Department of Forestry and Natural Resources, West Lafayette, IN 47907.

2 USDA Forest Service, Northern Research Station, 1561 Lindig Ave, St. Paul, MN 55108.

Corresponding author: jrmckenn@purdue.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Towards the Development of a Laurel Wilt 

Screening Program in Redbay (Persea borbonia)

Marc Hughes1 and Jason Smith2

Abstract

Laurel wilt is a highly destructive disease of redbay (Persea borbonia (L.) Spreng.) and other Lauraceous 

natives in the southeastern United States. The disease and associated vector, the redbay ambrosia beetle 

(Xyleborus glabratus), has spread through the United States coastal plain. The presence of surviving and 

asymptomatic individuals in severely-affected stands illustrates the possibility of natural resistance by surviving 

redbays. 

In 2008, a field survey was initiated to locate and identify healthy, asymptomatic redbays in areas of severe 

mortality. Six heavily-affected sites were chosen along the redbay-dense barrier islands of Florida, Georgia, and 

South Carolina. Over 80 trees with a 7.6 cm or larger diameter at breast height were selected as putatively 

resistant candidates. A 0.7 ha plot was then established around each candidate tree along with measurements of 

redbay plot disease severity and redbay ambrosia beetle activity, based on trapping studies. Branch cuttings 

were taken and used in novel experiments to investigate methods of redbay vegetative propagation and disease 

resistance screening. A mean of 25 percent rooting was achieved in the propagation experiment, although 

genotype had a large effect on rooting success. The vegetatively propagated clones of live parent trees are 

currently being tested for resistance/tolerance to the laurel wilt fungus by inoculation experiments. A 

preliminary study tested 10 clones; a single clone (FG-C1) has survived at 10 months post-inoculation, 

suggesting a possible tolerance response. Further testing of FG-C1, and over 50 new clones, is under way. 

Additional studies on the genetic/pathogenic variability of the pathogen and effects of inoculum concentration 

on symptom expression are underway. 

 1 Department of Plant Pathology, University of Florida, Gainesville, FL 32611.

2 School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611.

Corresponding author: mhughes741@ufl.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

211

Breeding Eucalyptus for Disease Resistance

Edival A.V. Zauza,1 Acelino Couto Alfenas,2 Lúcio Mauro da Silva Guimarães,

1

and João Flávio da Silva1

Abstract

Eucalyptus plantations cover about 1.5 percent of the agricultural area in Brazil, and contribute to 4 percent of 

GDP and 3 percent in exports of forest products. Technological and research advances in silviculture and 

genetic improvement have increased productivity up to 80 m3 of wood/ha/year, with an average of 35 to 45 

m3/ha/year. The greatest challenge to the introduction and commercial use of exotic species is the adaptation of 

species to climatic conditions. Even when maladapted species are able to survive, they are subjected to 

continuous stress that limits the expression of their maximum genetic potential. The incidence of insects and 

diseases frequently represent another limiting factor, because the exotic tree species have not co-evolved with 

most of the local insect and pathogen pests. In recent decades, the expansion of Eucalyptus plantations in Brazil 

has been frequently associated with frequent disease outbreaks. Prominent diseases, such as Ceratocystis wilt, 

Ralstonia wilt, rust, and bacterial/fungal leaf blights, have limited the establishment and growth of plantations 

that contain susceptible species or clones in regions favorable to disease establishment. Plantations of hybrid 

clones or elite-resistant varieties represent the most efficient strategy for disease control. In general, fungicide 

application is restricted to control of nursery diseases, and rarely for rust control in the field. Eucalyptus clonal 

forests offer potential to establish homogeneous, disease-free stands of high yield. Currently, the hybrid clones 

of E. urograndis (E. urophylla S.T. Blake x E. grandis Hill ex Maiden) are among the most planted material in 

Brazil because of their high adaptability, wood quality for pulping, and resistance to Chrysoporthe canker.

Other interspecific hybrids have been developed in an attempt to incorporate specific traits from other species, 

such as drought and frost tolerance, disease resistance, and high pulp yield. Primary breeding strategies are 

based on selecting species/provenances, coupled with individual genetic selection within populations, to capture 

the natural variability among and within populations. The recurrent selection method and its variants are applied 

to genetic improvement of Eucalyptus to obtain new clones of interspecific and intraspecific hybrids.

Backcrosses, mutation induction, and polyploidy have also been applied to develop new genotypes. These 

strategies help ensure the sustainability of commercial plantations and the continual contributions from the 

improvement program through the selection in advanced generations and superior clones. With interspecific 

breeding to incorporate contrasting traits, it is usually necessary to make continued backcrosses to the species of 

interest as the recurrent donor to recover the desired forest features and industrial properties. For each breeding 

generation, it is essential to perform disease screening under controlled conditions to select disease-resistant 

genetic materials for commercial cloning or identifying sources of resistance for other crosses. For diseaseresistance evaluations, reliable inoculation protocols are essential. In recent years, approximately 90 percent of 

elite-clones were susceptible to at least one of the main diseases tested using artificially inoculations under 

controlled conditions. Determining the genetic basis and the mode of inheritance of disease resistance is an 

essential step to obtain disease-resistant plant material for the breeding program. The breeding strategy varies 

according to the inheritance model. To reduce losses caused by rust (Puccinia psidii), efforts have been 

conducted to select and plant rust-resistant clones and determine the mode of inheritance. In E. grandis, the 

segregation pattern of resistance to rust is controlled by one locus with major effect, Ppr-1 (P. psidii resistance, 

gene 1), which is tightly linked to RAPD marker AT9-917. Controlled inoculations of other families allowed 

the identification of a homozygous, rust-resistant mother parent of E. grandis, currently used to obtain rustresistant progenies, regardless of pollen source. Screen tests for resistance to Ceratocystis wilt (caused by 

Ceratocystis fimbriata) showed a continuous variation in resistance, ranging from highly susceptible to highly 

resistant, indicating a pattern typical of horizontal resistance. Additional studies are needed to determine the 

genetic basis, the new sources of resistance, and the inheritance patterns of resistance of Eucalyptus spp. to 

other diseases. Furthermore, a better understanding of the population genetic structure of pathogens is needed to 

determine the role of pathogen variability in disease resistance that is targeted by the breeding strategies. 

 1 Suzano Papel e Celulose.

2 Departamento de Fitopatologia/BIOAGRO, Universidade Federal de Viçosa (UFV), 36570-000, Viçosa, MG, Brazil.

Corresponding author: edivalzauza@suzano.com.br.
213

Fusiform Rust Resistance -

Southern Pines

Fusiform rust canker on loblolly pine.

(Photo: J. Bronson)

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

215

Identification of Pathogen Avirulence Genes in the 

Fusiform Rust Pathosystem

John M. Davis,1,2 Katherine E. Smith,2,3 Amanda Pendleton,2 Jason A. Smith,1

and C. Dana Nelson3

Abstract

The Cronartium quercuum f.sp. fusiforme (Cqf) whole genome sequencing project will enable identification of 

avirulence genes in the most devastating pine fungal pathogen in the southeastern United States. Amerson and 

colleagues (unpublished) have mapped nine fusiform rust resistance genes in loblolly pine, suggesting that at 

least nine corresponding avirulence genes likely exist in the fungus. Identification of these avirulence genes 

would greatly facilitate selection of resistant pine genotypes for deployment to forest plantations. Based on 

work in other rusts, we anticipate avirulence genes may encode secreted effector proteins that interact directly 

or indirectly with host resistance proteins. As a step toward testing this hypothesis, we genetically mapped the 

Cqf avirulence gene Avr1, which specifically interacts with the Fr1 resistance gene, with the goal of integrating 

this map with the Cqf genome sequence assembly so that Avr1 can be identified. Once Avr1 is identified, we 

want to determine its allele frequency across geographically defined hazard maps for fusiform rust, to help 

guide genotype deployment by growers. We view identification of Avr1 as our initial case study, guiding our 

strategy to efficiently identify additional avirulence genes in Cqf that in turn can be used to guide plantation 

deployment of all genotypes currently represented in southern pine breeding programs.

 1 School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611. 2 Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, FL 32611.

3 Southern Institute of Forest Genetics, U.S. Forest Service, Saucier, MS 39574.

Corresponding author: jmdavis@ufl.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Tandem Selection for Fusiform Rust Disease 

Resistance to Develop a Clonal Elite Breeding 

Population of Loblolly Pine

Steve McKeand,

1 Saul Garcia,1 Josh Steiger,1 Jim Grissom,1 Ross Whetten,1

and Fikret Isik1

Abstract

The elite breeding populations of loblolly pine (Pinus taeda L.) in the North Carolina State University 

Cooperative Tree Improvement Program are intensively managed for short-term genetic gain. Fusiform rust 

disease, caused by the fungus Cronartium quercuum f. sp. fusiforme, is the most economically important 

disease in pine plantations across the southeastern United States, causing multi-million dollar annual losses. In 

order to cull susceptible crosses and progeny prior to field testing, seedling progeny of 51 crosses were 

challenged with bulked inocula of the fusiform rust fungus at the U.S. Department of Agriculture Forest Service 

Resistance Screening Center in Ashville, North Carolina. We used a high spore load (50,000 spores/ml) of a 

broad-base field inoculum that covered the entire range of deployment for these pine genotypes. Through this 

culling, we eliminated genotypes that may be susceptible in the field to rust, and we are able to focus on 

selection for growth and quality traits in the remaining population. Overall mean disease incidence was 48 

percent, which was ideal for assessing genetic variation for a binary trait. Family disease means ranged from 5 

percent to 81 percent. We fit a generalized linear mixed model and logit link function to partition observed 

variance in the response variable (1=gall, 0=no gall) into causal (additive and dominance) components. The 

genetic control for disease incidence was very high, with family mean heritability of 0.95, and the individual 

tree narrow-sense heritability was 0.5. Substantial genetic gains can be achieved for fusiform rust disease 

incidence in loblolly pine in a relatively short time (6 months) using greenhouse screening protocols.

Subsequent to the greenhouse screening, we cloned 2,442 full-sib progeny that were non-galled for field testing to 

carry out selection for growth and stem quality traits. Cloned progeny have been established in various locations in 

2009 and 2010 using a resolvable alpha incomplete block design. 

 1 North Carolina State University, Cooperative Tree Improvement Program, Raleigh, NC 27695-8002.

Corresponding author: Steve_McKeand@ncsu.edu. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

217

Bulked Fusiform Rust Inocula and Fr Gene 

Interactions in Loblolly Pine

Fikret Isik,

1 Henry Amerson,1 Saul Garcia,1 Ross Whetten,1 and

Steve McKeand1

Abstract

Fusiform rust disease in loblolly (Pinus taeda L.) and slash (Pinus elliottii Engelm. var elliottii) pine

plantations in the southern United States causes multi-million dollar annual losses. The disease is 

endemic to the region. The fusiform rust fungus (Cronartium quercuum sp. fusiforme) infects young 

pine trees and develops galls on stems and branches. Single pathogen gene and single host (pine) gene 

interactions can predict phenotype (disease expression) with high accuracy (Wilcox et al. 1996).

There are nine known pathotype-specific Fr genes in seven loblolly pine families. The R alleles 

condition resistance to specific genotypes of the fungal pathogen while r alleles do not condition for 

resistance.

We investigated interactions of R genotypes (seedlings bearing R allele) and r genotypes (seedling 

bearing r allele) of seven families with 10 bulked inocula. Our specific objectives were to (i) 

understand the response of pine R and r genotypes to different bulked inocula and (ii) assess the level 

of virulence in the fungus population that are able to overcome resistance in pine families. Such 

results would have significant effect on the breeding and disease management in pine plantations.

Half-sib progeny of seven loblolly pine families were challenged with 10 different bulked inocula of 

the fusiform rust fungus and assessed for disease incidence in greenhouse. Progeny of these families 

were genotyped to identify carriers of known alleles, “R” for resistance or “r” for lack of 

resistance/susceptibility.

Significant differences were detected among bulked inocula with regards to their ability to incite 

disease at the family level (F test Pr<0.001). Bulked inocula were also significantly different in 

virulence against R (Pr=0.002) and r genotypes (Pr=0.007). Across the inocula, disease levels differed 

significantly among half-sib families (Pr<0.001). Seedlings that carried only r alleles typically 

exhibited higher disease rates than did carriers of R alleles within each family. Interaction of bulked 

inocula with R genotypes from seven families was highly significant (<0.001), but it was not 

significant with r genotypes because regardless of inocula, r genotypes developed high levels of 

disease. However, the response of R genotypes varied across inocula. The magnitude of difference 

(odds ratio) between the R vs. r genotypes for disease incidence within each family varied from 1.0 

times to 32 times. The results demonstrate that greenhouse assessments of pathogen virulence against 

known fusiform rust resistance alleles can detect virulence variation among inocula. Such virulence 

assessments should be effective guides for the field deployment of plants carrying specific R alleles to 

regions where inocula samples are observed to show low or no corresponding virulence.

Literature Cited

Wilcox, P.L.; Amerson, H.V.; Kuhlman, E.G.; Liu, B.H.; O’Malley, D.M.; Sederoff, R.R. 1996. Detection 

of a major gene for resistance to fusiform rust disease in loblolly pine by genomic mapping. Proceedings of 

the National Academy of Science USA. 93: 3859–3864.

 1 North Carolina State University, Cooperative Tree Improvement Program, Raleigh, NC 27695.

Corresponding author: fisik@ncsu.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

218

Selection of Loblolly Pine Varieties Resistant to 

Fusiform Rust for Commercial Deployment

Andy Benowicz1 and Robert J. Weir2

Abstract

Commercial production of loblolly pine (Pinus taeda L.) varieties through somatic embryogenesis has been 

increasing significantly over the last several years. Large-scale operational plantations have been established 

since 2004 across the Southeastern United States, while the oldest field tests are now at mid-rotation. Extensive 

surveys of fusiform rust (Cronartium quercuum f.sp. fusiforme) infection rates in over 80 operational varietal 

plantations and demonstration plots were conducted in eight southeastern states. The surveyed stands were 3- to 

9-years old. The operational data confirm that the selection program for rust resistance in loblolly pine varieties 

has been very successful. Genotypes characterized by fast growth rate and tentative resistance to fusiform rust 

are identified at a young age in several field test sites. Cryogenic tissue from the selected genotypes is used to 

produce seedlings for tests involving artificial inoculations. Artificial inoculation tests of CellFor-produced 

loblolly pine clones have been performed by the U.S. Department of Agriculture resistance screening center in 

Asheville, North Carolina over the last 8 years. A number of loblolly pine clones of Atlantic Coastal Plains and 

Western Gulf origin were exposed to high concentrations of fusiform rust inocula. The inocula were developed 

from aeciospores collected in three regions representing the eastern, central and western distribution range of 

loblolly pine. The inocula from the three regions were used in separate tests. Commercial varieties ranked as 

resistant or very resistant, based on field data and artificial inoculation tests, show less than 0.5 percent stem 

infection rates, based on all 56 surveyed operational plantations and demonstration plots they were present in. 

 1 CellFor, 4-6772 Oldfield Road, Victoria, BC, V8M 2A3. 2 CellFor, Portland, Maine.

Corresponding author: abenowicz@gmail.com.
219

Resistance Breeding Programs

Resistance to Ceratocystis wilt in eucalyptus, caused by C. fimbriata in a 5

year-old plantation, in Minas Geratis, Brazil. A resistant clone is on the left. On 

the right is a susceptible clone (Eucalyptus grandis x urophylla hybrid clones).

(Photo: A.C. Alfenas)

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

221

The American Chestnut Foundation

Breeding Program

F.V. Hebard1

Introduction

Chestnut blight, incited by Cryphonectria parasistica, devastated American chestnut (Castanea 

dentata (Borkh.) Marsh) in the first half of the 20th century, killing approximately 4 billion dominant 

and codominant trees. Millions of small sprouts still persist throughout the botanical range of C. 

dentata. Most are not infected and do not flower, except for short periods, before the shoot is killed 

by blight. Around the fringes of the botanical range, isolated trees can escape infection for prolonged 

periods, reaching diameters of about 50 cm at breast height (dbh). In the heart of the range, fewer than 

20 large (>33 cm dbh) trees are known to persist that have survived blight infection for longer than 10 

years (Griffin et al. 1983). Those trees are termed large, surviving American chestnut trees. Some 

have low levels of blight resistance, but not enough for very many of their progeny to persist. They

currently are being bred for higher levels of blight resistance (Griffin 2000). 

Without active control measures, such as breeding for resistance and introduction of hypoviruses, 

it generally has been assumed that American chestnut will become extinct in its native range due to 

blight and the paucity of reproduction. However, the rate of extinction is quite low and numerous

sprouts persist throughout the range (Scrivani 2011). These often flower for short periods when 

exposed to high light levels and serve as a reservoir of germplasm. However, the persistence of this 

reservoir is not assured in the face of the changing environment, and continued monitoring is 

necessary. Increasing deer populations currently are a major threat to long-term survival of American 

chestnut sprout clumps (Burke and Wilber, unpublished 2

).

Breeding for blight resistance was initiated around 1930, but those early programs were 

abandoned as hopeless in the early 1960s because they had been unable to combine the forest 

competitiveness of American chestnut with the chosen sources of blight resistance, oriental chestnut 

species. However, the early breeding programs did identify species with blight resistance and develop 

methods for making crosses, cultivating seedlings, and screening them for blight resistance.

Charles Burnham (1981) first hypothesized that the blight resistance of Chinese chestnut, C. 

mollissima Blume, could be backcrossed into American chestnut. Backcrossing is the method of 

choice for introgressing a simply inherited trait into an otherwise acceptable cultivar. One of 

Burnham’s assumptions in 1981 was that blight resistance is controlled by a single factor.

Subsequently, in 1986, Burnham, French, and Rutter (Burnham et al. 1986) accepted Clapper’s

(1952) conjecture that blight resistance is controlled by two incompletely dominant factors. Burnham, 

French, and Rutter were the principals who started the American Chestnut Foundation (TACF) in 

1983 to facilitate testing of the Burnham hypothesis.

Burnham recruited Lawrence Inman to help with design of the program. Inman (1987) proposed 

breeding populations of chestnut at multiple locations throughout the American chestnut range to 

preserve local adaptation and increase genetic diversity. He also proposed using multiple sources of 

blight resistance. Inman (1989) suggested restricting local collections to within a radius of 16 

kilometers. Following Namkoong (1991), Hebard (1994) proposed breeding each source of blight 

resistance with 20 different American chestnut trees for each cycle of backcrossing; on average, 20 

individuals would capture alleles occurring at frequencies greater than 0.05 (=1/20).

 1 The American Chestnut Foundation, 29010 Hawthorne Dr., Meadowview, VA 24361. Fred@acf.org 2 Burke, K.L.; Wilbur, H.M. Unpublished. Effects of white-tailed deer on growth and mortality of Castanea dentata and 

Acer pensylvanicum. 
GENERAL TECHNICAL REPORT PSW-GTR-240

222

Hebard (2006) specified that intercrossing after backcrossing, whose purpose is to restore true 

breeding by creating and identifying segregants homozygous for resistance alleles, should be 

restricted to single sources of blight resistance. It would be impossible to eliminate alleles for 

susceptibility at the F2 stage if different donor trees had different loci with alleles for resistance.

Hebard (2006) presented results to date for the TACF breeding program, and discussed methods in 

depth. The purpose of the present report is to update the overall results and to discuss population 

management aspects of the program. While the TACF breeding program occurs in multiple locations, 

the most advanced crosses are located at our principal research facility in Meadowview, Virginia

(fig.1). Results given below were gathered at Meadowview.

Figure 1—View of the American Chestnut Foundation’s Price Research Farm from its Bryan 

Research Farm. The Foundation has four farms in Meadowview covering about 150 acres; the other 

two farms are B3-F2 seedling seed orchards of about 15 acres each. 

Results and Discussion

Variation in Pathogenicity Between Virulent Strains of Cryphonectria 

parasitica

Blight resistance is evaluated using two strains of the blight fungus, both virulent, but one, SG2-3, of 

mild pathogenicity and one, Ep155, of high pathogenicity. Cankers incited by the highly pathogenic 

Ep155 differ in size most prominently between chestnut trees with high and intermediate levels of 

blight resistance; whereas cankers incited by the mildly pathogenic SG2-3 differ in size most 

prominently between trees with intermediate and non-existent levels of blight resistance (fig. 2).

Thus, a wider range of resistance can be distinguished using both strains rather than just one alone.

The experiment summarized in fig. 2 employed a set of trees of various ages and levels of blight 

resistance that were planted in a randomized, complete-block design over several years and 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

223

inoculated in 1 year, using Ep155 and SG2-3. The three main effects were highly significant (table 1).

Figure 2—Mean canker length versus resistance for chestnut trees of different ages inoculated with 

two virulent strains of Cryphonectria parasitica, one highly pathogenic (Ep155) and the other slightly 

pathogenic (SG 2-3).

The effect of age in the experiment was confounded with shading of young trees by older trees. This 

occurred because spacing (3.0 m x 1.5 m) had not been set widely enough to avoid shading. So “age” 

was treated as a fixed effect in analysis of variance. Cross type and inoculum pathogenicity also were 

Table 1—Analysis of variance of chestnut blight canker length 2 months after inoculation, 

testing the effect of blight resistance (cross type), tree age, and inoculum pathogenicity

Source DF Sum of Squares Mean 

Square F Ratio Prob > F

Block 5 45.99630 9.1993 3.9453 0.0026

Cross Type 2 286.67139 143.3357 61.4732 <.0001

Age 3 60.11460 20.0382 8.5939 <.0001

Cross Type*Age 6 39.19499 6.5325 2.8016 0.0144

Inoculum 1 346.72540 346.7254 148.7021 <.0001

Cross Type*Inoculum 2 19.39003 9.6950 4.1580 0.0183

Age*Inoculum 3 7.23204 2.4107 1.0339 0.3808

Cross Type*Age*Inoculum 6 14.61847 2.4364 1.0449 0.4007

Model 28 889.5520 31.7697 13.6253

Error 105 244.8262 2.3317

C. Total 133 1134.3782 <.0001
GENERAL TECHNICAL REPORT PSW-GTR-240

224

treated as fixed effects. Nevertheless, the “age” factor is an environmental effect, and illustrates (once 

again) that environment, broadly considered, has a major effect on expansion of chestnut blight 

cankers, and thus assessments of blight resistance.

The significant (p=0.014) interaction of cross type with age (table 1) occurred because age did not 

affect canker size on American chestnut as much as on Chinese chestnut and on their F1 hybrid (fig.

2). In this experiment, canker sizes were closer overall to American rather than Chinese chestnut. The 

reverse occurred in another experiment conducted in another year (described below), again 

illustrating effects of environment on canker expansion.

Inoculum pathogenicity affected canker size more in the F1s than the pure species (p=0.018), 

which is to be expected as rates of canker expansion reach lower and upper limits in Chinese and 

American chestnut, respectively. An idealized shape of canker expansion versus resistance is a 

sigmoid curve, where the plateaus are the upper and lower limits of canker expansion. Thus, even 

though rates of linear canker expansion have fairly uniform variances throughout their range, and are 

appropriate metrics for resistance, host variety x fungus strain interactions can occur (Huang et al.

1996); however, this is not host specificity associated with avirulence genes, which has not been 

detected yet for chestnut blight. Likewise, the overall non-linearity of canker size with resistance can 

also lead to association of quantitative trait loci (QTL) for blight resistance with only one strain of the 

blight fungus. For instance, in fig. 2, 3-year-old trees might show a resistance QTL with SG2-3 but 

not Ep155. That same locus might show a lack of “virulence” for SG2-3 but not Ep155 in 1-year-old 

trees. I conclude that care must be exercised in analyzing variety x strain interactions for chestnut 

blight.

Current Stage of Backcrossing Program

At Meadowview, we have almost completed advancing two sources of blight resistance, derived from 

the ‘Clapper’ and ‘Graves’ first backcrosses (Hebard 2006) into 30 B3-F2 lines each of American 

chestnut. We have planted about 58,600 such nuts in two seedling seed orchards; about 35,400 remain 

after rogueing (table 2). As selection and rogueing continue, these will be reduced to about 500 

plants, which should occur within 3 to 5 years.

Hopefully we will be able to select for true breeding for blight resistance in these B3-F2 trees.

Simulations based on patterns of inheritance of resistance in straight F2s and B1-F2s indicate that it 

would be difficult to select for homozygous resistance to blight caused by two loci and impossible 

with three loci, due to overlap of phenotypic resistance classes. Analysis of those populations and 

others suggested that two or three loci control blight resistance in Chinese chestnut (Hebard 2006, 

Kubisiak et al. 1997). Three QTLs for blight resistance were found in the F2 mapping population 

originally genotyped by Kubisiak et al. in 1997 when it was regenotyped with several thousand new 

markers (Kubisiak et al., unpublished3

). Additional populations have been genotyped and phenotyped 

and data are being analyzed. Markers fairly close to these loci might facilitate identification of trees 

homozygous for blight resistance. In the meantime, the most blight-resistant individuals are being 

selected based on results from inoculation of their open-pollinated B3-F3 progeny in orchard settings, 

after initial selection in the parents, also based on inoculation. B3-F3 progeny additionally are being 

evaluated in wooded settings for forest performance. It currently is unclear whether those forest tests 

will contribute to selection of B3-F2 parents.

The Rationale of Screening B3-F2s for Blight Resistance

We use a gradual process to select for blight resistance in our B3-F2 seed orchards. A more rapid 

process would have to be more stringent, which would kill too many good trees.

 3 Kubisiak, T.L.; Nelson, C.D.; Staton, M.E.; Zhebentyayeva, T.; Smith, C.; Olukolu, B.A.; Fang, G.C.; Hebard, F.V.; 

Anagnostakis, S.; Wheeler, N.; Sisco, P.; Abbott, A.G.; Sederoff, R.R. Unpublished. A transcriptome-based genetic map of 

Chinese chestnut, (Castanea mollissima), and identification of regions of segmental homology with peach (Prunus persica). 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

225

We start by inoculating 2-year-old seedlings with the SG2-3 strain of the blight fungus and 

selecting those with small cankers. Although virulent and capable of killing American chestnut trees, 

the SG2-3 strain is considerably less pathogenic than the Ep155 strain, as shown above. Previous 

tests, including that summarized in fig. 2 and table 1 above, indicated that even 2-year-old Chinese 

chestnut trees are most likely killed by Ep155, whereas most 2-year-old trees survive inoculation with 

SG2-3 if they possess levels of blight resistance equal to or greater than that of F1s between Chinese 

and American. Such F1s typically are intermediate in blight resistance between the two parents. The 

results of the SG2-3 inoculations enable us to eliminate about 60 to 70 percent of the B3-F2s, 

depending on the size of the tested trees and the year.

We do not include controls such as Chinese chestnut in the B3-F2 seed orchards. The small SG2-3 

cankers after the first season of canker expansion on the selections could not get any smaller on 

Chinese chestnut, so their inclusion as controls for that period would not be informative. Chinese 

chestnut trees could be informative in later years if left in the orchard, but would then start to produce 

undesired pollen. The B3-F2 orchards also are designed to produce seed with maximum genetic 

diversity over long periods by maximizing distance between sibs, and the design appropriate for that 

goal cannot provide statistically and experimentally sound evidence of blight resistance in the B3-F2s.

We make additional selections over the next few years following inoculation by examining 

cankers on the preliminary selections. However, as outlined above, we have been intending to make 

the final selections among the B3-F2 trees by testing the blight resistance and performance of their B3-

F3 progeny in orchard and, possibly, forest settings.

Results of the First Season of Canker Expansion in an Orchard Test of 

Blight Resistance in B3-F3 Chestnut Trees Planted Using a Formal 

Experimental Design

In 2008, we harvested our first crop of B3-F3s large enough to test formally in both the orchard and 

forest using an experimental design and control plants. The orchard test was planted in 2009 using a 

completely randomized design. In retrospect, an incomplete block design would have enabled more 

accurate selection of superior parents, and we subsequently changed to that. In June 2011, we 

inoculated that first test planted in 2009 and measured canker lengths in December 2011, using 

methods described by Hebard (2006).

Table 3 shows statistics for cankers incited by both Ep155 and SG2-3 on the different cross types 

in the experiment. The controls for canker sizes, ranked from susceptible to resistant, were American, 

B2, F1 plus B1-F2, B1xChinese, and Chinese. (The backcross controls are imperfect resistance 

standards, as mistakes in selection can lessen their resistance). We expected the B3-F2 selections in 

the seed orchard that produced these progeny to have blight resistance equal to or greater than that of 

F1s and B1-F2s, since those B3-F2s had been screened only using strain SG2-3. If this first expectation 

were met, one would then expect their B3-F3 progeny from open pollination to have mean canker 

lengths intermediate between the F1/B1-F2s and the B1xCs. However, instead, cankers on the B3-F3s 

were similar to or slightly longer than those on B1-F2s. Reasons this may have occurred include that 

pollen is still being produced by unselected as well as selected B3-F2 parents in the seed orchards, that

selection is not complete, and/or that there was some degradation of factors for blight resistance 

during backcrossing.

Out of 583 trees tested, there were 95 B3-F3s with small cankers for both strains Ep155 and SG2-3, 

where small cankers are those less than 5 cm in length. Normally, one would expect trees with such 

small cankers to have a high level of blight resistance. However, there was dominance toward small 

cankers in this test, leading to a higher frequency of trees with small cankers in the B1-F2 progenies 

than is observed in most tests of F2s. We observed 35 out of 171 B1-F2s with small cankers, where 

usually we would expect to observe about 10. We expect many of these small cankers on the B1-F2s 

to start expanding in 2012. How much that occurs also in the 95 B3-F3s with small Ep155 cankers 

will be interesting to follow.
GENERAL TECHNICAL REPORT PSW-GTR-240

226

The phenotypic dominance toward small cankers, after the first season of canker expansion in this 

test, also is reflected in the bimodal distribution of canker sizes. The bimodality is most evident for 

the Ep155 cankers in the B3-F3 and B1-F2 crosses in table 3, where there is a peak in the 0-5 cm class 

and another in the 10-15 cm class. The peak at 0-5 cm occurred because cankers in the 0-5 cm class 

cannot get any smaller, so their numbers pile up when phenotypic dominance is toward small cankers.

There was even more clustering in the small canker class for the SG2-3 cankers, which is expected 

given its lesser pathogenicity. However, the ranking of mean canker size for cross type was the same 

for both strains.

The predominance of small cankers led to a significant fungus strain by cross type interaction, 

since the SG2-3 and Ep155 cankers on Chinese chestnut were similar in size, while in the other cross 

types, SG2-3 cankers were about 6-8 cm shorter than Ep155 cankers. There was no interaction 

between strain and families nested within cross type. Another effect of very small cankers was that 

the variance of canker length was reduced in Chinese chestnut compared to the other crosses, 

especially for strain SG2-3. The unequal variances (heteroscedasticity) for strain SG2-3 increased the 

likelihood that differences between cross types and families would be declared statistically 

significant, making declarations of significance suspect for SG2-3.

Table 4 shows canker length statistics for individual families within the various cross types. The 

Ep155 cankers yielded significant differences in canker size for the more resistant trees while the 

SG2-3 cankers yielded significant differences for the more susceptible trees (although, again, the 

significance of the SG2-3 differences is suspect). Twenty-one of 36 B3-F3 families had significantly 

(p<0.05) smaller SG2-3 cankers than the American chestnut family. The B3-F2 parents of the 

remaining 16 families are candidates for rogueing.

Discussion of B3-F3 Canker Results

This crop of B3-F3s was significantly more blight resistant than American chestnut, roughly 

comparable in resistance to Chinese x American F1s or backcross F2s, in the aggregate. None of the 

families had as high a level of blight resistance as Chinese chestnut, but many contained highly 

blight-resistant individuals as of the end of the first season of canker expansion. This is roughly in 

accord with expectation, given that selection is not finished in the B3-F2 orchards from which the nuts 

were harvested. The best families had resistance roughly comparable to that of crosses of selected 

straight backcrosses with Chinese chestnut, which is what we would expect in crosses of Chinese 

chestnut with the pollen being produced in the orchards. However, we have not eliminated the 

possibility that some of the blight resistance of Chinese chestnut has been lost during backcrossing.

Characterization of Breeding Populations

The effective population size, denoted Ne, is used commonly to estimate minimal sizes needed to 

maintain long-term viability of populations, such as species. Using quantitative genetic 

considerations, Franklin (1980) estimated that an Ne of 50 is needed to avoid immediate collapse of a 

population due to inbreeding depression, and an Ne of 500 is needed to offset loss of alleles by 

genetic drift with recruitment of new alleles by mutation. Ne can be estimated as the harmonic mean 

of population sizes, given the other assumptions of Hardy-Weinberg equilibrium.

To compute Ne as a harmonic mean, we need the number of individuals at each generation. The 

size of the original American chestnut population can be considered infinite for this computation;

whereas, the population goes through a bottleneck at the straight backcross stage. The population size 

at straight backcross was set at 20 using Namkoong’s considerations, as mentioned above in the 

introduction. Hebard (2002) estimated from first principles that obtaining nine F2 progeny from a 

straight backcross mother tree gives a 95 percent chance of capturing all her alleles. Thus the size of 

the F2 generation is nine multiplied by 20 American chestnut lines per source of blight resistance, or 

180. The size of backcross F3 and subsequent filial generations also can be considered infinite, but 

were limited to several thousand in simulations of inbreeding to be discussed momentarily. Using 

these numbers of individuals, the Ne of 20 lines of American chestnut, computed from the harmonic 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

227

mean, is about 72. It would increase if more than one individual per line were retained as a parent of 

the backcross F2 generation (fig. 3), which is generally the case.

Figure 3—Effect of the number of siblings per B3 line on the inbreeding coefficient at B3-F4 versus the 

number of American chestnut lines at B3, for partial diallel mating at B3 with four lines per diallel, ten 

B3-F2 offspring per line, and for random mating thereafter.

The inbreeding effective population size can be estimated independently using the inbreeding 

coefficient of a population. For some breeding scenarios, a harmonic mean cannot be used to compute 

Ne, but inbreeding coefficients can. Hebard (2002) computed inbreeding coefficients by ‘brute force’

simulation. The results of similar simulations are shown in table 5, which illustrate the effect of 

TACF’s Chapter breeding program on the overall Ne of the breeding population at B3-F4 (each 

chapter has a goal of breeding 20 American lines for at least one source of blight resistance).

Two sources of blight resistance, derived from the ‘Clapper’ and ‘Graves’ trees, constitute most of 

the breeding stock in the TACF program. We probably will be able to breed at five locations for each 

source, which would make the Ne of the overall breeding stock about 500 (table 5). Of course, 

selection at loci for resistance will decrease Ne, as will other violations of Hardy-Weinberg 

assumptions, such as skewed numbers of progeny per parent. This degradation will be offset 

somewhat by the chapters using more advanced breeding stock than that simulated for table 5. We 

plan to measure Ne in the breeding stock to compare with these predictions and to guide further 

breeding. We have some baseline data from the native population of American chestnut from a rather 

thorough sampling of the species (Kubisiak and Roberds 2006). DNA from that sampling also is 

available for further probing.

A third source of blight resistance, derived from the Nanking cultivar of Chinese chestnut, is being 

advanced in 20 American lines at Meadowview and a few other chapters. This will increase the Ne of 
GENERAL TECHNICAL REPORT PSW-GTR-240

228

our breeding population. We also hope to expand other sources of blight resistance to an Ne of 72 at 

various chapters, as outlined by Hebard (2004).

The decision to introgress blight resistance into American chestnut from other species was based 

in part on the thought that no blight resistance existed in the native population. This does not appear 

to be the case (Griffin 2000). Burnham (1990) outlined a plan for utilizing this native resistance 

which we have been following in a breeding program at Meadowview separate from the backcrossing 

effort. Ironically, the relatively small progeny sizes required by the backcross method make it a more 

efficient means of generating a breeding population with large Ne than recurrent selection of large, 

surviving American chestnut trees after introgressing them into a broad enough base of American 

chestnut with no resistance to blight. 

As the backcross breeding program has developed, we have come to realize that our goal is to 

enable the American chestnut to resume evolving on its own, as a wild species. Our B3-F3

‘Restoration’ chestnut trees should have sufficient blight resistance to produce numerous viable 

offspring in natural settings, although it is still unclear whether they will have levels of blight 

resistance similar to Chinese chestnut and whether even that level of blight resistance will enable 

them to be dominant forest trees. Their blight resistance could be increased further, if need be, by 

allowing them to intercross in forested settings with other sources of blight resistance, such as from 

additional backcross lines produced by TACF, from large, surviving American chestnut trees, or from 

other sources, such as transgenics or cisgenics. The intercrossing and further increase of progeny 

could be facilitated as needed by silvicultural interventions, but further intensive breeding in an 

orchard setting would not occur with the new base population, only with these supplemental 

populations. Hypovirulence may also play a role in re-establishing a self-sustaining breeding 

population, as Griffin (2000) and others have pointed out. The important point is that we are close to 

developing a self-sustaining breeding population of predominately American chestnut with a large 

enough Ne to constitute a viable species.
230

229

Table 2—Type and number of chestnut trees and planted nuts at TACF Meadowview Research Farms in May 

2011, with the number of sources of blight resistance and the number of American chestnut lines in the breeding 

stock

Type of Tree

Number of

Nuts or

Trees

Sources of

Resistance

American 

Lines

American 1575 223

Chinese 1014 30

Chinese x American: F1 417 18 57

American x (Chinese x American): B1 646 11 24

American x [American x (Chinese x American)]: B2 1316 13 43

American x {American x [American x (Chinese x American)]}: B3 2158 10 92

Am x (Am x {Am x [Am x (Ch x Am)]}): B4 888 4 14

(Ch x Am) x (Ch x Am): F2 213 5 5

[(Ch x Am) x (Ch x Am)] x [(Ch x Am) x (Ch x Am)]: F3 5 1 1

[Am x (Ch x Am)] x [Am x (Ch x Am)]: B1-F2 625 7 10

{Am x [Am x (Ch x Am)]} x {Am x [Am x (Ch x Am)]}: B2-F2 667 9 12

B2-F3 31 1 1

(Am x {Am x [Am x (Ch x Am)]}) x (Am x {Am x [Am x (Ch x Am)]}): B3-F2 35394 2 51

B3-F3 3826 2 22

Clapper B3 x Graves B3: B3-I1 110 2 9

Chinese x [American x (Chinese x American)]: Chinese x B1 167 3 7

Ch x {Am x [Am x (Ch x Am)]}: Chinese x B2 72 1 2

Ch x (Am x {Am x [Am x (Ch x Am)]}): Chinese Test Suite x B3 286 5 16

Chinese Test Suite x Chinese 1471 67 67

Chinese Test Suite x Japanese 46 2 2

Chinese Test Suite x European 43 1 1

Chinese Test Suite x Large, Surviving American 149 7 7

European x American: F1 2 1 1

Japanese 3 1 1

Japanese x American: F1 8 1 1

[(Japanese x American) x American]: B1 5 1 1
230

229

(Table 2 continued) 142 1 1

Japanese x European 80 1 1

Japanese x Large, Surviving American 27 5 5

Castanea ozarkensis 21 1 2

Castanea pumila 44 3 3

Castanea seguinii 48 3 3

Seguin x American: F1 54 2 2

Large Surviving American: F1 486 12 37

Large Surviving American: B1 506 8 14

Large Surviving American: B2 72 3 4

Large Surviving American: B3 161 1 1

Large Surviving American: F2 266 13 11

Large Surviving American: F3 270 1 2

Large Surviving American: I1 1666 32 13

Large Surviving American: I2 303 11 1

Large Surviving American: I3 104 2 32

Large Surviving American advanced: F1 804 12 25

Other 3

Total 56194
Table 3

231

—Mean, standard deviation, and distribution of canker size classes (length in cm) for cankers incited 

by two strains of the blight fungus on cross types of American and Chinese chestnut in 2011

Cross

Type

Fungus

Strain N

Least

Squares 

Mean*

Standard

Deviation

Length Class

0-5 5-10 10-15 15-20 20-25 25-

American Ep155 20 17.4 A 3.4 1 4 11 4

B2 Ep155 44 14.5 AB 6.6 4 2 19 15 2 2

B3-F3 Ep155 583 11.7 BC 6.3 95 71 219 151 32 15

B1-F2 Ep155 171 10.9 BC 6.4 35 23 70 30 10 3

F1 Ep155 8 10.0 ABC

D 3.6 4 3 1

B1xC Ep155 39 8.3 CD 5.0 12 9 16 2

CxC Ep155 38 3.2 D 2.3 27 11

American SG2-3 17 11.0 A 7.8 3 5 7 2

B2 SG2-3 45 5.3 B 5.6 32 5 6 2

B3-F3 SG2-3 592 4.9 B 4.3 402 111 65 10 3 1

B1-F2 SG2-3 172 3.0 BC 3.9 126 26 18 2

F1 SG2-3 7 3.3 BC 1.7 6 1

B1xC SG2-3 39 1.8 BC 2.6 35 3 1

CxC SG2-3 38 1.6 C 0.6 38

* Means followed by the same letter are not significantly different at p<0.05 by a Tukey HSD test. The declarations are suspect for strain SG2-3 

due to heteroscedasticity.
233

Table 4—Means for length (in cm) of cankers incited by two strains of the blight fungus on individual families 

of American and Chinese chestnut in 2011

Least Square Means**

Cross

Type Mother* Father Source ofResistance N Grand Mean Strain Ep155 Strain SG2-3

American PL1-08 op none 16 14.4 17.4 A 11.4 A 

B3-F3 D5-26-54 op Clapper 14 12.5 17.0 AB 8.0 ABC 

B2 B2208 AN17 Nanking;none 5 11.7 16.4 ABCD 7.0 ABCD

B3-F3 D5-29-124 op Clapper 1 11.2 12.1 ABCDE 10.4 ABCD

B3-F3 D3-28-10 op Clapper 11 11.0 16.2 ABC 5.8 ABCD

B2 TM474 A1530 Nanking;none 8 10.8 15.0 ABC 6.6 ABCD

B3-F3 D5-17-89 op Clapper 20 10.6 13.8 ABC 7.5 AB 

B3-F3 D5-19-72 op Clapper 22 10.2 14.2 ABC 6.1 ABCD

B3-F3 D5-18-95 op Clapper 7 10.0 11.4 ABCDE 8.6 ABCD

B3-F3 D5-18-50 op Clapper 10 9.8 14.1 ABCD 5.4 ABCD

B3-F3 D5-27-108 op Clapper 17 9.3 13.8 ABC 4.7 BCD

B3-F3 D5-27-101 op Clapper 29 9.3 14.1 ABC 4.5 BCD

B3-F3 D5-26-131 op Clapper 32 9.3 13.1 ABC 5.4 BCD

B3-F3 D5-29-50 op Clapper 10 9.1 11.9 ABCDE 6.3 ABCD

B2 B210 A1117 Nanking;none 15 9.1 13.4 ABC 4.9 BCD

B3-F3 D5-25-49 op Clapper 18 9.1 12.2 ABCD 6.1 ABCD

B3-F3 D5-18-101 op Clapper 12 8.9 14.1 ABC 3.8 BCD

B3-F3 D5-29-3 op Clapper 23 8.9 13.5 ABC 4.2 BCD

B1-F2 TM538 TM158 Nanking;Nanking 7 8.8 14.8 ABCD 2.7 BCD

B3-F3 D5-22-17 op Clapper 15 8.8 13.1 ABCD 4.5 BCD

B3-F3 D5-27-36 op Clapper 24 8.8 13.0 ABC 4.6 BCD

B3-F3 D5-26-94 op Clapper 28 8.6 13.0 ABC 4.2 BCD

B3-F3 D6-26-27 op Clapper 37 8.6 12.9 ABC 4.3 BCD

B3-F3 D5-18-25 op Clapper 26 8.4 13.0 ABC 3.7 BCD

B3-F3 D5-17-122 op Clapper 1 8.2 10.4 ABCDE 6.0 ABCD

B2 CY554 MB190 Nanking;none 16 8.0 13.1 ABC 2.8 BCD

B3-F3 D2-29-44 op Clapper 24 8.0 11.6 ABCD 4.3 BCD

B1-F2 B2354 TM672 Nanking;Nanking 117 7.9 11.7 ABC 4.2 BCD

B3-F3 D5-30-24 op Clapper 20 7.7 11.7 ABCD 3.6 BCD

B3-F3 D5-27-95 op Clapper 32 7.4 11.7 ABCD 3.1 BCD

B3-F3 D4-28-31 op Clapper 4 7.2 12.0 ABCDE 2.4 ABCD

B1xC JB478 MuChin1 MuChinX;MuChin1 1 7.1 13.3 ABCDE 0.8 ABCD

B1-F2 TM158 TM538 Nanking;Nanking 21 7.1 10.5 ABCDE 3.6 BCD

B3-F3 D2-26-72 op Clapper 23 6.9 10.3 ABCDE 3.6 BCD

B3-F3 D1-27-25 op Clapper 12 6.8 11.7 ABCDE 2.0 BCD

F1 TA3 MB190 Kuling;none 1 6.7 9.5 ABCDE 3.8 ABCD

F1 KY106 MB190 Meiling;none 6 6.7 10.5 ABCDE 2.9 BCD

B3-F3 D5-22-86 op Clapper 17 6.7 11.2 ABCDE 2.2 BCD 232
233

(Table 4 continued)

B3-F3 D5-25-147 op Clapper 11 6.6 9.9 ABCDE 3.4 BCD

B3-F3 D5-18-2 op Clapper 7 6.5 9.1 ABCDE 3.8 ABCD

B3-F3 D5-30-11 op Clapper 22 6.4 10.5 ABCDE 2.3 CD

B3-F3 D9-26-36 op Clapper 3 5.9 7.4 ABCDE 4.4 ABCD

B1xC TM672 GR119 Nanking;Nanking 18 5.7 9.1 ABCDE 2.4 BCD

B1-F2 B2275 B293 72-211;72-211 10 5.5 9.3 ABCDE 1.7 BCD

B1-F2 B2430 B2275 72-211;72-211 13 5.5 8.4 ABCDE 2.6 BCD

B3-F3 D2-28-76 op Clapper 14 5.3 7.9 BCDE 2.8 BCD

B3-F3 D5-17-130 op Clapper 9 5.2 7.9 ABCDE 2.5 BCD

B3-F3 D8-26-69 op Clapper 16 5.2 8.0 CDE 2.4 BCD

B3-F3 D2-26-66 op Clapper 3 5.0 8.2 ABCDE 1.7 ABCD

B3-F3 D2-28-52 op Clapper 3 4.3 5.9 ABCDE 2.7 ABCD

B1xC B2239 GR119 Nanking;Nanking 6 4.2 6.4 ABCDE 1.9 BCD

B1xC JB5 MuChin1 MuChinX;MuChin1 3 4.1 5.9 ABCDE 2.3 ABCD

B1xC B2426 GR119 Nanking;Nanking 10 4.1 6.8 CDE 1.4 BCD

CxC GR119 SLR1T15 Nanking;Mahogany 20 2.6 3.7 E 1.5 D

CxC KY106 op Meiling;unknown 5 2.4 3.3 CDE 1.4 BCD

CxC KY75 SLR1T15 Meiling;Mahogany 10 2.4 3.2 DE 1.6 BCD

CxC TA3 op Kuling;Unknown 3 2.2 2.8 ABCDE 1.7 ABCD

* The first letter of the code for the B3-F3 crosses identifies the farm containing its B3-F2 parent. The first number identifies the block of trees 

containing the B3-F2 parent. The middle number identifies the plot within a block, and corresponds to the American great, great grandparent of 

the B3-F2. The last number is the tree number within a plot, within a block. In this set of crosses, there is only one open-pollinated B3 

grandparent of the B3-F3s in a plot, except that the B3 grandparent of D5-18-95 and D5-18-101 differs from the B3 grandparent of the other 

trees in plot 18.

** Within a column, means not followed by the same letter are significantly different at p<0.05 by a Tukey HSD test. The declarations are 

suspect for strain SG2-3 due to heteroscedasticity.

Table 5—Effect of adding sets of 20 B3-F2 progeny from the American Chestnut Foundation’s Chapter 

breeding program on inbreeding and effective population size for the same source of blight resistance 

(inbreeding effective population size doubles with each additional chapter if different sources of blight 

resistance are used)

Number of Chapters Inbreeding Coefficient Inbreeding Effective Population 

Size

1 0.0207 72

2 0.0115 130

3 0.0085 176

4 0.0070 214

5 0.0060 248
GENERAL TECHNICAL REPORT PSW-GTR-240

234

Literature Cited

Burnham, C.R. 1981. Blight-resistant American chestnut: there's hope. Plant Disease. 65: 459–460.

Burnham, C.R. 1990. Evaluation and use of large American chestnut survivors in blight areas. Journal of the 

American Chestnut Foundation. 4:43-45.

Burnham, C.R.; Rutter, P.A.; French, D.W. 1986. Breeding blight-resistant chestnuts. Plant Breeding 

Reviews. 4: 347–397.

Clapper, R.B. 1952. Relative blight resistance of some chestnut species and hybrids. Journal of Forestry. 50(6): 

453–455.

Franklin, I.R. 1980. Evolutionary change in small populations. In: Soule, M.E.; Wilcox, B.A., eds., 

Conservation biology: an evolutionary-ecological perspective. Sunderland, MA: Sinauer Associates: 135–

140.

Griffin, G.J. 2000. Blight control and restoration of the American chestnut. Journal of Forestry. 98: 22–27.

Griffin, G.J.; Hebard, F.V.; Wendt, R.W.; Elkins, J.R. 1983. Survival of American chestnut trees: 

evaluation of blight resistance and hypovirulence in Endothia parasitica. Phytopathology. 73: 1084–1092.

Hebard, F.V. 1994. The American Chestnut Foundation breeding plan: beginning and intermediate steps.

Journal of the American Chestnut Foundation. 8: 21–28.

Hebard, F.V. 2002. Meadowview Notes 2001-2002. Journal of the American Chestnut Foundation. 16: 7–18.

Hebard, F.V. 2004. Research objectives of the American Chestnut Foundation, 2004-2014. Summary of 

TACF’s 10-year plan. Journal of the American Chestnut Foundation. 18: 13–19.

Hebard, F.V. 2006. The backcross breeding program of the American Chestnut Foundation. In: Steiner, K.C.; 

Carlson, J.E., eds. Restoration of American chestnut to forest lands, proceedings of a conference and 

workshop, May 4-6, 2004, The North Carolina Arboretum, Asheville. Natural Resources Report 

NPS/NCR/CUE/NRR – 2006/01. Washington, DC: National Park Service: 61–77.

Huang, H.; Carey, W.A.; Dane, F.; Norton, J.D. 1996. Evaluation of Chinese chestnut cultivars for resistance 

to Cryphonectria parasitica. Plant Disease. 80(1): 45–47.

Inman, L.I. 1987. Proposed strategies to preserve and restore the American chestnut. Journal of the American

Chestnut Foundation. 2: 6–9.

Inman, L.I. 1989. Simultaneous breeding of the American chestnut for many traits. Journal of the American 

Chestnut Foundation. 4: 16–17.

Kubisiak, T.L.; Hebard, F.V.; Nelson, C.D.; Zhang, J.; Bernatzky, R.; Huang, H.; Anagnostakis, S.L.;

Doudrick, R.L. 1997. Molecular mapping of resistance to blight in an interspecific cross in the genus

Castanea. Phytopathology. 87: 751–759.

Kubisiak, T.L.; Roberds, J.H. 2006. In: Steiner, K.C.; Carlson, J.E., eds. Genetic structure of American 

chestnut populations based on neutral DNA markers. In: Restoration of American chestnut to forest lands, 

proceedings of a conference and workshop, May 4-6, 2004, The North Carolina Arboretum, Asheville.

Natural Resources Report NPS/NCR/CUE/NRR – 2006/01Washington, DC: National Park Service: 109–

122.

Namkoong, G. 1991. Maintaining genetic diversity in breeding for resistance in forest trees. Annual Review of 

Phytopathology. 29: 325–42.

Scrivani, J. 2011. Forest inventory and analysis. Journal of the American Chestnut Foundation. 25: 17–18.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

235

Breeding Strategies for the Development of Emerald 

Ash Borer - Resistant North American Ash

Jennifer L. Koch,1 David W. Carey,

1 Kathleen S. Knight,1 Therese Poland,2

Daniel A. Herms,

3 and Mary E. Mason3

Introduction

The emerald ash borer (Agrilus plannipennis; EAB) is a phloem-feeding beetle that is endemic to 

Asia. It was discovered in North America in 2002, found almost simultaneously near Detroit, 

Michigan and Windsor, Ontario, Canada. Adult beetles feed on ash (Fraxinus spp.) foliage, but larval 

feeding on phloem, cambium, and outer xylem is far more detrimental because it effectively girdles

the tree, disrupting nutrient and water transport and ultimately killing the tree. As of 2007, it was 

estimated that 53 million ash trees had been killed by EAB in North America (Kovacs et al. 2010).

Potentially, more than 8 billion ash trees could eventually be affected, or about 2.6 percent of the 

timber trees in the United States (Sydnor et al. 2007).

Breeding for pest resistance in forest trees is a proven approach for managing both native and nonnative insects and diseases (FAO UN 2011). We initiated a breeding program that employs two 

strategies to incorporate EAB resistance into North American ash species: hybrid and traditional 

breeding. In the hybrid breeding approach, we are looking for EAB resistance in Asian species of ash 

from EAB’s region of origin (sympatric resistance). Asian ashes resistant to EAB will be crossed with 

native North American species to create hybrids. The hybrids subsequently will be subjected to

rounds of testing, selection, and backcrossing to the native species that will be repeated until only the 

resistance genes from the exotic species are carried into the native population while all of the traits of 

the native species are retained, similar to the breeding program of the American Chestnut Foundation 

(Hebard 2006). For a more traditional breeding approach, we are searching for rare native individuals 

with resistance or tolerance to EAB (allopatric resistance). The following is an overview of the 

current status of our program.

Hybrid Breeding

Contrary to the situation in North America, severe EAB outbreaks on ash species native to Asia are a 

rarity. These tree species, having co-evolved with EAB, are likely to have developed mechanisms of 

resistance. This hypothesis is supported by a recent common garden study that reports significantly 

less EAB-induced mortality in Manchurian ash (F. mandshurica Rupr.) compared to green (F. 

pennsylvanica Marsh.) and white (F. americana L.) ash (Rebek et al. 2008). Evidence for the EAB

resistance of Korean ash (F. chinensis ssp. rhynchophylla (Hance) Hemsley) is described by Liu et al.

(2007), who reported a mixed plantation of Korean ash and green ash where 95 percent of all green 

ash was found to be EAB infested while the Korean ash of similar size and in close proximity 

remained free of EAB. More recently, a study by Dian et al. (2010) reported that in both Manchurian 

and Korean ash, more than 75 percent of EAB larvae dissected from the trees had been killed through 

“host plant resistance,” or the formation of callous by the host trees that encapsulated the larvae.

 1 USDA Forest Service, Northern Research Station, 359 Main Rd., Delaware, OH 43015. 2 USDA Forest Service, Northern Research Station, 220 Nisbet Bldg., Michigan State University, East Lansing, MI 48823.

3 The Department of Entomology, Ohio Agricultural Research and Development Center, The Ohio State University, 1680

Madison Ave., Wooster, OH 44691.

Corresponding author: jkoch@fs.fed.us.
GENERAL TECHNICAL REPORT PSW-GTR-240

236

To identify additional EAB-resistant species that may be used as parents in a hybrid breeding 

program, we have been accessioning Asian ash species across a wide geographical and ecological 

range. A summary of our current collections is outlined in table 1.

Table 1: Exotic ash (Fraxinus) collection, U.S. Department of Agriculture, Forest Service, NRS, 

Delaware, Ohio

Species Origin Unique accessions

F. aspertisuquamifera Japan 1

F. angustifolia Syria, Europe 2

F. chinensis China, Korea 26*

F. excelsior Europe 3

F. insularis China 6*

F. lanuginosa Japan 4*

F. longicuspis China 1*

F. mandshurica China, Korea, Russia, Japan 25*

F. ornus Europe 3

F. paxiana China 5*

F. sieboldiana China, Korea, Japan 3

F. sogdiana China 3

F. spaethiana Japan 1

F. stylosa China 4*

*Includes accessions from the Morton Arboretum including those from a recent North American Chinese Plant Exploration 

Consortium (NACPEC) collection.

Each accession is confirmed for proper species identity through the use of DNA-based

technologies, including internal transcribed spacer (ITS) sequencing and amplified fragment length 

polymorphism (AFLPs). The ITS sequences were compared to published sequences for each species 

(Wallander 2008). The AFLP data was assessed for its utility in verifying species identities relative to

control reference species previously confirmed with ITS data. Species confirmation is a critical step 

because currently there is no comprehensive global taxonomic key, and few traits are diagnostic 

between closely related but geographically isolated species. Our current exotic ash collection includes 

85 independent accessions, including 11 Asian species and 3 European species. While confirming 

species identities, we uncovered seven different Asian ash accessions from reputable gardens and 

arboreta that were incorrectly identified, illustrating the difficulty of distinguishing Fraxinus species

(table 2). Most of these errors were native North American green ash mislabeled as Asian species, but 

in a few cases, Asian species were misidentified as other Asian species, European species, and North 

American black ash. In addition, six different seed lots (representing both Asian and North American 

species) from two different commercial seed companies were found to be incorrectly identified as 

shown in table 3. To date, about 70 percent of all of the commercially obtained Fraxinus seed sources 

have been incorrectly identified at the species level and 15 percent of the Asian accessions.

After species identity is confirmed, the individual accessions are replicated through grafting and 

will be installed in a common garden study at the Ohio Agricultural Research and Development 

Center in Wooster, Ohio along with North American EAB-susceptible species. The long-term goal of 

this study is to assess EAB resistance as well other desired characteristics such as growth rate, form, 

color, tolerance to native pests and diseases, and site suitability. 

Inter-specific hybridization of Fraxinus species has not been widely reported, with the exception 

of hybridization of F. nigra and F. mandshurica that resulted in the release of two horticultural 

cultivars, Northern Gem and Northern Treasure (Davidson 1999). To determine what species 

combinations may produce successful F1 hybrids, we performed controlled cross-pollinations using 

as many combinations as possible (dependent on the availability of viable male and female flowers).

In 2010, 42 different species combinations were crossed and 15 successfully produced seed. Five of 

the species combinations have now produced seedlings, with family sizes ranging from 11 to 104.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

237

Backcrosses were also performed between the commercially available F1hybrid Northern Gem and 

both F. mandshurica and F. nigra. These crosses successfully produced seed, and the seed is 

beginning to germinate. These new germinants represent the most advanced Fraxinus interspecies 

pedigree, and demonstrate that F1 hybrids are reproductively competent providing evidence that the 

hybrid/backcross breeding approach has potential to be successful in ash. All resulting F1 and 

backcross seedlings will be screened for EAB resistance and their hybrid parentage confirmed using 

molecular techniques.

Table 2—Misidentified Fraxinus accessions from arboreta and botanical gardens

Species labeled 

as:

Number of 

incorrect 

accessions

Correct species 

IDs (based on ITS 

and/or AFLP)

Location of 

arboretum 

Origin of 

accessions

F. chinensis 3 F. pennsylvanica United States Chinese Botanical 

Garden, Chinese 

Forest Preserve

F. chinensis var.

accuminata

2 F. pennsylvanica Europe Chinese Botanical 

Garden

F. chinensis var. 

rhyncophylla

1 F. mandshurica United States China seed 

collection

F. mandshurica 6 F. pennsylvanica, F. 

excelsior, F. nigra, 

and F. chinensis

United States China seed 

collection, US 

nursery, US 

university

F. sogdiana 1 F. pennsylvanica United States unknown

Table 3—Commercially obtained Fraxinus seed lots that were incorrectly identified at the 

species level

Distributed as: Number of incorrect 

seed lots

Correct species ID 

(ITS and/or AFLP)

Number of commercial 

sources

F. bungeana 3 F. pennsylvanica 3

F. chinensis 3 F. pennsylvanica 2

F. chinensis var. 

rhyncophylla 1 F. pennsylvanica 1

F. nigra 2 F. excelsior 2
GENERAL TECHNICAL REPORT PSW-GTR-240

238

Traditional Breeding

Initial EAB infestations were in urban areas where a few ash cultivars were planted in high numbers.

Although a large number of these trees were killed, they represented a small number of genotypes. As 

EAB moved into natural stands with greater genetic diversity, small numbers of surviving ash trees

have been identified that persist in areas where more than 98 percent of the ash trees have been killed 

by EAB (Knight et al., these proceedings; Marshall et al. 2010). These “lingering ash” may be simply 

the last trees to die, or they may be trees with rare phenotypes that are less preferred by EAB or are 

tolerant or resistant to EAB infestation.

We have been working to identify such “lingering ash” and preserve and replicate them through 

grafting. Currently, we have 33 individual grafted lingering ash tree genotypes grafted (primarily 

green and white). Two of these genotypes flowered and controlled cross-pollinations were performed 

resulting in two small seedling families. In addition, 2011 was a good seed year for ash and we were 

able to collect open-pollinated seed from nine lingering ash located in the southeast Michigan survey 

site and seven lingering ash trees in the northwest Ohio survey site described by Knight et al. (these 

proceedings). Because of the high degree of ash mortality in these areas, the pollen donors are 

assumed to also be lingering ash, so these open-pollinated seed are likely the resulting progeny of two 

lingering ash parents. These families will be analyzed for segregation of EAB tolerance/resistance 

phenotypes and will give an indication of the potential for enhancing native resistance through 

breeding.

Currently, our work focuses on developing a screen to distinguish EAB-resistant and EABsusceptible phenotypes in grafted lingering ash ramets and seedling families. Adult EABs are being 

reared at the East Lansing, Michigan, Northern Research Station laboratory. Adult EAB that emerge 

from naturally infested logs are placed along with ash foliage in plastic containers covered by a paper 

coffee filter. Groups of approximately 10 females and 4 males are placed in each container; adults 

mate and the females lay eggs on the coffee filters. Small sections of coffee filter with eggs attached 

are cut and then placed on each tree and wrapped with cheesecloth to prevent scavenging by predators 

(fig. 1A).

Figure 1—Development of emerald ash borer (EAB)-resistance screen. A. Coffee filters with EAB 

eggs are placed against the tree and covered with cheesecloth. B. Neonate larvae emerging from 

their eggs on the coffee filter. C. 3rd instar larva and gallery 6 weeks after egg hatch.

Approximately 8 to 10 weeks after the expected egg hatch date, the coffee filters will be removed 

and inspected to determine the number of eggs that successfully hatched. At each egg placement site, 

the bark will carefully be peeled back to dissect out larvae, recording the number of larvae, instar 

A B C
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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development, larval weight, and any evidence for host defense response such as callous formation.

Data will be analyzed to discern any differences between lingering ash and controls regarding 

successful larval infestation.

Lingering ash grafted ramets and seedling families will also be installed in replicated field trials to 

assess differences in response to natural EAB infestation. Interestingly, the nursery growing area at 

the Delaware, Ohio, USDA Forest Service Laboratory was plagued by a naturally occurring EAB 

infestation in the summer of 2010. One of the infested nursery beds contained grafted lingering ash 

selections that remained uninfested while known susceptible grafted selections immediately adjacent 

to the lingering ash were infested, an observation that supports the hypothesis that lingering ash are 

less preferred by EAB.

Literature Cited

Davidson, C.G. 1999. ‘Northern Treasure’ and ‘Northern Gem’ hybrid ash. HortScience. 34: 151–152.

Dian, J.J.; Ulyshen, M.D.; Bauer, L.S.; Gould, J.; Van Driesche, R. 2010. Measuring the impact of biotic 

factors on populations of immature emerald ash borers (Coleoptera: Buprestidae). Environmental 

Entomology. 39(5): 1513–1522. 

Food and Agriculture Organization of the United Nations [FAO UN]. 2011. Selection and breeding for 

insect and disease resistance. http://www.fao.org/forestry/26445/en/. (25 October 2011).

Hebard, F.V. 2006. The backcross breeding program of the American Chestnut Foundation. Journal of the 

American Chestnut Foundation. 19: 55–77.

Kovacs, K.F.; Haight, R.G.; McCullough, D.G.; Mercader R.J.; Siegert, N.W.; Liebhold, A.M. 2010. Cost 

of potential EAB damage in US communities, 2009-1019. Ecological Economics. 69: 569–578.

Liu, H.; Bauer, L.S.; Miller, D.L.; Zhao, T.; Gao, R.; Song, L.; Lusan, Q.; Jin, R.; Gao, C. 2007. Seasonal 

abundance of Agrilus planipennis (Coleoptera:Buprestidae) and its natural enemies Oobius agrili 

(Hymentoptera: Encyritidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae) in China. Biological 

Control. 42: 61–71.

Marshall, J.M.; Storer, A.J.; Mech, R.; Katovick, S.A. 2010. Prospects for long-term ash survival in the core 

emerald ash borer mortality zone. In: McManus, K.A.; Gottschalk, K.W., eds. Proceedings. 21st U.S. 

Department of Agriculture interagency symposium on invasive species. Gen. Tech. Rep. NRS-P-75. 

Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 99. 

Rebek, E.J.; Herms, D.A.; Smitley, D.R. 2008. Interspecific variation in resistance to emerald ash borer 

(Coleoptera: Buprestidae) among North American and Asian ash (Fraxinus spp.). Plant-Insect Interactions. 

37: 242–246.

Sydnor, T.D.; Bumgardner, M.; Todd, A. 2007. The potential economic impacts of emerald ash borer 

(Agrilus planipennis) on Ohio, U.S., communities. Arboriculture & Urban Forestry. 33(1): 48–54.

Wallander, E. 2008. Systematics of Fraxinus (Oleaceae) and evolution of dioecy. Plant Systematics and 

Evolution. 273: 25–49.
GENERAL TECHNICAL REPORT PSW-GTR-240

240

Breeding for Disease Resistance in Hevea spp. -

Status, Potential Threats, and Possible Strategies

Chaendaekattu Narayanan1 and Kavitha K. Mydin1

Abstract

Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg., a forest tree native to the tropical rain forests of Central and 

South America, has only been recently domesticated outside its natural range of distribution. Almost all of the 

commercially cultivated clones of H. brasiliensis represent a very narrow genetic base since they originated 

through hybridization or selection from a few seedlings of so called Wickham germplasm. Hence, the 

commercial rubber cultivation, due to their genetic vulnerability, is under a constant threat of attack by native as 

well as exotic diseases and insects. Climate change, which is clearly felt in the traditional rubber growing 

regions of India, may possibly alter the host-pathogen interactions leading to epidemics of otherwise minor 

diseases. 

Pathogenic fungal diseases including Phytophthora-caused abnormal leaf fall (ALF) and shoot rot, pink disease

caused by Corticium salmonicolor, Corynespora-caused leaf disease, and powdery-mildew (Oidium sp.) are 

challenging diseases posing epidemic threats to rubber cultivation. South American leaf blight (SALB) is a 

devastating disease caused by Microcyclus ulei (=Dothidella ulei) which has prevented large-scale planting of 

rubber in Brazil due to epidemic outbreaks. The SALB is a looming threat to other rubber growing areas. 

Hence, it is essential that a global SALB resistance breeding program be implemented to tackle such future 

threats of epidemics. Hevea clones clearly exhibit variable levels of susceptibility to pathogenic diseases. Hevea 

clones have been tested for their capacity to produce phytoalexins; a strong correlation was observed between 

phytoalexin accumulation and clone resistance. More lignin accumulation was also often associated with clone 

resistance. Attempts have been made to identify possible disease resistance gene analogues in rubber. The role 

of M13-1bn marker (a putative quantitative trait locus) in screening for resistance to SALB had been 

investigated through genome mapping, but needs further validation. Earlier selection and breeding of Hevea 

clones resistant to M. ulei and Phytophthora sp. in Brazil led to screening of resistant clones. Most of the 

resistant material had been derived from H. benthamiana “F4542.” Few other attempts for inter-specific 

hybridization have been made, particularly for SALB resistance (H. camargoana x FX 4098), but they did not 

follow large-scale evaluations for field resistance. 

Many man hours of labor and enormous quantities of fungicidal chemicals are required every year for 

management of above diseases in vast areas of rubber plantations in India and other rubber growing countries. 

The cost of fungicides and their long-term effect on environment justify the need for breeding disease resistant 

trees. There are several theories for genetic basis for disease resistance (horizontal/vertical) in Hevea. 

Nevertheless, there is every possibility for breakdown of resistance due to ever-evolving pathogenic races 

coupled with climate change, which is exemplified by evolving SALB races. A multidisciplinary breeding 

program for development of disease resistant clones would have to continuously utilize Wickham resource as 

well as wild germplasm, in addition to other Hevea spp., in order to have sustainable rubber production.

Key words: Hevea brasiliensis, fungal diseases, South American leaf blight, Microcyclus ulei,

disease resistance breeding

Introduction

Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg. (family, Euphorbiaceae; diploid, 2n=36), the Para 

rubber tree, is a forest species native to the tropical rain forests of Central and South America. 

Though the tree has only been recently domesticated outside its natural range of distribution, it has 

great commercial and socio-economic significance in many countries (Narayanan and Mydin 2011). 

 1 Rubber Research Institute of India, Kottayam 686 009 Kerala, India.

Corresponding author: cnarayanan@rubberboard.org.in.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

241

Hevea brasiliensis is monoecious, entomophilic, and predominantly out-crossing, and continues to 

be the major source of natural rubber in the entire plant kingdom. Latex (essentially cytoplasmic 

fluid) is collected from laticifer cells located in the bark tissue through systematic manual incision 

(the entire process referred to as ‘tapping’) for various uses. Most of the cultivated clones of Hevea

have been derived either through selection or breeding among the few selected seedlings (or their 

progenies) which were reportedly collected from a small geographic location in Boim, near the 

Tapajos river in Brazil (Allen 1984, Schultes 1977, Wycherley 1968). Commercially cultivated 

clones of H. brasiliensis represent a very narrow genetic base possibly originating from few selected 

seedlings collected by Henry Wickham in 1876 (Baulkwill 1989).

Rubber cultivation is under a constant threat of attack by native as well as exotic pathogenic 

fungal diseases due to genetic vulnerability of the Hevea clones. Leaves, stems, and roots of Hevea 

are susceptible to fungal pathogens. Leaf diseases are caused by Oidium heveae, Colletotrichum spp., 

Phytophthora spp., Corynespora cassiicola, and Microcyclus ulei. The above pathogens cause 

abnormal leaf fall or leaf spot of young as well as mature leaves of Hevea. Among stem infections,

pink disease, caused by Corticium salmonicolor, is the most important, capable of infecting young as 

well as mature trees. Dry rot caused by Ustulina deusta, patch canker caused by Phytophthora 

palmivora, and black stripe caused by P. palmivora, P. meadii, or P. botryose, are other important 

diseases affecting the stem. White root rot caused by Rigidiporus lignosus, brown rot caused by 

Phellinus noxius, and red rot caused by Ganoderma philippii are notable diseases of roots. Among 

the above diseases, South American leaf blight (SALB), caused by M. ulei (=Dothidella ulei), is the 

most devastating. This disease caused several serious epidemics, almost leading to cessation of 

planting of Hevea in Brazil.

The change in weather parameters due to the increasing trend in climate change, which is clearly 

felt in the traditional rubber growing regions of India, may possibly alter the host-pathogen 

interactions. This will possibly lead to epidemic outbreaks of otherwise minor diseases. Besides, 

there is every possibility that hitherto unreported exotic pathogens may be favored by the altered 

weather parameters. Emergence of leaf disease caused by C. cassiicola as a major pathogen is a 

classic example. This pathogen is rapidly progressing into new areas, thus highlighting the need for a 

stronger and advanced resistance breeding approach.

Clonal Variation and Breeding for Disease Resistance

Hevea breeding primarily aims at developing clones with potential to produce more latex. 

Introduction, ortet (plus tree) selection, and hybridization followed by clonal selection, are the major 

methods of crop improvement in rubber. During various stages of evaluation, observations are made 

on various pest and disease incidences (Mydin and Saraswathyamma 2005). Better growth vigor, 

smooth and thick bark, good bark renewal after tapping, and tolerance to major diseases are 

considered as good secondary characteristics (Varghese 1992, Varghese and Mydin 2000). 

In Hevea, the breeding strategy follows the conventional method of cyclical ‘generation-wise 

assortative mating’ (GAM), where superior genotypes of one generation form parents for subsequent 

breeding programs (Simmonds 1989). Most of the cultivated clones have been developed through 

selection or hybridization from among few selected high-yielding popular clones. Since yield 

remained the primary trait for breeding, there had been limited selection for resistance genes and 

their transmission among the hybrid progenies, and hence, many of the Hevea clones exhibit variable 

levels of susceptibility to the pathogenic fungal diseases in the field. 

Although systematic breeding in Hevea started in early 1900s, studies on genetic parameters were 

initiated after 1970 (Tan and Tan 1996). Studies on genetic parameters, however, aimed at major 

economic traits like yield and vigor and there were very limited attempts to understand the genetic 

basis of disease resistance in Hevea. Subsequent to major disease epidemic outbreaks of SALB, 

efforts were made to understand the genetic basis of disease resistance in Hevea and specific 

resistance breeding programs were implemented to identify and breed resistant clones. 
GENERAL TECHNICAL REPORT PSW-GTR-240

242

Understanding the pattern of genetic inheritance of disease resistance is crucial for developing

successful resistance breeding programs. 

In a genetic analysis study, two populations viz. (i) 18 selected genotypes of various degrees of 

resistance, and (ii) 15 randomly selected progenies of a five-parent diallel cross, were screened for 

leaf disease resistance after artificial inoculation with Colletotrichum and natural infection of 

Corynespora and Oidium (Tan and Tan 1996). The results revealed that Colletotrichum and 

Corynespora resistance had higher heritability estimates than Oidium resistance. Hence, selection for 

genotypes with Colletotrichum and Corynespora would be more effective than the selection for 

Oidium-resistant genotypes. A considerable part of major genetic variation of Colletotrichum disease 

resistance was attributed to additive gene control; Corynespora and Oidium resistance was ascribed 

to non-additive gene control. There were high proportions of general combining ability (GCA) 

effects for the above three diseases. However, specific combining ability effect was significant only 

for Corynespora and Oidium. Earlier, degrees of resistance to Colletotrichum, Corynespora, and 

Oidium, and continuous form of variation, had indicated operation of polygenic inheritance of 

disease resistance in Hevea (Lim 1973, Tan et al. 1992, Wastie 1973). Since no adverse genetic 

associations have been found between disease resistance traits and latex yield or growth vigor, 

breeding for durable resistance contributed by polygenic inheritance has been recommended as the 

best resistance breeding strategy in Hevea (Ho 1986; Simmonds 1983, 1985, 1986, 1989, 1990; Tan 

1987).

Oidium-Caused Leaf Disease

Leaf fall resulting from Oidium infection is capable of causing extensive defoliation, particularly 

when trees refoliate after wintering, leading to serious retardation of growth and considerable loss in 

yield (Liyanage and Jacob 1992). Although one low-yielding clone from Sri Lanka, LCB 870, has 

been reported to possess resistance to the disease, most of the high-yielding clones are susceptible to 

the disease. While clones PB 86, GT 1, GL 1, PR 107, PB 5/139, RRIM 703, RRII 208, and PB 310 

show limited levels of resistance, other clones viz., Tjir 1, PB 5/51, and RRIM 605 are highly 

susceptible. RRII 105, the hybrid clone extensively cultivated for several years in India, PB 235, PB 

280, as well as RRII 118 and RRII 300 are also susceptible to the disease. Interestingly, RRII 105 is 

a hybrid between Tjir 1 and Gl 1 and the male parent has been reported to show some level of 

resistance. 

A study on Oidium-caused leaf fall sensitivity of 25 Hevea clones developed from India, 

Indonesia, China, Thailand, and Malaysia revealed that clones SCATC 93-114, RRIM 703, Haiken 

1, RRII 208, RRII 5, and PB 310 had comparatively stable tolerance towards Oidium-caused leaf fall 

disease. These clones have been suggested for use in breeding for disease resistance (John et al.

2001, Rajalakshmy et al. 1997). In another study carried out in a plantation with 20 clones, PB 86, 

RRIC 52, AC/S/12 42/186, PR 261, RO/CM/10 44/7, RRIM 703, AC/S/12 42/59, and IAN 45-873 

exhibited low levels of infection (John et al. 2000). High resistance in the wild genotypes viz., 

AC/S/12 42/59, AC/S/12 42/186, and RO/CM/10 44/7 has already been reported (Quiong 1993). Of 

the RRII 400 series clones developed by Rubber Research Institute of India (RRII), almost all 

recommended clones were affected by the disease. However, RRII 422 followed by RRII 414 

showed comparatively less infection (Varghese et al. 2009).

Based on disease intensity in the field, 3,561 wild germplasm accessions of Hevea were screened 

for tolerance to Oidium leaf infection (Mydin et al. 2011). The above study, which was carried out 

for 3 consecutive years, identified two potential accessions which had less than 10 percent disease 

incidence. Such tolerant accessions could be used in future resistance breeding programs. Similar 

field screening studies carried out in Tripura (north east state of India) could identify seven wild 

accessions viz., MT 4859, MT 5136, RO 3794, RO 5055, RO 5087, RO 5160, and RO 5365 tolerant 

to Oidium leaf infection. Similarly, in another north east state of Assam (India), field screening 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

243

revealed stable disease tolerance in three wild accessions viz., RO 1737, AC 587, and AC 5302 

(Mydin et al. 2011). 

Phytophthora Leaf Diseases

Although Hevea clones that evolved in various rubber growing regions show variable levels of 

susceptibility to abnormal leaf fall disease caused by Phytophthora spp., most of the high-yielding 

clones are susceptible. High yielding clones such as RRII 105, GT 1, Gl 1, PB 86, PB 217, PB 235, 

PB 260, PB 311, PB 28/59, RRIM 600, RRIM 628, RRIM 703, PR 255, PR 261, and Tjir 1 are 

reported to be affected by the disease. Some of the above clones showed higher leaf retention under 

prophylactic fungicidal spraying (Liyanage and Jacob 1992). Clones susceptible to abnormal leaf fall 

disease are also highly susceptible to shoot rot caused by Phytophthora spp. In India, among the 

RRII 400 series clones, RRII 414 and RRII 430 exhibited low incidences of abnormal leaf fall due to 

Phytophthora spp. 

Two trials consisting of 13 clones each were assessed for tolerance to the disease under 

prophylactic fungicidal spray (Mushrif et al. 2004). High leaf retention was observed in RRII 105 

and RRII 5. Clones RRIM 600, PB 260, and PB 280 along with RRIM 703 showed low leaf 

retention. It may be noted that RRIM 703 had been identified as a moderately tolerant clone in 

Malaysia (RRIM 1975). Interestingly, another clone, PR 255, showed significantly more leaf 

retention although its parents, namely Tjir 1 and PR 107, are susceptible. Since stomatal entry of the 

pathogen had been observed, attempts were made to identify anatomical factors, particularly petiolar 

stomata, influencing disease development in various clones (Premakumari et al. 1979, Premakumari 

and Panikkar 1984, Thankamma et al. 1975). Subsequent studies using various clones also indicated 

the frequency of petiolar stomata and the aperture index as potential criteria for selection of diseaseresistant clones (Premakumari et al. 1988). The above study showed that 68 percent variation in leaf 

retention after the incidence of Phytophthora-caused leaf fall disease could be explained by the 

characteristics of petiolar stomata.

About 2,691 accessions of wild germplasm of Hevea, originally collected from provenances viz., 

Mato Grasso, Acre and Rondonia were assessed for field level abnormal leaf fall disease (Mydin et 

al. 2011). Based on percent leaf retention after infection, 257 accessions were found to retain more 

than 75 percent of leaves, while more than 1,900 accessions shed leaves indicating varying levels of 

disease resistance operating among the wild germplasm accessions. Detached leaves of more than 

100 wild accessions were subjected to artificial laboratory inoculation using zoospore suspensions. 

Based on lesion size as an indicator of resistance, 18 accessions were rated as tolerant compared to 

check clone RRII 105. The tolerant accessions include one (RO 4423) from Rondonia, three (AC 

2016, AC 3146, and AC 462) from Acre, and 14 (MT 1617, MT 4494, MT 1631, MT 4436, MT 

1581, MT 1715, MT 2219, MT 2233, MT 3707, MT 4252, MT 1027, MT 900, MT 4702, and MT 

4874) from Mato Grosso (Mydin et al. 2011).

Corynespora-Caused Leaf Disease

Leaf disease caused by Corynespora infection was first reported from India in seedling nurseries

(Ramkrishnan and Pillay 1961). Subsequently, the disease has been reported from Malaysia, Nigeria, 

Indonesia, Sri Lanka, and Thailand. The disease has now spread to almost all rubber growing regions 

(Chee 1990). In Thailand, several cultivated clones were screened for disease resistance in the 

laboratory as well as the field using 24 isolates of C. cassiicola (Rodesuchit and Kajornchaiyakul

1996). For laboratory bioassay 10- to 12-day- old juvenile leaflets were used and for field assessment 

1-year-old budwood were inoculated with virulent isolates. Based on the laboratory bioassay and 

field inoculations, the study grouped clones into three classes viz., highly susceptible to susceptible, 

moderate, and resistant to highly resistant. While clones GT 1, RRIM 600, and KRS 225 were rated 

susceptible to highly susceptible in laboratory and field assays, clone KRS 226 which was rated as 
GENERAL TECHNICAL REPORT PSW-GTR-240

244

resistant to highly resistant in the laboratory assay was found to be highly susceptible after field 

inoculation. Similarly, clone PR 261 which was rated as highly susceptible to susceptible after 

laboratory inoculations later proved to be resistant to highly resistant based on field inoculations. In 

Malaysia, high variability for resistance to C. cassiicola races have been reported in clones (Hashim

2011). Clones PB 260, PB 350, PB 359, RRIM 929, and RRIM 2025 were highly susceptible to race 

2, but resistant to race 1. In contrary, clones RRIM 2009 and RRIM 2026 were resistant to race 2, but 

highly susceptible to race 1. It was also found that clone PB 355 was resistant to both the races. Also, 

clones RRIM 600, RRIM 928, and RRIM 2001 were highly susceptible to both the races. The above 

variability for resistance indicated possible operation of horizontal as well as vertical resistance in 

Hevea. While few other clones including RRIM 2024 and RRIM 2025 have been reported to be 

mildly infected by the pathogen (Murnita 2011), the disease reaction needs further long-term 

evaluation in multiple locations. These studies once again reiterate the need for a careful approach in 

breeding for disease resistance to Corynespora leaf infection in Hevea where laboratory assays 

should always be correlated with field-level resistance. In addition, multilocational field trials under 

different eco-climatic conditions are also needed before finally rating a clone as resistant or 

susceptible. 

In Malaysia, more than 90 clones and 47 wild germplasm accessions were screened for resistance 

under laboratory and field conditions (Chee 1988). This and other studies (Othman et al. 1996, Tan 

et al., 1992) showed that clones which were earlier rated as resistant subsequently became 

susceptible, possibly due to more virulent pathotypes of the pathogen. Specifically, clone RRIM 600 

which was relatively resistant to the disease eventually showed severe infection. Othman et al.

(1996) hypothesized allelic heterozygosity for Corynespora leaf disease in Hevea, since controlled 

crosses between susceptible parents produced moderately or less susceptible progenies. Also, they 

observed that RRIM 600 (Tjir 1 x PB 86) was severely infected; however, both the parental clones of 

RRIM 600 were almost free from fungal infection. 

In India, almost all clones were reported to be affected by the disease (Jacob 1997, Mathew

2006). While clone RRIC 103 has been reported to be highly susceptible, GT 1 is fairly tolerant. Sri 

Lankan clones viz., RRIC 104, RRIC 110, and RRIC 133 have been reported to be susceptible 

(Jayasinghe and Silva 1996). The disease has already been reported in many Malaysian clones 

including RRIM 600, PB 5/51, PB 235, and the Indonesian clone Tjir 1 (Tan 1990). AVROS 2037 

(Indonesia), BPM 24 (Indonesia), and RRIC 100 (Sri Lanka) have been reported to show tolerance to 

the disease under Indonesian conditions (Azwer et al. 1993). In India, most of the high-yielding 

clones, including RRII 105, RRIM 600, PB 260, and PR 107 are susceptible (Jacob 1997). Among 

RRII 400 series clones developed by RRII, RRII 414, and RRII 430 have been found comparatively 

less infected by the pathogen under the assistance of prophylactic fungicidal spray (Varghese et al.

2009). In order to screen tolerant genotypes, wild germplasm accessions were artificially inoculated 

with spore suspensions and monitored for lesion development compared to GT 1 and RRII 105 

(Mydin et al. 2011) and tolerant accessions were shortlisted for further evaluation. 

Molecular techniques have been used to study genetic variability in Corynespora affecting Hevea

(Atan and Hamid 2003, Darmano et al. 1996, Philip et al. 2004, Saha et al. 2000, Silva et al. 1998, 

Romruensukharom et al. 2005) using RAPD, rDNA-RFLP, etc. In general, studies revealed 

considerable levels of genetic variability among the isolates, which indicates the need for a 

molecular marker-assisted approach in breeding Hevea for resistance to Corynespora. Attempts have 

been made to develop Hevea clones resistant to the pathogen through genetic transformation 

techniques (Sunderasan et al. 2011) using anti-cassicolin-specific scFv gene. 

Colletotrichum-Caused Leaf Disease

Studies have shown that cultivated clones viz. PB 86, RRII 5, RRII 105, RRII 118, RRII 208, and 

RRII 300 are susceptible to the disease. Clones PB 217, PB 260, and RRIM 600 have been shown to 

have comparatively more tolerance. Preliminary studies on intensity and severity of Colletotrichum-
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

245

caused leaf disease in wild germplasm accessions helped in identification of accessions with varying 

levels of disease reaction (Mydin et al. 2011). While 161 accessions were almost disease free, more 

than 700 accessions showed severe infection in the form of spots and leaf fall. 

South American Leaf Blight

South American leaf blight (SALB), caused by M. ulei, is the most devastating disease of Hevea so 

far reported from the entire rubber growing region, capable of causing up to 90 percent loss in yield 

of rubber. The disease occurred in epidemic proportions in Brazil and adjacent regions leading to 

almost complete failure of rubber cultivation. The leaf blight is an obligate pathogen reported only in 

Hevea spp. and it occurs only in tropical America (Chee and Holliday 1986, Lieberei 2007). The 

pathogen has been recorded from four species of Hevea viz., H. brasiliensis, H. benthamiana, H. 

guianensis, and H. spruceana. Breeding for resistance to SALB is the only long-term strategy for 

disease management. Crown budding, a process where a resistant clone like FX 516 is budded on 

otherwise susceptible clone, has also been suggested as an interim strategy for management of the 

disease. Clones of Hevea have been screened for susceptibility to the SALB pathogen (Chee 1976). 

Several other trials are also underway in order to assess resistance of clones to SALB (Omokhafe

2011).

Hevea clones were tested for their capacity to produce a phytoalexin named scopoletin and to 

produce lignins in their infection sites (Garcia et al. 1999). A strong correlation was observed 

between scopoletin accumulation and clone resistance. Moreover, strong lignin accumulation was 

often associated with a longer stromatic generation period. These two physiological reactions could 

interfere by limiting fungal development in several clones. However, neither scopoletin nor lignin 

accumulation could individually explain the behavior of all clones (Garcia et al. 1999). Studies 

carried out in several accessions of wild germplasm using microsatellite markers indicated 

correlation between genetic resistance and geographical distribution (Guen 2011).

The earliest breeding program concentrated on breeding resistant genotypes using surviving trees 

identified from severely diseased plantations at Belterra and Fordlandia in Brazil (ANRPC 1995). 

The strategy involved crossing resistant genotypes like F 170, F 315, F 351, F 1425, and F 4542 with 

high-yielding clones viz., AVROS 49, AVROS 193, AVROS 363, PB 86, PB 186, and Tjir 1 

imported from Indonesia and Malaysia. Later, resistance breeding was continued in Brazil by 

crossing clones, including RRIM 600 and RRIM 501 (introduced from Malaysia, Sri Lanka, and 

Indonesia) with the primary Ford clones and also progenies of the Ford crosses. Although there have 

been several other reports about breeding for SALB resistance, none of the reported clones have 

been found resistant to the pathogen, possibly due to rapidly evolving pathogenic races. Another 

collaborative breeding program carried led to development and selection of 13 CMS clones with 

promising horizontal resistance to SALB (Mattos 2011). The resistant clones viz., CD 1174, CDC 

56, CDC 312, MDX 607, MDX 624, PMB 1, FDR 4575, FDR 5240, FDR 5283, FDR 5597, FDR 

5665, FRD 5788, and FDR 5802 were tested in multi-regional trials in Brazil, Ecuador, Colombia, 

Guatemala, and Peru. The above clones were also found to possess putative resistance to selected 

isolates of C. cassiicola (Murnita et al. 2011). However, further studies are required to assess the C. 

cassiicola resistance of the above clones through large-scale field testing. 

The mechanism of resistance of Hevea to M. ulei has not yet been fully understood, but two 

possible types of resistance viz., vertical (race specific) and horizontal (race non-specific), have been 

proposed (Hashim and Pereira 1989a, 1989b) based on field observations and laboratory assays. At 

least 11 physiological races of the pathogen have been reported so far. Hitherto, resistant clones were 

susceptible to new races of the pathogen. Among the species of Hevea, H. benthamiana clone F 4542 

was found resistant to Race 1 (wild race) of M. ulei. However, most of the progenies of F 4542 were 

susceptible to Race 2 and Race 3. 

Vertical resistance has been attributed to host cell death around the site of infection in clones 

which are resistant to some races of the pathogen. Breeding for vertical resistance led to clones 
GENERAL TECHNICAL REPORT PSW-GTR-240

246

which were resistant only to few races. Clones with vertical resistance subsequently succumbed to 

new races of the pathogen. Horizontal resistance has been suggested as more durable since it confers 

resistance to almost all races of the pathogen. Clones with horizontal resistance allowed fungal 

penetration, but prolonged the rate of spread, reduced the size of lesion, and minimized spore 

production. In order to simplify the field assessment method for SALB disease, Rivano et al. (2010) 

tested the resistance of eight rubber tree clones to M. ulei in Ecuador in a Fisher block design with 

four replicates per treatment and concluded that assessing the resistance of rubber tree clones to 

SALB in large-scale clone trials can be optimized to reduce the number of observation times by 50 

percent. 

In the lines of conventional breeding for disease resistance, intra- and inter-specific 

hybridizations were attempted for developing SALB resistance. While the initial intra-specific 

breeding utilized clones of H. brasiliensis, subsequent inter-specific hybridization involved another 

species, H. benthamiana (clone F 4542). Further search for resistance in other species led to the 

identification and use of H. pauciflora (clone P 10) since this species was free from SALB infection. 

Thus, H. pauciflora was hybridized with H. brasiliensis and H. benthamiana (Pinheiro and Libonati

1971). Subsequently, controlled crosses were also made between H. camargoana and H. brasiliensis

(H. camargoana x Fx 4098); Fx 4098 is a hybrid of PB 86 x FB 74, both primary clones of H. 

brasiliensis (developed in Malaysia and Brazil, respectively) and the resultant hybrid progenies were 

selected for resistance to SALB (Goncalves et al. 1982). However, the resistant hybrids were 

apparently not high-yielding. As indicated in several studies using wild genotypes as well as 

cultivated clones, durable resistance is possibly present in wild accessions of H. brasiliensis as well 

as other allied species (Priyadarshan and Goncalves 2003).

Stem Diseases

Pink disease, caused by C. salmonicolor, mainly affects young trees, but of late, trees up to 7-yearsold are also affected. Repeated infection of young branches lead to retardation of growth, thereby 

extending the period before the trees can be utilized for extraction of rubber through tapping. Clones 

PB 217, PB 311, and RRII 105 are highly susceptible to the disease. Among the other cultivated 

clones, Tjir 1, LCB 1320, RRIM 501, RRIM 701, etc., were also found affected. Another study 

indicated less susceptibility to the disease in clones viz., PB 86, RRIM 513, Gl 1, PR 107, GT 1, and 

PB 260 (Ramakrishnan and Pillay 1962, RRIM 1992). Among the RRII 400 series clones, RRII 430 

is least affected while RRII 429 is highly susceptible (Varghese et al. 2009).

With regard to other minor stem diseases, clones RRIM 600, PB 235, PB 311, and PB 28/59 are 

severely affected by black stripe due to Phytophthora spp.; PB 217 is moderately affected. With 

reference to patch canker caused by Phytophthora spp., clone PB 260 is highly susceptible 

(Kothandaraman and Idicula 2000).

Disease Resistance Genes, Molecular Mapping, and MarkerAssisted Selection

Attempts have been made to identify markers linked to disease resistance in Hevea. Chen et al.

(1994, 2003) used RAPD markers to identify Oidium leaf fall resistance gene and subsequently 

sequence one RAPD marker (OPV-10390) possibly linked to gene conferring resistance to 

Phytophthora. Studies have been carried out to identify possible disease resistance gene analogues in 

rubber (Licy et al. 2000). Eighteen primers, designed based on homologies between known 

resistance genes, were used in various combinations to amplify sequences from rubber cultivar FX 

516, which is resistant to Phytophthora-caused leaf fall disease and cultivar RRII 105, which is 

tolerant. Although none of the clones obtained had high homology to resistance gene sequences, the 

putative protein encoded by one sequence had some homology to hem N gene. Studies are being 

carried out at RRII on resistant gene analogues (RGAs) and their relationship with functional RGAs 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

247

in response to Corynespora infection (Saha et al. 2010). Using degenerated primers based on 

conserved motifs of NBS domains of known R-genes, a PCR-based approach was followed. RGAs 

of NBS-LRR class were cloned from H. brasiliensis and H. benthamiana and 22 transcriptionally 

active diverged RT-RGAs were identified. Characterization of these RGAs is required for better 

understanding plant reaction to Corynespora infection. 

Using a population of 192 progeny individuals derived from a cross between a resistant clone 

(RO 38; original name is FX3899, a low yielding inter-specific hybrid between H. brasiliensis and 

H. benthamiana) and a susceptible cultivated clone (PB 260), Guen et al. (2003) identified a major 

QTL named as M13-1bn, located on linkage group g13, that is responsible for 36 to 89 percent of the 

phenotypic variance of resistance. The role of M13-1bn in screening clones for resistance to SALB 

needs validation through large-scale multilocational field testing.

Discussion and Conclusion

Field evaluation through visual observation and laboratory assays through excised leaf inoculation 

have led to screening of putatively resistant Wickham clones and wild germplasm. However, 

laboratory bioassays are preliminary and may not ensure actual field-level resistance as observed for 

diseases of other forest trees. Hevea clones have also been tested for their capacity to produce 

phytoalexins; a strong correlation was observed between phytoalexin accumulation and clone 

resistance. More lignin accumulation was also often associated with clone resistance. With regard to 

wild germplasm, earlier studies indicated considerable variation in growth and disease resistance 

among and within genotypes collected from various provenances of Brazil. 

Many man hours of labor and enormous quantities of fungicidal chemicals are required every 

year for management of the above diseases in vast areas of rubber plantations in India and other 

rubber growing countries. The cost of fungicides and their long-term effect on the environment 

justify the need for breeding disease-resistant trees. There are several theories for genetic basis for 

disease resistance (horizontal/vertical) in Hevea. Nevertheless, there is every possibility for 

breakdown of resistance due to ever-evolving pathogenic races coupled with climate change, which 

is exemplified by evolving SALB races. Multidisciplinary breeding programs for development of 

disease-resistant clones would have to continuously utilize the Wickham resource, as well as wild 

germplasm, in addition to other Hevea spp. in order to have sustainable rubber production. The 

present germplasm resource of Hevea is predominantly constituted by the domesticated clones and 

more than 4,000 wild germplasm accessions collected from Acre, Rondonia, and Mato Grosso. 

Disease resistance in Hevea, like other forest trees, is apparently polygenic, as indicated by 

varying levels of resistance in domesticated and wild germplasm. Elaborate studies are needed to 

understand the molecular basis of resistance and mechanisms of inheritance of disease resistance in 

Hevea. Molecular markers, wherever feasible, should be integrated into breeding programs to 

accelerate selections. Once the germplasm accessions are characterized for disease resistance, 

association mapping strategies could be employed for identifying markers linked to resistance which 

would ultimately help in molecular-assisted selection/breeding (MAS/MAB) in Hevea.

Acknowledgments

The first author expresses his sincere thanks to: Chairman, Rubber Board, Government of India and Director, 

Rubber Research Institute of India, India, for granting sponsorship; Department of Science and Technology, 

Government of India, for the travel grant; Richard Sniezko, Geneticist and Workshop Chair, Dorena Genetic 

Resource Center, Oregon; Katie Palmieri and Janice Alexander, California Oak Mortality Task Force, 

University of California; and many others at the workshop for all their help. 
GENERAL TECHNICAL REPORT PSW-GTR-240

248

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Developing a Disease Resistance Research 

Program for Tanoaks

Katherine J. Hayden,1 Jessica W. Wright,2 Richard S. Dodd,1 and 

Matteo Garbelotto1

Abstract

The introduced pathogen Phtyophthora ramorum has had a devastating impact on populations of tanoak 

(Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H.Oh) in California and Oregon, where it 

causes the disease known as sudden oak death. Tanoaks are a keystone species ecologically, and one of the few 

species that are both killed by P. ramorum and contributes to its spread. While ecologically important, tanoaks 

were little studied prior to the onset of the sudden oak death epidemic, resulting in few intellectual or material 

resources on which to base a disease resistance research program.

We report on progress to date on a project funded since 2006 by the U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station, with the goal to collect the information required to understand 

what role resistance might play in the disease dynamics and/or management of tanoak populations. A common 

garden population of open-pollinated tanoak seed families has been established. These trees have been 

evaluated for variation in neutral genetic markers, growth and morphology, and quantitative resistance to 

laboratory inoculations. Simultaneously, a field disease trial, using the same families of trees, was initiated to 

validate and expand on laboratory observations, under natural disease pressures. 

The work thus far has resulted in a significant expansion of the understanding of tanoak phylogeography and 

population genetics, the discovery of heritable quantitative resistance to disease in the lab, and potential links 

between morphology and disease resistance in the lab and field. Here, we tie together the investigations, 

presenting 3 years of data from laboratory and field resistance trials, along with their correlations with growth 

traits, in light of the understanding of tanoak population dynamics generated from neutral markers. We show the 

relevance of laboratory studies to field outcomes, and discuss the potential for disease resistance in driving the 

management or evolution of the species in response to this considerable threat.

 1 University of California, Berkeley, Environmental Science, Policy, and Management, Berkeley, CA 94702.

2 USDA Forest Service, Pacific Southwest Research Station, Davis, CA.

Corresponding author: khayden@berkeley.edu.
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Genetic Variation in Resistance to Pine Pitch 

Canker and Western Gall Rust in Monterey Pine 

(Pinus radiata D. Don): Results From a ThreeCountry Collaborative Field Trial

A.C. Matheson,

1 W.R. Mark,2 G. Stovold,3 C. Balocchi,4 N Smith,2 and 

C. Brassey2

In 1998, Australia, Chile, and New Zealand agreed to work together in a program designed to test 

their elite breeding lines and to test for the genetics of resistance to pitch canker (causative 

organism Fusarium circinatum). Pitch canker was first discovered in the United States in 1946 

and in California in 1986. The first discoveries in California were in ornamental plantings, and the 

disease was first found in native stands of Monterey pine (Pinus radiata D. Don) in 1992. By 

1997, pitch canker was found in 22 counties in California, and the Board of Forestry established a 

“Zone of Infestation.” Early estimates of resistance levels were as low as 3 percent. Pitch canker 

is also found in Spain, South Africa, Chile, Haiti, Mexico, Portugal, and Chile. The pathogen has 

not been found in Australia or New Zealand, and its introduction could result in an economic 

disaster to the respective softwood forest industries. Due to the widespread use of Monterey pine 

around the world (table 1), there was interest in assessing the risk of forest plantations outside the 

United States for pitch canker.

Table 1—Estimated land area used for Monterey pine plantations worldwide in 1999 

(adapted from Balocchi, et al. 1999) 

Country Area ( 000 ha) Production (1000m3/yr)

Chile 1 380 18 548

Australia 1 338 17 000

New Zealand 642 10 400

Spain 237 2 000

South Africa 66 486

Other 100 NA

Totals 3 763 48 434

The first step was a conference (called IMPACT Monterey) held at Monterey, California to 

bring together present knowledge of the disease, its epidemiology, known vectors, as well as host 

responses. Proceedings were published by the Australia Commonwealth Scientific and Industrial 

Research Organization (CSIRO) (Devey et al. 1999). The planned effort included collaboration in

California for greenhouse trials and field trials of greenhouse screened lines.

Phase 1 of the IMPACT Project, a greenhouse trial, was conducted at Pebble Beach, 

California, comparing the responses of more than 500 open- or control-pollinated families to 

inoculation by F. circinatum. Results using a t-test showed there were significant differences 

between families for the length of the lesion developed 6 weeks following inoculation (Matheson 

et al. 2006). Estimates of heritability, using the statistical software package ASReml, in the 

different populations are presented in table 2. There were also significant differences between 

male parents, but apparently not between female parents (table 3). The next step was to test the 

 1 CSIRO, Kingston, Australia. 2 California Polytechnic State University, San Luis Obispo, CA, USA. 3 Scion, Rotorua, New Zealand. 4 Bioforest, SA, Concepcion, Chile.

Corresponding author: wmark@calpoly.edu. 
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same families in the field to see if the family rankings would match those obtained in the 

greenhouse.

Table 2—Individual tree heritability in the analysis of lesion length (adapted from Matheson 

et al. 2006)

Population Heritability SE (Heritability) Residual variance

Chile CP 0.34 0.06 192

Año Nuevo 0.78 0.29 206

Monterey 0.46 0.26 169

Australian crosses 0.34 0.05 162

NZ OP 0.49 0.08 158

Table 3—Heritability estimates for lesion length separated by parent sex (adapted from 

Matheson et al. 2006)

Term

Variance 

component

Standard error 

of variance 

component

Heritability 

estimate

Standard error 

of heritability 

estimate

Female 1.9 3.7 0.04 0.08

Male 16.8 6 0.38 0.12

Combined

female X male

11.4 5.4 0.21 0.07

In 2005, Phase 2 of the IMPACT Project, a field trial was set up at Davenport, California at the 

Swanton Pacific Ranch, owned by California Polytechnic State University. A total of 264 seedlots 

were available to test disease scores in the same environment as a disease-affected natural stand 

of the species. Seedlots from Chile were selected from among those tested in Phase 1, so as to 

cover the range of response; those from New Zealand were selected from among the better 

performing families in Phase 1. New seedlots were included from Australia to include reciprocal 

pairs of controlled pollinations, but also included seedlots with a range of performance from 

Phase 1. The trial was designed to be removed before pollen from the trial would materially affect 

the gene pool of the native stand nearby. Environmental factors, including wet areas in the 

planting site and deer damage, were analyzed to eliminate those factors from the analysis. The 

analysis of the deer damage showed that the damage was associated with replicate (location in the 

planting site) and height of the trees at the time they were damaged. High mortality following the 

planting in 2005 resulted in the mapped wet areas.

The field trial was assessed annually for disease symptoms until the final assessment in 

February 2011, after which the trial was removed. Apparent quite early in the life of the trial were 

galls produced by the western gall rust (causative organism Endocronartium harknessii), acquired 

both at the Institute of Forest Genetics at Placerville, where many of the trial seedlings 

overwintered, as well as at Swanton, following planting. Other disease symptoms included those 

caused by Diplodia pinea, particularly in the 2009 assessment. Pitch canker was confirmed

present in the trial in 2009, and became more severe in 2010 and 2011.

Western gall rust findings were analyzed to look for differences among families (seedlots) in 

resistance. A mixed model, treating seedlots and replicates as fixed rows and columns (random),

was used to obtain an estimate of the statistical significance of the differences between families.

The differences between seedlots using a logarithmic transformation are highly statistically 

significant (P<0.001). The fact that the difference between families is so strong suggests that the 

heritability will be high. A proper genetic analysis is not done yet, so no estimate for the 

heritability is available at this time.

Diplodia blight outbreaks in Monterey pine in the United States have not been considered 

significant, but have caused significant damage in Monterey pine plantations in the Southern 

Hemisphere (Peterson 1981). The native population has extensive pycnidia presence, but no other 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

255

symptoms. The plantation suffered extensive tip mortality, resulting in stunting, excessive 

branching and forked tops. Little, if any, mortality was recorded due to Diplodia blight. In this 

experiment, symptomatic trees were found to have a significantly clustered distribution, which is 

consistent with rain splash being the primary mechanism of spore dispersal in D. pinea.

Preliminary analysis of the pitch canker data used only the blocking structures (replicates, 

rows, and columns) provided for in the original design and included the tree height as a covariate.

Replicates were treated as fixed effects, the rows and columns as random effects. A binomial 

model with a logistic link function rather than models using normal distribution suggests the 

heritability is 15.21 percent ± 9.44 percent (or 0.1521 ± 0.0944). A fitted individual-tree mixed 

model in ASReml was utilized to get the BLUP (Best Linear Unbiased Predicted) breeding values 

for the parents of the trees involved in the trial. The correlation with the greenhouse results is 

0.284, using the Chilean and New Zealand families only.

Some potential explanations of poor correlation with greenhouse results are as follows: 1)

Presence-absence data collected in the field versus measured values for lesion lengths in the 

greenhouse; 2) Artificial inoculation in the greenhouse versus natural infection from vectors or 

wounds in the field trial; 3) Escape of susceptible individuals in the field trial; and 4) Difficulty in 

accurate assessment of large trees in the field.

Work on a complete analysis of the heritability of resistance to western gall rust and pitch 

canker is continuing.

Acknowledgments

Funding for this study came from The Agricultural Research Initiative, California State University; CSIRO, 

Australia; Scion, New Zealand; and Bioforest SA, Chile. Thanks to Swanton Pacific Ranch, California 

Polytechnic Corporation for land use, equipment, and personnel.

Literature Cited

Balocchi, C.; Ahumada, R.; Ramirez, O. 1999. Present and future of radiata pine in Chile. In: Devey, M.;

Matheson, C.; Gordon, T.R., eds. Current and potential impacts of pitch canker in radiata pine. Proc. 

IMPACT Monterey Workshop. Tech. Rep. No 112. Canberra, Australia: CSIRO, Forestry and Forest 

Products: 1–4.

Devey, M.E.; Matheson, A.C.; Gordon, T.R., eds. 1999. Current and potential impacts of pitch canker in 

radiata pine. Proc. IMPACT Monterey Workshop. Tech. Rep. No 112. Canberra, Australia: CSIRO, 

Forestry and Forest Products. 120 p. 

Matheson, A.C.; Devey, M.E.; Gordon, T.R.; Balocchi, C.; Carson, M.J.; Werner, W. 2006. The 

genetics of response to inoculation by pine pitch canker (Fusarium circinatum Nirenberg and 

O'Donnell) infection by seedlings of radiata pine (Pinus radiata D. Don). Australian Forestry. 69(2): 

101–106.

Peterson, G. 1981. Diplodia blight of pines. Forest insect and disease leaflet 161. Washington, DC: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 8 p.
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Genetic Selection in Coastal Douglas-fir for 

Tolerance to Swiss Needle Cast Disease

Keith J.S. Jayawickrama,1 David Shaw,

2 and Terrance Z. Ye1

Introduction

Swiss needle cast (SNC) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), caused by the 

ascomycete fungus Phaeocryptopus gaeumannii, is associated with significant volume growth losses 

(20 to 50 percent) along the Oregon Coast. Although the pathogen is endemic, disease symptoms 

have intensified in coastal forests of Oregon and Washington since the early 1990s, reaching a peak 

of 177,691 symptomatic hectares detected in the 2011 aerial survey of western Oregon (fig. 1). This 

symptom expression is the highest in 16 years of survey, despite the conversion of tens of thousands 

of pure Douglas-fir plantations to mixed or alternate species over that period. Regular surveys have 

not been done in Washington State in the same way, but in 1999 and 2000, about 75,000 ha with 

visible symptoms were detected, and the disease continues to affect stands. 

Figure 1—Area of Douglas-fir forest in western Oregon with symptoms of Swiss needle cast detected 

during aerial surveys conducted from April-June, 1996-2011. (Figure from Alan Kanaskie and Mike 

McWilliams, Oregon Department of Forestry)

Coastal Douglas-fir is an enormously important timber species for Oregon; consequently, there 

has been considerable interest and investment in understanding the disease, quantifying its impact, 

finding silvicultural and management solutions, and developing seed sources suitable for this zone. 

The Swiss Needle Cast Cooperative (SNCC) based at Oregon State University (OSU) and several 

breeding and testing cooperatives (supported by the Northwest Tree Improvement Cooperative also 

based at OSU) are working toward these goals. 

 1 Department of Forest Ecosystems and Society, Oregon State University (OSU), Corvallis, Oregon.

2 Department of Forest Engineering, Resources, and Management, OSU.

Corresponding author: jayawickrama@oregonstate.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

257

Douglas-fir everywhere are naturally infected with P. gaeumannii, yet the disease only develops in 

certain geographic settings where pseudothecia are produced on 1- and 2-year-old needles. Needle 

retention (NR) is directly related to tree productivity. Trees with less than 2 years NR are significantly 

less productive than normal NR (3 years or more). Severely infected trees have <1 year foliage 

retention. Fungal infection and needle colonization occur passively, through needle stomata and the 

needle apoplast, respectively, and neither process requires physical or enzymatic penetration of host 

tissue that might trigger a host defense response. Climatic conditions near the Oregon coast are often 

conducive to SNC disease development. Mild winter temperatures and spring/summer leaf wetness 

are key. 

There are no known resistance mechanisms, as all foliage and all trees are susceptible given the 

right climatic conditions during spore dispersal and colonization. No significant differences were 

found between families in fungal DNA content (Temel 2002). Nonetheless, tolerance to the disease 

has been well documented, with some families continuing to grow well in the presence of SNC, and 

adequate family-mean narrow-sense heritabilities for foliage traits are present. 

Work in First Generation Programs

Several studies have centered on the Nehalem progeny trial series of 400 first-generation families 

established on 10 sites in the northern Oregon coast range in 1986 and subject to moderate to heavy 

SNC disease pressure. This series was measured for the fourth time in summer 2010, and age-26 

diameter growth (DBH) from 200 families on five sites were obtained. Data have also been collected 

from other first-generation programs near the coast. Some key conclusions from Nehalem and other 

first-generation programs are as follows:

(1) Foliage traits are heritable (individual h2 ≈ 0.10 to 0.23), but less heritable than growth traits 

(h2 ≈ 0.3) (table 1). This could be from foliage traits being subjectively scored.

(2) Crown density was seen as a better indicator of genetic tolerance than NR (Johnson 2002), 

but due to extensive use of NR by the SNCC it has been adopted in tree improvement 

programs as well. 

(3) Growth after age-11 was more strongly associated with age-11 tree size (type-B correlations 

≈ 0.7) than with foliage traits (type B correlations ≈ 0.2 to 0.5). However, rank changes have 

taken place for DBH between age-11 and age-26.

(4) Usable gains for DBH were seen even at age-26. The top 10 percent of parents for DBH-26 

had predicted gains of 18.2 percent over population mean, and predicted gains of 23.7 percent 

over population mean for DBH-18 to -26 increments.

(5) In areas with >2 years of foliage, gains from selection for DBH should be adequate to offset 

volume growth losses from SNC and keep Douglas-fir as a viable plantation species. 

(6) Selecting on an index of age-11 crown density + age-11 DBH (≈13.8 percent gain) was only 

slightly better than age-11 DBH alone (≈ 13.4 percent gain) in improving age-26 DBH. 

(7) Selecting for NR alone, foliage color or crown density at age-11 gave little to modest (≈ 1 

percent, 2.2 percent, and 5.7 percent) gains in age-26 DBH. 
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Table 1—Heritabilities obtained from the Nehalem first-generation Douglas-fir program

Trait Across sites

Individual Family mean

Height 11 0.30 0.81

Volume index 11 0.29 0.80

DBH 11 0.27 0.79

DBH 17/18 0.35 0.82

DBH 26 0.36 0.70

DBH increment (age-11 to 17/18) 0.32 0.80

Needle retention: 1993 secondary laterals 0.23 0.77

Crown density 0.18 0.71

Foliage color 0.11 0.59

Second-Cycle Trials

One second-cycle program has been established in Washington (Washington coast), and four on the 

Oregon coast: 

(1) South Central Coast (SCC) in the south, with the mainline in the core Douglas-fir 

area, and a smaller satellite program in the zone most affected by SNC. 

(2) Plum Creek’s CL98 program adjacent to South Central Coast. 

(3) Trask in the north, with a coast program (moderately affected by SNC), and an inland 

program (little affected by SNC).

(4) Plum Creek’s Toledo program in the same geographic area as Trask coast.

These five programs have 40 progeny test sites planted between 1997 and 2008. Key points about 

the second-cycle work to date:

(1) Needle retention is being assessed at age-7 and age-12 and DBH is being assessed at 

age-7 and age-12 so that an age-7 to age-12 DBH increment can be calculated. There 

may be additional (later) DBH assessments to confirm continued growth in DBH. 

(2) Ability to maintain height and diameter growth, and good needle retention scores,

will be the primary selection criteria within these trials. 

(3) Relative heritability patterns for growth traits vs. NR are similar to the firstgeneration (NR is less heritable than growth). 

(4) No molecular marker work has been done to date or is currently planned. It is

speculated that the ability to grow in the presence of SNC is controlled by many 

small-effect genes. 

(5) A cloned breeding population might provide additional gains compared to a seedling 

breeding population, but due to the difficulty of rooting Douglas-fir, it would be 

expensive.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

259

(6) No early (e.g., age-2) testing has been implemented. Research done by Temel et al. 

(2005) indicated that such testing would provide gains in foliage traits, but little in 

the way of growth gains even at age-10.

(7) Tested 2nd-cycle families have shown large realized gains in growth over 

unimproved Douglas-Fir in the SNC zone. In SCC, the tested families had 39.9

percent age-7 volume index (DBH2 x height) gains over unimproved controls (on 

SNC sites) and 2.8 percent gains for NR. For Trask, the corresponding numbers for 

the top 50 families were 48.2 percent and 5.7 percent.

Seed Orchards and Deployment

Orchard blocks (such as Georgia-Pacific’s Toledo block, the Forest Service’s Hebo block, and the 

Nehalem block at the Schroeder seed orchard complex near St Paul, Oregon) serving the northern

Oregon coast were established between the 1970s and 1990s. The Oregon Department of Forestry 

(ODF) had a strong commitment to growing a proportion of Douglas-fir on the coast, and a large area 

under timber management. The ODF saw tree improvement as the main viable tool to counteract SNC 

and continue growing Douglas-fir, and took the lead in developing a “needlecast-tolerant” orchard 

block around 2002 built around the Nehalem orchard block. 

This orchard incorporated elite selections from the Nehalem and other coastal breeding units on 

the Oregon coast. The orchard is 2.4 ha in extent, and other cooperators have subsequently joined. 

Other blocks are being established, and ability to grow in the presence of SNC will remain a priority 

as selections are added from the 2nd-cycle and beyond. Tree-form, wind-pollinated orchards are 

likely to be the predominant engine for producing tolerant seed. Mixtures of family lots are used. The 

goal is to develop seed sources capable of acceptable growth rates on sites averaging ≥2 years of 

needle retention. 

Alternate Species

In areas with higher disease pressure and foliage retention ≤2 years, it may be prudent to use alternate 

timber species such as western hemlock, western red cedar, red alder, western white pine, and Sitka 

spruce. Many forest owners have been making this switch during the past 10 to15 years. Either 

through complete replacement as described above or through mixed species plantings with a 

component of Douglas-fir (in moderate SNC areas); pure Douglas-fir stands are gradually being 

replaced. One consequence of SNC infection has been strong support for cooperative genetic 

improvement of western hemlock (Jayawickrama 2003) which is now entering into a third cycle of 

breeding and testing. 
GENERAL TECHNICAL REPORT PSW-GTR-240

260

Figure 2—Second-cycle Douglas-fir test sites established on the Oregon and Washington coasts.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

261

Literature Cited

Jayawickrama, K.J.S. 2003. Genetic improvement and deployment of western hemlock in Oregon and 

Washington: review and future prospects. Silvae Genetica. 52: 25–36.

Johnson, G.R. 2002. Genetic variation in tolerance of Douglas-fir to Swiss needle cast as assessed by symptom 

expression. Silvae Genetica. 51: 80–86.

Temel, F. 2002. Early testing of Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) for Swiss 

needle cast tolerance. Corvallis, OR: Oregon State University. 177 p. Ph.D thesis.

Temel, F.; Johnson, G.R.; Adams, W.T. 2005. Early genetic testing of coastal Douglas-fir for Swiss needle 

cast tolerance. Canadian Journal of Forest Research. 35: 521–529.
GENERAL TECHNICAL REPORT PSW-GTR-240

262

White Pine Blister Rust Resistance in Pinus 

monticola and P. albicaulis in the Pacific Northwest 

U.S. – A Tale of Two Species

Richard A. Sniezko,

1 Angelia Kegley,1 and Robert Danchok1

Western white pine (Pinus monticola Dougl. ex D. Don) and whitebark pine (P. albicaulis Engelm.) 

are white pine species with similar latitudinal and longitudinal geographic ranges in Oregon and 

Washington (figs. 1 and 2). Throughout these areas, whitebark pine generally occurs at higher 

elevations than western white pine. Both of these long-lived forest tree species are highly susceptible 

to white pine blister rust, caused by the non-native fungus Cronartium ribicola, and both have 

suffered extensive mortality in many parts of their range (Aubry et al. 2008, Fins et al. 2001, Geils et 

al. 2010, Schwandt et al. 2010). The high susceptibility of these two species to blister rust has limited 

their use in reforestation and restoration. In July 2011, due to multiple threats, including blister rust, 

whitebark pine was added as a candidate species eligible for protection under the United States

Endangered Species Act and assigned a listing priority number of 2, which means the threats are of 

high magnitude and are imminent (U.S. Fish and Wildlife Service 2011). Gene conservation efforts 

with whitebark pine are underway (Mangold 2011; Sniezko et al. 2011b).

Genetic diversity and genetic resistance to pathogens and insects are a species’ primary defense 

and avenue to evolving in the face of threats such as blister rust and climate change. Several 

operational programs in forest tree species to utilize this natural genetic resistance to help mitigate the 

impacts of invasive pathogens are well underway (Sniezko 2006; Sniezko et al. 2011a). Active 

research and management to identify and utilize the low frequency of genetic resistance to blister rust 

within western white pine and whitebark pine populations offers the best potential for successful 

long-term reforestation or restoration. Planting resistant seedlings will likely be necessary to retain 

these white pine species as viable components in many ecosystems (Fins et al. 2001, Harvey et al. 

2008, Keane and Schoettle 2011) and to help increase the frequency of the resistance genes 

throughout the range. In the Pacific Northwest, an operational program to find and utilize naturally

occurring genetic resistance in western white pine has been ongoing for more than 50 years, while the 

program for whitebark pine has been in progress for only a decade (Kegley and Sniezko 2004; King 

et al. 2010; McDonald et al. 2004; Sniezko et al. 2007, 2011a). However, the program for whitebark 

pine has benefited greatly from the existence of facilities and expertise in use for the western white 

pine program.

For both species, parent trees are rated for resistance based on performance of their seedling 

progeny in artificial inoculation trials. In the first cycle of selection and testing, using wind-pollinated 

seed from selections in the forest, progeny of over 4,200 parent trees of western white pine have been 

evaluated for resistance, and progeny of 360 parent trees of whitebark pine have been evaluated or are 

currently in testing. Resistance screening of hundreds of additional whitebark pine seedling families 

is anticipated in the next 5 years. After inoculation, seedlings are assessed annually for up to 5 years 

to examine genetic variation in a range of resistance types, including number of needle lesions 

(‘spots’), type of needle spots (normal or hypersensitive-like response, HR), number of stem 

symptoms, type of stem symptoms (normal cankers or bark reactions), timing of stem symptom 

appearance, and severity of infection (Kegley and Sniezko 2004; Sniezko et al. 2007, 2011a). 

 1 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon, USA.

Corresponding author: rsniezko@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

263

Although western white pine and whitebark pine inhabit similar geographic distributions in the 

Oregon and Washington portions of their geographic ranges, there are both similarities and 

differences in their resistance to blister rust. Nearly 100 percent of the seedlings of both species 

develop needle spots in artificial inoculation trials at the U.S. Department of Agriculture, Forest 

Service’s Dorena Genetic Resource Center (DGRC) in Cottage Grove, Oregon. Ninety to 100 percent 

of the seedlings in the most susceptible families develop stem infections, usually within 8 to 24 

months after inoculation (Kegley and Sniezko 2004; Sniezko et al. 2007, 2011a). Progeny of a small 

percentage of western white pine parents from western Oregon show signs of a hypersensitive-like 

response in the needles (HR) (Kinloch et al. 1999, 2003), but no whitebark pine with HR have been 

noted to date (Sniezko, unpublished data2

). In western white pine, HR appears to be present only in 

western Oregon and in parts of California, and virulence to this resistance has been documented in 

parts of the range (Kinloch et al. 2003, 2004).

Figure 1—Range maps for (a) western white pine (Pinus monticola) and (b) whitebark pine (P. 

albicaulis) (adapted from U.S. Geological Survey 1999).

Moderate levels of other types of resistance, such as canker-free (and not HR) and bark reaction,

are relatively rare in wind-pollinated seedling families of western white pine from parents selected in 

Pacific Northwest forests (Kegley and Sniezko 2004). Only a few non-HR families of western white 

pine show canker-free levels of >30 percent in the seedling testing at DGRC (Kegley and Sniezko 

2004, Sniezko 2006; Sniezko and Kegley 2003a, 2003b). This is in contrast to whitebark pine, which 

appears to have a higher frequency of progenies with moderate levels of canker-free seedlings 

(Kegley et al., Blister rust resistance among 20 families of whitebark pine, Pinus albicaulis, from 

Oregon and Washington – early results from an artificial inoculation trial, these proceedings; Sniezko 

et al. 2007, 2011a). Preliminary summary of the early trials of whitebark pine families indicate that 

the highest frequency of parents with moderate levels of canker-free seedlings occur in the Cascade 

Range from central Oregon to central Washington, with a much lower frequency of resistance in 

eastern Oregon (Kegley et al., Blister rust resistance among 20 families of whitebark pine, Pinus 

albicaulis, from Oregon and Washington – early results from an artificial inoculation trial, these 

proceedings; Sniezko et al. 2007, 2011a). In some of these areas, many of the families tested have 20 

to 50 percent (or more) canker-free seedlings (Sniezko, unpublished, see footnote 2). The highest 

resistant (and non-HR) families of both species show several common resistance attributes including 

 2 Unpublished data from blister rust testing at Dorena GRC – 2004 and 2007 trials. On file: USDA Forest Service, Dorena 

Genetic Resource Center, Cottage Grove, OR 97424.
GENERAL TECHNICAL REPORT PSW-GTR-240

264

a lower frequency of seedlings with cankers, more latent infections, more bark reactions, and longer 

time to mortality (Sniezko and Kegley 2003a, Sniezko et al. 2007). Only a small number of seedlots 

of whitebark pine from outside Oregon and Washington have been tested at DGRC and none of these 

seedlots show the level of resistance exhibited by many of the seedlots from the Cascade Range 

(Sniezko et al. 2011a; Sniezko, unpublished, see footnote 2).

Figure 2—(a) whitebark pine (WBP) at Crater Lake National Park, (CRLA), (b) western white pine 

(WWP) at CRLA, (c) WBP in eastern Oregon, (d) WBP with blister rust at CRLA, (e) blister rust 

susceptible and resistant WBP seedling families in rows in rust resistance testing, (f) dead WWP with 

>100 rust cankers, directly adjacent to WWP field trial in southern Oregon. (Photo credits: Richard 

Sniezko: a, b, d, e, f; Chris Jensen: c)

It is encouraging that both western white pine and whitebark pine show family variation in 

resistance to C. ribicola. For both species, more information is needed on the number of resistant 

mechanisms and their inheritance as well as their expected efficacy and durability in the presence of 

an evolving pathogen and a changing climate. For western white pine, breeding zones are established, 

seed orchards are producing seed, breeding to increase resistance is underway, and a large number of 

field trials have been established (see Sniezko et al., White pine blister rust resistance of 12 western 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

265

white pine families at three field sites in the Pacific Northwest, these proceedings, for results from 

some field trials). For whitebark pine, seed zones have been established (Aubry et al. 2008) and land 

managers will be notified about which parent trees show resistance to facilitate additional seed 

collection for restoration; a few small field trials have recently been established. For both species, the 

field trials will serve to help validate the results of artificial inoculation trials, to provide land 

managers with updated information on efficacy of rust resistance over a range of sites, and to monitor 

for changes in durability of resistance or changes in general health of the species with changing 

climate. The parent trees selected previously in the field will be rated for resistance based on their 

progeny rust resistance, and they can serve as valuable long-term monitors of changes in efficacy of 

resistance over time or changes in virulence of the pathogen.

Literature Cited

Aubry, C.; Goheen, D.; Shoal, R.; Ohlson, T.; Lorenz, T.; Bower, A.; Mehmel, C.; Sniezko, R. 2008. 

Whitebark pine restoration strategy for the Pacific Northwest Region 2009-2013. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Region. 212 p. 

http://www.fs.usda.gov/detail/r6/landmanagement/resourcemanagement/?cid=stelprdb5278980. (02 January 

2012).

Fins, L.; Byler, J.; Ferguson, D.; Harvey, A.; Mahalovich, M.F.; McDonald, G.; Miller, D.; Schwandt, J.; 

Zach, A. 2001. Return of the giants: restoring white pine ecosystems by breeding and aggressive planting of 

blister rust-resistant white pines. Station Bulletin 72. Moscow, ID: University of Idaho, College of Natural 

Resources. 20 p. http://www.fs.fed.us/rm/pubs_other/rmrs_2001_fins_l001.pdf . (02 January 2012).

Geils, B.W.; Hummer, K.E.; Hunt, R.S. 2010. White pines, Ribes, and blister rust; a review and synthesis. 

Forest Pathology. 40: 147–185.

Harvey, A.E.; Byler, J.W.; McDonald, G.I.; Neuenschwander, L.F.; Tonn, J.R. 2008. Death of an 

ecosystem: perspectives on western white pine ecosystems of North America at the end of the twentieth 

century. Gen.Tech. Rep. RMRS-GTR-208. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 10 p.

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsm9_018495.pdf. (02 January 2012).

Keane, R.E.; Schoettle, A.W. 2011. Strategies, tools, and challenges for sustaining and restoring high 

elevation five-needle white pine forests in western North America. In: Keane, R.E.; Tomback, D.F.; Murray, 

M.P.; Smith, C.M., eds. 2011. The future of high-elevation, five-needle white pines in western North 

America: Proceedings of the high five symposium. Proceedings RMRS-P-63. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station: 276–294. 

http://www.fs.fed.us/rm/pubs/rmrs_p063/rmrs_p063_276_294.pdf. (02 January 2012).

Kegley, A.; Sniezko, R.A. 2004. Variation in blister rust resistance among 226 Pinus monticola and 217 P. 

lambertiana seedling families in the Pacific Northwest. In: Sniezko, R.A.; Samman, S.; Schlarbaum, S.E.; 

Kriebel, H.B., eds. Breeding and genetic resources of five-needle pines: growth, adaptability, and pest 

resistance. Proceedings RMRS-P-32. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station: 209–226.

http://www.fs.fed.us/rm/pubs/rmrs_p032/rmrs_p032_209_226.pdf. (01 January 2012).

King, J.N.; Noshad, D.A.; Smith, J. 2010. A review of genetic approaches to the management of blister rust in 

white pines. Forest Pathology. 40: 292–313.

Kinloch, B.B., Jr.; Sniezko R.A.; Barnes, G.D.; Greathouse, T.E. 1999. A major gene for resistance to white 

pine blister rust in western white pine from the western Cascade Range. Phytopathology. 89(10): 861–867.

Kinloch, B.B., Jr.; Sniezko, R.A.; Dupper, G.E. 2003. Origin and distribution of Cr2, a gene for resistance to 

white pine blister rust in natural populations of western white pine. Phytopathology. 93(6): 691–694.

Kinloch, B.B., Jr.; Sniezko, R.A.; Dupper, G.E. 2004. Virulence gene distribution and dynamics of the white 

pine blister rust pathogen in western North America. Phytopathology 94(7): 751–758.

Mangold, R.D. 2011. The U.S. Forest Service's renewed focus on gene conservation of five-needle pine 

species. In: Keane, R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. 2011. The future of highelevation, five-needle white pines in western North America: Proceedings of the high five symposium. 
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Proceedings RMRS-P-63. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station: 151. http://www.fs.fed.us/rm/pubs/rmrs_p063/rmrs_p063_151.pdf. (02 January 

2012).

McDonald, G.I.; Zambino, P.J.; Sniezko, R.A. 2004. Breeding rust-resistant five-needled pines in the western 

United States: lessons from the past and a look to the future. In: Sniezko, R.A.; Samman, S.; Schlarbaum, 

S.E.; Kriebel, H.B., eds. Breeding and genetic resources of five-needle pines: growth, adaptability and pest 

resistance. IUFRO Working Party 2.02.15. Proceedings RMRS-P-32. Fort Collins, CO: U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station: 28–50.

http://www.fs.fed.us/rm/pubs/rmrs_p032.html. (02 January 2012).

Schwandt, J.W.; Lockman, I.B.; Kliejunas, J.T.; Muir, J.A. 2010. Current health issues and management 

strategies for white pines in the western United States and Canada. Forest Pathology. 40: 226–250.

Sniezko, R.A. 2006. Resistance breeding against nonnative pathogens in forest trees—current successes in 

North America. Canadian Journal of Plant Pathology. 28: S270–S279.

Sniezko, R.A.; Kegley A. 2003a. Blister rust resistance experiences in Oregon and Washington: evolving 

perspectives. In: Stone, J.; Maffei, H., comps. Proceedings of the 50th Western International Forest Disease 

Work Conference. Bend, OR: U.S. Department of Agriculture, Forest Service, Central Oregon Service 

Center: 111–117.

Sniezko, R.A.; Kegley, A.J. 2003b. Blister rust resistance of five-needle pines in Oregon and Washington. In: 

Xu, M.; Walla, J.; Zhao, W., eds. Proceedings of the second IUFRO rusts of forest trees working party 

conference. Forest Research. 16 (Suppl.): 101–112.

Sniezko, R.A.; Kegley, A.J.; Danchok, R.S.; Long, S. 2007. Variation in resistance to white pine blister rust 

among 43 whitebark pine families from Oregon and Washington—early results and implications for 

conservation. In: Goheen, E.M.; Sniezko, R.A., tech. coords. Proceedings of the conference whitebark pine:

Whitebark pine: a Pacific Coast perspective; R6-NR-FHP-2007-01. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Region: 82–97.

http://www.fs.fed.us/r6/nr/fid/wbpine/papers/2007-wbp-wpbr-resist-sniezko.pdf. (01 January 2012).

Sniezko, R.A.; Mahalovich, M.F.; Schoettle, A.W.; Vogler, D.R. 2011a. Past and current investigations of 

the genetic resistance to Cronartium ribicola in high-elevation five-needle pines. In: Keane, R.E.; Tomback, 

D.F.; Murray, M.P.; Smith, C.M., eds. 2011. The future of high-elevation, five-needle white pines in 

western North America: Proceedings of the high five symposium. Proceedings RMRS-P-63. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 246–264.

http://www.fs.fed.us/rm/pubs/rmrs_p063/rmrs_p063_246_264.pdf. (01 January 2012).

Sniezko, R.A.; Schoettle, A.; Dunlap, J.; Vogler, D.; Conklin, D.; Bower, A.; Jensen, C.; Mangold, R.; 

Daoust, D.; Man, G. 2011b. Ex situ gene conservation in high elevation white pine species in the United 

States: a beginning. In: Keane, R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. 2011. The future of 

high-elevation, five-needle white pines in western North America: Proceedings of the high five symposium. 

Proceedings RMRS-P-63. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station: 147–149. http://www.fs.fed.us/rm/pubs/rmrs_p063/rmrs_p063_147_149.pdf. 

(02 January 2012).

U.S. Fish and Wildlife Service. 2011. Endangered species: whitebark pine. http://www.fws.gov/mountainprairie/species/plants/whitebarkpine/. (01 January 2012).

U.S. Geological Survey. 1999. Digital representation of Atlas of United States trees, by Elbert L. Little Jr.;

http://esp.cr.usgs.gov/data/atlas/little/. (01 January 2012).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Shoot Winter Injury and Nut Cold Tolerance: 

Possible Limitations for American Chestnut 

Restoration in Cold Environments?

Thomas M. Saielli,

1 Paul G. Schaberg,2 Gary J. Hawley,3 Joshua M. Halman,3

and Kendra M. Gurney4

Abstract

Approximately 100 years ago, American chestnut (Castanea dentata (Marsh.) Borkh.) was rapidly removed as 

an overstory tree by the fungal pathogen Cryphonectria parasitica (the causal agent of chestnut blight). 

Currently, the most effective method of restoration involves the hybridization of American chestnut with the 

highly blight-resistant Chinese chestnut (Castanea mollissima Blume), with subsequent backcrossing of 

resistant stock to American chestnut sources. However, preliminary evidence suggests that backcross material 

may not have the cold hardiness needed for restoration in the north. Two factors that can significantly influence 

cold tolerance are plant genetics and environmental parameters (e.g., cold exposure of plant tissues). Also, the 

cold tolerance of nuts is of concern because reproductive tissues are particularly sensitive to freezing damage. 

To contribute to the successful restoration of American chestnut in the north, the focus of this research was to 

analyze the cold tolerance of American chestnut through 1) an assessment of first-year growth and shoot winter 

injury of a range of American and Chinese chestnut and red oak (Quercus rubra L., a native competitor) 

seedlings under three silvicultural treatments (open, partial, and closed canopies) in the Green Mountain 

National Forest, Vermont, and 2) the comprehensive evaluation of nut cold tolerance for a range of American 

and Chinese chestnut nuts and red oak acorns. We examined American chestnut sources by temperature zones

(warm, moderate, or cold) that differentiated sources based on winter low temperatures in the areas where they 

originated. 

Seedlings grown under open canopies exhibited greater growth than seedlings grown under partial and closed 

canopies, but also experienced increased shoot winter injury. Chinese chestnut seedlings had significantly 

greater growth, but also experienced greater winter injury than American chestnut and red oak seedlings. 

Among American chestnut sources, seedlings from sources from warmer, low-elevation southern and central 

locations grew more, but experienced greater winter injury than seedlings from sources from the colder north. 

Additionally, nuts of Chinese chestnut were significantly less cold-tolerant than either American chestnut nuts 

or red oak acorns. Among American chestnut sources, nuts from warm and moderate temperature zones 

exhibited similar levels of cold tolerance, but were significantly less cold tolerant than nuts from the cold 

temperature zone. There were significant differences among sources within the warm and moderate temperature 

zones, but not among sources within the cold temperature zone. We believe that the temperature zone index 

may provide a reliable guide for targeting sources with lower winter shoot injury and greater nut cold tolerance. 

Our results suggest that both silvicultural treatment and genetic selection can influence growth and winter injury 

of American chestnut at the northern limit of its range. There was also a strong correlation between nut cold 

tolerance and winter shoot injury, suggesting that nut cold tolerance measurements (that can be obtained in 

weeks rather than years) may be a reasonable screening tool for identifying sources with greater shoot 

hardiness.

 1 The American Chestnut Foundation, 160 Zillicoa Street, Suite D, Asheville, NC 28801. 2 U.S. Department of Agriculture, Forest Service, Northern Research Station, 705 Spear Street, South Burlington, VT 05403.

3 University of Vermont, Rubenstein School of Environment and Natural Resources, 105 Carrigan Drive, Burlington, VT 

05405.

4 The American Chestnut Foundation, 705 Spear Street, South Burlington, VT 05403.

Corresponding author: tom@acf.org.

269

Posters

P. strobiformis with 

100’s of needle 

spots

P. albicaulis with ‘needle 

shed’

Infected P. 

Infected primary needles of flexilis

P. monticola

susceptible

HR

White Pine Blister Rust Testing at Dorena Genetic

Resource Center - 5 host pines shown – with 

needle spots and stem infections (cankers & bark 

reactions)

P. monticola

P. albicaulis

P. aristata with 

bark reaction

P. monticola

(Photo credit: R.Sniezko)

Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

271

Screening Sitka Spruce for Resistance to Weevil 

Damage in British Columbia

René I. Alfaro1 and John N. King2

Abstract

The white pine weevil, Pissodes strobi (Coleoptera, Curculionidae), has serious impacts on Sitka (Picea

sitchensis (Bong.) Carrière), Engelmann (P. engelmannii Parry ex Engelm.), and white spruce (P. glauca

(Moench) Voss) plantations in British Columbia (BC), Canada. This weevil attacks the terminal leader of the 

tree, causing significant growth loss and deformities. Genetic resistance to this insect was demonstrated in early 

provenance trials in BC. This encouraged us to initiate a systematic search for resistant trees to confirm this 

resistance and to improve parent source selections, especially in the Sitka spruce breeding program. Test 

plantations were initiated and, to accelerate the screening process and create a uniform weevil pressure, insect 

populations were artificially augmented at many of these trial sites. Artificial infestation provided quick and 

effective screening and allowed us to proceed with the construction of an F-1 population, and to understand the 

mechanisms and heritability of resistance. Seed orchards have been established and weevil resistant seedlings 

are now available for operational planting.

Key words: spruce, weevil, genetic resistance, tree improvement, pest management

Introduction

The white pine weevil, Pissodes strobi (Colepoptera, Curculionidae), is an important native pest that 

limits Sitka (Picea sitchensis (Bong.) Carrière), Engelmann (P. engelmannii Parry ex Engelm.), and 

white spruce (P. glauca (Moench) Voss) reforestation in western Canada. This insect lays its eggs on 

the uppermost shoot or leader of the tree. Upon hatching, the larvae mine under the leader bark, 

causing the destruction of the leader. Although this insect does not destroy the entire tree, a weevil 

attack leads to deformity and a reduction of growth, thereby making the tree economically unsuitable 

as a timber resource. By preventing restocking with spruce, the natural species for certain sites, the 

weevil also affects non-timber values such as biodiversity. 

In order to find a natural solution to the problem, we searched for trees that are naturally resistant 

to the weevil. Several spruce provenances and genotypes with heritable resistance were found and are 

now included in seed orchards and reforestation programs in British Columbia (BC). To further 

understand the inheritance of resistance, we investigated resistance of progeny from selected crosses 

(F1) from parents with different levels of resistance. We also conducted studies aimed at 

understanding the basis for the observed resistance. 

Materials and Methods

Sources Tested for Resistance

Seed was collected by BC Ministry of Forests and Range personnel from potentially resistant trees 

throughout the range of Sitka spruce in Coastal BC and the United States in Alaska, Washington, 

Oregon, and California. Seedlings produced from the collected seed were planted and tested in field 

trials (fig. 1) located in BC and Oregon and screened by Canadian Forest Service personnel.

Plantations included block and family replication to allow statistical comparisons and heritability 

calculations. 

 1 Canadian Forest Service, Pacific Forestry Centre, Victoria, BC, Canada. 2 BC Ministry of Forests and Range (retired), Victoria, BC, Canada.

Corresponding author: ralfaro@pfc.forestry.ca.
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Figure 1—Locations of Sitka spruce screening trials on the west coast of North America.

Screening Methods

Screening of over 29,000 trees was conducted by artificially releasing laboratory-reared weevils (fig. 

2) into the test plantations (for methods, see Alfaro et al. 2008) and then conducting annual 

evaluations of attack rates on the tested genotypes. Statistical analysis determined if particular 

families were resistant to attack and the heritability statistics.

Figure 2—Weevil release in Sitka spruce screening trial to screen genotypes for resistance to white 

pine weevil.
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Results

Our trials indicated significant provenance variation in weevil resistance (fig. 3), with some openpollinated parents consistently producing offspring with statistically demonstrable resistance to P.

strobi (Alfaro et al. 2008). These parents are now in seed orchards and producing resistant seed for 

reforestation. We have also determined that resistance is both heritable and stable.

Figure 3—Range of Sitka spruce in western North America (green). Dots indicate sources tested for 

genetic resistance to weevil damage. Red dots indicate sources found to consistently produce 

progeny with resistance.

Artificial infestations allowed us to quickly and effectively proceed with the construction of an F1 

population, to determine the heritability of resistance, and to further our understanding of the 

mechanisms that underlie weevil resistance. This F1 population is now screened (Moreira et al. 2011)

using the same methodology as is used for the parent populations. Weevil resistance in the F1 spruce 

progeny demonstrated that progeny from resistant parents (R × R progeny) sustained significantly 

fewer weevil attacks than progeny from susceptible parents (S × S progeny) or progeny with one 

resistant and one susceptible parent (R × S progeny). Individual and family heritability estimates of 

the weevil resistance were 0.5 and 0.9, respectively. We also related the level of resistance in the F1 

crosses to two constitutive mechanisms of resistance: the density of cortical resin canals and the 

amount of sclereid cells in the leader cortex (Moreira et al. 2011). Constitutive defenses were 

significantly higher in R × R progeny than in R × S or S × S progeny. We observed a negative 

correlation between the percentage of trees attacked in each cross and the average density of the resin 

canals or sclereid cells for each cross.

Our results indicated that effective screening for weevil resistance can be accomplished by using 

artificial weevil infestation. At the time of weevil release, trees need to be of susceptible height, i.e., 

outplanted for approximately 3 to 4 years in coastal BC. Once plantations are artificially infested, 

they should be monitored for at least 4 years to ensure that resistance is stable, i.e., that the selected 

genotypes sustain consistently low attack rates. This is a fairly quick turnaround time for studying 
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resistance of forest trees from temperate regions. This is a promising result, which should encourage 

resistance studies for other regeneration pests.

Production and Deployment of Resistant Trees

The resistant genotypes identified through this program now form the basis for the successful 

establishment of Sitka spruce plantations in BC. Using our selections of weevil resistant parents, seed 

orchards have been established and are producing regeneration material for operational planting 

(seeds and seedlings) (fig. 4). 

Figure 4—Annual amount resistant seedlings produced in British Columbia at Western Forest 

Products nursery in Saanich, BC.

While in the past, BC provincial coastal reforestation guidelines for high weevil hazard areas have 

recommended exclusion or limited planting of spruce, current guidelines indicate that up to half of the 

stand could be planted with Sitka spruce in moderate- or high-hazard areas if “A+” seed (from 

selected orchard-grown, weevil-resistant trees [R+87]) is used, and about a third if “B+” seed is used 

(from naturally resistant stands [R+64]) (Heppner and Turner 2006). 

Conclusions

• The screening of putative parents for weevil resistance has been successfully completed (over 

29,000 trees screened).

• Resistant Sitka spruce parents have been selected.

• Resistance is heritable and stable. 

• An F1 progeny test was produced from control crosses of Resistant x Resistant, Resistant x 

Susceptible, and Susceptible x Susceptible parents.

• Breeding for resistance: Individual and family heritability estimates of weevil resistance were 

0.5 and 0.9, respectively. 

• Seed orchards are now producing the first generation of resistant seed for BC’s next forest 

crop.

Acknowledgments

We thank George Brown, Forest Technician, Canadian Forest Service (retired) for the detailed assessment of 

the Sitka spruce test plantations over more than a decade. 
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Literature Cited

Alfaro, R.I.; King, J.N.; Brown, R.G.; Buddingh, S.M. 2008. Screening of Sitka spruce genotypes for 

resistance to the white pine weevil using artificial infestations. Forest Ecology and Management. 255: 1749–

1758.

Heppner, D.; Turner, J. 2006. British Columbia’s coastal forests: spruce weevil and western spruce budworm 

forest health stand establishment decision aids. BC Journal of Ecosystems and Management .7(3):45–49. 

http://www.forrex.org/publications/jem/ISS38/vol7_no3_art6.pdf. (29 January 2012).

Moreira, Xoaquín; Alfaro, R.I.; King, J.N. 2011. Constitutive defenses and damage in Sitka spruce progeny 

obtained from crosses between white pine weevil resistant and susceptible parents. Forestry.

doi:10.1093/forestry/cpr060.
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Developing Clones of Eucalyptus cloeziana

Resistant to Rust (Puccinia psidii)

Rafael F. Alfenas,1 Marcelo M. Coutinho,2 Camila S. Freitas,1 Rodrigo G. 

Freitas,1 and Acelino C. Alfenas1

Abstract

Besides its high resistance to Chrysoporthe cubensis canker, Eucalyptus cloeziana F. Muell. is a highly valuable 

tree species for wood production. It can be used for furniture, electric poles, fence posts, and charcoal. 

Nevertheless, it is highly susceptible to the rust caused by Puccinia psidii, which limits its growth in areas 

favorable to infection. Since E. cloeziana does not interbreed naturally with other Eucalyptus species, its 

seedling plantations in Brazil are relatively uniform. Thus, the selection and multiplication of rust- resistant 

genotypes could constitute the best strategy for rust control. However, it is recalcitrant to rooting, which limits 

its large-scale clonal multiplication of resistant genotypes for planting, as has been done with E. grandis Hill ex 

Maiden, E. urophylla S.T. Blake and their hybrids. Aiming to obtain the greatest possible number of rustresistant genotypes for cloning, about 3,500 seedlings of several seed lots of different origins were sprayinoculated with an inoculum suspension of 2x104 urediniospores/ml of the single pustule isolate (EUBA-1, race 

4) of P. psidii according to the standard procedures used in our laboratory. The assessment of disease severity 

on each seedling was performed 12 and 20 days after inoculation using the rust severity scale of Junghans et al. 

(2003, Fitpatologia Brasileira. 28: 261–265). Eucalyptus grandis x E. urophylla hybrid clones C1179 (resistant) 

and C1183 (susceptible) were used as controls. Only about 2 percent of the inoculated seedlings were resistant 

and clonable. Attempts to multiply all 69 resistant clones are being made in order to have enough plants for 

clonal trials to evaluate their silvicultural performance and wood properties.

 1 Departamento de Fitopatologia, Universidade Federal de Viçosa, Viçosa, MG 36570-000, Brasil. 2 Clonar Resistência a Doenças Florestais, Viçosa, MG 36570-000, Brasil.

Corresponding author: aalfenas@ufv.br.
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Screening for Resistance to Fusiform Rust in 

Southern United States Forest Trees

Josh Bronson1

Abstract

The Resistance Screening Center (RSC) is operated by the Forest Health Protection unit of the U.S. Department 

of Agriculture, Forest Service, Southern Region, State and Private Forestry. The RSC is located at the Bent 

Creek Experimental Forest near Asheville, North Carolina. The center evaluates seedlings for resistance to 

disease, primarily fusiform rust (caused by Cronartium quercuum f. sp. fusiforme) and pitch canker (caused by 

Fusarium circinatum) as a service to tree improvement specialists, seed orchard managers, scientists, 

government agencies, research institutions, universities, and private industry. Testing enables clients to obtain 

information on the relative resistance of their materials in much less time than is possible in field progeny tests.

The RSC has the flexibility to modify current screening procedures to accommodate specialized requests, such 

as unique species or inoculation procedures. This allows researchers to use the RSC as an additional 

experimental tool. In a research assistance capacity, the RSC has played an important role in newly developed 

understanding of genetic interactions in the pine-fusiform rust pathosystem, and will continue to do so in the 

foreseeable future. By using information from the resistance screening center tests, trees producing resistant 

progeny can be identified, or questions may be answered, concerning such things as the nature of pathogen 

variation or the effectiveness of fungicides. The RSC remains open to service screening work or research 

endeavors in an effort to improve forest health.

 1 USDA Forest Service, Resistance Screening Center, 1579 Brevard Road, Asheville, NC 28805.

Corresponding author: jjbronson@fs.fed.us.
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Variation in the Development of Current Season 

Needle Necrosis on Noble, Nordmann, and Turkish 

Fir Christmas Trees in the 

United States Pacific Northwest

Gary A. Chastagner,

1 Kathy Riley,1 and Chal Landgren2

Introduction

Current season needle necrosis (CSNN) is a poorly 

understood disease that affects a number of Abies spp. that 

are grown as Christmas trees (fig. 1). The disease has been 

reported on noble (A. procera Rehder), Nordmann (A. 

nordmanniana (Steven) Spach), and grand fir (A. grandis

(Douglas ex D. Don) Lindl.) in Europe and these species, 

plus Turkish fir (A. bornmuelleriana Mattf.), in the United 

States Pacific Northwest (Chastagner 1997, Chastagner et 

al. 1990, Talgø et al. 2010). The development of CSNN 

varies by site and there is tremendous tree-to-tree variation 

in susceptibility to this disease (Chastagner and Landgren 

1998; Chastagner et al. 1990, 2008). In an effort to 

determine year-to-year variation in the development of 

CSNN and identify sources of trees with resistance to this 

disease, data have been collected from a series of genetic 

trials at Washington State University (WSU), Puyallup. 

Methods

In 2002 and 2004, a series of replicated genetic field trials 

were established at the WSU Research Center in Puyallup, 

Washington. This is a low elevation site that is very 

conducive to the development of CSNN. These trials 

contain 91 sources of noble fir, 15 sources of Nordmann 

fir, and 4 sources of Turkish fir. Over a 4- to 6-yearperiod, symptom severity was rated on a scale of 0 to 10 

during late summer/early fall. 

Results and Discussion

There was significant yearly variation in the overall average CSNN ratings for the trees in these trials. 

Over 6 years, ratings for the 37 sources of noble fir in the 2002 planting ranged from an average low 

of 1.5 in 2009 to an average high of 3.4 in 2006. Over 5 years, the yearly overall average rating for 

 1 Washington State University, Research and Extension Center, 2606 West Pioneer, Puyallup, WA 98731. 2 Oregon State University, North Willamette Research and Extension Center, 15210 NE Miley Rd, Aurora, OR 97002. 

Corresponding author: chastag@wsu.edu.

Figure 1—Current season needle 

necrosis (CSNN) symptoms on a noble 

fir Christmas tree at Washington State 

University (WSU), Puyallup.
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the 54 sources of noble fir in the 2004 trial ranged from a low of about 1.1 in 2005 to a high of 3.3 in 

2008. Compared to the noble fir, limited CSNN developed on the Nordmann and Turkish fir trees. 

Over 4 years, the average ratings for the sources of Nordmann and Turkish fir ranged from 0.3 to 0.9. 

In the 2002 noble fir plot, the percentage of trees that were resistant to CSNN ranged from 5 to 80

percent depending on the source (fig. 2). In the 2004 noble fir plot, the range was 4 to 65 percent. No 

CSNN developed on over 80 percent of the trees from seven of the sources of Nordmann/Turkish fir. 

Although there was significant yearly variation in the severity of CSNN, Spearman rank order 

correlation analysis indicated that there was a highly significant correlation between the yearly 

susceptibility rankings of the noble, Nordmann, and Turkish fir sources in each of these plots.

The effect of the site on CSNN development was examined by comparing the severity of CSNN 

development on trees in the 2004 noble fir genetic test plot at Puyallup to the same sources at Silver 

Mountain Christmas Trees near Sublimity, Oregon in 2009. The overall plot CSNN rating for the 

trees at Silver Mountain was much lower than Puyallup. Over 94 percent of the trees at Silver 

Mountain had no CSNN, compared to only 23 percent at Puyallup. Even though much less CSNN 

developed at Silver Mountain, Spearman rank order analysis indicated that there was a highly 

significant correlation in susceptibility rankings of the sources at both sites. Regression analysis of 

CSNN ratings from Nordmann and Turkish fir planted in a “valley” and “hill” at Puyallup also 

indicated that there was a highly significant correlation between the 4-year-average ratings of the 

individual sources at the valley and hill sites. 

These results indicate that there is considerable variation in the susceptibility of different sources 

of noble, Nordmann, and Turkish fir to CSNN and that the relative susceptibility of different sources 

of trees to CSNN can be determined after 1 or 2 years at conducive sites. 

Literature Cited

Chastagner, G.A., ed. 1997. Christmas tree diseases, insects, and disorders in the Pacific Northwest: 

identification and management. Cooperative Ext. Publication MISC0186. Pullman: Washington State 

University. 154 p.

Chastagner, G.A.; Landgren, C. 1998. Genetic variation and the influence of fertilization on the development 

of current season needle necrosis on noble fir Christmas trees. In: Leflamme, G.; Berube, J.A.; Hamelin, 

R.C., eds. Foliage, shoot and stem diseases of trees. Quebec: Natural Resources Canada, Canadian Forest 

Service: 219–226.

Figure 2—Five trees from a resistant source of trees 

in the 2002 noble fir trial at WSU, Puyallup.
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Chastagner, G.A.; Riley, K.L.; Landgren, C. 2008. Variation in the susceptibility of noble fir to current 

season needle necrosis. In: Thomsen, I.M.; Rasmussen, H.N.; Sørensen, J.M., eds. Proceedings of the 8th 

international Christmas tree research and extension conference. Hørsholm, Denmark: University of 

Copenhagen: 87–88. www.ps-xmastree.dk and www.iufro.org. (29 January 2012).

Chastagner, G.A.; Staley, J.M.; Riley, K.L. 1990. Current season needle necrosis: a needle disorder of 

unknown etiology on noble and grand fir Christmas trees in the Pacific Northwest. In: Merrill, W.; Ostry, 

M.E., eds. Recent research on foliar diseases. Gen. Tech. Rept. WO-56. Washington, DC: U.S. Department 

of Agriculture, Forest Service: 38–42.

Talgø, V.; Chastagner, G.A.; Thomsen, I.M.; Cech, T.; Riley, K.L.; Lange, K.; Klemsdal, S.S.; Stensvand, 

A. 2010. Sydowia polyspora associated with current season needle necrosis (CSNN) on true fir (Abies spp.). 

Fungal Biology. 114: 545–554.
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Frequency of Hypersensitive-Like Reaction and Stem 

Infections in a Large Full-Sib Family 

of Pinus monticola

Robert S. Danchok,

1 R.A. Sniezko,1 S. Long,1 A. Kegley,1 D. Savin,1 J.B. Mayo,1

J.J. Liu,

2 and J. Hill3

Introduction

Western white pine (WWP) (Pinus monticola Douglas ex D. Don) is a long-lived forest tree species with 

a large native range in western North America. The tree species is highly susceptible to the non-native 

fungal pathogen, Cronartium ribicola, the causative agent of white pine blister rust (WPBR).

Several types of genetic resistance to WPBR are present in WWP, of which the best documented is a 

hypersensitive-like reaction (HR) in the needles that conveys complete resistance (generally no stem 

infection) and is conditioned by a single dominant gene (Cr2). The HR resistance is rare, and its 

occurrence is geographically limited (Kinloch et al. 1999, 2003). Virulence to Cr2 in the rust (vcr2) is 

known (Kinloch et al. 2004). Most screening trials examine relatively few seedlings for any one family 

for HR. We report on the frequency of HR (using needle phenotypes) in a large full-sib family (3,592

individuals) in a cross between two putative Cr2 heterozygotes (this is part of a larger genetic study to 

map Cr2); the frequency of stem symptoms and mortality in HR and non-HR seedlings; and the number 

of stem symptoms per seedling for HR and non-HR phenotypes.

Materials and Methods

Western white pine (WWP) seed of a single full-sib family (15045-862 x 15045-837) was stratified for 

120 days and then sown in June 2010. The 3,592 seedlings from this 1991 seed collection were grown in 

a greenhouse at Dorena Genetic Resource Center (DGRC), Cottage Grove, Oregon in the summer of 2010

(fig. 1). Both parents are heterozygotes (Cr2cr2) for HR resistance and originated from the Champion 

Mine area on the Cottage Grove Ranger District, Umpqua National Forest in Oregon. This stand had 

previously been noted for having the highest incidence of parents with Cr2 of any areas tested (Kinloch et 

al. 1999, 2003). Seedlings had both primary needles and cotyledons present at inoculation (only a few 

seedlings displayed any secondary needles). Seedlings were inoculated with C. ribicola in September 

2010 using standard DGRC protocols (Danchok et al. 2004). For artificial inoculation, infected Ribes spp. 

leaves (the alternate host of C. ribicola) from geographical areas outside of the areas of known occurrence 

of vcr2 were collected and suspended above the pines under optimal conditions for spore drop and 

germination. Inoculum density for the trial averaged 6,805 basidiospores/cm2

, and spore germination 

averaged 89 percent. After inoculation, seedlings were placed in an unheated greenhouse for nursery 

culture and subsequent development of rust symptoms.

Seedlings were assessed at periodic intervals when needle spots and stem symptoms were evident. The 

assessments for spot type included rating each seedling for whether all spots were susceptible (code 1) or 

whether all spots were HR type (code 4). Seedlings with a combination of HR and susceptible spots (or 

some of the seedlings with HR-like spots, but stem symptoms at the early assessments) were initially 

assigned a separate category (code 2), as were seedlings where the spot phenotype was ambiguous (code 

 1 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon, USA. 2 Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, British Columbia, Canada. 3 National Park Service, Southeast Utah Group, Moab, Utah, USA.

Corresponding author: rsniezko@fs.fed.us.
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3). The presence of stem symptoms was also recorded, and in March and April 2011, a count of stem 

symptoms was made. Mortality was also recorded. In June 2011, the spot type was re-evaluated on the 33 

seedlings coded as ‘3’ to classify into type 1, 2, or 4. A subset of 150 seedlings was examined for type of 

stem symptoms in July 2010.

Results

The artificial inoculation was very successful, with most seedlings having dozens to hundreds of needle 

spots (fig. 1). All seedlings were ultimately classified as having type 1, 2, or 4 spots. All 3,592 seedlings 

showed needle spots by April 2011, and 36 percent had only susceptible (S), non-HR spots (fig. 1). The 

64:36 ratio (HR spots: S spots) differed significantly from the expected 3:1 ratio (Chi-square p-value < 

0.0001) and also from a 1:1 ratio (p-value < 0.0001). 

Figure 1—March 2011 photos (6 months after inoculation): (a) overview of P. monticola trial; (b) hundreds 

of needle spots (HR) on inoculated P. monticola seedling; (c) resistant (HR) and susceptible (S) spots on 

P. monticola; note many infections per needle, (d) needles with S and HR spots (note incomplete 

development at this point of HR spots on one needle). 

By April 2011, 94.4 percent of the category 1 (S) seedlings had stem symptoms, versus 29.5 percent 

for the HR seedlings (groups 2 and 4) (57.4 percent and 13.8 percent of the category 2 and 4 (HR) 

seedlings, respectively) (figs. 1 to 3). By November 2011, stem symptoms had increased to 97.5 and 53.1 

percent for S and HR seedlings, respectively. The non-HR (S) seedlings averaged 4.6 stem symptoms per 

infected tree in April, while the HR seedlings averaged 1.6 (type 2 and 4 seedlings averaged 1.7 and 1.4

stem symptoms per infected tree, respectively (fig. 4)). This difference was significant (two-sided p-value 

< 0.0001 from a likelihood ratio test); the mean number of stem symptoms per infected tree is estimated 

to be 2.9 times greater for S seedlings than HR seedlings (95 percent confidence interval from 2.7 to 3.1 

times greater) (figs. 2 and 4). Stem symptoms generally appeared earlier on non-HR than on the HR 

seedlings: 77.9 percent of non-HR had SS in March, while only 14.1 percent and 3.1 percent of the HR 

seedlings in categories 2 and 4, respectively, had SS (fig. 3).

Overall mortality (seedlings with and without stem symptoms) by November 2011 was 92.2 and 72.6 

percent for S and HR seedlings, respectively; the odds of mortality for S seedlings were estimated to be 

3.6 times to 5.6 times as large as the odds of mortality for HR seedlings (approximate 95 percent 

confidence interval). The mortality for the subset of seedlings with stem symptoms was 92.2 and 77.0

percent for S and HR seedlings, respectively. For seedlings with no visible stem symptoms, mortality was 

also high: 90.9 and 67.5 percent for the S and HR groups.

Discussion

Based on the needle phenotypes following artificial inoculation, the ratio of HR:non-HR seedlings 

(64:36) was less than the 75:25 ratio anticipated in a cross of two Cr2 heterozygotes. In a previous trial, 

progenies of some of the parents from the Champion Mine stand have showed reduced penetrance of 

Cr2– expressed by altered Mendelian ratios (mostly less-than-expected resistant phenotypes) in specific 

combinations of certain parents, indicating the potential presence of modifier genes with effects that 

ranged from mild to almost complete suppression of Cr2 (Kinloch et al. 1999). This may explain the

(a) (b) (c)

(d)
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Figure 2—Stem infections observed on seedlings with S spots (a, c, and e) and on seedlings with HR 

spots (b and d). 

Figure 3—Percentage of Pinus monticola seedlings with stem symptoms by assessment date for the 

three needle spot types: 1: all susceptible (S) spots, 2: at least one resistant (HR) spot, 4: all (HR) spots. 

(a) (b) (c)

(d) (e)
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Figure 4—Mean number of stem symptoms (per infected tree) by assessment date for the three needle

phenotype category types: 1: all susceptible (S) spots, 2: at least one HR spot, 4: all HR spots. 

slightly lower than expected frequency of HR phenotypes in this full-sib family. Previous tests in 2002 

and 2008 using this full-sib family from five different seed collection years (1974, 1980, 1988, 1989, 

1991) showed similar results, with ratios in all tests exhibiting <75 percent HR type spots (unpublished 

data). There was no evidence of a difference in the ratio of HR:non-HR seedlings for any of the five 

different seed collection years (p-value = 0.14 from a likelihood ratio test). This suggests that inadvertent 

contamination of this full-sib seedlot by other pollens or seedlots is unlikely, but further investigation is 

needed to discern whether this is a possibility. Other possible explanations are discussed below.

The number of HR seedlings with stem symptoms was unexpected. In previous trials, including the 

2002 and 2008 trials with this same family, few or no seedlings of WWP with HR had stem symptoms in 

the absence of known vcr2 sources of rust. The reason for the high incidence of stem infection of HR 

seedlings in this trial is unknown, but it is possible that a very low frequency of vcr2 was present in the 

inoculum and coupled with a very large number of needle infections per seedling resulted in stem 

infections on some seedlings. Another possible explanation is that there could be ‘leakage’ in the HR, at 

least under this high-effective infection level and some environments. Many seedlings had several 

hundred needle spots with some individual needles having more than 15 needle spots (fig.1). It was 

observed that the timing of development of the necrotic bands around the yellow spots could vary even 

within a seedling and that on several seedlings the necrotic band did not fully encircle the spot, suggesting 

that HR was sometimes only partially effective. Some of these seedlings showed stem symptoms at the 

base of such needles. The HR in some plant pathosystems is a temperature-dependent trait (Goodman and 

Novacky 1994, Wang et al. 2009) and that may have had some influence here, but investigations of the 

temperature sensitivity of HR in WWP (and other white pines) have not been done. However, relatively 

high levels of stem symptoms of HR seedlings are not unprecedented. Kinloch and Comstock (1980) 

reported that about 32 percent of sugar pine (P. lambertiana Dougl.) seedlings with HR showed evidence 

of stem symptoms, and that this infection was generally only apparent on seedlings with primary needles 

(versus cotyledons or secondary needles). In sugar pine HR seedlings, the stem infections were abnormal 

and never sporulated.

The latency in stem symptom development observed on HR seedlings in this WWP trial is similar to 

that reported on HR seedlings in sugar pine by Kinloch and Littlefield (1977). They reported that 27 

percent of HR (‘fleck’) phenotypes developed atypical bark symptoms from primary needle infection. The 
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285

bark symptoms resulting from primary needle infection on fleck phenotypes of sugar pine developed at a 

slower rate and were always incompatible and healed by the second year after inoculation. It was 

hypothesized that primary needles may react differently or have a somewhat lower resistance than 

secondary needles (Kinloch and Comstock 1980). Primary needles were the predominant needles present 

on the WWP in this study. In this study, in contrast to S seedlings, the HR seedlings with stem symptoms 

tended to show SSs later, had fewer SSs, and had lower mortality of both seedlings with SSs and those 

showing no SSs. This trial had a very high number of needle spots on the primary needles and a high level 

of stem symptoms on HR seedlings, as well as an unexpected and relatively high mortality of seedlings 

with no stem symptoms (reason unknown). In the earlier 2002 and 2008 trials of this full-sib family, very 

few of the HR seedlings developed stem symptoms and there was little mortality of HR seedlings. Thus, 

if very young WWP seedlings are used, needle spot type would generally be a more reliable indicator of 

presence or absence of HR in WWP than relying on whether stem symptoms are present. 

The nature of the resistance for seedlings showing an HR-like needle spot is still unknown. Some 

recent histological investigations indicate that it is not the classic, very rapid, hypersensitive response that 

often occurs within hours of infection; further histological work is underway to further characterize the 

nature of this needle resistance in WWP (Sweeney et al., Are needle reactions in resistance to Cronartium 

ribicola a hypersensitivity response?, these proceedings). Tissue from the seedlings in this WWP study 

will be used by Dr. Liu’s group for molecular characterization by RNA-seq for discovery of single 

nucleotide polymorphisms (SNPs) between phenotypic groups, with some future work using high 

throughput SNP genotyping technique to find functional genes tightly linked to Cr2 in this full-sib family 

for positional characterization of Cr2.

Acknowledgments

We thank Brian Luis for assistance with assessments and Bohun B. Kinloch Jr. for review of an earlier version of 

this paper.

Literature Cited

Danchok, R.; Sharpe, J.; Bates, K.; Fitzgerald, K.; Kegley, A.; Long, S.; Sniezko, R; Danielson, J.; Spence, R. 

2004. Operational manual for white pine blister rust inoculation at Dorena Genetic Resource Center. 31 p. On 

file with: USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon.

Goodman, R.N.; Novacky, A.J. 1994. The hypersensitive reaction in plants to pathogens—a resistance 

phenomenon. St. Paul, MN: APS Press. 244 p.

Kinloch, B.B., Jr.; Comstock, M. 1980. Cotyledon test for major gene resistance to white pine blister rust in sugar 

pine. Canadian Journal of Botany. 58: 1912–1914.

Kinloch, B.B., Jr.; Littlefield, J.L. 1977. White pine blister rust: hypersensitive resistance in sugar pine. Canadian 

Journal of Botany. 55: 1148–1154.

Kinloch, B.B. ,Jr.; Sniezko, R.A.; Barnes, G.D.; Greathouse, T.E. 1999. A major gene for resistance to white 

pine blister rust in western white pine from the western Cascade Range. Phytopathology. 89(10): 861–867.

Kinloch, B.B., Jr.; Sniezko, R.A.; Dupper, G.E. 2003. Origin and distribution of Cr2, a gene for resistance to 

white pine blister rust in natural populations of western white pine. Phytopathology. 93(6): 691–694.

Kinloch, B.B. ,Jr.; Sniezko, R.A.; Dupper, G.E. 2004. Virulence gene distribution and dynamics of the white pine 

blister rust pathogen in western North America. Phytopathology. 94(7): 751–758.

Wang, Y.; Bao, Z.; Zhu, Y.; Hua, J. 2009. Analysis of temperature modulation of plant defense against biotropic 

microbes. Molecular Plant-Microbe Interactions. 22(5): 498–505.
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Operational Disease Screening Program for 

Resistance to Wilt in Acacia koa in Hawaii1

Nick Dudley,2 Robert James,3 Richard Sniezko,4 Phil Cannon,5 Aileen Yeh,2

Tyler Jones,

2 and Michael Kaufmann2

Introduction

In Hawaii, koa (Acacia koa A. Gray) is a valuable tree species economically, ecologically, and 

culturally. With significant land use change and declines in sugarcane, pineapple, and cattle 

production, there is an opportunity and keen interest in utilizing native koa in reforestation and 

restoration efforts. However, moderate to high mortality rates in many of the low to moderate 

elevation plantings have impeded past efforts (fig. 1). The primary cause for this mortality, 

particularly in young plantings, is thought to be koa wilt, caused by Fusarium oxysporum f. sp. koae

(FOXY) (Gardner 1980). Fusarium oxysporum is a relatively common agricultural and nursery 

fungus, but the origin of strains of FOXY virulent to koa in Hawaii is unknown.

Figure 1—Koa stand killed by Fusarium oxysporum f. sp. koae (FOXY).

 1 With the exception of minor word changes and the addition of photos, this paper was previously published in 

the Hawaii Forestry Association August 29 2009 issue of the Association Newsletter. 2 Hawaii Agriculture Research Center, Forestry, Honolulu, HI, USA. 3 Plant Disease Consulting Northwest, Vancouver, WA, USA. 4 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, OR, USA. 5 USDA Forest Service, Forest Health Protection, Vallejo, CA, USA.

Corresponding author: ndudley@harc-hspa.com.
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Solution

Identifying and developing koa populations that are genetically resistant to virulent strains of FOXY 

may be the key to successful koa restoration and reforestation (Sniezko 2006). Great differences in 

mortality among seed sources in young koa field trials planted in the 1990s were the impetus for 

developing a seedling screening test and investigating genetic resistance to FOXY (Sniezko 2003).

Status of Resistance Breeding Program

A statewide survey was conducted to determine distribution of koa wilt/dieback disease across the 

four main Hawaiian Islands: Kauai, Maui, Oahu, and Hawaii (fig. 2). A total of 386 samples were 

taken at 46 different sites covering approximately 5,597 ha of natural and planted koa forest. Koa 

trees and seedlings infected by F. oxysporum were found on all of the major islands in forest tree 

seedling nurseries and in natural and plantation forests. From these samples, more than 500 isolates of 

F. oxysporum were obtained. Of these, 160 isolates have been tested for virulence on koa seedlings in 

controlled greenhouse inoculation tests. From isolate screening tests, 10 highly virulent isolates have 

been identified for use in screening selected koa families for disease resistance (Dudley et al. 2007).

Figure 2—Koa concentrations and sites with Fusarium oxysporum f. sp. koae (FOXY) in Hawaii.

Between 2006 and 2011, more than 270 koa families were evaluated for their potential FOXY 

resistance in greenhouse tests (fig. 3). Most of the seed lots came from wild populations. However, 

several seed lots were from survivors of family level progeny trials at the Hawaii Agriculture 

Research Center’s (HARC) Maunawili Field Station (fig. 4). All seed lots were open-pollinated. A 

composite of five virulent isolates of FOXY were used for inoculation (Dudley et al. 2007). Seedling 

wilting and mortality in the greenhouse was monitored over a 90-day period for each test. Seedling 

mortality among seed lots varied widely (4 to 100 percent) and averaged 60 percent (Dudley et al. 

2009). These initial results indicate that natural resistance to FOXY is low within native koa 

populations.
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Figure 3—Koa disease screening trial at the Hawaii Agriculture Research Center (HARC).

Figure 4—Koa seed orchard at the Hawaii Agriculture Research Center (HARC).

Continued screening of additional koa families for pathogen resistance, retesting putative resistant 

families, and developing koa seed orchards with disease-resistant stock are either ongoing or planned. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Literature Cited

Dudley, N.S.; James R.L.; Sniezko R.A.; Yeh A. 2007. Investigating koa wilt and dieback in Hawai’ipathogenicity of Fusarium species on Acacia koa seedlings. Native Plants. : 259–266.

Dudley, N.S.; Sniezko, R.A.; James, R.L.; Cannon, P.; Jones, T.; Yeh, A.; Kaufman M. 2009. Developing 

resistant koa-early results from survey to seedlings resistance testing in Hawaii. In: Cram, M.M., ed. 

Proceedings of the 7th meeting of IUFRO Working Party 7.03.04. Diseases and insects in forest nurseries.

Forest Health Protection Report 10-01-01. Atlanta, GA: U.S. Department of Agriculture, Forest Service, 

Southern Region: 39–47. 

Gardner, D.E. 1980. Acacia koa seedling wilt caused by Fusarium oxysporum. Phytopathology. 70: 594–597.

Sniezko, R.A. 2003. Potential for selecting for genetic resistance to F. oxysporum (koa wilt) in koa for 

conservation, restoration and utilization in Hawaii. Trip report (8/23/2003). Cottage Grove, OR: U.S.

Department of Agriculture, Forest Service, Dorena Genetic Resource Center. 13 p. 

Sniezko, R.A. 2006. Resistance breeding against nonnative pathogens in forest trees–current successes in North 

America. Canadian Journal of Plant Pathology. 28: S270–S279.
GENERAL TECHNICAL REPORT PSW-GTR-240

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A First Look at Genetic Variation in Resistance to 

the Root Pathogen Phytophthora cinnamomi using 

a Range-wide Collection of Pacific Madrone 

(Arbutus menziesii)

Marianne Elliott,

1 Gary A. Chastagner,1 Annie DeBauw,1 Gil Dermott,1 and 

Richard A. Sniezko2

Introduction

Phytophthora cinnamomi (Oomycetes) causes root disease and basal canker on a number of 

hardwood and conifer hosts, including Pacific madrone (Arbutus menziesii Pursh) (figs. 1, 2), a 

broadleaf evergreen species whose range extends from coastal British Columbia to southern 

California (Reeves 2007). Increasing mortality of Pacific madrone and the related shrub species 

manzanita (Arctostaphylos spp.) has been seen in California forests (Fichtner et al. 2009) in recent 

years.

Considered to be one of the world’s most invasive species, P. cinnamomi is thought to have 

originated in southeast Asia and spread worldwide, causing decline in Australian eucalypt forests, 

crop losses on avocado in California and Europe, and on Christmas tree plantations in the southeast 

United States. It has a wide host range and infects many ornamental trees and shrubs in landscape 

plantings. In this situation, it is thought that the pathogen moves in contaminated soil on nursery stock 

(Smith 1988).

Materials and Methods

Between 2006 and 2008, Pacific madrone (Arbutus menziesii Pursh) fruit was collected from 237 

trees at various sites throughout its natural distribution range in British Columbia, California, Oregon, 

and Washington and extracted seed was added to the Washington State University (WSU) Puyallup 

madrone seed collection. From this collection, seedlings were grown from 34 families representing 

seven ecoregions for the study (table 1). One to five families per geographic location within one or 

two locations in each ecoregion were sampled. Twelve one-year-old seedlings per family were 

arranged randomly in rows on the greenhouse benches. Six seedlings per family were inoculated with 

an isolate of P. cinnamomi collected from madrone roots in Sonoma County, California, and the 

remaining six were not inoculated. After inoculation, all plant pots were placed in 946 ml plastic cups 

which were then flooded with water. Plants remained submerged in water for 2 days.

Seedlings were then observed for wilting and browning symptoms (fig. 3) over a 6- week period 

after which roots and stems of inoculated and check plants were sampled for dry weight (table 2). 

Principal components analysis was performed on mean values of all variables arranged by seedlot

(family) after standardization (z-score). Non-parametric ANOVA (Kruskal-Wallis test) was 

performed on the original variables grouped by site and ecoregion, and Dunn’s post-hoc test was 

performed if the differences were significant at P = 0.05.

 1 Washington State University, Research and Extension Center, 2606 West Pioneer, Puyallup, WA 98731. 2 USDA Forest Service, Dorena Genetic Resource Center, 34963 Shoreview Road, Cottage Grove, OR 97424. 

Corresponding author: melliott2@wsu.edu.
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Figure 1 (left)—Phytophthora cinnamomi in V8 culture The organism can be identified by clusters of 

chlamydospores and coralloid hyphae. (Photo: Jeff Kepley)

Figure 2 (right)—Madrones in Soquel State Demonstration Forest, California. These trees have 

symptoms of crown dieback and general decline. Phytophthora cinnamomi was baited out of soil from 

this area. Horsetail (Equisetum spp.) is growing on the site, which is near the bottom of a slope, and 

is indicative of a wet site. 

Table 1—Sites and ecoregions in western North America where Pacific madrone seed sources 

used in this experiment originated

Site Ecoregion Number of seed sources

BC16 Coastal western hemlock/Sitka spruce 1

BC14 Pacific and Nass ranges 1

FB, FW Strait of Georgia, Puget lowland 10

WV Willamette Valley 2

CP, KL Klamath mountains 10

PL Sierra Nevada 5

CC, SC California coastal sage, chaparral, and oak woodlands 5
GENERAL TECHNICAL REPORT PSW-GTR-240

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Figure 3—Madrone seedlings in greenhouse 21 days after inoculation. Some variability in symptom 

expression can be seen. The seedling on the left is dead where the seedling on the right is relatively 

healthy.

Table 2—Data collected on Pacific madrone seedlings inoculated with Phytophthora

cinnamomi in the greenhouse trial

Variable Description

BR Disease severity - 0-10: 0 = 0% brown foliage, 1= 1-10% brown, 2=11-20% brown, 10= 91-

100% brown, dead plant.

WR Wilt rating - 0-4: 0 = no disease, 1= slight disease, 2 = Moderate disease, 3 = severe, 4= 

dead plant.

RR Root ball size rating - 1-5: 1 = smallest, 5 = largest

RDW Root dry wt, grams

SDW Stem dry wt, grams

Results and Discussion

Variability within seed sources was high for root dry weight in both inoculated and check treatments 

and lower for other variables. Phytophthora cinnamomi was isolated from the roots of all inoculated 

plants and not from the roots of check plants. 

No relationship between ecoregion and any of the variables was observed. There were significant 

differences in disease severity on inoculated plants among sites, with the least disease seen on 

seedlings obtained from parent trees at Fidalgo Bay, Washington (fig. 4). There was no significant 

difference in disease severity among families when considered separately; however, agglomerative 

hierarchical clustering produced three groups with low, intermediate, and high disease severity (table 

3). 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Figure 4—Severity of browning and wilting symptoms on madrone seedlings from seed collected at 

10 sites. (Kruskal-Wallis test with Dunn’s multiple comparisons. Bars with different letters are 

significantly different at p = 0.05).

Table 3—Groupings of families by severity of browning symptoms on plants inoculated with 

Phytophthora cinnamomi using agglomerative hierarchical clustering (AHC) 

Class Disease severity 

ratinga

Families

1 6.0 FB3, FB5, FB8, FW10, KL8

2 7.5 BC14, CC2, CC3, CC7, CP18, CP21, CP23, CP24, FB1, FB6, 

FW5, FW7, FW9, KL1, KL2, PL11, PL4, SC9, WV2

3 8.9 BC16, CC1, CP11, FW3, KL10, KL7, PL3, PL5, PL9, WV5

a Disease severity rating is given as the class centroid for each group.

Root ball size rating (RR) and root dry weight (RDW) of inoculated plants were negatively 

correlated with the disease severity rating (r = -0.459 RR, -0.424 RDW). The root dry weight, stem 

dry weight, and root ball size rating on check plants showed no relationship with disease ratings on 

inoculated plants from the same seed sources. The first two principal components accounted for 63% 

of the variation in the data set (Figure 5). The first principal component was related to root and shoot 

weights and root ball rating (inoculated and check plants), and the second to disease severity 

(inoculated plants). Thus, plant size was not a predictor of disease development in this experiment.

This initial study indicated that while madrones from some seed sources had less severe symptoms 

than others, there was no significant relationship of disease severity with plant characteristics

measured, such as root ball size and stem dry weight. To confirm these results, this experiment was 

repeated in fall 2011 using seedlings from the same seedlots. Results from 2011 were similar to those 

from 2010.

0 1 2 3 4 5 6 7 8 9 10

Santa Clara, CA

China Camp SP, CA

Placerville, CA

Klamath Nat'l...

Central Point, OR

Willamette Valley, OR

Federal Way, WA

Fidalgo Bay, WA

Gold River, BC

Saltery Bay, BC

Disease severity rating...

AB

AB

AB

AB

AB

AB

B

B

B

A

B
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Figure 5—Scatter plot showing results of principal components analysis on the variables measured in 

this study. Individual seed sources are labeled by site and tree number.

Literature Cited

Fichtner, E.J.; Rizzo, D.M.; Swiecki, T.J.; Bernhardt, E.A. 2009. Emergence of Phytophthora cinnamomi in 

a sudden oak death-impacted forest. In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010.

Proceedings of the sudden oak death fourth science symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, 

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 378 p.

Reeves, S.L. 2007. Arbutus menziesii. In: Fire effects information system, [online]. U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer).

http://www.fs.fed.us/database/feis/. (06 October 2011).

Smith, P.M. 1988. Phytophthora cinnamomi. In: Smith, I.M.; Dunez, J.; Lelliott, R.A.; Phillips, D.H.; Archer, 

S.A., eds. European handbook of plant diseases. Oxford, UK: Blackwell Scientific Publications: 213–215.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Range-wide Genetic Variability in Pacific Madrone 

(Arbutus menziesii): Examining Disease Resistance, 

Growth, and Survival in a Common Garden Study

Marianne Elliott,

1 Gary A. Chastagner,1 Gil Dermott,1 Alan Kanaskie,2 

Richard A. Sniezko,

3 and Jim Hamlin4

Introduction

Pacific madrone (Arbutus menziesii Pursh, Ericaceae) is an important evergreen hardwood species in 

Pacific Northwest (PNW) forests that provides food and habitat for wildlife and has high value in 

urban environments. Reeves (2007) indicates that Pacific madrone provides habitat for numerous 

wildlife species, especially cavity-nesting birds. Its evergreen foliage provides browse, especially in 

the winter, for a number of animals. The berries are an important food for deer, birds, and other small 

mammals because they are produced in large quantities and may persist on the tree in winter when 

alternative food sources are limited (Dayton 1931). Reeves (2007) also indicates that it provides 

excellent erosion control and slope stabilization and is highly prized as an ornamental species for its 

crooked beauty, colorful bark, showy flowers, and brightly colored fruits. Pacific madrone is 

relatively drought tolerant, which makes it desirable in urban habitats. Native American tribes have 

also used various portions of this tree for food, utensils, and medicinal purposes (Arno et al. 1977, 

Dayton 1931).

The species has been in decline for several decades due to a combination of factors such as climate 

change, forest management, and several endemic fungal pathogens. Several diseases affect the health 

of Pacific madrone throughout its range from southern British Columbia (lat. 50  ̊N) to southern 

California (lat. 33  ̊N). These include the endemic canker disease caused by species of Fusicoccum 

(F. arbuti and F. aesculi), numerous foliar pathogens, and introduced diseases such as Phytophthora 

ramorum and P. cinnamomi. Changes in climate over the past 100 years as well as the introduction of 

P. ramorum have resulted in an increase in disease incidence and tree mortality in western North 

American forests. Phytophthora ramorum, F. arbuti, and Phacidiopycnis washingtonensis, a newly 

identified potential foliar pathogen, are cool-temperature organisms, having optimum temperatures 

for growth below 25  ̊C (Farr et al. 2005, Werres et al. 2001, Xiao et al. 2005). Damage from foliar 

blight attributed to P. washingtonensis was especially severe in 2010, which was a strong La Niña 

year resulting in extended periods of cool, wet weather in the Pacific Northwest.

Many foliar and canker diseases are more severe during periods of increased wetness and warm 

temperatures. An example of increased foliar disease due to climate change is the Dothistroma needle 

blight outbreak on lodgepole pine in Canada (Woods et al. 2005). The higher incidence of observed 

foliar disease and new canker infections on Pacific madrone may be attributed to the increased 

frequency of warmer, wetter spring weather during the last half of the 20th century. Phytophthora 

cinnamomi, which causes root disease, is favored by high soil temperatures and has increased in 

incidence in California forests. The incidence and severity of these diseases is likely to be affected by 

 1 Washington State University, Research and Extension Center, 2606 West Pioneer, Puyallup, WA 98731. 2 Oregon Department of Forestry, 2600 State Street, Salem, OR 97310. 3 USDA Forest Service, Dorena Genetic Resource Center, 34963 Shoreview Road, Cottage Grove, OR 97424. 4 USDA Forest Service, Umpqua National Forest, 2900 NW Stewart Parkway, Roseburg, OR 97471.

Corresponding author: melliott2@wsu.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

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changes in temperature and precipitation and may cause certain populations of Pacific madrone to go 

extinct under predicted climate change scenarios. A better understanding of the epidemiology of these 

pathogens on Pacific madrone and the level of genetic resistance among different seed sources is 

needed to assess this risk.

Very little is known about the range of genetic variability in Pacific madrone. A Canadian study 

found that genetic diversity was low within populations in the northern part of the range (Beland et al. 

2005), and we believe this will be the first common garden study of the species. Establishing field 

provenance trials of this species over multiple sites will give baseline information on the genetic 

variation in a range of adaptive traits and will provide guidance for its management under changing 

climate.

Project Description and Objectives

This project will use a range-wide collection of Pacific madrone to examine genetic variability in a 

range of traits. Using material from the Washington State University Pacific madrone seed collection 

(seed collected from 2006 to 2010), common garden plantings consisting of 105 families collected 

from seven ecoregions were planted in five locations in California, Oregon, and Washington during 

fall 2011 and winter 2012 (fig. 1). A separate sowing is planned for a planting site in British 

Columbia. Smaller demonstration plantings with a few trees taken from a range of widely different 

geographic seed sources, and including other Arbutus species from the western states, are planned at 

other locations to be determined.

Specific objectives include: 

1) Screen for resistance to multiple pathogens such as P. ramorum, P. cinnamomi, and endemic 

canker and foliar pathogens. 

2) Examine variation in growth and adaptive traits in the nursery and at multiple field sites. 

3) Identify seed sources or populations that may contain individuals that are best adapted to 

climate change and for urban and restoration plantings.

Seedling Production

Seed from 125 families of Pacific madrone representing seven ecoregions were cleaned and weighed 

in lots of 100 seed each prior to sowing in plug trays. Plug trays were watered and placed into a 1.1 

 ̊C cooler for stratification in February 2011. Trays were removed from the cooler in April 2011 and 

placed in a greenhouse. Fertilizer was applied twice a week at the rate of 200 ppm N with Technigro 

20-9-20 soluble. Seedling germination data was collected in June 2011. No significant relationship 

between seed weight and percent germination was seen.

Plugs were transplanted into Treepots with media consisting of 50 percent Specialty Soils standard 

greenhouse mix + 50 percent fine fir bark + 3 kg/m slow release Plantacote 14-9-15. The seedlings 

were grown outside until planting (fig. 2). Common garden sites in the United States were planted in 

fall 2011/winter 2012. Seed was shipped to British Columbia for sowing and planting in fall 

2012/winter 2013.

Data collected on these seedlings prior to planting included height, stem diameter, leaf area, and 

leaf color (fig. 3). Preliminary data collected on a subset of the seedlings in August 2011 and on 

seedlings from a 2008 sowing of some of the same families indicates that there is some variability in 

these traits within and among families.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

297

Figure 1—Map showing Pacific madrone range, ecoregions, seed collection sites, and locations for 

common garden plantings in western North America. The range of Pacific madrone extends to San 

Diego County, California, but the species occurs rarely in the far southern portion of the range.
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Figure 2—Madrone seedlings in the container nursery at WSU-Puyallup after transplanting from plug 

trays. These seedlings were planted in the common garden sites in the United States in winter 2011.

Figure 3—Differences in leaf color in Pacific madrone seedlings. The seedlings on the left have more 

red pigment than those on the right. Quantitative differences in color were measured using a Minolta 

CR200b Chroma Meter.
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Transplanting Madrone Seedlings

Pacific madrone has a reputation for being difficult to transplant since its roots react poorly to 

disturbance. Before transplanting into the common garden sites, a study was undertaken to determine 

the best methods for planting madrone seedlings. Two-year-old seedlings from three families were 

subjected to five treatments based on methods used in Hummel et al. (2008) and planted in December 

2010. Seedlings were examined in May 2011. Treatments that involved washing the roots resulted in 

the most seedling mortality (fig. 4). Seedling mortality was between 5 and 10 percent in the unwashed 

treatments and 30 to 50 percent in the two treatments that involved washing the roots. Resprouting 

was observed on some seedlings that had died back, indicating that the root system may still be 

functioning. Total mortality and final measurements of height, stem diameter, and dry weight will be 

taken after two growing seasons.

Figure 4—Effects of several transplanting methods on the growth of two-year-old Pacific madrone 

seedlings. Bars with different letters are significantly different at p = 0.05, Kruskal-Wallis test with 

Dunn’s multiple comparisons.

Summary

We believe this project represents the first common garden study involving a range-wide collection of 

Pacific madrone. This project will provide baseline information on the genetic variation in a range of 

adaptive traits and will provide guidance for the management of this species under changing climatic 

conditions. Preliminary data on the seedlings grown for the common garden study indicates that 

variability is present in a number of plant characteristics. Other studies to assess the range of 

resistance to various pathogens, such as P. cinnamomi and P. ramorum, have been conducted in the 

greenhouse and biocontainment facilities at WSU Puyallup.

0.0 1.0 2.0 3.0 4.0 5.0

Cut off bottom 1/3 of root system

Cut and spread roots

Plant directly from pot

Wash potting medium from root system

Wash and cut off bottom 1/3 of roots

Seedling health rating

Madrone transplanting study

Healthy Dead

A

A

B

B

B
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In addition to the common garden field plots, a series of long-term disease monitoring plots in 

naturally occurring stands of Pacific madrone near the common garden test sites are planned to 

determine which pathogens are present near these sites. These plots will be monitored at several 

intervals to observe changes in pathogen populations and disease severity due to fluctuations in 

climate or other factors. For example, it is known that P. ramorum and P. cinnamomi have been 

detected in the vicinity of the California site. Damage attributed to the foliar pathogen P.

washingtonensis and others has been observed throughout the range of the species. This provides an 

opportunity to formally document portions of the range and the incidence of these pathogens and to 

study field resistance to several important pathogens of Pacific madrone.

The potential for many projects including molecular phylogenetic studies of Pacific madrone 

exists and we invite other scientists with an interest in working on special assessments from these 

tests to contact us. More information and updates on the project will be available on the website: 

http://www.puyallup.wsu.edu/ppo/madrone/.

Acknowledgments

The authors wish to thank everyone who helped to collect seed, identify, plant, and provide possible sites for the 

common garden and demonstration plantings (B.C. Ministry of Forests, BLM, Starker Forests, CalFire, WSU, 

Willamette University), and with growing and maintaining the seedlings. Funding for this project was provided 

by the USDA Forest Service, Forest Health Protection.

Literature Cited

Arno, S.F.; Hammerly, R.P. 1977. Northwest trees. Identifying and understanding the region's native trees. 

Seattle, WA: The Mountaineers. 222 p. 

Dayton, W.A. 1931. Important western browse plants. Misc. Publ. No. 101. Washington, DC: U.S. Department 

of Agriculture. 214 p.

Beland, J.D.; Krakowski, J.; Ritland, C.E.; Ritland, K.; El-Kassaby, Y.A. 2005. Genetic structure and

mating system of northern Arbutus menziesii (Ericaceae) populations. Canadian Journal of Botany. 83: 

1581–1589.

Farr, D.F.; Elliott, M.; Rossman, A.Y.; Edmonds, R.L. 2005. Fusicoccum arbuti sp. nov. causing cankers on 

Pacific madrone in western North America with notes on Fusicoccum dimidiatum, the correct name for 

Scytalidum dimidatum and Nattrassia mangiferae. Mycologia. 97(3): 730–741.

Hummel, R.L.; Evans, E.; Riley, R. 2008. Effect of mechanical root disruption at transplant on short- and 

longer-term growth and survival of Scotch pine (Pinus sylvestris) and shore pine (Pinus contorta var. 

contorta). In: Watson, G.; Costello, L.; Scharenbroch, B.; Gilman, E., eds. The landscape below ground III. 

Proceedings of an international workshop on tree root development in urban soils. Champaign, IL; ISA:

211–221.

Reeves, S.L. 2007. Arbutus menziesii. In: Fire effects information system. [Online]. U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). 

http://www.fs.fed.us/database/feis/. (06 October 2011).

Werres, S.; Marwitz, R.; Man in 't Veld, W.A.; De Cock, A.W.; Bonants, P.J.M.; De Weerdt, M.; 

Themann, K.; Ilieva, E.; Baayen, R.P. 2001. Phytophthora ramorum sp. nov: a new pathogen on 

Rhododendron and Viburnum. Mycological Research. 105(10): 1155–1165.

Woods, A.; Coates, K.D.; Hamann, A. 2005. Is an unprecedented Dothistroma needle blight epidemic related 

to climate change? BioScience. 55(9): 761–769.

Xiao, C.L; Rogers, J.D.; Kim, Y.K.; Liu, Q. 2005. Phacidiopycnis washingtonensis – a new species 

associated with pome fruits from Washington State. Mycologia. 97(2): 464–473.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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Development of Screening Trials to Rank Pinus 

radiata Genotypes for Resistance to Defoliation by 

Monterey Pine Aphid (Essigella californica)

Stephen Elms,1 Peter Ades,2 and Nick Collet3

Abstract

The Monterey pine aphid (Essigella californica) is a recent arrival in Australia, having first been detected in 

1998. It quickly spread throughout the national radiata pine (Pinus radiata D. Don) plantation estate, 

causing seasonal defoliation and compromising tree growth in many areas. Selection of resistant radiata 

pine for deployment commenced in 2002, based on heritable defoliation in mature progeny trials. Further 

development of the breeding program requires efficient methods for screening later generation germplasm.

Glasshouse and nursery-based screening trials, involving infestation of potted radiata pine plants with 

aphids, were used to develop guidelines for operational scale screening of defoliation resistance. Three 

types of plant material were tested: physiologically aged clonal cuttings (10 to 30 years); 1-year-old clones 

derived from somatic embryogenesis; and 1-year-old seedling families. All of the plant types examined 

supported dramatic increases in aphid populations under suitable environmental conditions. Variability 

between plants within clones and between clones was high. For example, over a period of 28 days, one m 

tall aged cuttings infested with 30 aphids yielded between 26 and 3,804 aphids (mean 584). Significant and 

repeatable differences in aphid population counts among different radiata pine genotypes and families were 

obtained using trial durations of 18 to 35 days.

Treatment options, including environmental conditions, initial infestation levels, and replication 

requirements, were examined. Specific attention needs to be paid to managing peak aphid production levels 

by infestation level and experiment duration in order to minimize alate initiation. 

Glasshouse screening results were tested by counting aphids on 1-year-old clonal cuttings in the field, and 

on 1-year-old stool beds. Both showed good correlation with glasshouse trial results (r=0.45 and 0.93). 

Damage to foliage and eventual loss of foliage, was observed where aphid counts were high, suggesting that 

identification and deployment of genetic material which does not support rapid aphid population buildup is 

likely to result in reduced defoliation and reduced growth losses in plantations due to Essigella activity. 

 1 HVP Plantations, PO Box 385, Churchill, Victoria, Australia 3842. 2 University of Melbourne, Victoria, Australia 3010.

3 DPI Victoria, Melbourne, Victoria, Australia.

Corresponding author: selms@hvp.com.au.
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Tree-Mediated Interactions Between the Jack Pine 

Budworm and a Mountain Pine Beetle Fungal 

Associate

Nadir Erbilgin1 and Jessie Colgan1

Abstract

Coniferous trees deploy a combination of constitutive (pre-existing) and induced (post-invasion) structural and 

biochemical defenses against invaders. Induced responses can also alter host suitability for other organisms 

sharing the same host, which may result in indirect, plant-mediated, interactions between different species of 

attacking organisms. Current range and host expansion of the mountain pine beetle (Dendroctonus ponderosae, 

MPB) from lodgepole pine (Pinus contorta Douglas ex Loudon)-dominated forests to the jack pine (Pinus 

banksiana Lamb.)-dominated boreal forests provides a unique opportunity to investigate whether the 

colonization of jack pine by MPB will be affected by induced responses of jack pine to a native herbaceous 

insect species, the jack pine budworm (Choristoneura pinus pinus, JPBW). We simulated MPB attacks with one 

of its fungal associates, Grosmannia clavigera, and tested induction of either herbivory by JPBW or inoculation 

with the fungus followed by a challenge treatment with the other organism on jack pine seedlings and measured 

and compared monoterpene responses in needle. There was clear evidence of an increase in jack pine resistance 

to G. clavigera with prior herbivory, indicated by smaller lesions in response to fungal inoculations. In contrast, 

although needle monoterpenes greatly increased after G. clavigera inoculation and continued to increase during 

the herbivory challenge, JPBW growth was not affected. However, JPBW increased feeding rate to possibly 

compensate for altered host quality. Jack pine responses varied greatly and depended on whether seedlings were 

treated with single or multiple organisms, and their order of damage.

 1 University of Alberta, 4-42 ESB, Edmonton, Alberta, Canada T6G2E3.

Corresponding author: erbilgin@ualberta.ca.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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The Chapter Breeding Program of the 

American Chestnut Foundation

Sara Fitzsimmons,1 Kendra Gurney,2 William White,3 and Katy McCune4

Abstract

A unique feature of the American Chestnut Foundation breeding program is the use of volunteers to conduct 

most of the regional breeding that will help increase genetic diversity and preserve local adaptation in the 

products of our program. This effort is coordinated by the four authors of this abstract, who are employees of 

the Foundation. The Foundation has 16 state chapters spread over almost the entire range of the American 

chestnut (Castanea dentata (Marsh.) Borkh.). Fourteen chapters have active breeding programs. The chapter 

with the most advanced program, Pennsylvania, has been planting a B3-F2 seedling seed orchard for a number 

of years, and is starting to harvest B3-F3 seed from the selections. The stages of breeding at other chapters 

range from making B3 crosses, to making selections, to beginning to plant B3-F2 orchards.

The chapters make one backcross onto local American chestnut trees using pollen from B2 and B3 trees at the 

Foundation’s professionally operated Meadowview facilities. Most chapters have been using the ‘Graves’ and 

‘Clapper’ sources of blight resistance, since these exist in about 30 unique American backgrounds each. The 

chapters endeavor to cross pollen from 20 backgrounds onto 20 local chestnut trees; 20 backgrounds yields a 

reasonable prospect of avoiding population collapse from inbreeding depression, with an inbreeding effective 

population size of about 72. Collectively across all units, the inbreeding Ne (effective breeding number) for 

‘Graves’ and ‘Clapper’ combined exceeds 500.

Full recovery of local adaptation may not occur with only one cross onto local trees, but may enable that 

adaptation to be recovered by natural selection. As the number of regional science coordinators has increased, 

and the collective knowledge of the chapters has increased, the ability of chapters to embark on more complex 

breeding endeavors has increased. Increasing additional sources of resistance into 20 American backgrounds 

with two cycles of backcrossing should be possible.

The regional science coordinators also are developing a curriculum to train volunteers to plant and monitor 

forest test plantings. This will need to be a focused, long-term, effort should the performance of the trees be 

sufficient. It should be an interesting process, hopefully increasing awareness of the forest as well as producing 

valuable data.

 1 206 Forest Resources Lab, Pennsylvania State University, University Park, PA 16802. 2 U.S. Forest Service, Northern Research Station, 705 Spear St., South Burlington, VT 05403.

3 Department of Biology and Environmental Science, University of Tennessee - Chattanooga, 615 McCallie Ave., 

Chattanooga, TN 37403.

4 Virginia Department of Forestry Central Office, 900 Natural Resources Dr., Charlottesville, VA 22903.

Corresponding author: Fred@acf.org.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Needle Terpenoid Composition of Pinus halepensis 

(Mill.) Trees Infested by the Scale Insect Marchalina 

hellenica (Genn.) in Greece

Athanassios Gallis,1 Carlos Arrabal,2 Aristotle C. Papageorgiou,3 and 

Maria C. Garcia-Vallejo4

Abstract

Needle terpenoid composition was determined by using GLC-MS in Pinus halepensis (Mill.) trees that were

infested and not infested by the scale insect Marchalina hellenica. The study area was within the Forest 

National Park of the Cape Sounion, southern Attica region, Greece. A total of 43 compounds, 32 of which were

identified, were detected, including monoterpenes, sesquiterpenes, and neutral diterpenes. The healthy trees 

showed higher mean concentration for the monoterpenes fraction as well as for the diterpene fraction than the 

mean concentration of infested trees; whereas, the concentration for the sesquiterpene fraction was more or less 

similar in infested and non-infested trees. The statistical analysis of terpene data showed the existence of 

quantitative differences between healthy and infested trees mainly for the components β-caryophyllene, 

neoabietal, α- humulene, cembrene, and neoabietol. A Ward cluster analysis based on selected major 

compounds classified all trees in two chemotypes, with the majority of healthy trees belonging to one 

chemotype and most of the infested trees belonging to the second.

Key words: Pinus halepensis, Marchalina hellenica, needle terpenoids

Introduction

Marchalina hellenica is a scale insect which in Greece infests mainly Pinus halepensis (Mill.) and 

Pinus brutia (Ten.) (Avtzis 1985). The insect attack results in the production of honeydew, which is 

used as a feeding substrate by honeybees and converted into honey (Erlinghagen 2001, Gounari 

2006). During the mid-1990s, M. hellenica was artificially introduced into the pinewoods of the 

Attica region in Greece in order to increase the total honey yield. The artificial infestations resulted in

an ecological imbalance in the ecosystems of regional Aleppo pine. Current field observations show 

the overpopulated occurrence of the insect in the pinewoods of the area (Gallis 2007). Terpene 

composition in conifers is usually strongly inherited, not greatly affected by environmental 

conditions, and offers a valuable tool to study several scientific problems such as: hybrid 

identification, introgression, tree resistance to insect attacks and diseases, and others (Hannover 1992, 

Squillace 1987). The objectives of this study are: a) to determine the qualitative and quantitative 

terpenoid composition in the needles of Aleppo pine trees that are infested and not infested by the 

scale insect M. hellenica, and b) to investigate if the needle terpenoid composition could be used to 

study the parasitism of the scale insect to Aleppo pine trees.

Materials and Methods

All trees investigated were sampled from two natural stands of Aleppo pine: the “Markati” and “Agia 

Triada” in the area of the National Park of Cape Sounion, southern Attica region, Greece. A total of 

 1 Decentralized Administration of Macedonia and Thrace, 11, Professor Rossidis Street, GR-54008, Thessaloniki, Greece. 2 Departamento de Ingenieria Forestal, ETSI Montes, Universidad Polytechnica de Madrid, Cuidad Universitaria, 28040, 

Madrid, Spain.

3 Department of Forestry, Environment and Natural Resources, Democritus University of Thrace, P.O. Box 129, Pantazidou 

193, GR – 68200, Orestiada, Greece. 4 Departmento de Industrias Forestalaes, INIA-CIFOR, Apartado 8188, 28080, Madrid, Spain.

Corresponding author: tgallis@hotmail.com.
Proceedings of the 4the International Workshop on Genetics of Host-Parasite Interactions in Forestry

305

22 trees, including 11 trees infested by M. hellenica and 11 healthy trees (not infested), were sampled 

in late March 2006. The sampling of the infested trees in the field depended on the macroscopic 

evaluation of the insect’s attack on the tree as described in the literature (Erlinghagen 2001, Gounari 

2006), including: white cottonish secretions, the degree of tree desiccation, and the existence and the 

extent of drained needles. The trees were of similar age (over 30 years old) in both stands, have been 

subjected to the same silvicultural treatments, and so far have been protected effectively from forest 

fires. As mentioned above, healthy and infested Aleppo pine trees occurred in the same area in both 

sampled stands in our study. Gounari (2006) states that it is normal that both infested and noninfested Aleppo pine trees occur in the same area. This differentiation is due to phenol-immunity 

phenomenon. The differentiation could also be permanent or temporary and periodic, with permanent

immunity controlled genetically (Smirnoff and Valero 1975).

From each tree, 1-year-old needles were collected from the upper 1/3 part of the crown and from 

the same side to avoid any epigenetic variation in terpene composition due to tissue age, crown 

position, grown conditions, etc. After collection, the samples were put in plastic bags and stored in a 

refrigerator at -20 o

C for about 2 months. The needle samples were transferred into a portable ultra 

low freezer with CO2 (dry ice) and transported the same day by plane to Spain. Chemical analysis 

was conducted by the Department of Forestry Engineering, Escuela Tecnica Superior de Ingenieros 

(ETSI), Montes, Polytechnic University of Madrid, Spain. 

The needles were cut into small pieces (2 to 4 mm). A known weight of needles (3 g approx.) was 

extracted with 5 ml petroleum ether/diethyl ether (1:1) for 24 hours at 4 oC. Isobuthylbencene (60 

μg/ml), heptadecane (60 μg/ml), and heptadecanoic acid (60 μg/ml) were used as internal standards.

One μl of the mixture was injected into a gas chromatograph. The samples were analyzed with an

HP5890A gas chromatograph and connected to an HP 5971 mass detector (EI, 70 eV). The following 

temperature program was used: the oven temperature was initially at 60 oC, increasing at a rate of 4 o

C/minutes to a final temperature of 270oC, and was held for 10 min. The injector’s temperature was 

set at 260 oC and the detector’s temperature was set at 300 oC. A DB-5 capillary column, 30 m x 0.25

mm (0.25 μm film thickness), was used for component separation. Helium was used as a carrier gas 

with 15 psi column head pressure and a split ratio of 1:50. For quantitative measurements, by the 

internal standard method, additional injections of replicate samples were made using a flame 

ionization detector under the same working conditions. All compounds were identified on the basis of 

their retention time and their electron impact (EI) mass spectra by comparing them with those in the 

database and literature. Hewlett Packard Chemstation software was used for peaks integration.

Components were quantified as a percentage contribution of each peak to the total terpenoids found in 

the chromatogram, i.e. total oleoresin basis. The concentration of each terpenoid fraction 

(monoterpenes, sesquiterpenes, and neutral diterpenes) was also calculated, using the internal 

standard method. For statistical analysis, a Statistical Packages for Social Sciences statistical package 

(SPSS/PC) was used. Thirteen components were selected due to their presence in amounts more than 

2 percent in most samples. To separate the infested and healthy pine trees, a cluster analysis (Ward, 

squared Euclidian distance) on the basis of the percentage amounts of the selected components was 

used. A discriminate analysis was performed to see which components have the largest effect in 

classifying trees as healthy or infested. Τhe percentages of the components were transformed into 

arcsine - square root functions (Kung 1988) before statistical analysis. 

Results 

A total of 43 compounds were detected in current year needles of Aleppo pine trees analyzed by Gas 

Liquid Chromatography Mass Spectrometry (GLC-MS), including monoterpenes, sesquiterpenes, and

neutral diterpenes (table 1).
GENERAL TECHNICAL REPORT PSW-GTR-240

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Table 1—Terpenoid composition (total oleoresin basis) in Pinus halepensis needles

Components Healthy Trees Wounded Trees

% mean std % mean std

1 α-thujene 0.13 0.19 0.19 0.15

2 ●a α-pinene 5.30 1.96 4.52 2.25

3 camphene 0.95 1.03 0.01 0.02

4 ● sabinene 2.41 4.19 2.01 1.04

5 β-pinene 0.78 0.69 0.47 0.26

6 ● myrcene 4.87 4.68 5.88 4.43

7 3-δ-carene 0.31 0.55 0.62 0.57

8 α-terpinene 0.20 0.23 0.10 0.22

9 limonene+phellandrene 0.51 0.79 0.31 0.21

10 trans-β-ocimene 0.15 0.21 0.17 0.18

11 γ-terpinene 0.13 0.19 0.14 0.14

12 ● terpinolene 2.47 2.82 2.03 0.97

13 terpine-4-ol 0.05 0.12 0.02 0.04

14 α-terpineol 0.01 0.04 0.03 0.10

15 bornyl acetate 0.02 0.05 0.11 0.20

16 α-copaene 0.24 0.12 0.22 0.22

17 ● β-caryophyllene 11.89 1.96 14.01 2.42

18 ● α-humulene * 2.03 0.32 2.39 0.41

19 germacrene-D 1.23 1.05 0.96 0.32

20 ● phenylethyl isovaleranate 3.99 1.33 3.31 1.59

21 α-Muurolene 0.31 0.20 0.19 0.14

22 δ- cadinene 0.39 0.25 0.49 0.19

23 β-cadinene 0.22 0.18 0.17 0.19

24 α-bisalolene 0.17 0.09 0.13 0.12

25 β-elemene 0.25 0.13 0.08 0.07

26 M+220 0.24 0.13 0.21 0.19

27 M+222 0.30 0.20 0.33 0.15

28 M+222 0.26 0.16 0.26 0.05

29 ● cembrene 33.03 11.27 24.69 9.22

30 M+286 (Diterpenic aldehyde) 1.09 1.21 0.90 0.35

31 dehydroabietal 1.23 0.92 1.38 0.48

32 ● methyl levopimarate 1.88 1.52 3.72 3.45

33 ● methyl dehydroabietate 1.81 1.31 2.28 0.90

34 ● neoabietal 1.43 1.05 2.77 1.82

35 ● methyl 8.13(15) abietadien 18 oate 2.57 1.77 2.78 1.36

36 ● neoabietol 2.10 1.42 3.79 2.31

37 M+332 0.92 0.64 1.71 1.19

38 M+ 314 1.91 1.07 1.66 1.35

39 M+330 1.35 0.69 1.60 1.24

40 M+332 0.83 0.49 1.53 1.03

41 M+330 (C21H30O3) 1.12 1.16 1.81 1.00

42 M+316 0.74 0.55 1.43 0.83

43 M+406 0.62 0.63 1.16 0.58

% monoterpenes total 19.68 19.41

% sesquiterpenes total 23.31 23.76

% diterpenes total 57.01 56.82

Mean concentration monoterpenes (mg g-1 ) 1.25 1.10

Mean concentration sesquiterpenes (mg g-1 ) 1.50 1.55

Mean concentration diterpenes (mg g-1 ) 1.27 0.89 a Components selected for further evaluation.
Proceedings of the 4the International Workshop on Genetics of Host-Parasite Interactions in Forestry

307

The components α-pinene, sabinene, myrcene, and terpinolene were the major constituents in the 

monoterpenes fraction of all the samples analyzed. The sesquiterpene fraction is characterized by the 

high amounts of β-caryophyllene found to be in higher levels (14.01 percent) in infested trees 

compared with those (11.89 percent) in healthy trees. The macrocyclic diterpene cembrene is the 

major component among all the samples analyzed in our investigation. Cembrene is found with 

higher amounts (33.03 percent) in healthy trees than in infested trees (24.69 percent). The mean (mg 

g-1 dry weight of needles) concentration of monoterpene, sesquiterpene, and diterpene fractions for 

infested and healthy trees was also calculated too (table 1). The healthy trees showed higher mean 

concentration (1.25 mg g -1

) for the monoterpenes fraction in comparison to mean concentration (1.10 

mg g -1

) of infested trees. The concentration of the sesquiterpene fraction was similar in infested (1.55 

mg g -1

) and in healthy trees (1.50 mg g -1). Concerning the diterpene fraction, the differences between 

healthy (1.27 mg g -1

) and infested (0.89 mg g -1) trees are larger.

Compounds selected for statistical analysis were α-pinene, sabinene, myrcene, terpinolene, βcaryophyllene, α-humulene, phenylethyl isovaleranate, cembrene, methyl 8, 13 (15) abietadien 18 

oate, neoabietol, methyl levopimarate, methyl dehydroabietate, and neoabietal. The last three 

compounds were present in amounts less than 2 percent in healthy trees, but were also included 

because they occurred in amounts more than 2 percent in infested trees. The dendrogram produced by 

hierarchical cluster analysis revealed the classification of all Aleppo pine trees in two major clusters 

(clusters A and B). Cluster A contains 12 trees in total (54.54 percent), while cluster B contains 10 

pine trees in total (45.45 percent). The frequencies of clusters in each group of trees differ 

significantly. A total of 81.81 percent of the infested trees (nine trees) included in our analysis belong 

to cluster A, and 72.72 percent of the healthy trees (8 trees) belong to cluster B (table 2). The two 

Table 2—Mean (percent) terpene composition of clusters, distribution, and percent frequency 

in healthy and wounded Aleppo pine trees

Components Cluster A (mean %) Cluster B (mean %)

1 α-pinene 7,28 5,69

2 sabinene 2,89 3,08

3 myrcene 6,28 8,26

4 terpinolene 3,76 2,15

5 β-caryophyllene 19,86 14,36

6 α-humulene 3,40 2,44

7 phenylethyl isovaleranate 4,94 6,46

8 cembrene 29,35 48,51

9 methyl levopimarate 5,04 2,22

10 methyl dehydroabietate 3,61 1,70

11 neoabietal 4,14 1,37

12 methyl 8, 13 (15) abietadien 18 oate 3,74 3,43

13 neoabietol 5,67 1,99

Total trees 12 10

% total frequency 54,54 45,45

Healthy trees 3 8

% frequency of healthy trees 27,27 72,72 

Wounded trees 9 2

% frequency of wounded trees 81,81 18,18

clusters (A and B) differ in the amounts of the most components. The components: β-caryophyllene, 

α-humulene, neoabietal, cembrene, and neoabietol showed differences between clusters. Results of 

discriminate analysis showed that the above mentioned five compounds have the largest discriminate

ability between healthy and wounded trees. The quantitative terpene composition of clusters

expressed as percent contribution on the basis of the total area of 13 components is shown in table 2.
GENERAL TECHNICAL REPORT PSW-GTR-240

308

Discussion 

The literature concerning the relation of terpenes in P. halepensis with an attack by M. hellenica are 

rather rare. Mitta et al. (2002) analyzed cortical oleoresin terpenes in Aleppo pine trees infested by 

the insect from Crete Island, Greece. The authors reported that sensitivity of the pine to attack by M.

hellenica was significantly correlated with high levels of α- pinene and low levels of limonene and α -

terpinyl acetate. In our study, statistically significant differences in quantitative terpene composition 

between infested and non-infested trees were found for β-caryophyllene, α-humulene, neoabietal, 

cembrene, and neoabietol. A Ward cluster analysis, based on relative (percent) quantity of 13 selected 

components, revealed that arrangement of all trees into two clusters occurred at different frequencies. 

The quantitative terpene composition of cluster A is characterized by high amounts of β -

caryophyllene (19.86 percent) and the composition of cluster B by very high amounts (48.51 percent) 

of cembrene (table 2). With respect to the limited number of trees analyzed from our results, it could 

be suggested that the presence of cluster A indicates infested trees, while the presence of cluster B 

indicates healthy ones. To conclude, the needle terpenoid analysis by gas chromatography seems to 

be a valuable tool to help the scientists study the parasitism of M. hellenica to Aleppo pine. However,

further research will be required, with analysis of a larger number of samples by GLC-MS, including

trees from different seasons of the year as well from several locations around the area, to clarify the

influence of M. hellenica on terpenoid content and composition of P. halepensis in order to study the 

interaction between insect and pine tree as well the parameters involved in this complicated

phenomenon. 

Acknowledgments

The authors thank Dr. Paz Andres for performing the laboratory preparation of needle samples and Dr. Sofia

Gounari for supplying valuable literature. Special thanks to Mr. Konstantinos Priftis for his great assistance 

during field sampling. The study was conducted under a short-term official educational license of the first 

author by the Authority of the Region of Attica, Greece, which also funded the travel expenses.

Literature Cited

Avtzis, N. 1985. Marchalina hellenica (Monophlebus hellenicus) Genn. Greece’s most important honeybearing-insect. Forestry Research. 6 (1): 51–64 (in Greek). 

Erlinghagen, F. 2001. Portrait of an insect: Marchalina hellenica (Genn). (Sternorrhyncha: Coccina:

Margarodidae), important producer of honeydew in Greece. Apiacta. 36: 131–137.

Gallis, A.T. 2007. Evaluation of the damage by insect Marchalina hellenica (Genn.) in eastern Attica, Greece. 

Conclusions for sustainable management of forest ecosystems. In: (eds unknown). Proceedings of the 10th

international conference on environmental science and technology, (place of publication unknown): GNEST and University of Aegean: B191–B196.

Gounari, S. 2006. Studies on the phenology of Marchalina hellenica (Hemiptera: Coccoidea, Margarodidae) in 

relation to honeydew flow. Journal of Apicultural Research. 45(1): 8–12.

Hanover, J.W. 1992. Applications of terpene analysis in forest genetics. New Forests. 6: 159–178.

Kung, F.H. 1988. Application of data transformation in forest genetics. Silvae Genetica. 37(2):45–49.

Mitta, E.; Tsitsimpikou, C.; Tsiveleka, L.; Petrakis, P.; Ortiz, A.; Vagias, C.; Roussis, V. 2002. Seasonal 

variation of oleoresin terpenoids from Pinus halepensis and Pinus pinea and host selection of the scale 

insect Marchalina hellenica (Homoptera, Coccoidea, Margarodidae, Coelostonidiinae). Holzforschung. 56: 

572–578.

Squillace, A.E. 1987. Monoterpene composition in forest genetics research. Naval Stores Review. 97(1): 12–

15.

Smirnoff, W.A.; Valero, L. 1975. Effects au moyen de la fertilization par ure ou par potassium sur Pinus 

nanksiana L. et le comportment de ses insects devastateurs: tel que Neodiprion swainei et Toumeyella 

numismaticum. Canadian Journal Forest Research. 5: 236–244.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

309

Genetic Variation of Piperidine Alkaloids in Pinus 

ponderosa From a Common Garden

Elizabeth A. Gerson,

1 Rick G. Kelsey,1 and J. Bradley St. Clair1

Abstract

Most species of pine and spruce synthesize and accumulate variable quantities of alkaloids in their tissues.

These compounds express numerous types of biological activities in bioassay and could potentially offer

resistance against enemies, although this function has never been confirmed for any known enemies of pine or 

spruce under natural conditions. The present study takes a complementary, and intensive, common garden 

approach to examine genetic variation in Pinus ponderosa Lawson & C. Lawson var. ponderosa C. Lawson 

alkaloid production. It also investigates the potential trade-off between seedling growth and alkaloid production, 

and associations between topographic/climatic variables and alkaloid production. Piperidine alkaloids were 

quantified in foliage of 501 nursery seedlings grown from seed sources in west-central Washington, Oregon,

and California, roughly covering the western half of the native range of ponderosa pine. A nested mixed model 

was used to test differences among broad-scale regions and among families within regions. Alkaloid 

concentrations were regressed on seedling growth measurements to test metabolite allocation theory. Likewise, 

climate characteristics at the seed sources were also considered as explanatory variables.

Qualitative variation in alkaloid profiles was low. However, quantitative variation from seedling to seedling was 

high, and regional variation exceeded variation among families. Regions along the western margin of the 

species range exhibited the highest alkaloid concentrations, while those further east had relatively low alkaloid 

levels. All measures of seedling growth related negatively to alkaloid concentrations on a natural log (ln) scale;

however, coefficients of determination were low. At best, annual height increment explained 19.4 percent of the 

variation in ln(total alkaloids). Among the climate variables, temperature range showed a negative, linear 

association that explained 41.8 percent of the variation. Given the wide geographic scope of the seed sources 

and the uniformity of resources in the seedlings’ environment, observed differences in alkaloid concentrations 

are evidence for genetic regulation of alkaloid secondary metabolism in ponderosa pine. The theoretical tradeoff with seedling growth appeared to be real, however slight. The climate variables provided little evidence for 

adaptive alkaloid variation, especially within regions. This study has been published in full (Gerson, E.A.; 

Kelsey, R.G.; St Clair, J.B. 2009. Genetic variation of piperidine alkaloids in Pinus ponderosa: a common 

garden study. Annals of Botany. 103: 447–457.).

 1 USDA Forest Service, PNW Research Station, 3200 Jefferson Way, Corvallis, OR 97331.

Corresponding author: rkelsey@fs.fed.us.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Gene Expression in the TanoakPhytophthora ramorum Interaction

Katherine J. Hayden,1 Matteo Garbelotto,

1 Hardeep Rai,2 Brian Knaus,3 

Richard Cronn,3 and Jessica W. Wright4

Abstract

Disease processes are dynamic, involving a suite of gene expression changes in both the host and the pathogen, 

all within a single tissue. As such, they lend themselves well to transcriptomic analysis. Here we focus on a 

generalist invasive pathogen (Phytophthora ramorum) and its most susceptible California Floristic Province 

native host, tanoak (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H. Oh). The advent of 

new sequencing technologies has made the study of non-model systems possible at a scale never before 

possible. We argue that this non-model system is ideal for studying the interactions between host and pathogen 

using massively parallel mRNA sequencing, in part because the P. ramorum genome has been fully sequenced. 

We present early data in a project developed to elucidate the molecular genetic interactions between these two 

species. We used the Illumina Genetic Analyzer system to sequence all mRNA present in a single tanoak 

genotype after inoculation with P. ramorum, and from a non-inoculated control. We separated sequences from 

the dataset that originated from the pathogen, or from highly conserved regions, by aligning the reads to the P. 

ramorum genome; this first set includes the genes expressed in vivo by the pathogen. The remaining sequences 

were used to generate a de novo reference transcriptome for tanoak, and then to subsequently pinpoint gene 

regions of interest with increased or decreased expression after infection. Each of the three outcomes—the set 

of expressed P. ramorum gene regions, the tanoak gene regions showing changes in expression, and the tanoak 

transcriptome reference itself—are an important step in understanding the interactions of forest pathogens and 

their hosts at the molecular level.

 1 University of California, Berkeley, Environmental Science, Policy, and Management, 137 Mulford Hall, Berkeley, CA 

94720. 2 Utah State University, Logan, UT. 3 USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR.

4 USDA Forest Service, Pacific Southwest Research Station, Davis, CA.

Corresponding author: khayden@berkeley.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

311

Blister Rust Resistance Among 19 Families of 

Whitebark Pine, Pinus albicaulis, From Oregon and 

Washington – Early Results From an Artificial 

Inoculation Trial

Angelia Kegley,

1 Richard A. Sniezko,1 Robert Danchok,1 and Douglas P. Savin1

Whitebark pine is considered one of the most susceptible white pine species to white pine blister rust, 

the disease caused by the non-native pathogen Cronartium ribicola. High mortality from blister rust 

and other factors in much of the range in the United States and Canada have raised serious concerns 

about the future viability of this high-elevation forest tree species (USFWS 2011). A major effort is 

now underway to collect seed for gene conservation, to evaluate trees for genetic resistance to blister 

rust, and to establish restoration plantings (Aubry et al. 2008, Mangold 2011; Sniezko et al. 2011a, 

2011b).

Seed collected from 19 whitebark pine parent trees at four locations in Oregon and Washington 

were sown in 2005 (fig. 1). The number of families represented in this study varied by geographic 

source: eight from Mt. Rainier National Park, three from the Colville National Forest, two from the 

Deschutes National Forest, and six from the Fremont National Forest. The number of seedlings 

available per family for blister rust inoculations ranged from 21 to 60.

Figure 1—Parent tree locations for 19 whitebark pine families sown in 2005 for a white pine blister 

rust resistance screening trial. 

 1 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon, USA.

Corresponding author: rsniezko@fs.fed.us. 
GENERAL TECHNICAL REPORT PSW-GTR-240

312

A randomized complete block design was used, with each of the 19 families distributed randomly

in row plots in six blocks. Within each family, the number of seedlings in each row plot was made as 

equal as possible between the blocks, but varied between families (from 3 to 10). Seedlings were 

inoculated with C. ribicola twice, initially in 2006, and again in 2009, using the well-established 

protocols at Dorena Genetic Resource Center (DGRC), Cottage Grove, Oregon. The first inoculation 

was very effective in producing needle spots (on >99 percent of the seedlings), but, unexpectedly, less 

than 10 percent of the seedlings later developed cankers. The second inoculation was also very 

effective in producing needle spots (on >99 percent of the seedlings), and subsequently led to the 

expected high levels of stem infection (fig. 2). This second inoculation will be particularly interesting 

to follow; the trees were 5 years old and much larger than any whitebark pine previously subjected to 

artificial inoculation at DGRC. 

Figure 2—(a) Seedlings of whitebark pine in April 2010 (following September 2009 inoculation); (b) 

needle lesions in June 2010 from blister rust inoculation; (c) and (d) many stem infections on 

seedlings in early November 2010. (Photo credits: Richard Sniezko)

As of November 2010, 76.6 percent of trees had stem symptoms (SSp). The Deschutes and 

Fremont locations from eastern Oregon averaged 97 percent and 96 percent SSp, respectively; the

Colville location (northeastern Washington) averaged 71 percent SSp, and the Mt. Rainier location

(west-central Washington) averaged 58 percent SSp. Family means ranged from 46.9 percent (a Mt. 

Rainier family) to 100 percent (four families, all in the Fremont population).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

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The number of stem symptoms per infected tree (excludes trees with needle spots, but no stem 

symptoms) at the November 2010 assessment averaged 16.4, with family mean number of stem 

symptoms per infected tree ranging from 7.3 (a Mt. Rainier family) to 28.3 (also a Mt. Rainier 

family). One individual seedling had 90 stem symptoms. In addition to normal cankers, some bark 

reactions (complete and partial) were apparent; families from the Colville and Mt. Rainier locations

had the highest levels of bark reaction.

To estimate the effect of geographic source (population) and family within a source on tree 

resistance, analyses of variance (ANOVAs) were conducted on row plot means using SAS PROC 

GLIMMIX (SAS 2008). For these analyses, population and family within population were considered 

fixed effects, while block and associated interactions were treated as random effects.

The first response examined was the presence or absence of stem symptoms. A binomial model 

with logit link was applied to the data. In cases where the proportion of trees with a stem symptom 

was 0 or 1 in a row plot, an adjustment of +0.01 or -0.01 was applied to the row plot mean, 

respectively. The analysis indicates that there is strong evidence of an association between the odds of 

getting a stem symptom and the geographic source of the family parent (p<0.0001, fig. 3). In 

addition, within a source there is suggestive evidence of a family difference in the odds of getting a 

stem symptom (p=0.055). It is estimated that the odds of getting a stem symptom for seedlings with 

parents in the Deschutes and Fremont National Forests are 19.6 and 30.7 times higher (respectively)

than for seedlings with parents in Mt. Rainier National Park; the 95 percent confidence intervals with 

Tukey-Kramer adjustment are from 3.9 times higher to 99.2 times higher and from 7.0 times higher to 

134.3 times higher for Deschutes and Fremont, respectively.

The second response examined was the number of stem symptoms per infected tree. A negative 

binomial model with log link was applied to the data. There is suggestive but inconclusive evidence 

of an association between the number of stem symptoms on an infected tree and the geographic 

source of origin of the family parent (p=0.057, fig. 3). However, within sources, there is convincing 

evidence of family differences in the mean number of stem symptoms per infected tree (p<0.0001). If 

all trees (including those with no stem symptoms at this stage) are included in the analysis, then the 

evidence of an association between the number of stem symptoms and the geographic source of origin 

is highly significant (p=0.0002), with families from the two northern sources generally having much 

lower means than those from the Deschutes and Fremont National Forests.

The early results for this trial show similar trends to those observed from previous inoculation

trials at DGRC. Families from Mt. Rainier were highest in resistance: all families from Mt. Rainier 

showed fewer seedlings with stem symptoms and tended to have fewer stem symptoms than those 

from Deschutes and Fremont. The two northern-most sources (of the four tested here), Mt. Rainier 

and Colville, in this trial have the highest level of resistance. Other recent trials have suggested 

possible geographic trends in the percentage of seedlings developing stem symptoms (Sniezko et al.

2007; Sniezko, unpublished). 

This trial included relatively few families from each of the four populations, but the trends for SSp

observed in this trial are consistent with population results from other trials at DGRC (Sniezko,

unpublished). However, screening of more families from each population will likely identify families 

with some level of resistance even amongst populations such as Deschutes and Fremont that have 

very high SSp in this trial. 

At this stage of the trial, there are differences in both the percent of trees with stem symptoms 

(between populations and between families within populations) as well as in the number of stem 

symptoms per tree (between families). Both of these traits may have utility in selecting parents that 

will be most resistant to blister rust in the field. The first field trials for whitebark pine have recently 

been established and will serve to help validate seedling resistance screening as well as monitor the 

durability of rust resistance.
GENERAL TECHNICAL REPORT PSW-GTR-240

314

Figure 3—Least squares means and 95 percent confidence intervals for (a) the percent of seedlings 

with stem symptoms, and (b) the number of stem symptoms per infected seedling for each source

using the final models described in the text. Locations sharing the same letter indicate no significant 

difference at the α=0.05 level after Tukey-Kramer adjustment.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

315

Canker severity is currently low (3 on a 9 point scale), but is expected to increase dramatically 

over the next year. Mortality from blister rust is currently low (3.7 percent), but is expected to 

continue, with very high mortality expected over the next 2 years. Seedlings will be assessed annually 

over the next several years for additional stem symptom development and mortality. The high levels 

of canker-free seedlings in some families at this stage are encouraging, but at least one additional year 

of assessments will be needed to discern latent stem symptoms. The parents identified with genetic 

resistance to blister rust in seedling screening trials will be a key component to successful restoration 

of whitebark pine. Seedlings from top families can be grafted for gene conservation, and land 

managers will be contacted concerning the potential to collect seed from the top parents to use for 

restoration.

Acknowledgments

We would like to acknowledge the USDI National Park Service (NPS), Mt. Rainier National Park, and USDA 

Forest Service, Pacific Northwest Region (Region 6), particularly the Genetic Resources and Forest Health 

Programs, which provided funding for this project; also the USDI NPS, Region 6 Geneticists and Tree 

Improvement personnel for tree selection and cone collection; DGRC technicians and staff for inoculating the 

seedlings with blister rust, providing nursery culture after inoculation, and assessing the seedlings. We would 

also like to thank Jim Hamlin for invaluable assistance with the analysis and reviewing of a previous version of 

this manuscript. 

Literature Cited

Aubry, C.; Goheen, D.; Shoal, R.; Ohlson, T.; Lorenz, T.; Bower, A.; Mehmel, C.; Sniezko, R. 2008.

Whitebark pine restoration strategy for the Pacific Northwest Region 2009-2013. Portland, OR: U.S. 

Department of Agriculture, Forest Service, Pacific Northwest Region. 212 p. 

http://www.fs.usda.gov/detail/r6/landmanagement/resourcemanagement/?cid=stelprdb5278980 (22 March

2012).

Mangold, R.D. 2011. The U.S. Forest Service's renewed focus on gene conservation of five-needle pine 

species. In: Keane, R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. The future of high-elevation, 

five-needle white pines in western North America: proceedings of the high five symposium. Proceedings 

RMRS-P-63. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station: 151.

SAS Institute Inc. 2008. SAS® Component Language 9.2: Reference. Cary, NC: SAS Institute Inc.

Sniezko, R.A.; Kegley, A.J.; Danchok, R.S.; Long, S. 2007. Variation in resistance to white pine blister rust 

among 43 whitebark pine families from Oregon and Washington—early results and implications for 

conservation. In: Goheen, E.M.; Sniezko, R.A., tech. coords. Proceedings of the conference whitebark pine: 

whitebark pine: a Pacific Coast perspective; R6-NR-FHP-2007-01. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Region: 82–97. (22 March 2012).

Sniezko, R.A.; Mahalovich, M.F.; Schoettle, A.W.; Vogler, D.R. 2011a. Past and current investigations of 

the genetic resistance to Cronartium ribicola in high-elevation five-needle pines. In: Keane, R.E.; Tomback, 

D.F.; Murray, M.P.; and Smith, C.M., eds. The future of high-elevation, five-needle white pines in western 

North America: proceedings of the high five symposium. Proceedings RMRS-P-63. Fort Collins, CO: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station: 246–264. 

Sniezko, R.A.; Schoettle, A.; Dunlap, J.; Vogler, D.; Conklin, D.; Bower, A.; Jensen, C.; Mangold, R.; 

Daoust, D.; Man, G. 2011b. Ex situ gene conservation in high elevation white pine species in the United 

States: a beginning. In: Keane, R.E.; Tomback, D.F.; Murray, M.P.; Smith, C.M., eds. The future of highelevation, five-needle white pines in western North America: proceedings of the high five symposium. 

Proceedings RMRS-P-63. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station: 147–179. 

U.S. Fish and Wildlife Service. 2011. Endangered species: whitebark pine. http://www.fws.gov/mountainprairie/species/plants/whitebarkpine/. (22 March 2012).
GENERAL TECHNICAL REPORT PSW-GTR-240

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Interaction of an Invasive Bark Beetle with a Native 

Forest Pathogen: Potential Effect of Dwarf Mistletoe 

on Range Expansion of Mountain Pine Beetle in 

Jack Pine Forests

Jennifer Klutsch1 and Nadir Erbilgin1

Abstract

In recent decades, climate change has facilitated shifts in species ranges that have the potential to significantly 

affect ecosystem dynamics and resilience. Mountain pine beetle (Dendroctonus ponderosae) is expanding east 

from British Columbia, where it has killed millions of pine trees, primarily lodgepole pine (Pinus contorta

Douglas ex Loudon) over the last 10 years. In Alberta, mountain pine beetle is in portions of the lodgepole pine 

x jack pine hybrid zone and was recently intercepted at the western edge of jack pine (P. banksiana Lamb.) 

forests. There is a potential threat that mountain pine beetle will expand into the naïve host, jack pine, which 

extends from Alberta into eastern Canada and the Great Lakes Region in the United States. A successful 

invasion of jack pine by mountain pine beetle could generate serious ecological and economic problems 

throughout the boreal forest. Therefore, studies conducted during this initial establishment of mountain pine 

beetle in jack pine forests, and their applied implications for controlling the invasion, are highly relevant for the 

rest of jack pine and eastern pine forests. If done proactively, such studies may identify jack pine stands 

susceptible to mountain pine beetle and help forge pre-emptive management strategies prior to mountain pine 

beetle arrival. The further easterly expansion of mountain pine beetle might be constrained by low winter 

temperatures, by the amount of weakened or stressed trees, and by low, endemic, beetle populations. The native 

parasitic plant, dwarf mistletoe (Arceuthobium americanum), induces stress in jack pine and causes extensive 

damage throughout its range. In this study, we will investigate whether infection of jack pine by the dwarf 

mistletoe will influence jack pine susceptibility to mountain pine beetle. The work that started in 2011 will 

evaluate how dwarf mistletoe-induced chemical and physiological changes in jack pine affect growth and 

development of mountain pine beetle and its associated fungi. Interactions between dwarf mistletoe and 

mountain pine beetle on jack pine will be identified by addressing a number of research objectives, focusing 

primarily on how changes in jack pine defenses mediate mountain pine beetle-dwarf mistletoe interactions. 

These investigations are important because the eastward expanding wave of mountain pine beetle will likely 

first encounter, and colonize, these highly abundant dwarf mistletoe-infected jack pine trees. 

 1 University of Alberta, Department of Renewable Resources, ESB 442, Edmonton, Alberta T6G 2E3.

Corresponding author: klutsch@ualberta.ca.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

317

Using Dutch Elm Disease-Tolerant Elm to Restore 

Floodplains Impacted by Emerald Ash Borer

Kathleen S. Knight,

1 James M. Slavicek,1 Rachel Kappler,2 Elizabeth 

Pisarczyk,2 Bernadette Wiggin,2 and Karen Menard2

Abstract

American elm (Ulmus Americana L.) was a dominant species in floodplains and swamps of the Midwest before 

Dutch elm disease (DED) (Ophiostoma ulmi and O.novo-ulmi) reduced its populations. In many areas, ash 

(Fraxinus spp.) became dominant in these ecosystems. Emerald ash borer (EAB) (Agrilus planipennis), an 

introduced insect, is now spreading through the Midwest and killing up to 99 percent of ash trees in infested 

areas. In spring 2011, we began a restoration experiment to study reforestation of ash-dominated floodplains 

impacted by EAB through plantings of native tree species, including DED-tolerant American elm, sycamore 

(Platanus occidentalis L.), and pin oak (Quercus palustris Münchh.). We are testing the effect of planting trees 

before, during, or after ash mortality by planting in sites across a gradient of EAB infestation duration. Initial 

causes of seedling damage differed among sites. At sites that experienced flooding, many seedlings had wilted 

leaves. However, at the site that did not experience flooding, most seedlings had insect herbivory. Initial 

seedling mortality was low and differed among sites and species. American elm seedlings tolerant to DED 

performed as well as or better than the pin oak and sycamore seedlings planted in this experiment. Future results 

of this experiment will provide recommendations to managers for methods to restore EAB-impacted floodplain 

forests with DED-tolerant American elm and other tree species.

Key words: American elm, Dutch elm disease, emerald ash borer, floodplain restoration

Introduction

Dutch elm disease (DED) is caused by two nonnative fungal pathogens (Ophiostoma ulmi and O. 

novo-ulmi) of elm trees (Ulmus spp.) (Brasier 1991), which are spread by elm bark beetle species 

(primarily Scolytus multistriatus and Hylurgopinus rufipes) in North America (Schreiber and Peacock 

1979). The first epidemic of O. ulmi was noticed in North America in the 1930s (Schreiber and 

Peacock 1979), and the second, more aggressive epidemic of O. novo-ulmi probably spread during the 

1940s. The two epidemics decimated populations of American elm (Ulmus americana) (Gibbs 1978).

While it was a popular street tree, American elm also was an important tree species in many riparian 

areas and swamps (Barnes 1976). The disease killed almost all of the large American elm trees, 

restricting elm populations to small diameter trees, while ash (Fraxinus spp.) and a few other tree 

species replaced American elm as dominant canopy species in these habitats (Barnes 1976).

Emerald ash borer (EAB) (Agrilus planipennis), an introduced insect pest, has killed millions of 

ash trees in the midwestern United States and adjacent Canada and is spreading rapidly (Cappaert et 

al. 2005). The impact of EAB is greatest in riparian areas and swamps where ash is the most abundant 

tree genus, sometimes making up 50 percent or more of the basal area of these stands (Knight,

unpublished data). As the ash trees are killed, large canopy gaps provide an opportunity for both 

native plants and invasive plants to respond to the increased light in the understory (Knight et al.

2010). In turn, these changes reverberate through the ecosystem, affecting other organisms and 

ecosystem processes. In some areas, there is very little regeneration of native tree species. Invasive 

plants such as reed canary grass (Phalaris arundinacea L.) and common buckthorn (Rhamnus 

 1 USDA Forest Service, Northern Research Station, Delaware, OH. 2 Metroparks of the Toledo Area, Toledo, OH.

Corresponding author: ksknight@fs.fed.us.
GENERAL TECHNICAL REPORT PSW-GTR-240

318

cathartica L.), which inhabit these ash-dominated areas (Knight, unpublished data), may inhibit 

future tree establishment.

Restoration strategies for ash-dominated riparian and swamp habitats are needed. Ongoing work to 

develop DED-tolerant American elm trees has generated several genotypes of DED-tolerant

American elm trees from large-scale screening operations (Smalley and Guries 1993; Townsend and 

Douglas 2001; Townsend et al. 1995, 2005). Ash mortality provides an opportunity to introduce 

DED-tolerant American elm into these areas, re-establishing a canopy tree species that had been lost 

from these forests. This is also an important opportunity to diversify these forests by planting other 

native forest tree species, with a long-term restoration goal of creating resilient, functional floodplain 

forests.

We are conducting a restoration experiment to understand tree seedling growth and survival in 

floodplains, to study the performance of planted DED-tolerant elms, and to determine optimal 

restoration techniques for these ecosystems. Our long-term research is focused on two main 

questions:

• What factors affect the growth and survival of planted tree seedlings in Ohio floodplains 

impacted by EAB?

• What factors affect DED incidence and survival of planted elms?

These questions will be answered over several years. However, for the initial spring planting, we 

have examined the following questions:

• What are the major causes of initial damage or mortality for the planted tree seedlings?

• Does damage or mortality differ among species, sites, and sizes of planted tree seedlings?

Methods

Three riparian sites in Ohio were selected for this 

study (fig. 1). The Swan Creek floodplain in Oak 

Openings Preserve Metropark in northwest Ohio 

has been infested by EAB for the longest time 

period. The first EAB exit holes were observed in 

2005 and 97 percent of the mature ash trees were 

dead by 2009. Green ash (F. pennsylvanica

Marsh.) was the most abundant tree species in the 

floodplain, followed by American elm, boxelder 

(Acer negundo L.), and silver maple (A.

saccharinum L.). The Tuscawaras River 

floodplain in the Clinton Conservation Area in 

northeast Ohio is currently uninfested by EAB 

(no exit holes have been observed), and the 

majority of the ash trees are healthy. Green ash is 

the most abundant tree species in this floodplain, 

followed by silver maple, red maple (A. rubrum

L.), and boxelder. The Spring Creek floodplain 

in Sharon Woods Metro Park in central Ohio 

has been recently infested by EAB. The first 

EAB exit holes were observed in 2009 and 33 percent ash mortality in the floodplain was recorded in 

2011. White ash (F. americana L.) is the most abundant species in the floodplain, followed by 

American elm, sugar maple (A. saccharum L.), and blue ash (F. quadrangulata Michx.). 

Pin oak (Quercus palustris Münchh.) and sycamore (Platanus occidentalis L.) seedlings grown 

from seeds collected in Ohio were purchased from Riverside Native Trees (Delaware, Ohio).

American elm seedlings were produced by the USDA Forest Service from crosses between Valley 

Figure 1—Restoration planting sites in Ohio, USA.

Oak Openings

Sharon Woods

Clinton 

Conservation Area
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

319

Forge and R18-2, two cultivars with known tolerance to DED. Valley Forge is a selection from a 

chemical test that showed high levels of DED tolerance that was identified by Alden Townsend and 

Lawrence Schreiber. R18-2 is one of 17 survivors out of 21,000 seedlings screened for DED tolerance 

by Cornell University and the Boyce Thompson Institute. The geographic origin of these trees is 

unknown. Crosses between these disease-tolerant cultivars have been shown to have high levels of 

tolerance to DED (Slavicek and Knight, Generation of American elm trees with tolerance to Dutch 

elm disease through controlled crosses and selection, these proceedings).

At each site, plots were located in ash-dominated areas that were easily accessible. Three “large 

tree plots” were planted at each site with containerized seedlings 0.5 to 2.5 m tall on a 6 x 7 m grid 

(fig. 2). Two sites—Oak Openings Preserve Metropark and Clinton Conservation Area—also had 

three “small tree plots” planted with containerized seedlings 0.3 to 1 m tall on a 2.5 x 2.5 m grid.

American elm, pin oak, and sycamore containerized tree seedlings were planted in random order on a 

grid pattern at each plot with half of the trees planted in spring 2011 (late May and early June) and the 

other half in fall 2011 (September) (fig. 2). Half of the large trees received cages to prevent deer 

browsing and rubbing. The DED-tolerant cultivars Valley Forge (VF), Princeton (PRN), and New 

Harmony (NH) were obtained from JLPN Inc. (Salem, OR) as bare-root stock, grown for 3 weeks in 

containers, and then planted in rows outside of each plot (fig. 2). These known DED-tolerant cultivars 

were planted to compare their survival to those elms from the USDA Forest Service crosses.

Figure 2—Planting design for one of the large tree plots, with trees planted on a 6 x 7 m grid. Valley 

Forge (VF), Princeton (PRN), and New Harmony (NH) are the Dutch elm disease-tolerant cultivars 

planted outside the plot. (F) represents cages randomly assigned to half of the trees.

One to 2 days after spring planting, Oak Openings Preserve Metropark and Clinton Conservation 

Area experienced heavy flooding. Trees were submerged for 1 to 4 days. The duration and depth of 

flooding was not measured, but appeared to be greater at Clinton Conservation Area. Approximately 

2 weeks to 1 month after spring planting, we measured the following variables for each planted 

seedling: initial damage, survival, diameter of the stem at 2.5 cm height, seedling stem height to 

highest live leaf or bud, and canopy openness using a concave spherical densiometer.

A general linear multivariate mixed model using a binomial logit distribution was used to analyze 

mortality and the incidence of wilting, deer browsing, and insect herbivory. The main effects of site, 

tree size class (small seedlings or large seedlings), and species (elm, oak, sycamore) were tested.

Analyses comparing tree size classes included both small tree and large tree plots at Oak Openings 

Preserve Metropark and Clinton Conservation Area. Sharon Woods Metropark data was analyzed in 

separate analyses using only large trees for all three sites, because only large trees were planted at 

Sharon Woods. These results were similar to the results of the first analyses and are not reported here.

Post-hoc Tukey tests were used to determine differences among class variables.

VF (F) (F) (F) (F)

VF (F) (F) (F)

VF (F) (F) (F) (F) (F)

NH (F) (F) (F)

NH (F) (F) (F) (F) (F) (F) (F) (F)

PRN (F) (F) (F) (F) (F)

PRN PRN (F) (F) (F) (F) (F)

VF NH (F) (F) (F)

Red = elm planted in spring

Orange = oak planted in spring

Yellow = sycamore planted in spring

Green = elm planted in fall

Blue = oak planted in fall

Pink = sycamore planted in fall
GENERAL TECHNICAL REPORT PSW-GTR-240

320

Results

Damage

Leaves that were submerged during flooding wilted and died, then the seedlings sprouted new leaves 

(fig. 3) or died. Tall seedlings often exhibited wilted lower leaves and healthy upper leaves, probably 

because the upper leaves were not submerged long enough to kill them. Wilting differed among sites 

(p<0.0001) and species (p<0.0001). Wilting was most common at Clinton Conservation Area, with 

>50 percent of the trees of all species exhibiting wilting (fig. 4). Wilting was also observed at Oak 

Openings Preserve Metropark, but was uncommon at Sharon Woods Metropark. The incidence of 

wilting was similar for small and large trees (p>0.05). Differences among species were inconsistent 

between sites and size classes, but in general, sycamore seedlings exhibited more wilting than oak 

seedlings.

Insect herbivory differed among sites (p=0.006) and was most common at Sharon Woods 

Metropark, where almost 100 percent of the trees exhibited some herbivory (fig. 5). Herbivory was 

more common on large trees than small trees (p=0.0003) and was less common on elms than other 

species (p<0.0001). Browsing by deer was uncommon at all sites during the spring when the data was 

collected (fig. 6), but differed significantly among sites (p<0.0001) and species (p=0.0004). However, 

browsing was more common in the fall (data not shown), and may have a greater impact than the 

initial results suggest. Raccoons or groundhogs dug up or destroyed some of the small seedlings 

planted at Oak Openings Metropark and Clinton Conservation Area; seedlings that survived this 

disturbance were replanted.

Figure 3—Elm with wilted lower leaves; re-sprouting new upper leaves (left). Elm with dead lower 

leaves that fell off, surviving upper leaves (center). Sycamore with dead lower leaves and surviving 

upper leaves (right).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

321

Figure 4—Wilting differed among sites and species.

Figure 5—Insect herbivory differed among sites and species.

0

10

20

30

40

50

60

70

80

90

100

Clinton Oak Openings Sharon

Percent trees wilted

Site

large Ulmus

large Quercus

large Platanus

small Ulmus

small Quercus

small Platanus

0

10

20

30

40

50

60

70

80

90

100

Clinton Oak Openings Sharon

Percent trees with herbivory

Site

large Ulmus

large Quercus

large Platanus

small Ulmus

small Quercus

small Platanus
GENERAL TECHNICAL REPORT PSW-GTR-240

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Figure 6—Browsing by deer was uncommon.

Mortality

Initial mortality was greater for small seedlings than large seedlings (p<0.0001), and differed among 

species and sites (p<0.0001) (fig. 7). Small sycamore seedlings exhibited the greatest mortality. Initial 

mortality was greatest at Oak Openings Preserve Metropark, and there was no initial mortality at 

Sharon Woods Metropark. It was difficult to determine the cause of mortality among seedlings. Some 

seedlings may have died from submergence under water at the flooded sites. During the fall planting, 

additional mortality, especially of small sycamore seedlings at Clinton Conservation Area, was 

recorded (data not shown). A second flood in July at Clinton Conservation Area may have caused the 

additional mortality.

Figure 7—Mortality differed among sites, species, and size classes.

0

10

20

30

40

50

60

70

80

90

100

Clinton Oak Openings Sharon

Percent trees with browse

Site

large Ulmus

large Quercus

large Platanus

small Ulmus

small Quercus

small Platanus

0

2

4

6

8

10

12

14

16

18

20

clinton oak openings sharon

Percent mortality

Site

large Ulmus

large Quercus

large Platanus

small Ulmus

small Quercus

small Platanus
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

323

Discussion

Mixed plantings of DED-tolerant American elm, sycamore, and pin oak showed good initial survival.

Damage for all three species was mostly due to wilting from flooded conditions or herbivory at the 

site that was not flooded. The incidence of wilting seems to reflect the severity of flooding with >50

percent of the seedlings wilted at the site with the greatest depth and duration of flood waters.

However, many of the wilted seedlings resprouted. At the site that did not experience flooding, 

herbivory by insects was common. It is possible that insect populations are greater at that site or that 

herbivory was inhibited by flooding. DED-tolerant elm seedlings (VF x R18-2) performed as well as 

or better than the other two floodplain tree species, which is a promising result for the use of DEDtolerant elm in restoration. We will continue to monitor the survival and growth of the planted 

seedlings. The future results of this study will allow us to make recommendations to managers for 

restoration of EAB-impacted floodplains using DED-tolerant American elm and other tree species. 

Acknowledgments

We thank the Metroparks of the Toledo Area, MetroParks Serving Summit County, and Columbus Metro Parks 

for their collaboration in establishing the restoration experiment at their parks. The American Recovery and 

Reinvestment Act and the USDA Forest Service provided funding to support this research. Kyle Costilow, 

Charles Flower, Stephanie Fluke, Stephanie Smith, and Lawrence Long helped establish the plots and collect 

the data. Eileen Sawyer, Reid Plumb, and Daniel Spalink helped to collect data on tree composition at Oak 

Openings. John Stanovick assisted with statistical analysis. Suggestions from Shivanand Hiremath and Linda 

Haugen improved this manuscript.

Literature Cited

Barnes, B.V. 1976. Succession in deciduous swamp communities of southeastern Michigan formerly 

dominated by American elm. Canadian Journal of Botany. 54: 19–24.

Brasier, C.M. 1991. Ophiostoma novo-ulmi sp. nov., causative agent of current Dutch elm disease pandemics. 

Mycopathologia. 115: 151–161.

Cappaert, D.; McCullough, D.G.; Poland, T.M.; Siegert, N.W. 2005. Emerald ash borer in North America: a

research and regulatory challenge. American Entomologist. 51: 152–165.

Gibbs, J.N. 1978. Intercontinental epidemiology of Dutch elm disease. Annual Review of Phytopathology. 16:

287–307.

Knight, K.S.; Herms, D.A.; Cardina, J.; Long, R.P.; Rebbeck, J.; Gandhi, K.J.K.; Smith, A.; Klooster,

W.S.; Herms, C.P.; Royo, A.A. 2010. EAB aftermath forests: the dynamics of ash mortality and the 

responses of other plant species. In: Michler, C.H.; Ginzel, M.D., eds. Proceedings of the symposium on ash 

in North America. Gen. Tech. Rep. NRS-P-72. Newtown Square, PA. U.S. Department of Agriculture,

Forest Service, Northern Research Station: 11.

Schreiber, R. R.; Peacock, J.W. 1979. Dutch elm disease and its control. Agriculture Information Bulletin No. 

193. Washington, DC: U.S. Department of Agriculture, Forest Service.

Smalley, E.B.; Guries, R.P. 1993. Breeding elms for resistance to Dutch elm disease. Annual Review of 

Phytopathology. 31: 325–352.

Townsend, A.M.; Bentz, S.E.; Douglass, L.W. 2005. Evaluation of 19 American elm clones for tolerance to 

Dutch elm disease. Journal of Environmental Horticulture. 23(1): 21–24.

Townsend, A.M.; Bentz, S.E.; Johnson, G.R. 1995. Variation in response of selected American elm clones to 

Ophiostoma ulmi. Journal of Environmental Horticulture.13: 126–128.

Townsend, A.M.; Douglass, L.W. 2001. Variation among American elm clones in long-term dieback, growth, 

and survival following Ophiostoma inoculation. Journal of Environmental Horticulture. 19(2): 100–103.
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Quantitative Trait Loci for Resistance to Two Fungal 

Pathogens in Quercus robur

Cécile Robin,

1 Amira Mougou-Hamdane,1 Jean-Marc Gion,1 Antoine Kremer,1

and Marie-Laure Desprez-Loustau1

Abstract

Powdery mildew, caused by Erysiphe alphitoides (Ascomycete), is the most frequent disease of oaks, which are 

also known to be host plants for Phytophthora cinnamomi (Oomycete), the causal agent of ink disease.

Components of genetic resistance to these two pathogens, infecting either leaves or root and collar, were

investigated in a full-sib family of Quercus robur L, that was vegetatively propagated by cuttings. 

Resistance to powdery mildew was assessed by two methods. First, inoculations with E. alphitoides were 

performed under controlled conditions on excised leaves removed from cuttings grown in the greenhouse. The 

level of host-pathogen compatibility was assessed by recording infection success and mycelial growth. Second, 

the progeny, planted in a comparative test, were assessed for susceptibility to powdery mildew using field 

evaluation under natural infection conditions over 3 years. Resistance to ink disease was estimated by 

inoculating P. cinnamomi on stems of 2-year-old cuttings grown in the glasshouse, and by measuring the length 

of the induced lesion in two experiments.

Preliminary results showed that quantitative trait loci (QTL) associated with the response to both pathogens 

were located on the genetic linkage maps available for the two parents of the F1 family. However we could not 

identify QTL involved in both diseases.

Although the genetic architecture of resistance to E. alphitoides varied between years and infection conditions, 

stable QTL were detected. Because infection by this fungus is strongly dependent on the phenological status of 

its host, co-locations between QTL for resistance and QTL for phenology were studied.

 1 INRA, UMR 1202 BIOGECO, INRA-Université Bordeaux 1, 69 route d’Arcachon, 33612 Cestas, France.

Corresponding author: robin@bordeaux.inra.fr.
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Testing Resistance to Chestnut Blight of Hybrid 

Chestnuts

Cécile Robin,1 Xavier Capdevielle,1 Gilles Saint-Jean,1 and Teresa Barreneche2

Abstract

Castanea sativa Mill. is an ecologically and economically important species in Europe, not only as a forest tree, 

but also as a fruit tree. It is dramatically threatened by ink disease caused by Phytophthora spp., introduced 

during the nineteenth century. To limit its impact, C. mollissima and C. crenata, which proved to be tolerant 

species, were imported from China and Japan to Europe at the beginning of the twentieth century, together with 

the agent of chestnut blight (Cryphonectria parasitica) which is now spreading throughout Europe. A clonal 

selection has resulted in the use of a few inter-specific hybrids as rootstocks tolerant to ink disease, and onto 

which fruit cultivars are grafted, as fruit cultivars for the fruit quality and as forest clones selected for growth 

characteristics. New chestnut forest plantations are clonal in Spain, but not yet in France. However, with the 

threat of increasing loss due to ink disease and chestnut blight, hybrid clones are seen as a promise by some 

foresters. Our aim was to test the resistance to chestnut blight of hybrid chestnuts which are used as rootstocks 

or grafted varieties for fruit production, or inter-specific full sib families used for building chestnut maps. Field 

testing revealed significant differences among genotypes: some fruit cultivars being less susceptible than the 

large majority, in agreement with field observations. With excised stem assays, a large phenotypic variation 

within the F1 progeny was observed.

 1 INRA, UMR 1202 BIOGECO, INRA-Université Bordeaux 1, 69 route d’Arcachon, 33612 Cestas, France. 2 INRA, UREF INRA UREF, 71, Avenue Edouard-Bourlaux, B.P.81- 33883 Villenave d'Ornon Cédex, France.

Corresponding author: robin@bordeaux.inra.fr.
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The Italian Elm Breeding Program for Dutch Elm 

Disease Resistance

Alberto Santini,

1 Francesco Pecori,1 and Luisa Ghelardini1

Abstract

In the 20th century, elms across Europe and North America were devastated by two pandemics of Dutch elm 

disease (DED), caused by the introduction of two fungal pathogens: Ophiostoma ulmi, followed by O. novoulmi. At the end of 1920s, research into a resistance to DED began in Europe and then in the United States.

No worthwhile resistance was ever found within the native European or American elms. The Dutch team 

resorted to hybridizing European elms, from which the first cultivars exhibiting a resistance were obtained 

during the 1930s, but these clones did not have sufficient resistance to the second pandemic. It was the use of 

Asian species, with their higher resistance to DED, which accelerated progress on both sides of the Atlantic. 

Eventually a number of second-generation resistant clones derived from hybrids of native and Asian species 

were released to commerce.

Many efforts were dedicated to control the disease, such as the elm breeding program started in Italy in 1975 

by the Institute of Plant Protection (IPP). This program was born of a conviction that the Mediterranean 

environment would demand its own selections. In Italy, the favorable adaptation of species such as the 

Siberian elm (U. pumila L.), and the unsuitability of the Dutch selections to the more arid regions, encouraged 

the wider assessment of the Asian elms. The purpose was twofold: to examine more fully adaptability to the 

climate and to broaden the genetic base of the native species. 

Selection of superior genotypes ostensibly reduces genetic variation in cultivated species. However, when 

breeding is designed for obtaining plants adapted to different environmental conditions and for different uses, 

the outcome can actually result in increased variation. The case of elm breeding for resistance to DED is 

paradigmatic. The uses of elm are in fact manifold. For this reason, breeding for resistance is not enough. 

Many other features are required, including fast growth, and aesthetic factors such as attractive shape and 

foliage. To satisfy all these needs, whilst maintaining enough genetic variability to buffer the effects of 

climate change and possible arrival of new strains of DED or other diseases, it was decided to undertake a 

fundamental broadening of genetic resources, or, as it will be called later, “incorporation.” A base of native 

elms with enough good characters to act as parents was hybridized with those disease-resistant Asian species 

able to acclimatize.

This program has produced DED-resistant elm varieties able to adapt to arid conditions, yet endowed with 

some remarkable ornamental characteristics. Five of these clones have already been patented and released to 

commerce: ‘San Zanobi,’ ‘Plinio,’ ‘Arno,’ ‘Fiorente,’ and, more recently, 'Morfeo.' 'Morfeo' is a robust, 

attractive tree that is extremely resistant to DED. It is also fast-growing and tolerant of both summer drought 

and winter floods, thus proving as well adapted to the climate of northwestern Europe as that of the 

Mediterranean. Indeed, following trials in England, ‘Morfeo’ is now considered potentially the most important 

cultivar in the conservation of several invertebrates there endangered by the consequences of DED.

Key words: Dutch elm disease, Ophiostoma, Italian elm breeding program

Introduction

In the 20th century, elms across Europe and North America were devastated by two pandemics of 

Dutch Elm Disease (DED), caused by the introduction of two fungal pathogens with very different 

aggressiveness: Ophiostoma ulmi circa 1910, followed 60 years later by the three times more lethal 

 1 Institute of Plant Protection – C.N.R Via Madonna del Piano, 10, 50019 Sesto fiorentino, Italy. 

Corresponding author: a.santini@ipp.cnr.it .
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327

O. novo-ulmi (Brasier 2000). The gravity and impressiveness of the damages caused by the disease 

stirred up the interest of researchers and public opinion, such as to necessitate a solution to the 

problem. The elm is, in fact, a plant of great beauty, has significant historic and artistic value, and 

adapts exceptionally well to stress and to difficult sites, such as those occurring in cities, alongside 

roads. and in windswept coastal areas. Thanks to these features, the elm is an important and 

characteristic component of the cities’ tree-lined roads and of the rural landscape in several 

European and North American countries.

The idea of searching for resistance to DED through breeding arose in 1928 at the Willie 

Commelin Scholten Phytopathological Laboratorium in Baarn (The Netherlands). There, the first 

studies were performed on the etiology of the disease that had been killing elms in western Europe 

since the end of World War I. The causal fungus was firstly isolated by Dina Spierenburg (Buisman 

1921), then described and named by Marie B. Schwarz (Schwarz 1922); later followed by the 

development of a reliable inoculation method by Christine Buisman (Westerdijk et al. 1931). The 

path-breaking research conducted by the two latter scientists—both of whom were working at the 

aforesaid Laboratorium—constituted the fundamental requisite for starting up a breeding program

(Holmes 1993).

At the beginning, research focused on selecting resistant individuals within the native species. 

During this phase, a couple of clones were selected into the species of field elm (Ulmus minor

Miller) and named ‘Christine Buisman’ (1936) and ‘Bea Schwarz’ (1947). However, these proved 

disappointing because of their slow growth, poor shape, and susceptibility to a branch canker caused 

by the Nectria cinnabarina fungus. In order to combine resistance mechanisms of different species 

and enhance the growth rate, Dutch researchers started crossing different elm species. In addition to 

resistance to DED, the long-term goals of the program were resistance to coral spot (caused by N. 

cinnabarina), to frost, and to wind. Fast growth, good form, decorative leaves, and valuable timber 

were also considered. The first two releases, the ‘Commelin’ (1960) and ‘Groeneveld’ (1963) 

clones, were first-generation hybrids between European elm species. Initially, they proved to be a 

great success. However, the arrival in the late 1960s of the new, more aggressive species, O. novoulmi, to which ‘Commelin’ was particularly susceptible, inflicted a hard blow on the Dutch breeding 

program. Although decades of breeding have shown that it was possible to slowly accumulate 

resistance in second or third generation clones of purely European elms (Heybroek, personal 

communication), complete DED resistance was not found in European nor in American native elms, 

but individuals highly resistant to DED have nevertheless been identified (Townsend et al. 2005).

At that time, it was noticed that the most resistant clones were second generation hybrids which 

contained one grandparent of Asian provenance. 

Therefore, since then, Asian DED-resistant elm species have generally been crossed to native 

elms to speed up the selection of resistant trees. A base of native elms with desirable characteristics 

was bred with Asian elms species that, besides a fair level of DED resistance, showed the ability to 

adapt to a range of climatic conditions and environments. This artificial base broadening of the 

genetic resources, or “incorporation” (Simmonds 1993), was planned in order to satisfy all 

traditional elm uses, so taking into account not only DED resistance, but also fast growth, tree 

silhouette, leaf and bark color, leaf shape, and overall tree size. The resulting progeny were

expected to combine the DED resistance of Asian species with the superior growth, ornamental 

value, and environmental adaptability of European elms.

The risk of adopting this kind of strategy was to go toward a genetic bottle-neck, since selection 

of superior genotypes reduces genetic variation (Simmonds 1993, Tanksley and McCouch 1997). 

However, when breeding, as in this case, is designed to obtain plants adapted to different 

environmental conditions and for different uses, the outcome could result in an increase of 

variability (Cox and Wood 1999).

The case of elm breeding for resistance to DED is, therefore, paradigmatic. The uses of elm are 

manifold: from wood to fodder, from medicine to urban silviculture, and many others. For this 

reason, breeding for resistance is not enough; many other features are requested as tree silhouette, 
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fast growth, leaf and bark color, and leaf shape and dimensions. In order to satisfy all these needs, 

maintaining at the same time enough genetic variability to buffer the rising of possible new stresses, 

such as new forms of the disease or other diseases and climate change, we decided to initiate a base 

broadening of the genetic resources, or, as it will be called later, “incorporation” (Simmonds 1993).

The Italian Program

A second elm breeding program in Europe was initiated in the late 1970s in Florence by the Institute 

of Plant Protection (IPP), part of the Italian National Research Council (C.N.R.), when the second 

more destructive DED pandemic incited by O. novo-ulmi reached Italy. The goal of the Italian elm 

breeding program was the selection of elm cultivars resistant to DED, but suited to the 

Mediterranean climate, unlike the Dutch selections intended for northern Europe. European elm 

species and selections with desirable morphological and physiological characteristics were 

hybridized with DED-resistant Asian species (Smalley and Guries 1993) that had thrived in the 

Mediterranean climate, broadening genetic resources (Simmonds 1993). The breeding strategy

program is reported in figure 1.

Figure 1—Italian elm breeding strategy program.

The Italian program is indebted to the work of the Dutch researchers for the breeding strategy 

and for much of the material used in the crossings. Other materials came from native species and 

from extant plantations of Siberian elm, as well as through exchanges with Americans and other

research Institutes. The inoculation and crossing techniques used in Florence were also derived from 

the Dutch experience, with the introduction of a few improvements, such as the realization of 

pollination without having to lift the isolation sack, by blowing the pollen into the sack 

(Mittempergher and La Porta 1991).
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Material and Methods

Collection of Material

In late 1970s, a wide collection of elm species and provenances from all over the world with a 

particular preference for Asian species, generally more resistant to DED (Smalley and Guries 1993),

was set up. Collections were established with two aims:

1) to check adaptation to a new environment, constituted by the Mediterranean climate and by 

all the biotic and abiotic damage agents that may affect introduced species under these conditions, 

and, as a consequence the new hybrid clones; and

2) to obtain adult plants for breeding work.

Hybridization Studies

Crossability among elm species which involved the use of several European and Asian species of 

different taxonomic sections was checked under Mediterranean climatic conditions (Mittempergher 

and La Porta 1991). Pollen of the desired species was obtained from cut branchlets held in vases 

with water during the pollen dispersal phase. Different species and individuals were kept in separate 

rooms of a greenhouse to avoid contamination. The collected pollens were conserved at 3° to 4°C 

and dehydrated at 10 percent relative humidity (RH) when used within a short period of time (few 

days or weeks). When pollen had to be stored for 6 months (Asian Autumn flowering species), or 

for about 1 year (to cross the later pollen donor with the earlier spring flowering species), it was 

conserved at -20 °C and 10 percent RH. Pollen vitality was checked before pollination by using the 

Fluoro-chromatic Reaction technique (Heslop-Harrison and Heslop-Harrison 1970, Heslop-Harrison 

et al. 1984).

Flower pollination was carried out by injecting pollen into the pollination bags with forced air. 

At least three bags were used for each single cross, and, at least three control bags on each mother 

tree did not receive foreign pollen in order to check the selfing for each mother tree. Matured seeds 

were harvested and sown in open air nursery beds, monitoring germination. The seedlings were 

checked morphologically during the first and second growing season in order to ascertain their 

hybrid nature. The percent of viable seedlings from full seeds sown was scored at the end of the 

first growing season. 

Screening Disease Resistance

The seedlings were evaluated for DED resistance. Selection for resistance was operated following a 

two-step protocol:

1) Mass inoculation. Three-year-old elm seedlings obtained by controlled crosses were grown in 

the IPP nursery and planted in the field. The following year, during the third week of May, which is 

the time of beetle flight in the area of Florence, trees were inoculated. Inoculation was performed 

with a single wound per plant, using a knife blade carrying two drops of a 0.2 ml of a 1x106 ml-1

yeast phase cells, consisting of two tester isolates of the subsp. novo-ulmi and subsp. americana of 

O. novo-ulmi (Brasier and Kirk 2001), so that the inoculum would be absorbed by the tree’s rising 

sap. Symptoms of disease (percent of defoliation and percent of dieback) were observed after 4 

weeks and at 3 and 8 months from inoculation date and assessed by three independent assessors.

2) Selection. Seedlings with less than 10 percent dieback were vegetatively propagated by 

hardwood cuttings and planted out the following year in a randomized complete block design. 

Clones showing less than 25 percent dieback were considered resistant, and were evaluated for other 

characters. Twelve rooted cuttings per clone, divided into three blocks, were used. Inoculations and 

disease evaluations were performed 2 years after, as described above, and the symptoms were 

compared with those expressed by clones having known DED responses acting as controls, 

generally the Dutch clones ‘Commelin’ and ‘Lobel’, highly susceptible and intermediately resistant, 

respectively. 
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Adaptation Trials

In order to check the possible effect of phenotypic plasticity and to determine the best 

environmental condition for growth of each, a series of traits were evaluated in different ecological 

conditions. Several field trials were planted in different climates in Italy. In each trial, a randomized 

complete block design was used with generally three blocks of four ramets for each clone. Traits 

were measured each year and a final evaluation was performed at the end of the trial, generally after 

6 to10 years.

Clones showing DED resistance were also evaluated for:

1) leaf shape: length, breadth, and slenderness, with the favorite shape being that of U. minor

leaves which are rounder than Asian elms.

2) leaf color: dark green was considered preferable for being similar to the native field elm.

3) shape of the crown: columnar with a monocormic straight trunk and slender branches is the 

favorite shape.

4) height and diameter growth. 

Clones were ranked for these traits and resistance according to a 5-step scale by three 

independent observers. This datum is reported together with the clone code as a variable number of 

marks. The scale goes from no marks = not an eligible clone, to four marks = clone that 

accomplishes all the requested characteristics: resistance, adaptation, leaf color, trunk and crown 

shape.

Results and Discussion

More than 50,000 hybrid seedlings have been raised and tested, of which circa 80 individuals 

totaled a very high score. A wide range of resistant elm clones with different parentage and valuable 

characters are going to be released. This should help further safeguard against the appearance of 

new and even more aggressive strains of the pathogen, as occurred in the 1970s when the O. novoulmi appeared in Europe, or against different unpredictable risks (Santini et al. 2008).

Five DED-resistant elm clones have been patented and released to the market. ‘San Zanobi’ (Pat. 

n. RM97NV0006) and ‘Plinio’ (Pat. n. RM97NV0005) (Santini et al. 2002), both obtained by a 

controlled crossing of the Dutch hybrid ‘Plantijn’ with two different individuals of U. pumila, were 

released in 1997. The resistance levels of the clones were significantly higher than the resistance 

levels of ‘Lobel’ and other reference clones. 

‘San Zanobi’ is monocormic and shows exceptionally rapid growth on fertile soils and in 

temperate climates, suggesting that it could be used also for production of construction timber 

(Santini et al. 2002, 2010). Its habit is cone-shaped with pronounced apical dominance, resulting in 

limited lateral branching on the developing shoots of the current season’s growth (fig. 2). The crown 

is therefore narrow and columnar. Apical dominance is so marked that seedlings rarely need pruning 

or training. The wood characteristics of this clone are not different from those known for European

field elm (Ulmus minor Mill.) (Santini et al. 2004).

‘Plinio’ grows rapidly, although slightly slower than ‘San Zanobi,’ roughly similar to that of 

fast-growing benchmark ‘Lobel.’ ‘Plinio’ can be used as an ornamental shade tree (fig. 2). The 

crown is vase shaped. In isolated trees, the width of the crown can be up to 70 percent of its height. 

The trunk is straight, at times slightly sinuous and short.
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Ulmus ‘San Zanobi Ulmus ‘Plinio’

Figure 2—Characteristics of two Dutch elm disease-resistant clones.

In 2006, ‘Arno’ (Community Plant Variety Right n. 27598) and ‘Fiorente’ (Community Plant 

Variety Right n. 27599) (Santini et al. 2007) were released. The first is a full sib of ‘Plinio,’ while 

the second is a first generation hybrid between a U. pumila with a U. minor that originated in Italy. 

The resistance levels of the clones are comparable to that of ‘Lobel.’ 

‘Arno’ is single stemmed (monocormic), with erect habit, ascending branches, and an upright 

oval crown (fig. 3). Field trials indicate that ‘Arno,’ although slower than ‘Fiorente,’ is among the 

fastest growers ever tested. The trunk is straight, branching at a height of 3 m. The wood 

characteristics of this clone are not different from those known for field elm (Santini et al. 2004).

‘Fiorente’ is monocormic and shows exceptionally rapid growth, significantly greater than the 

other cultivars planted at the same site, suggesting that it could also be used for timber production. 

Its habit is conical with pronounced apical dominance, a result of the limited lateral branching on 

the developing shoots of the current season’s growth (fig. 3). The crown is therefore slender and 

columnar. The trunk is straight and long.
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Ulmus ‘Arno’ Ulmus ‘Fiorente’

Figure 3—Characteristics of two Dutch elm disease-resistant clones.

In 2010, the Italian elm breeding program raised a new variety by crossing a specimen of U. 

chenmoui W. C. Cheng with the Dutch hybrid clone '405.' This new release, named 'Morfeo'

(Community Plant Variety n. 2011/0223) (Santini et al. 2011), is extremely resistant to DED and 

has an attractive form and foliage. It is also fast-growing, able to free stand at a very early age, and 

tolerant of drought and soils waterlogged in winter (fig. 4). Therefore, it is proving well adapted to 

the maritime, wet winter climate, where temperatures are moderated by the gulfstream of

northwestern Europe, as well as the Mediterranean climate. Following trials in England, ‘Morfeo’ is 

considered of potential importance in the conservation of several invertebrates endangered by the 

consequences of DED.
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333

Ulmus ‘Morfeo’

Figure 4—Characteristics of the Dutch elm disease resistant clone ‘Morfeo’.

Regarding genotype x environment interaction (GxE), results show that some DED-resistant elm

clones showed superior growth at all experimental sites. These clones are likely suitable for 

successful planting in a range of different environments. In addition, several resistant clones showed 

a more marked GxE interaction having high-growth parameters at a particular site (Santini et al.

2010). Results deriving from biometric studies strongly support our belief that these elm clones can 

be successfully used for timber and biomass production.

The introduction of non-native elm species from different continents is one of the main features

of the elm breeding program for DED resistance. It may involve the risk that local parasites of 

minor importance toward native species might cause some damage to new the hosts. For instance, a 

disease named ‘Elm Yellows’ caused by phytoplasmas, which until 1985 had been regarded of 

American origin, was found to be harmful and even deadly for a number of Asian elm species 

resistant to DED and for their hybrids (Mittempergher 2000). This disease commonly kills the 

American elm (U. americana L.); therefore, its outcome in this case does not differ from the results 

of DED. Yet in Europe, Elm Yellows is tolerated by the populations of native elms, with only a few 

individuals showing symptoms of yellowing, witches’ brooms, growth retardation, or a general 

decline (Mittempergher 2000) and sometimes death.

Numerous insects are also known to damage European elms. Among these, the elm leaf beetle 

(Xantogalerucha luteola) and the goat moth (Cossus cossus) have to be mentioned. The various 

Asian elm species used in breeding programs because of their resistance to DED show varying 

susceptibility to these insects. For example, in our experience, the Chinese species U. laciniata

(Trautv.) Mayr is so susceptible to leaf beetle that it is very difficult to raise it in central Italy 

without chemical control, whereas U. parvifolia Jacq. and U. wilsoniana Schneid., are scarcely 
GENERAL TECHNICAL REPORT PSW-GTR-240

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damaged. The IPP has thus set up a rating program to assess the extent to which the commonly used 

Asian species may be susceptible to Elm Yellows and to the elm leaf beetle. 

Multiple resistances to all of these different threats is taken into account when the clones are in 

adaptation trial fields by scoring their natural susceptibility. It is very conceivable that the climate 

could play a primary role in the build-up of insect populations and to phytoplasma infection, which 

is vectored by some species of insects belonging to phloem-feeding Hemiptera. 

Moreover, the strategy adopted for elm genetic improvement had produced a new population that 

has a high proportion of unique, exotic-derived alleles that indeed broaden the elm genetic base, so 

that variance will be enhanced (Simmonds 1993). The gain in genetic richness and variability 

should encompass the strong bottleneck that, for the use of exotic species, even if hybridized with 

endogenous germplasm, is represented by adaptation to new environments. In conclusion, these 

results open the possibility for elms to overcome the condition of shrubs to which DED seemed to 

have destined these magnificent trees, and to be cultivated again for providing us not only with 

beauty, but also with new and different possible uses.

Acknowledgments

The authors wish to thank Prof. Lorenzo Mittempergher, who lead the Italian elm breeding program until 

1999; Mr. Fabio Ferrini, Mr. Alberto Fagnani, and Mr. Abdellah Dahmani, without whom the results of this 

program would not be achieved.

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Simmonds, N.W. 1993. Introgression and incorporation. Strategies for the use of crop genetic resources. 

Biological Reviews. 68: 539–562.

Smalley, E.B.; Guries, R.P. 1993. Breeding elms for resistance to Dutch elm disease. Annual Review of

Phytopathology. 31: 325–352.

Spierenburg, D. 1921. Een onbekende ziekte in de iepen. Tijdschrift Over Plantenziekten. 18: 53–61.

Tanksley, S.D.; McCouch, S.R. 1997. Seed banks and molecular maps: unlocking genetic potential from the 

wild. Science. 277: 1063–1066.

Townsend, A.M.; Bentz, S.E.; Douglass, L.W. 2005. Evaluation of 19 American elm clones for tolerance to 

Dutch elm disease. Journal of Environmental Horticulture. 23: 21–24.

Westerdijk, J.; Ledeboer, M.; Went, J. 1931. Mededeelingen omtrent gevoeligheidsproeven van iepen voor 

graphium ulmi Schwarz, Gedurende 1929 en 1930. Tijdschrift Over Plantenziekten. 37: 105–110.
GENERAL TECHNICAL REPORT PSW-GTR-240

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Patterns of Resistance to Cronartium ribicola in 

Pinus aristata, Rocky Mountain Bristlecone Pine

A.W. Schoettle,

1 R.A. Sniezko,2 A. Kegley,2 R. Danchok,2 and K.S. Burns3

Abstract

The core distribution of Rocky Mountain bristlecone pine, Pinus aristata Engelm., extends from central

Colorado into northern New Mexico, with a disjunct population on the San Francisco Peaks in northern 

Arizona. Populations are primarily at high elevations and often define the alpine treeline; however, the species 

can also be found in open mixed conifer stands with ponderosa (Pinus ponderosa Lawson & C. Lawson) and/or 

pinyon (Pinus edulis Engelm.) pines in some locations. On dry, exposed sites the stands are open and sparse and 

Rocky Mountain bristlecone (hereafter referred to as bristlecone) is commonly the only species present. The 

combination of the pine’s adaptive traits with infrequent disturbance has enabled trees on these sites to attain 

ages of over 2,500 years. These same traits and conditions, which contribute to a long generation time, will 

inevitably hinder the ability of bristlecone pine to adapt to novel anthropogenic stresses such as climate change 

and infection by the non-native pathogen (Cronartium ribicola) that causes the lethal disease white pine blister 

rust (WPBR). Infection of bristlecone pine by C. ribicola was first documented in the field in 2003 in southcentral Colorado. Rapid climate warming and the associated increase in mountain pine beetle activity are also 

affecting these high elevation ecosystems. These threats and the species’ unique aesthetic and ecological roles 

make bristlecone pine a species of conservation interest. Blister rust invasion is relatively recent compared to 

the generation time of bristlecone pine, thus we still have a window of opportunity to gain and utilize new 

knowledge of this species and their ecosystems under natural conditions and develop proactive conservation 

strategies.

Genetic resistance to WPBR is a key factor that will affect the trajectory of bristlecone pine populations in the 

future. Preliminary results of studies to identify and quantify rust resistance in bristlecone pine families are 

reported here. Seeds collected in 2001 from 184 individual trees across 11 sites along the full latitudinal 

gradient in Colorado were sown in 2002. The 3-year-old seedlings were inoculated with C. ribicola at Dorena 

Genetic Resource Center in 2005. Seeds for an additional smaller test, also conducted at Dorena Genetic 

Resources Center, were sown in April 2009 and seedlings inoculated with C. ribicola in September 2009.

Needle infection lesions (spots) were easily identified on needles of the seedlings inoculated at a young age 

(2009 trial), but were less obvious on needles of the seedlings inoculated at an older age (2005 trial). However, 

WPBR stem symptoms did develop on many of the older seedlings in the 2005 trial for which infection spots 

were not observed, suggesting that needle lesions are not a good early measure of infection in older bristlecone 

pine seedlings. Multiple WPBR resistant phenotypes are expressed by bristlecone pine including canker-free 

seedlings, seedlings with partial bark reactions, and seedlings developing stem infections later or showing 

longer survival with stem infections. Survivorship of seedlings in the large study, 5 years post-inoculation, 

range from 0 percent to 92 percent among families, and 17 percent to 60 percent among geographic areas.

 1 USDA Forest Service, Rocky Mountain Research Station, Ft. Collins, CO. 2 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, OR.

3 USDA Forest Service, Forest Health Management, Lakewood, CO.

Corresponding author: aschoettle@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

337

Development of Methods for the Restoration of the 

American Elm in Forested Landscapes

James M. Slavicek1

Abstract

A project was initiated in 2003 to establish test sites to develop methods to reintroduce the American elm 

(Ulmus americana L.) in forested landscapes. American elm tree strains with high levels of tolerance to Dutch 

elm disease (DED) were established in areas where the trees can naturally regenerate and spread. The process of 

regeneration will allow the American elm to co-evolve with the DED fungal pathogen to ensure this valuable 

tree species will not be lost from the landscape. This effort in Ohio is being carried out in partnership with the 

Ohio Department of Natural Resources-Division of Forestry, Franklin County Metro Parks, Worthington City 

Parks, and The Wilds. The project was expanded in 2005 by establishment of restoration sites in Iowa;

Wisconsin; and Minnesota in partnership with the USDA Forest Service Northern Area State and Private 

Forestry; Luther College, Iowa; the U.S. Army Corps of Engineers; and the Carpenter St. Croix Valley Nature 

Center in Minnesota. In 2007, a site was established at Dago Slough near Cassville, Wisconsin in partnership 

with the U.S. Department of the Interior, Fish and Wildlife Service, and a test planting was established at the 

Bad River Indian Reservation in Wisconsin to assess tree cold hardiness. The restoration effort was expanded in 

2010 in partnership with The Nature Conservancy through establishment of three sites in Vermont. To date, 12 

American elm restoration test sites have been established in five states. 

Key words: Dutch elm disease, American elm, restoration

Introduction

The American elm (Ulmus americana L.) was once widely distributed throughout the eastern United 

States and was a preferred tree for use along city streets and in the yards of many homeowners. The 

Dutch elm disease (DED) fungal pathogen Ophiostoma ulmi was introduced into the United States in 

1930 and O. novo-ulmi in the early 1940s (Brasier 2000). In the subsequent years, these pathogens 

have destroyed millions of American elm trees in the United States and Canada. By 1976, only 34 

million of the estimated 77 million elms present in the urban landscape before the introduction of the 

DED pathogen remained, and far fewer are still present today.

One line of research on the American elm from the 1970s to the present focused on the 

identification of American elm isolates that could withstand the DED pathogen. Over 100,000 

American elm trees were tested for resistance to DED. No trees were found that were resistant; 

however, a few were identified that exhibited high levels of tolerance (approximately 60 to 96 percent

survival after fungal inoculation) to the disease (Sherald 1993, Smalley and Guries 1993). Out of over 

100,000 American elm trees screened for DED resistance, a very small number of trees (about seven)

were identified that exhibit the necessary levels of DED tolerance to withstand the disease. Five of 

these selections include trees that have been released to the nursery industry: Valley Forge, Princeton, 

and New Harmony, and two other selections, the R18-2 and Delaware 2 (Smalley et al. 1993, 

Townsend et al. 1995, Townsend and Douglass 2001, Townsend et al. 2005). These five selections 

were planted at experimental test plots to assess fitness for selected sites and to develop methods for 

restoration of the American elm.

 1 Northern Research Station, Forestry Sciences Laboratory, 359 Main Road, Delaware, OH 43015.

Corresponding author: jslavicek@fs.fed.us. 
GENERAL TECHNICAL REPORT PSW-GTR-240

338

Materials and Methods

The origin of the following American elm selections used in the test plots are:

• Princeton: A cultivar originating from Princeton, NJ that exhibits high tolerance to DED

identified in 1922.

• R18-2: One of 17 survivors out of 21,000 seedlings screened for DED tolerance by Cornell 

University and the Boyce Thompson Institute reported in 1993.

• Delaware 2: One of the two most DED-tolerant trees identified in a screen of 35,000 

seedlings by the U.S. Department of Agriculture, Bureau of Plant Industry selected in the 

1940s.

• New Harmony: A clone of an ancient tree near Springfield, Ohio that was identified by 

Denny Townsend and Larry Schreiber in 1975 and exhibits high DED tolerance .

• Valley Forge: A selection out of hundreds that was identified by Denny Townsend and Larry 

Schreiber in 1975 and showed high levels of DED tolerance .

• Valley Forge x New Harmony seedling trees.

Additional selections will be added to the test sites as they become available. Test sites established to 

date are listed in table 1.

Table 1. Test sites established as of June 2012

Site Location Year established Site description

Highbanks Metro Park Delaware County, OH 2003 Upland, heavy clay

Mohican Memorial State 

Forest Ashland County, OH 2003 Upland, loam

The Wilds Muskingum County, OH 2003 Reclaimed coal mine

Maumee State Forest Fulton County, OH 2004 Lowland, sand

Glacier Ridge Metro Park Union County, OH 2004 Upland, heavy clay

Carpenter St. Croix Valley 

Nature Center Hastings, MN 2005 Lowland, loam

Eagle Island Stoddard Islands, WI 2005 Island within the 

Mississippi River

Luther College Decorah, IA 2005 Upland, loam

Dago Slough Cassville, WI 2007 Mississippi River 

floodplain

Olentangy River Park Worthington, OH 2009 Olentangy River 

floodplain

Otter Creek Cornwall, VT 2010 Otter Creek floodplain

Connecticut River Maidstone, VT 2010 Connecticut River 

floodplain

LaPlatte River Shelburne, VT 2010 LaPlatte River floodplain

American elm selections were propagated by vegetative cuttings, grown in 7.6 L pots for 2 years, 

and were about 2 m in height at planting. Valley Forge x New Harmony seedlings were generated by 

controlled pollinations, the seedlings were grown in 7.6 L pots for 1 year, and were about 1.3 m in

height at planting. A 0.9 m diameter space was cleared of vegetation, 400 cm3 of 10-10-10 fertilizer 

and 20 cm3 of trace elements was added to the bottom of the planting hole, the trees were planted, and 

watered if the soil was dry. Mulch was added to the base around the trees with the exception of trees 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

339

planted in 2003 where weed barriers of recycled plastic were used. Wire enclosures 0.76 m in 

diameter were constructed of poultry wire, 12 gauge woven wire, or plastic deer fencing and were 

placed around the trees to prevent deer browse and deer rub. A variety of site types were selected 

including upland, lowland, floodplain, and island sites as indicated in table 1 above. Trees were 

planted in a grid pattern or in clusters with spacing of individual trees from approximately 8 to 11 m

apart. Annual measurements of trunk diameter at 1.2 m in height and height were made in early 

spring before leaf expansion. Instances of mortality were noted and the cause determined when 

possible.

Results and Discussion

All deer exclusion methods employed were effective; however, the wire enclosures made of 12 gauge 

woven wire were the most durable and needed no attention after placement. Mulching the base of 

trees provided effective weed/plant control for about 2 to 3 years after planting. Weed barrier mats 

provided long-term effective weed/plant control; however, they also provided nesting cover for mice 

which fed on bark during winter causing the loss of a few trees. The mats were removed in the spring 

of 2005.

At the Mohican Memorial State Forest site, average yearly growth ranged from 0.3 to 1.02 m in 

height for the clonally propagated selections, and from 0.25 to 0.9 m for Valley Forge x New 

Harmony seedling trees (fig. 1). Cumulative growth to date resulted in an average height of 6.2 m for 

the clonal selections and 3.5 m for the seedling trees (fig. 2). Average yearly growth ranged from 7.5 

to 15 mm in stem diameter of the clonally propagated selections, and from 0.25 to 14.5 mm of Valley 

Forge x New Harmony seedling trees (fig. 3). Cumulative stem growth to date resulted in an average 

stem diameter of 80 mm for the clonal selections and 30 mm for the seedling trees (fig. 4). 

Figure 1—Average growth in American elm tree height of trees planted in 2003 at the Mohican 

Memorial State Forest in Ohio. The average growth plus the standard deviation for each year is

shown.
GENERAL TECHNICAL REPORT PSW-GTR-240

340

Figure 2—Cumulative growth in American elm tree height of trees planted at the Mohican Memorial 

State Forest in Ohio. The average cumulative height +/- the standard deviation for each year is 

shown.

Figure 3—Average growth in American elm tree trunk diameter at 1.2 m in height of trees planted in 

2003 at the Mohican Memorial State Forest in Ohio. The average growth plus the standard deviation 

for each year is shown.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

341

Figure 4—Cumulative growth in American elm tree trunk diameter at 1.2 m in height of trees planted 

in 2003 at the Mohican Memorial State Forest in Ohio. The average cumulative growth +/- the 

standard deviation for each year is shown.

Literature Cited

Brasier, C.M. 2000. Intercontinental spread and continuing evolution of the Dutch elm disease pathogens. In: 

Dunn, C.P., ed. The elms: breeding, conservation and disease management. Boston: Kluwer Academic 

Publishers: 61–72.

Sherald, J.L. 1993. Demands and opportunities for selecting disease-resistant elms. In: Sticklen, M.B.; Sherald, 

J.L., eds. Dutch elm disease research: cellular and molecular approaches. New York: Springer-Verlag: 60–

68.

Smalley, E.B.; Guries, R.P. 1993. Breeding elms for resistance to Dutch elm disease. Annual Review of 

Phytopathology. 31: 325–352.

Smalley, E.B.; Guries, R.P.; Lester, D.T. 1993. American Liberty elms and beyond: going from the 

impossible to the difficult. In: Sticklen, M.B., Sherald, J.L., eds. Dutch elm disease research: cellular and 

molecular approaches. New York: Springer-Verlag: 26-45.

Townsend, A.M.; Bentz, S.E.; Douglass, L.W. 2005. Evaluation of 19 American elm clones for tolerance to 

Dutch elm disease. Journal of Environmental Horticulture. 23: 21–24.

Townsend, A.M.; Bentz, S.E.; Johnson, G.R. 1995. Variation in response of selected American elm clones to 

Ophiostoma ulmi. Journal of Environmental Horticulture. 13: 126–128.

Townsend, A.M.; Douglass, L.W. 2001. Variation among American elm clones in long-term dieback, growth, 

and survival following Ophiostoma inoculation. Journal of Environmental Horticulture. 19(2): 100–103.
GENERAL TECHNICAL REPORT PSW-GTR-240

342

Generation of American Elm Trees with Tolerance to 

Dutch Elm Disease Through Controlled Crosses and 

Selection

James M. Slavicek1 and Kathleen S. Knight1

Abstract

The goal of our research and development efforts is to generate new and/or improved selections of the 

American elm (Ulmus americana L.) with tolerance/resistance to Dutch elm disease (DED). The approaches we 

are taking for this effort include: 1) controlled breeding using known DED -tolerant selections, 2) controlled 

breeding using DED-tolerant selections and local escape/survivor trees from specific geographic areas to 

generate site-adapted and DED-tolerant selections, 3) screening of escape/survivor trees for tolerance to DED, 

and 4) identification of additional American elm escape/survivor trees across its range. Our efforts in these 

areas are described. Progeny trees generated from crosses of Delaware 2 x New Harmony, Valley Forge x New 

Harmony, R18-2 x New Harmony, and Valley Forge x R18-2 exhibited a significantly lower mean of foliar 

symptoms at 8 to 9 weeks compared to the susceptible control.

Key words: American elm, Ulmus americana, Dutch elm disease resistance, tolerance

Introduction

Dutch elm disease (DED) has devastated North American species of elm, nearly eliminating the use 

of American elm (Ulmus americana L.) as an urban shade tree. There is a need for multiple, 

genetically diverse, American elm cultivars with superior tolerance to DED to be used by arborists 

and nurseries for urban plantings as shade trees and for the restoration of the American elm in forests.

A few American elm selections have been identified and released to the nursery industry. The 

Princeton cultivar is the most sold cultivar by far in the United States (Sherald 1993, Smalley et al.

1993, Townsend and Douglass 2001, Townsend et al. 1995, Townsend et al. 2005).

Previous attempts at selective breeding of American elm resulted in low levels of resistance 

(Lester 1969a, 1969b). In Wisconsin, a group led by Smalley and Guries (1993) crossed 29 trees 

yielding more than 3,000 progeny. After inoculation screenings of vigorous individuals multiplied for 

clonal trials, six trees were selected to comprise the American Liberty Multiclone and one of these 

was patented as the Independence elm. Unfortunately, the DED tolerance of American Liberty has 

subsequently been extensively tested and found to be unexceptional (Townsend and Douglass 2001, 

Townsend et al.1995, Townsend et al. 2005).

Several American elm clones have tested with higher DED tolerance than the American Liberty.

Valley Forge showed the best tolerance of all clones, but significant tolerance was also measured for 

New Harmony, Princeton, R18-2, and Delaware 2 (Townsend and Douglass 2001, Townsend et al. 

1995, Townsend et al. 2005). The different levels of foliar symptoms and crown dieback may suggest 

that multiple distinct genes are involved in conferring tolerance to DED. If complementary DED 

tolerance genes from both parental trees were placed in a progeny tree through crossing, the progeny 

could exhibit greater tolerance to DED than the parents.

There have been about 80 years of DED pressure on American elms in the landscape.

Consequently, there is an increasing likelihood that the remaining large trees (0.9 to 1.2 m diameter at 

breast height) have tolerance to DED and the screening of these trees could be fruitful.

 1 Northern Research Station, Forestry Sciences Laboratory, 359 Main Road, Delaware, OH 43015.

Corresponding author: jslavicek@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

343

Materials and Methods

Controlled breeding was carried out in the laboratory with cut branches and/or with field-planted 

trees. The selections listed in table 1 have been used to date.

Table 1—DED tolerant, escape/survivor, and elm yellow escape/survivor selections used

Known DED tolerant Escape/survivor trees on the 

Chippewa NF

Escape/survivor trees in 

Massachusetts & Connecticut

Princeton (Pr) Black Duck (BD) Hadley (HA)

Valley Forge (VF) SO 1 Rainbow Bridge 1 (RB1)

New Harmony (NH) SO 2 Goff (GF)

Delaware 2 (Del2) Walker (WR) Rainbow Bridge 2 (RB2)

R18-2 (18-2)

8630 (elm yellows survivor) 

Pollination bags were placed on branches on trees in the field prior to flower bud opening.

Branches from trees were collected, brought into the laboratory, and pollen was collected after pollen 

drop. Pollen was used to pollinate flowers within bagged branches on trees in the field and flowers on 

branches in the laboratory. The controlled pollinations performed to date include:

VF ♀ x 18-2 ♂ , 18-2 ♀ x VF ♂ , VF ♀ x SO 2 ♂ , 18-2 ♀ x SO 2 ♂ , 18-2 ♀ x GF ♂ 

VF ♀ x NH ♂ , NH ♀ x Del2 ♂ , VF ♀ x WR ♂ , Del 2 ♀ x BD ♂ , 18-2 ♀ x RB 2 ♂ 

R18-2 ♀ x NH ♂ , VF ♀ x 8630 ♂ , 18-2 ♀ x SO 1 ♂ , Pr ♀ x SO 2 ♂ , VF ♀ x HA ♂ 

Del2 ♀ x NH ♂ , VF ♀ x BD ♂ , 18-2 ♀ x WR ♂ , 18-2 ♀ x HA ♂ , VF ♀ x RB 1 ♂ 

Pr ♀ x VF ♂ , VF ♀ x SO 1 ♂ , 18-2 ♀ x BD ♂ , 18-2 ♀ x RB 1 ♂ , VF ♀ x GF ♂ 

VF ♀ x Pr ♂ , VF ♀ x RB 2 ♂ , Pr ♀ x RB 1 ♂ , Pr ♀ x HA ♂ , Pr ♀ x GF ♂ 

Pr ♀ x RB 2 ♂ 

Progeny generated from controlled pollinations of Del2 x NH, VF x NH, R18-2 x NH, and VF x 

R18-2 were established in field plots at the Delaware, Ohio Forestry Sciences Laboratory, Northern

Research Station, using a randomized complete block design. The plot design consists of 10 blocks of 

3.05 m x 3.05 m plantings with a total of 100 progeny of each of the four crosses; ramets of each 

parent and a susceptible clone, NA57845; and unselected seedlings (total n = 720). When trees 

reached 5 to 7 years old, each was inoculated with O. novo-ulmi strains PMP1 and H961, and the O. 

ulmi strain PG442, using a mixed inoculum consisting of equal counts of conidia of the three strains.

The mixed inoculum resolves degrees of tolerance among American elm clones better than a single 

strain inoculum (Townsend, ARS, US National Arboretum, retired, personal communication). An 

aliquot of approximately 0.15 ml of an aqueous stock suspension with a concentration of 4 million 

spores/ml was applied to a hole drilled with a 0.2 cm diameter bit to a 2.54 cm depth in the lower 

third of the trunk, and the hole was wrapped with parafilm. Foliar symptoms of DED include wilting, 

chlorosis, and necrosis. For each tree, the combined percentage of the crown exhibiting wilting, 

chlorosis, or necrosis was visually estimated (consensus rating by a pair of evaluators) to the nearest 5 

percent at 8 to 9 weeks, 1 year, and 2 years after inoculation. In addition to the 5 percent intervals, an 

additional interval of < 5 percent was used to indicate trace symptoms that were significantly less 

than 5 percent.
GENERAL TECHNICAL REPORT PSW-GTR-240

344

Results and Discussion

Controlled Crosses of Chippewa National Forest, Connecticut, and 

Massachusetts Escape/Survivor American Elms

The results of field pollinations performed in 2011 are shown in table 2. The number of seeds 

generated per flower ranged from 2.1 to 19.0. The control bags produced essentially no viable seeds.

The germination rate of seed from the crosses ranged from 70 to 85 percent. 

Table 2—Results of controlled pollinations performed in 2011

Cross Seeds/Flower Bud Cross Seeds/Flower Bud

18-2 ♀ x HA ♂ 7.2, 8.9, 6.0 Pr ♀ x HA ♂ 10.1

18-2 ♀ x RB 1♂ 3.2, 8.0, 2.2 Pr ♀ x RB 1♂ 8.5, 11.4

18-2 ♀ x GF ♂ 7.4, 6.4, 4.6 Pr ♀ x GF ♂ 8.7

18-2 ♀ x RB 2 ♂ 4.6, 6.9, 2.1 Pr ♀ x RB 2 ♂ 10.5

VF ♀ x HA ♂ 2.1 18-2 ♀ control 0, 0, 0.3, 0

VF ♀ x RB 1♂ 13.4, 12.2 VF ♀ control 0.1, 0, 0, 0

VF ♀ x GF ♂ 10.0, 8.8 Pr ♀ x 18-2 ♂ 10.2

VF ♀ x RB 2 ♂ 3.0 18-2 ♀ x VF ♂ 9.9, 19.0 

Seedlings generated from crosses between known DED-tolerant and Chippewa National Forest

elm trees, and known DED-tolerant and Massachusetts and Connecticut trees, have been planted in 

test sites at the Chippewa National Forest and in Connecticut. When the trees are approximately 5 to 

7 years old they will be inoculated with O. ulmi and O. novo-ulmi to test for DED tolerance.

DED Testing of Progeny Trees From Controlled Crosses of DED 

Tolerant Selections

The Del 2 x NH, VF x NH, R18-2 x NH, and VF x R18-2 crosses yielded seedlings with lower-mean 

foliar symptoms at 8-9 weeks compared to the susceptible control. The distributions of symptom 

severity for the progeny trees of the VF x R18-2 cross and the clonally propagated DED susceptible 

control (NA54845) are shown in fig. 1. The progeny trees from the other crosses exhibited a similar

symptom response and distribution compared to the progeny trees from the VF x R18-2 cross (data 

not shown). Valley Forge x R18-2 yielded the highest number of individuals with few to no 

symptoms compared to the other crosses (data not shown). The crosses showed no correlation 

between tree height or caliper and foliar symptoms (data not shown). These results show that it may 

be possible to generate new American elm clones with high levels of DED tolerance using these 

selected crossings. Fifteen progeny trees that showed little or no DED symptoms were clonally 

propagated, the clones were established in field plots, and the trees will be challenged with O. novoulmi as described above in 2 to 3 years. 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

345

 Percentage foliar symptoms at 8 weeks post inoculation

Figure 1—(A) Dutch elm disease symptoms on progeny trees from the R18-2 x Valley Forge cross, 

and (B) on clonally propagated susceptible control NA54845.

Identification of Large American Elm Escape/Survivor Trees

Twenty-five large escape/survivor American elms have been identified in Michigan, Ohio, Indiana, 

and Illinois. These trees were clonally propagated, established in replicate blocks at Delaware, Ohio, 

and will be tested for DED tolerance in 2 to 3 years. Additional escape/survivor trees are being 

identified with the help of interested foresters and others as discussed below.

A new effort was initiated to identify additional large surviving American elms through the 

generation of a website where information on these trees can be submitted 

(http://nrs.fs.fed.us/SurvivorElms). Approximately 605 elm trees throughout the region have been 

submitted as of June 5, 2012. We are asking for the help of state foresters, park employees, and the 

interested public to identify large American elm trees on their landscapes.

Potential DED-tolerant elm trees identified by cooperators (fig. 2) will be compiled in a database 

and prioritized for sampling. When funding becomes available, scion wood will be collected from the

identified high-priority trees and used to clonally propagate these trees initially through bud grafts 

and then vegetative cuttings. When the trees reach susceptible size, they will be screened for DED 

tolerance, and selections with tolerance will be released to state tree nurseries for generation of elm 

seed orchards and to the commercial nursery trade. In addition, seed collected from the seed orchards 

created at the Delaware, Ohio Forest Service laboratory will be used in landscape restoration efforts.

0

2

4

6

8

10

12

14

16

0 <5 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Number of Trees Exhibiting Symptoms

0

2

4

6

0 <5 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

A

B
GENERAL TECHNICAL REPORT PSW-GTR-240

346

Figure 2—Locations of submitted escape/survivor American elm trees as of August 17, 2011.

Literature Cited

Lester, D.T. 1969a. Genetics and breeding of American elm. In: Proceeding of the 16th northeastern forest tree

improvement conference. Ottawa: Canada Department of Forestry: 9–13.

Lester, D.T. 1969b. Self-compatibility and inbreeding depression in American elm. Forest Science. 17: 321–

322.

Sherald, J.L. 1993. Demands and opportunities for selecting disease-resistant elms. In: Sticklen, M.B.; Sherald, 

J.L., eds. Dutch elm disease research: cellular and molecular approaches. New York: Springer-Verlag: 60–

68.

Smalley, E.B.; Guries, R.P. 1993. Breeding elms for resistance to Dutch elm disease. Annual Review of 

Phytopathology 31: 325–352.

Smalley, E.B.; Guries, R.P.; Lester, D.T. 1993. American Liberty elms and beyond: going from the 

impossible to the difficult. In: Sticklen, M.B.; Sherald, J.L., eds. Dutch elm disease research: cellular and 

molecular approaches. New York: Springer-Verlag: 26–45.

Townsend, A.M.; Bentz, S.E.; Johnson, G.R. 1995. Variation in response of selected American elm clones to 

Ophiostoma ulmi. Journal of Environmental Horticulture. 13: 126–128.

Townsend, A.M.; Bentz, S.E.; Douglass, L.W. 2005. Evaluation of 19 American elm clones for tolerance to 

Dutch elm disease. Journal of Environmental Horticulture. 23: 21–24.

Townsend, A.M.; Douglass, L.W. 2001. Variation among American elm clones in long-term dieback, growth, 

and survival following Ophiostoma inoculation. Journal of Environmental Horticulture. 19: 100–103.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

347

Breeding for Resistance to Adelgids in Abies 

fraseri, Tsuga canadensis, and T. caroliniana

Ben Smith,

1 Fred Hain,2 and John Frampton2

Abstract

The balsam woolly adelgid (BWA; Adelges piceae) and hemlock woolly adelgid (HWA; Adelges tsugae) have 

had a tremendous impact on native ecosystems with Fraser fir (Abies fraseri (Pursh) Poir), eastern hemlock 

(Tsuga canadensis (L.) Carrière), and Carolina hemlock (T. caroliniana Engelm. ) in the eastern United States 

since their introduction from Asia. They have also caused serious economic damage in Christmas tree 

plantations and nurseries. The Alliance for Saving Threatened Forests is engaged in research to complement 

other methods of adelgid control through the development of genetically resistant Fraser fir and hemlocks. The 

approach focuses on the identification, testing, and breeding of resistance or tolerance found within natural 

populations. In addition, development of interspecific hybrids is underway, incorporating both traditional 

breeding techniques and somatic embryogenesis.

 1 North Carolina State University, Alliance for Saving Threatened Forests, 239 Test Farm Rd., Waynesville, NC 28786. 2 North Carolina State University, Raleigh, NC.

Corresponding author: threatenedforests@ncsu.edu.
GENERAL TECHNICAL REPORT PSW-GTR-240

348

Nine Year Survival of 16 Phytophthora lateralis

Resistant and Susceptible Port-Orford-Cedar 

Families in a Southern Oregon Field Trial

Richard A. Sniezko,

1 Jim Hamlin,2 Everett M. Hansen,3 and Sunny Lucas1

Abstract

Port-Orford-cedar (Chamaecyparis lawsoniana) has suffered high mortality from the pathogen Phytophthora 

lateralis in portions of its natural range in southwest Oregon and northwest California, as well as in horticultural 

plantings in North America, and more recently in Europe. A program to develop genetically resistant 

populations of Port-Orford-cedar is underway. This operational program began in 1996 and utilizes artificial 

inoculation of the roots of young seedlings (or rooted cuttings) to rate parent trees for resistance. A key step to 

any such program is to establish field trials: to validate the results of artificial inoculation trials; to examine 

resistance at a range of sites and environments conducive to the pathogen; and to monitor the durability of 

resistance. The field trial (‘Foggy Eden’ site) examined here consists of 16 Port-Orford-cedar families 

established in southern Oregon in 2002 at a site on the Rogue River-Siskiyou National Forest. The Foggy Eden 

(FE-02) site was chosen because of the notable recent heavy P. lateralis related mortality of the natural PortOrford-cedar trees in the immediate vicinity. Sixteen families had been selected to cover a range from highly 

susceptible (100 percent mortality) to highly resistant (0 percent mortality) in a previous short-term greenhouse 

artificial inoculation (‘root dip,’ RD) trial. Some of the families were also included in two short-term raised bed 

(RB) trials, where the raised bed had been previously infested with P. lateralis. By summer 2010, overall 

mortality at FE was 42.7 percent (263 of 616 trees). Most of the mortality occurred by 2006 (244 trees). Family 

variation in survival ranges from 20.8 to 93.8 percent. The top three families for survival involve a common 

parent and all have less than 11 percent mortality. The comparison of the results from FE-02 with the short-term 

‘root dip’ and raised bed tests showed a strong relationship between these trials. The results are encouraging at 

this point, but the trial needs to be followed longer, and the results from the additional trials in other 

environments conducive to P. lateralis need to be examined to determine the efficacy of resistance in different 

environments. Resistant seed from wind-pollinated orchards is now available to aid restoration and 

reforestation.

Key words: Port-Orford-cedar, Chamaecyparis lawsoniana, Phytophthora lateralis, genetic 

resistance, field trials

Introduction

Port-Orford-cedar (POC) (Chamaecyparis lawsoniana (A. Murr.) Parl.), a long-lived conifer, has 

experienced high mortality from the non-native root pathogen Phytophthora lateralis (Tucker and 

Milbrath) in portions of its natural range in southwest Oregon and northwest California (USDA FS

and USDI BLM 2004), as well as in horticultural plantings in North America and Europe. The use of 

growing stock with genetic resistance is key to restoration of POC in these areas as well as in general 

reforestation and horticultural plantings where P. lateralis may be problematic. An applied program 

to develop genetically resistant populations of POC is underway (Sniezko 2006, Sniezko et al. 2009;

Sniezko et al., Operational program to develop Phytophthora lateralis resistant populations of PortOrford-cedar (Chamaecyparis lawsoniana), these proceedings). This resistance program utilizes 

several methods of short-term testing to rapidly evaluate hundreds of field selections for resistance.

However, there are few reports detailing differential survival of resistant and susceptible POC 

 1 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon, USA.

2 USDA Forest Service, Umpqua National Forest, Roseburg, Oregon, USA. 3 Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, USA.

Corresponding author: rsniezko@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

349

families in field tests or the correspondence between genetic resistance in short-term seedling tests 

and field tests (Sniezko et al. 2006).

In this paper, we examine field survival 9 years after the planting of 16 POC families at a site 

where P. lateralis mortality of the existing POC in the forest stand had been moderately high. We 

also examine the relationship between field mortality in this trial with seedling mortality in four shortterm greenhouse and raised bed trials.

Materials and Methods

Seedlings for all trials were grown for 1 year in 164 ml tubes using a soilless nursery medium 

(mixture of peat, vermiculite, composted fir bark, perlite, and pumice) in a greenhouse at Dorena 

Genetic Resource Center (DGRC) in Cottage Grove, Oregon. The ‘Foggy Eden’ field site (42°

48.195’ N, 123° 53.340’ W, 731 m elevation; fig. 1) on the Powers Ranger District of the Rogue 

River-Siskiyou National Forest was planted at 1.83 m x 1.83 m spacing on April 5th and 6th 2002 

using 616 1-year- old seedlings from 16 families (representing 20 parents). Seedlings were planted in 

family row plots with 4 to 12 trees per family in each of the four blocks of the randomized complete 

block design. The parent trees were forest selections from within varying parts of the natural range in 

Oregon and California. The nine open-pollinated (wind-pollinated seed lots from a clone bank at 

DGRC) and six full-sib families had been chosen to represent a wide range of resistance (0 to 100 

percent survival) based on a previous short-term (7 month) seedling test (RD-00, see below). The 

Foggy Eden site (FE-02) was chosen due to the presence of recent mortality from P. lateralis of 

approximately 75 percent of the POC in the overstory and surrounding area. For each of the seven 

assessments since the 2002 planting, seedlings were rated as alive, ‘fading,’ or dead. We report on the 

cumulative mortality of POC through the July 2010 assessment. 

Figure 1—Port-Orford-cedar Phytophthora lateralis resistance trial planted in 2002 in understory of 

forest canopy at Foggy Eden (FE-02) site on Siskiyou National Forest in Oregon. (Photo credit: Eric 

Martz)

Subsets of the 16 families planted in FE-02 were also tested as part of four short-term (7-month to 

2-year duration) greenhouse or raised bed trials (table 1) at Oregon State University in 2000 and 
GENERAL TECHNICAL REPORT PSW-GTR-240

350

2002. The two greenhouse trials (RD-00, RD-02) utilized the ‘root dip’ technique where roots that 

had emerged from the bottom of the 164 ml tubes were clipped and the bottom 1 cm of the upright 

tubes were immersed in a zoospore suspension for 48 hours. The suspension was a mix of two 

standard P. lateralis isolates, ‘366’ and ‘368.’ The seedlings remained in the greenhouse for 

monitoring and assessments over the subsequent 5 to 7 months (table 1; fig. 2). In the raised bed trials 

(RB-00, RB-02), seedlings were planted in a raised bed (fig. 3) in which the soil had been previously 

infested with the same two isolates of P. lateralis. Mortality in all four seedling trials was recorded 

over the ensuing 5 to 18 months as seedling foliage began to wilt or turn chlorotic (table 1). At each 

assessment, fading seedlings were scraped at the root collar to check for the presence of the 

characteristic cinnamon-colored stain (fig. 3). Phytophthora lateralis was also confirmed on a small 

subset of seedlings using a combination of culture and PCR techniques. Selective culture and PCR 

methods were adapted from Winton and Hansen (2001). Further details are discussed elsewhere (Oh 

et al. 2006). Further details on methods for root dip and raised bed testing are reported by Hansen et 

al., Methods for screening Port-Orford-cedar for resistance to Phytophthora lateralis, these 

proceedings.

Figure 2—Port-Orford-cedar seedlings in greenhouse following ‘root dip’ inoculation with 

Phytophthora lateralis. (Photo credit: Richard Sniezko)

Figure 3—Raised bed trial of Port-Orford-cedar for Phytophthora lateralis resistance and seedling 

showing brown lesion from P. lateralis at root collar. (Photo credit: Richard Sniezko)

The design for all trials was a randomized complete block, with the number of seedlings per 

family in each block varying by trial (table 1). The 2000 trials (RD-00 and RB-02) had relatively few 

seedlings per family compared to the other trials (table 1). Survival in all trials was analyzed with 

blocks considered random and families as fixed effects using SAS (SAS Institute Inc., V9.2, 2008). 
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351

Survival at the termination of the four short-term trials (RD-00, RD-02, RB-00, RB-02) and field 

survival through summer 2010 for FE-02 (9 years after planting) were used for all analyses. Family 

differences in survival were analyzed using family-plot means in Proc Glimmix (binomial 

distribution); plot means were adjusted by 0.01 for those values of 1.0 (adjusted to 0.99) and 0.0 

(adjusted to 0.01) in order to avoid the non-convergence issue in Proc Glimmix when numerous 

values of 1 or 0 are encountered in the dataset. Proc Corr (product moment correlation) was used to 

examine family mean correlations among the respective experiments, and Proc Reg (regression) was 

used to plot the family mean survival among experiments. All statistical analyses were conducted 

using SAS (SAS Institute Inc., V9.2, 2008).

Table 1—Background information on five Phytophthora lateralis resistance trials: field trial 

(FE-02), raised bed trials (RB-00, RB-02), and root dip trials (RD-00, RD-02)

Trial No. 

Families 

(F)

No. 

Blocks (B)

No. 

Seedlings/F/B

First 

Assessment

Final 

Assessment

No. of 

Assessments

FE-02 16 4 4 to 12 2002 2010 7

RD-00 15 6 1 to 6 04/2000 01/2001 22

RD-02 9 4 6 to 12 04/2002 09/2002 5

RB-00 15 6 1 to 4 04/2000 01/2001 20

RB-02 9 4 6 to 12 05/2002 11/2003 8

Results

Overall survival of POC was 57.3 percent through summer 2010 at FE-02. Most of the mortality 

occurred within 4 years of planting (fig. 4). Visual symptoms (cinnamon-colored staining of the 

phloem tissue at the base of the tree after scraping away the bark) or DNA testing using PCR 

indicated that P. lateralis was the probable cause of mortality, but because the field site was only 

assessed periodically, definitive confirmation of cause of death for some seedlings was not possible.

A few seedlings in the 2003 examination showed the lesion only at ground level (not downward into 

the roots) indicating that there may have been overland water flow infecting the seedlings right at the 

soil line, and not through the root tips, thus, potentially bypassing some resistances.

Significant differences (p<.05) among families were found at FE-02 with survival varying from 

20.8 to 93.8 percent (fig. 4; table 2). The three families involving parent 117490 and the two families 

involving CF1 (both resistant parent controls) had high survival at FE-02 (table 2). Significant 

differences were also found among families in all of the raised bed and root dip trials (RB-00, RB-02, 

RD-00, RD-02). There was a moderate to high correlation between trials for the five tests, with only 

the correlations of RB-00 with RB-02 and RD-02 being non-significant (table 3; fig. 5). Families 

involving parents 117490, CF1, 117486, 510015, and 510005 generally ranked high for survival in 

the trials, although a few of these families showed higher mortality in the raised bed trials (table 2). In 

contrast, families involving relatively susceptible female parents 118463, D-70013, D-70103, and D70119 have low survival across all tests. The remaining six families generally showed intermediate 

survival across the tests (table 2).
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Figure 4—Survival from 2002 to 2010 for 16 Port-Orford-cedar families in Foggy Eden 2002 (FE-02) 

Phytophthora lateralis resistance planting.

Figure 5—Regression plot between family mean survival in Foggy Eden field test (FE2002) and 

survival in 2002 root dip test (RD2002) for Phytophthora lateralis resistance (using eight common 

families).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

353

Discussion

After 9 years in the field, we found very large differences in survival among the 16 families in the 

FE-02 trial. Visual examination or PCR testing confirmed the likelihood of P. lateralis as the 

causative agent of mortality and no other apparent cause of mortality was noted at this site. Little 

mortality has occurred in the last 5 years, suggesting that the genetic resistance present may be 

Table 2—Mean survival proportions (and number of seedlings tested) by family for five trials: 

field test (FE-02), two raised bed tests (RB-00, RB-02), and two root dip tests (RD-00, RD-02)

Test

Family FE-02 RB-00 RB-02 RD-00 RD-02

117490 x CF1 0.94 (16) nt 0.77 (47) nt 1.00 (48)

117490 x w 0.94 (48) 0.33 (6) 0.50 (48) 1.00 (6) 0.92 (24)

117490 x 117503 0.90 (48) 0.72 (18) nt 1.00 (18) nt

CF1 x CF2 0.83 (24) 0.67 (12) nt 0.83 (12) nt

117486 x 117851 0.79 (24) 0.92 (12) nt 0.92 (12) nt

510005 x w 0.77 (48) 0.67 (12) nt 0.75 (12) nt

510015 x w 0.67 (36) 0.29 (24) 0.21 (18) 0.58 (12) 0.75 (48)

117341 x w 0.67 (48) 0.44 (18) 0.33 (48) 0.50 (18) 0.75 (48)

117344 x w 0.54 (24) 0.25 (12) 0.23 (48) 0.50 (12) 0.25 (24)

510041 x w 0.50 (48) 0.33 (6) 0.17 (48) 0.60 (6) 0.38 (48)

70024 x 70028 0.49 (49) 0.33 (6) 0 (48) 0 (6) 0.33 (48)

117852 x 510049 0.42 (24) 0.39 (22) nt 0.58 (24) nt

118463 x w 0.35 (48) 0 (12) 0.02 (48) 0 (12) 0.17 (48)

70013 x 117343 0.28 (36) 0.12 (24) nt 0.08(24) nt

70103 x w 0.21 (47) 0 (12) 0.04 (24) 0 (12) 0.21 (24)

70119 x w 0.21 (48) 0 (12) nt 0 (12) nt

Note: nt = not tested; w= wind-pollinated.

Table 3—Family mean correlations (number of families; significance value) for survival 

between five tests for Phytophthora lateralis resistance: Foggy Eden field trial (FE-02), two

raised bed trials (RB-00, RB-02), and two root dip trials (RD-00, RD-02)

RB-00 RB-02 RD-00 RD-02

FE-02 0.81 (15; <0.001 ) 0.89 (9; 0.001) 0.91 (15; 0.001) 0.93 (9; <0.001)

RB-00 0.60 (8; 0.118) 0.82 (15; <0.001) 0.68 (8; 0.066)

RB-02 0.93 (8; <0.001) 0.87 (9; 0.002)

RD-00 0.79 (8; 0.019)

durable or that the environmental conditions present now are less conducive to further infection and 

impact by P. lateralis. None of the families, including those with 100 percent mortality in some of the 

earlier seedling screening trials, have reached 100 percent mortality at FE-02 suggesting that microsite or other factors may at least temporarily reduce the impacts by P. lateralis in this area. This site 

will continue to be monitored for changes in status of the remaining trees, and especially the more 

resistant families.

In many resistance programs, short-term testing of seedlings is a common procedure for evaluating 

resistance of parent trees or their progenies. However, verification of seedling test results in field 

trials is essential to confirm the relationship with seedling tests and to examine durability of 

resistance. In this series of tests examining genetic variation in P. lateralis resistance, there is 

generally good correspondence between survivals in the four short-term tests with that in the FE-02 

field trial through the 2010 assessment. Similar results have also been found in early assessments 

from several other field trials and these seedling trials (Sniezko et al. 2006). The somewhat higher 

than expected mortality of a few resistant families in the two raised bed trials warrants further 

examination. These families were selected from among the first tests of P. lateralis resistance, and 
GENERAL TECHNICAL REPORT PSW-GTR-240

354

more recent trials will be examined to further evaluate resistance in the different trial types. Future 

updates from FE-02 and the earlier reported trials (Sniezko et al. 2006) will also be of interest.

The seedlings planted at FE-02 are in the understory of an existing forest stand and are growing 

slowly. Although this may be somewhat typical for some stands with POC, and the survival results 

are very encouraging, data from additional field sites are needed to more fully examine the 

effectiveness of P. lateralis resistance under a wider range of growing conditions. Several other field 

trials were planted in Oregon in 2002 using a subset of these 16 families, and they show little or no 

mortality from P. lateralis at this point (Harrington et al. 2012), indicating that many sites may be 

free of P. lateralis, at least initially. Classification of sites for P. lateralis hazard (low, moderate, 

high, and extremely high) would be helpful in establishing any further locations or interpreting results 

from current field trials as well as in making recommendations to land managers on the use of 

resistant POC seed. Additional field trials have been established in areas where P. lateralis is known 

to be active, and results from these trials should greatly expand the scope of reference regarding 

effectiveness of P. lateralis resistance.

Resistant POC seed is now available from seed orchards for several breeding zones and is in use 

by several organizations. Resistant seed lots are generally bulk collections and would be expected to 

give some intermediate level of field survival in the presence of P. lateralis as opposed to the high 

survival shown by the top families in the FE-02 trial. Further testing of these bulk open-pollinated 

seed orchard lots at multiple sites is needed to define the level of survival to be expected for general 

restoration and reforestation. Breeding work in POC has begun and further increases in resistance are 

expected in future seed orchard populations.

A genetic resistance program offers a natural method of coping with major pathogen and insect 

problems in forest trees. The POC program is one of the few resistance programs for non-native 

pathogens or insects in forest trees to reach the level of producing seed for operational use (FAO 

2008, Sniezko 2006). However, as discussed at this conference, there is more need and interest than 

ever in using this approach. Existing programs such as the one for POC can provide insights to help 

increase the efficiency of efforts in other species. Regional centers of expertise and facilities can also 

facilitate efficient development of resistance in multiple species. Sustained support is essential to help 

ensure success in resistance programs.

Acknowledgments

Eric Martz, Holly Looney, Leslie Elliott, Scott Kolpak, Don Goheen, and other USDA Forest Service personnel 

for help in planning, planting, or assessments of FE-02; OSU personnel for inoculation and assessments of the 

short-term tests; and Douglas Savin for assistance with generating graphs. Funding from the USDA Forest 

Service and USDI Bureau of Land Management and the help of Dorena Genetic Resource Center personnel are 

gratefully acknowledged. We thank Dave Shaw, Phil Cannon, Tom Blush, and Arnaldo Ferreira for their 

reviews of an earlier version of this paper.

Literature Cited

FAO. 2008. Selection and breeding for insect and disease resistance. http://www.fao.org/forestry/26445. (30 

December 2011).

Harrington, C.A.; Sniezko, R.; Gould, P. 2012. Growth and survival of Port-Orford-cedar families on three 

sites on the south Oregon coast. Western Journal of Applied Forestry, 27:156-158.

Oh, E.; Hansen, E.M.; Sniezko, R.A. 2006. Port-Orford-cedar resistant to Phytophthora lateralis. Forest 

Pathology. 36: 385–394.

Sniezko, R.A. 2006. Resistance breeding against nonnative pathogens in forest trees — current successes in 

North America. Canadian Journal of Plant Pathology. 28: S270–S279.

Sniezko, R.A.; Elliott, L.E.; Hansen, E.M.; Betlejewski, F.; Goheen, D.J.; Casavan, K.; Kolpak, S.E.; 

Frank, C.L.; Angwin, P.A.; Hamlin, J. 2009. Operational program to develop resistance to P. lateralis in 
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Port-Orford-cedar (Chamaecyparis lawsoniana): the first ten years. In: Goheen, E.M.; Frankel, S.J., tech. 

coords. Phytophthoras in forests and natural ecosystems: Proceedings of the fourth meeting of the 

International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09. Gen. Tech. Rep. 

PSW-G–TR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station: 158–160.

Sniezko, R.A.; Kolpak, S.E.; Hansen, E.M.; Goheen, D.J.; Elliott, L.J.; Angwin, P.A. 2006. Field survival 

of Phytophthora lateralis resistant and susceptible Port-Orford-cedar families. In: Brasier, C.; Jung, T.; 

Osswald, W., eds. Progress in research on Phytophthora diseases of forest trees: Proceedings of the third 

international IUFRO working party S07.02.09. Farnham, UK: Forest Research: 104–108.

U.S. Department of Agriculture, Forest Service; U.S. Department of Interior, Bureau of Land 

Management [USDA FS and USDI BLM]. 2004. Management of Port-Orford-cedar in southwest Oregon: 

final supplemental environmental impact statement. BLM/OR/WA/PL-04/005-1792, Portland, OR, USA.

http://www.fs.usda.gov/detail/rogue-siskiyou/landmanagement/resourcemanagement/?cid=stelprdb5316256.

(31 December 2011).

Winton, L.M.; Hansen, E.M. 2001. Molecular diagnosis of Phytophthora lateralis in trees, water, and foliage 

baits using multiplex polymerase chain reaction. Forest Pathology. 31: 275–283.
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White Pine Blister Rust Resistance of 12 Western 

White Pine Families at Three Field Sites in the 

Pacific Northwest

Richard A. Sniezko,

1 Robert Danchok,1 Jim Hamlin,2 Angelia Kegley,1 Sally 

Long,

1 and James Mayo1

Abstract

Western white pine (Pinus monticola Douglas ex D. Don) is highly susceptible to the non-native, invasive 

pathogen Cronartium ribicola, the causative agent of white pine blister rust. The susceptibility of western white 

pine to blister rust has limited its use in restoration and reforestation throughout much of western North 

America. Fortunately, some genetic resistance to blister rust exists in western white pine, and several 

operational programs to select for resistance and develop orchards to produce resistant seed exist. These 

programs, such as that of the U.S. Department of Agriculture, Forest Service (USDA FS) Pacific Northwest 

Region, utilize short-term artificial inoculation trials of seedlings to evaluate resistance. However, until 

recently, there have been very few multi-site field trials to examine resistance closely. In this paper, we report 

on 12 western white pine families common to three field trials in western Oregon and northern California. The 

sites have been assessed for white pine blister rust infection and mortality at multiple ages since they were 

planted from 1996 to 2003. The 12 families had been selected based on results in previous artificial inoculation 

trials of seedlings of more than 4,000 parent trees at Dorena Genetic Resource Center (DGRC). Most of the 

families were from wind-pollinated seedlots from forest stands and represent some of the top ranked families 

from a first round of selection. Timing and frequency of field trial assessments varied to coincide with the 

appearance and development of rust infection at the three sites. By 2010, all three sites had moderate to high 

levels of blister rust infection, and moderate levels of mortality were present at some sites. The susceptible 

control family had 92 to 100 percent of trees with stem symptoms at the three sites, while only 29 to 43 percent 

of the trees in the top resistant family showed stem symptoms in those trials. The high level of stem symptoms

in the two families with the Cr2 gene, which conditions a hypersensitive-like reaction (HR) in the needles,

suggests that it is likely that a virulent vcr2 strain of rust is present at all three sites. A wide range in family 

variation is present at all three sites for the proportion of trees with stem symptoms. Mortality lags behind stem 

infection, and more time is needed to assess the full impact of current stem infections. Infected trees in families 

with putative partial resistance traits such as bark reaction noted in previous screening trials will continue to be 

followed to examine their subsequent survival under field conditions. In general, moderate to high correlations 

were found between current levels of resistance at the three sites and seedling screening results from DGRC, 

providing some of the first field validation of results of short-term testing. These current results and future 

assessments will provide valuable information on the level of rust resistance and survival that land managers 

can expect over a range of sites. Long-term monitoring of these sites will be essential to evaluate durability of 

the various resistance types. Ongoing breeding efforts are aimed at increasing the levels of resistance to provide 

greater opportunities for successful reforestation and restoration use.

Key words: Pinus monticola, Cronartium ribicola, rust resistance, field trials, operational program

Introduction 

Western white pine, Pinus monticola Dougl. ex D. Don, a wide ranging forest tree species in western 

North America, is highly susceptible to the non-native, invasive pathogen Cronartium ribicola, the 

fungus causing white pine blister rust (Geils et al. 2010). Fortunately, some natural genetic resistance 

 1 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, Oregon, USA. 2 USDA Forest Service, Umpqua National Forest, Roseburg, Oregon, USA.

Corresponding author: rsniezko@fs.fed.us.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

357

exists in white pine populations (Bingham 1983, Kegley and Sniezko 2004, King et al. 2010, Kinloch 

et al. 1999, Sniezko 2006). The development of populations of white pine with genetic resistance to 

white pine blister rust offers an invaluable management tool to restore this species and its utilization 

in managed forests. In the Pacific Northwest (Oregon and Washington), the U.S. Department of 

Agriculture, Forest Service (USDA FS) has a regional program at Dorena Genetic Resource Center 

(DGRC) in Oregon to develop genetically resistant populations of western white pine to blister rust. 

Resistance is identified through artificial inoculation of 2-year-old seedlings and subsequent 

assessments, using seed collected from parent trees selected in the forest or from control crosses 

among selected parents. Thousands of parent trees have been evaluated since the 1960s and several 

types of complete and partial resistance have been documented in these seedling tests (Kegley and 

Sniezko 2004; Kinloch et al. 1999, 2003; Sniezko and Kegley 2003a, 2003b; Sniezko 2006).

Western white pine is a long-lived conifer, and in many environments it will be exposed to C. 

ribicola for decades or even hundreds of years. Field testing is essential to validate the results of 

short-term rust resistance screening by artificial inoculation on very young seedlings and to monitor

the durability of blister rust resistance. There are few summaries of field trials that examine white 

pine blister rust resistance in individual families of western white pine in the Pacific Northwest, and 

generally these reports encompass only one field site or have a more limited range of resistance types 

available (Hunt 2002, Kinloch et al. 2008, King et al. 2010, Kolpak et al. 2008; Sniezko et al. 2000, 

2004a, 2004b). Until relatively recently, there were few multi-site field trials that closely monitored 

the field performance of families with the different types of putative resistance observed in artificial 

inoculation trials of seedlings. However, a series of field trials have been established in the Pacific 

Northwest since 1996 to examine the effectiveness and durability of resistance under field conditions. 

In this paper, we examine the incidence of blister rust stem symptoms and mortality in a common set 

of 12 western white pine families established in three field trials between 1996 and 2003 in western 

Oregon and northern California. These trials include families from diverse geographic and resistance 

backgrounds. We also examine the correlation of field performance to seedling test results following 

artificial inoculation at DGRC.

Materials and Methods

Seed from western white pine parents with varying types and levels of resistance observed in previous 

seedling artificial inoculation trials at DGRC were used to establish a series of field trials since 1996. 

We examine the subset of 12 families that are common to three trials in northern California and 

western Oregon. The seedlots include 11 half-sib families from wind pollination of parent trees in 

natural stands from a wide range of geographic areas in Oregon and Washington (10 families), or in 

an orchard at DGRC (one family); the remaining family is a full-sib from a control cross between two 

parent trees in an orchard at DGRC (table 1). The families were chosen to represent varying levels 

and types of complete and partial resistance identified in seedling screening, and include a susceptible 

control (family 4) that showed little or no resistance in previous testing at DGRC. In addition, two of 

the 12 families (families 11 and 12) have R gene resistance conveyed from the Cr2 gene, which 

imparts complete resistance (from a hypersensitive-type reaction in the needles, HR) in the absence of 

a strain of rust with corresponding virulence gene, vcr2 (Kinloch et al. 1999, 2003, 2004). The 

inheritance of HR resistance is due to a single dominant gene and, based on results from a previous 

artificial inoculation trial at DGRC, these families would be expected to have 75 percent or more of 

their progeny canker free in the absence of a virulent strain of rust (Sniezko et al. 2004b). The 

remaining nine families had previously displayed varying types and levels of partial resistance in 

seedling tests at DGRC (see Kegley and Sniezko 2004, Sniezko et al. 2004a; Sniezko and Kegley 

2003a, 2003b). Seedlings with partial resistance often develop stem symptoms, but not to the degree 

of the most susceptible seedlings. In the seedling trials at DGRC most wind-pollinated families with 

partial resistance show high levels of stem symptoms (70 to 100 percent of seedlings), but in contrast 

to the high-susceptible families, partial-resistance families often have fewer stem symptoms per 
GENERAL TECHNICAL REPORT PSW-GTR-240

358

seedling, more of the stem symptoms appear at a later assessment (latent), more stem symptoms 

manifest as bark reactions (complete or partial bark reactions) rather than normal cankers, and there is 

longer survival of seedlings with stem symptoms. More information is needed on the underlying basis

of partial resistance(s) and its inheritance. Family 6 displays many of the partial resistant traits, but in 

the seedling testing it also has a relatively high level of seedlings with no stem symptoms (Sniezko et 

al. 2004a; Sniezko and Kegley 2003a, 2003b).

Table 1—Family identities, origin, and resistance type for 12 western white pine families: 

11 wind-pollinated and one control crossed 

Family code Family Parent origin Resistancea

1 11053-552 x w OR pr

2 03023-509 x w WA pr

3 03024-510 x w WA pr

4 03024-532 x w WA susceptible control

5 03024-793 x w WA pr

6 05081-003 x w WA high canker free, non-HR

7 06023-521 x w OR pr

8 18034-140 x w OR pr

9 18035-150 x w OR pr

10 21105-052 x w WA high bark reaction

11 (15045-816 x 15045-841) x wso OR HR 

12 15045-823 x 15045-840 OR HR 

w= wind pollinated (in forest stand); wso =wind pollinated in orchard; OR=Oregon, WA=Washington; HR= hypersensitivelike reaction in needles; pr= low levels of partial resistance (Sniezko et al. 2004a). a Based on previous screening trials at Dorena Genetic Resource Center.

Periodic visits to each field site were made, but formal assessments began only after blister rust 

infection was noted in each trial (figs. 1 and 2). Trees were assessed in multiple years for growth as 

well as number of stem symptoms (on main bole or branches), types of stem symptoms (incipient 

infections that appear as small orange circles at the base of some infected needles, normal cankers, 

bark reactions, partial bark reactions), severity of infection, and mortality. For this paper, we report 

on family variation, 8 to 11 years after planting (depending on site), only on the proportion of trees 

(1) with stem symptoms from blister rust and (2) surviving. Analyses of the other traits such as 

number of stem symptoms are pending. We report on cumulative proportion of stem symptoms, since 

some stem symptoms present in 1 year may not be seen in some future years (e.g., the tree may 

outgrow small bark reactions or branches with stem symptoms may self prune).

Happy Camp (HC)

The site is located on the Happy Camp Ranger District on the Klamath National Forest in northern 

California (N41° 53' 08.51’; W123° 24 09.82’, 777 m elevation). This location is the main field test 

site for sugar pine (Pinus lambertiana Dougl.) for the USDA FS, Pacific Southwest Region’s blister 

rust resistance program, but western white pine has also previously been tested on the site (Kinloch et 

al. 2008). Seedlings from the spring 1995 sowing of 12 sugar pine and 13 western white pine families 

were planted in April 1996 in a randomized complete block (RCB) design with 12 blocks and up to 

four trees per family per block in row plots. Trees were planted at 0.60 x 0.75 m spacing. Ribes 

sanguineum, an alternate host to C. ribicola, was inter-planted among the pines to augment existing 

sources of rust spores and to help ensure uniform exposure to rust infection. Assessments of blister 

rust stem symptoms, mortality, and growth were done annually from 1999 to 2003, and also in 2005 

and 2007. Additional background and earlier results are presented elsewhere (Sniezko et al. 2000, 

2004a).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

359

Kerbluey (K)

The site is located on the Cottage Grove Ranger District (CGRD), Umpqua National Forest, in 

southern Oregon (N 43°39.765’, W 122° 39.711’, 974 m elevation). Western white pine occurs 

naturally as well as in reforestation areas. Ribes species are present in the area, and white pine blister 

rust had previously been documented in the area. The parents for the two families with HR resistance 

were selected during the 1950s on CGRD. Seedlings sown in 1995 for the same 25 families used for 

HC were planted in June 1997 (blocks 1 to 4) and October 1997 (blocks 5 to 8) in a RCB design; a 

few trees that died in blocks 1 to 4 were replaced in October 1997. In most cases, there were four 

trees in each family row plot per block (varied from two to five). Eleven families are included in all 

eight blocks, and one of the 12 families is included only in four blocks. Trees are planted at 3 x 3 m

spacing. Assessments of blister rust infection and mortality were done in 1999, 2001 to 2005, and 

2008.

Figure 1—Western white pine trial at Coast Plum site in 2010/2011: (a) overview, (b) incipient stem 

symptom with the ‘point-of-entry’ needle still alive in the center, (c) expanding canker, (d) older canker 

with aecia and girdling stem, (e) partial bark reaction, (f) partial bark reaction. (Photo credits: Richard 

Sniezko (a to e), Robert Danchok (f))

Coast Plum (CP)

The site is located on Plum Creek Corporation land in the Coast Range of Oregon (N 44° 49.966’, W 

123° 44.790’, 533 m elevation). One-year-old seedlings of 28 western white pine families, sown in 

early March 2002, grown in containers at Plum Creek Corporation’s nursery in Cottage Grove, 

Oregon were planted in early February 2003 in a RCB design with nine blocks. Nine of the 12 

(a) (b)

(d) (e) (f)

(c)
GENERAL TECHNICAL REPORT PSW-GTR-240

360

families are included in all nine blocks, one family is included in seven blocks, and two families are 

included in six blocks. Seedlings were planted in family row plots with generally five trees per family 

plot (varied from three to six trees per family row plot). Trees were planted at 1.8 x 3 m spacing. 

Blister rust incidence was very low until the first assessment in 2010 (figs. 1 and 2); the trial was also 

assessed in 2011. There is little or no other known western white pine in the general vicinity, but 

abundant R. sanguineum, an alternate host to C. ribicola, was noted in the surrounding areas.

DGRC Seedling Artificial Inoculation Trial (DGRC1995)

All 12 families had previously been evaluated for rust screening in various seedling trials at DGRC, 

and 10 of the families are included in a common test sown in 1995 (inoculated in 1996). Two-yearold seedlings are used in the artificial inoculation trials, and these seedlings often have primary 

needles as well as secondary needles. Kegley and Sniezko (2004) provide detailed information on 

seedling screening trials at DGRC. We use the results from the 1995 test to examine the relationship 

between short-term seedling screening and field results at the three sites (Sniezko et al. 2004a). The 

two families with HR resistance are not included in the 1995 seedling test. The three variables used in 

this paper from the screening trial results include needle lesion mean (NLC), proportion of trees with 

stem symptoms (SSp), and proportion of trees dead from rust (RMORT) 5 years after inoculation.

NLC is based upon number of needle lesions or “spots” on all secondary needles, and ranges from 0 

to 4 for individual seedlings. Generally, the scale is setup, after examining a subset of seedlings, to 

have approximately 25 percent of the seedlings in each needle lesion class; seedlings with no needle 

lesions are usually rare and are assigned to class 0. The NLC and a preliminary estimate of SSp were 

used in an earlier summary of early mortality at the Happy Camp site (Sniezko et al. 2004a). 

Statistical analysis procedures were conducted using SAS (SAS Institute Inc., V 9.2, 2008) where 

the trials were analyzed as a RCB. Family differences in survival (and proportion of trees with stem 

symptoms) were analyzed using family-plot means in Proc Glimmix (binomial distribution); plot 

means were adjusted by 0.01 for those values of 1.0 (adjusted to 0.99) and 0.0 (adjusted to 0.01) in 

order to avoid the non-convergence issue in Proc Glimmix when numerous values of 1 or 0 are 

encountered in the dataset. Site, family, and site x family interaction were considered as fixed effects 

in the model. Proc Corr (product moment correlation) was used to examine family mean correlations 

among the respective field trials as well as between these three field trials and the 1995 seedling 

screening trial at DGRC.

Figure 2—Time trend for proportion of white pines with (a) stem symptoms and (b) surviving, at three 

sites: Happy Camp (planted 1996), Kerbluey (planted 1997), and Coast Plum (planted 2003). 
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

361

Results

A moderate (0.60 at HC) to high proportion (0.78 at CP and 0.84 at K) of trees displayed stem 

symptoms (SSp) by the latest assessment (fig. 2; table 2) at the three sites. The temporal dynamics of 

blister rust infection varied among the three sites. For the 1996 HC trial, over 35 percent of trees had 

stem symptoms by 1999, but there has been only a gradual increase in infection through 2007; for the 

1997 K planting, over 35 percent of trees had stem symptoms by 2001 and there has been a continued 

steady increase in infection through the last assessment (2008); for the 2003 CP planting, over 60 

percent of trees had infection by the first assessment in 2010 and a further jump in infection was 

recorded in 2011 (fig. 2). The severity codes associated with the first assessments at each site 

indicated that only 7 (of 311) trees at K, and 14 (of 556) trees at HC had larger and presumably older 

active cankers at their first assessments, while 105 trees (of 465) at CP had relatively large active 

cankers at that site’s first assessment (fig. 1). Most of the symptoms noted at CP in 2010 were on the 

2007 and 2008 portions of the main stem or branches. At their first assessment, 2, 6, and 8 trees were 

dead from blister rust at K, HC, and CP, respectively. The number of stem symptoms per living tree 

varied by site and year from a maximum of 0.60 at HC to 7.18 at K, and 9.53 at CP (table 3).

Table 2—Overall site means (and range in variation among the 12 families) for proportion of 

trees with blister rust stem symptoms and survival at three field sites

Site Stem symptom Survival 

Coast Plum (CP) .78 (.37 to 1.0) .95 (.90 to 1.0)

Kerbluey (K) .84 (.43 to 1.0) .64 (.19 to .94)

Happy Camp (HC) .60 (.29 to. 92) .75 (.43 to 1.0)

Highly significant differences (p < .0001) existed among sites and families in the inter-site 

analysis for SSp. In addition, there was a significant site x family interaction (p = .0001). This 

interaction appears to be influenced by the relatively low proportion of trees in families 11 and 12 

with stem symptoms at HC relative to that at CP and K (fig. 3). When these two families with HR 

resistance are excluded from the inter-site analysis, the site x family interaction is not significant for 

SSp (p = .056). A wide range of family variation in SSp was evident at all three sites (table 2, fig. 3).

Table 3—Number of stem symptoms (SS) per living tree at several assessments for three 

western white pine blister rust resistance trials

Site Year Number of 

living trees

Mean number of 

stem symptoms (SS) 

per living tree

Range in SS 

per tree

Happy Camp 1999 551 0.60 0 to 9

Happy Camp 2003 438 0.15* 0 to 4

Kerbluey 2001 305 1.08 0 to 11

Kerbluey 2005 244 2.39 0 to 20

Kerbluey 2008 192 7.18 0 to 48

Coast Plum 2010 457 4.95 0 to 91

Coast Plum 2011 449 9.53 0 to 80

Happy Camp planted in 1996; Kerbluey in 1997; Coast Plum in 2002.

*Decrease in SS at HC reflects mortality of many of the trees with early stem symptoms.

SSp for the susceptible control (family 4) was very high (0.92 to 1.0) at all sites (fig. 3; table 2). At 

HC, the difference in SSp between the susceptible family and the other 11 families was particularly 

notable (fig. 3). At K, four families showed 100 percent stem symptoms, including the susceptible 

control (family 4), the two families with HR resistance (families 11 and 12), and family 2. Family 2 

was the only one to have 100 percent stem symptoms at CP, which was slightly higher than the 98 

percent for the susceptible control (fig. 3). Family 6 had the lowest proportion of trees with stem 
GENERAL TECHNICAL REPORT PSW-GTR-240

362

symptoms (0.29 to 0.43) at all three sites (fig. 3). At K, the two families (11 and 12) with HR 

resistance showed delayed, but then rapid, steady increase in infection over time (fig. 4).

Despite the moderate to high level of stem infection, the proportion of trees surviving (SURV) 

was moderate (0.64 at K, 0.75 at HC) to high (0.95 at CP) at the three sites (table 2, fig. 3), with 

family survival varying from 0.19 to 1.0 (table 2, fig. 3). Through the latest assessments, mortality 

has just begun at CP, it has nearly plateaued at HC, and it appears to be steadily increasing at K (fig. 

2). There were significant differences among both sites and families (p < .0001), but the site x family 

interaction for SURV was not significant (p=.50). The susceptible control (family 4) has the lowest 

survival at all sites (fig. 3). Family 6 had the highest survival at CP and K, and second highest at 

survival HC (fig. 3). 

Survival of the 12 families was significantly correlated among the three sites (0.64 to 0.81), but for 

SSp, only the family mean correlation between CP and K (0.92) was significant (table 4). SSp at HC 

Figure 3—Family means at last assessment over three sites for 12 families for (a) proportion of trees 

with stem symptoms and (b) proportion of trees surviving. 

Figure 4—Time trend for 12 western white pine families for 1997 Kerbluey planting for proportion of 

trees with stem symptoms from blister rust infection (first assessment in 1999). 

was significantly correlated with survival at all three sites (-0.78 to -0.88); there was a moderate 

family mean correlation of SSp at CP with survival at both K (- 0.59, p = .04) and HC (-0.56, p

= .06); there was a moderate, but non-significant correlation of SSp and survival at K (-0.51, p = .09

(table 3, fig. 4)). At K, the majority of the stem symptoms on families 11 and 12, the two families 

with HR resistance, appear much later than other families, but reached 100 percent by 2008 (fig. 4). 

When families 11 and 12 are excluded (n=10), many of the family mean correlations for SURV and 

SSp were higher and additional ones were significant (14 of the 15 correlations ranged from 0.59 to 

0.92, with only SSp at K with Surv at CP being somewhat lower (-0.40)).
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

363

For the 10 families included in the DGRC artificial inoculation trial, there were significant

moderate- to high-family mean correlations (0.60 to 0.87) of SSp and RMORT at DGRC with both 

SSp and SURV at each of the three field trials (table 5, fig. 5). The correlations of seedling results at 

DGRC (SSp and RMORT) were slightly higher for SSp (0.82 to 0.87) at the field sites than for SURV 

(-0.60 to -0.78) (table 4). The correlations of seedling needle lesion class (NLC_DGRC) with SSp and 

SURV in the field were only low to moderate, and were non-significant (table 5).

Table 4— Family mean correlations (p-values) between three field trials for proportion of trees 

with white pine blister rust stem symptoms (SSp) and survival (Surv)

Surv_CP SSp_K Surv_K SSp_HC Surv_HC

SSp_CP -0.42 (0.178) 0.92 (0.0001) -0.59 (0.044) 0.45 (0.138) -0.56 (0.059)

Surv_CP -0.19 (0.546) 0.64 (0.025) -0.78 (0.0028) 0.76 (0.004)

SSp_K -0.51 (0.088) 0.34 (0.271) -0.31 (0.329)

Surv_K -0.88 (0.0001) 0.81 (0.0015)

SSp_HC -0.85 (0.0005)

Based on 12 families.

HC, K, and CP field trials assessed through 2007, 2008, and 2011, respectively.

Figure 5—Relationship between proportion of seedlings with stem symptoms in artificial inoculation 

trial at DGRC (SY95_SS) and proportion of trees with stem symptoms at Kerbluey field trial (K_Inf) for 

10 western white pine families (excludes two families with HR resistance).

Table 5—Family mean correlations (p-values) between results from the 1995 artificial 

inoculation test of seedlings at DGRC and proportion of trees with white pine blister rust stem 

symptoms (SSp) and survival (Surv) at three field trials

SSp_CP Surv_CP SSp_K Surv_K SSp_HC Surv_HC

NLC_DGRC 0.34 (.338) -0.33 (.346) 0.46 (.185) -0.59 (.074) 0.48 (.157) -0.35 (.318)

SSp_DGRC 0.82 (.004) -0.61 (.059) 0.86 (.001) -0.72 (.019) 0.84 (.002) -0.60 (.067)

MORT_DGRC 0.87 (.001) -0.72 (.018) 0.83 (.003) -0.78 (.008) 0.84 (.002) -0.68 (.029)

10 families included (excludes the two families with HR resistance, refer to text for details).

NLC = needle lesion rating after artificial inoculation (see text for details), MORT=mortality proportion 5 years after 

inoculation.
GENERAL TECHNICAL REPORT PSW-GTR-240

364

Discussion

As evidenced by the high proportion of stem symptoms on the susceptible control (family 4), all three 

sites have had moderate to heavy exposure to blister rust infection. The incidence of rust at the Coast 

Plum 2003 planting was low until the first assessment in 2010. From the early data, CP would have 

appeared to be a low rust hazard, but the abundance of Ribes spp., the alternate host of blister rust,

and favorable climate eventually led to a large jump in stem symptoms by 2010 and again in 2011.

The rust hazard of this site likely increased since 2003 as tree harvest operations in the surrounding 

areas removed the forest canopy, permitting more abundant natural regeneration of Ribes, the 

alternate host. The levels of infection and family variation in proportion of trees with stem symptoms 

in these trials appears to be somewhat comparable to results from other field trials with mostly 

different families (at Happy Camp and in Idaho) discussed by Kinloch et al. (2008), in British 

Columbia (Hunt 2002), and somewhat lower than the 0.93 (six years after planting) for another trial 

on CGRD (Kolpak et al. 2008). It will be important to monitor these sites over time since western 

white pine is a long-lived tree species and exposure to blister rust infection will continue to occur at 

periodic intervals. 

There are large differences in blister rust resistance among the 12 families tested. In the absence of 

the vcr2 strain of rust, families 11 and 12, with HR resistance, were expected to show little or no 

incidence of stem symptoms. The moderate to high incidence of stem symptoms on families 11 and 

12 confirms the presence of the vcr2 strain of rust at all three sites. Several other trials on CGRD also 

show a very high incidence of stem symptoms on families with HR resistance (Kolpak et al. 2008,

Sniezko et al. 2004b). Previous monitoring has indicated that a low to high frequency of this strain of 

rust is present in much of western Oregon, as well as at HC (Kinloch et al. 2004). At two of the three 

sites, K and CP, the proportion of trees with stem symptoms for these two families is now very 

similar to that of the susceptible control. At the third site, HC, the level of stem symptoms in these 

two families has increased since the last report (Sniezko et al. 2004a). At this stage, the survival of the 

two families with HR resistance is relatively high, but this may be at least partially a function of the 

more recent infection at CP and the relatively recent increase in incidence of vcr2 at K. Future 

monitoring of these families at these sites will indicate whether they have any resistance beyond HR. 

The airborne spread of rust spores and the current geographic distribution of vcr2 will limit the utility 

of HR resistance in many areas of western Oregon. At some sites where vcr2 is not yet present, HR 

resistance may offer utility in delaying stem infections until trees are larger which may benefit overall 

survival. Combining HR resistance with partial resistances would offer an attractive option and has 

been implemented in the design of current seed orchards.

The other nine families are thought to have varying levels of partial resistance. Two of these, 

families 6 (05081-003 x w) and 10 (21105-052 x w), have been tested many times and have 

repeatedly shown moderate to high resistance in artificial inoculation trials of seedlings at DGRC

(Kegley and Sniezko 2004; Sniezko and Kegley 2003a, 2003b). In the DGRC trials, these two 

families have been among the most outstanding for partial resistances. These two wind-pollinated 

families are probably in the top 1 percent for partial resistance of the thousands of forest selections 

evaluated in seedling testing. In these three field trials, these two families are also among the top for 

rust resistance. In seedling testing at DGRC, these two families show a higher incidence of seedlings 

free of stem symptoms and seedlings with bark reactions than do many other families (Kegley and 

Sniezko 2004; Sniezko and Kegley 2003a, 2003b). They also show comparable or higher levels of 

resistance in rust screening at DGRC to the full-sib checklots from the Interior West rust resistance 

program based in Idaho (see Table 5 in Kegley and Sniezko 2004) and higher resistance in the field at 

the CP site than the full-sib seedlots from the British Columbia program (Sniezko et al. 2010).

Progeny of these parents have been grafted into orchards, and clone banks, pollinations made with 

them, and other clones will help increase resistance and help us understand more about inheritance of 

some of the mechanisms involved.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

365

The significant correlations among some resistance traits at DGRC and those in the field are

encouraging, and validate short-term testing. Only a low, non-significant correlation of the needle 

spot class trait (at DGRC) with the proportion of trees infected or dead was found in this study; a

somewhat similar result has been found elsewhere (Hunt 2002). However, in another study, needle 

spot class correlated with number of stem symptoms in the field (Kolpak et al. 2008). Data analyses 

for the relationship between needle lesion mean (at DGRC) and number of stem symptoms at these 

three field sites has not yet been done. A significant amount of additional data to examine this 

relationship will also be available in the near future as assessments on many other field trials are 

completed and analyzed.

Continued observations from these field trials will help confirm the type of resistances and their 

effectiveness on larger trees under repeated exposure to C. ribicola infection. Of particular interest 

will be what level of bark reaction in these families is manifested in the field and whether it 

contributes significantly to survival. Some further discussion of possible resistances and their 

inheritance in western white pine is reported for an older trial at HC (Kinloch et al. 2008) and from 

the early results from another trial on CGRD (Kolpak et al. 2008). However, more research is needed 

into the underlying mechanisms and the inheritance of partial resistance. 

Mortality from blister rust usually lags at least several years behind stem infection, so future 

assessments will be needed to provide a more definitive indication of the full impact of blister rust 

infection. Most infections at CP were observed for the first time in 2010 or 2011, 7 and 8 years after 

planting, and it is expected to be several years before these larger, older trees show much mortality. 

Coast Plum showed a relatively large jump in stem symptoms from 2010 to 2011, probably due to 

some latent infections not being visible during the February 2010 assessment on trees with partial 

resistance as well as new infections from what now appears to be a fairly high-hazard site for rust. 

The incidence of stem symptoms and mortality at HC has been relatively static between 2002 and 

2007, and many of the previous infections appear to be inactive. At CP and K, the incidence of stem 

symptoms has been increasing and a commensurate increase in mortality may occur depending on the 

effectiveness of the partial resistances.

Somewhat surprisingly, blister rust mortality is only low to moderate at HC and K. The 

susceptible control shows the highest mortality of the 12 families, but only at one of the three sites is 

its mortality greater than 60 percent. The effectiveness of partial resistance may depend on several 

factors, including the size and age of the seedlings when infection occurs and the rust hazard of the 

site. In the three trials reported here, there is a moderate to high level proportion of trees with stem 

symptoms and a low to moderate number of stem symptoms per tree, but most of the symptoms occur 

several years or more after planting. On an extreme high-hazard site, there may be high incidence of 

infection events and a much higher number of stem infections per tree. Experience with 2-year-old 

seedlings in artificial inoculation trials at DGRC shows much higher mortality in most families, 

usually within 2 to 4 years of appearance of stem symptoms. Small seedlings in the field that are 

repeatedly infected or trees with dozens to hundreds of stem symptoms may not survive. Rating sites 

for rust hazard is still problematic in many cases, and more information is needed to provide land 

managers rust hazard ratings for lands being considered for planting western white pine.

These trials are the first tests in the Pacific Northwest to closely follow field performance of 

specific families at multiple sites. Results from short-term screening using artificial inoculation of 2-

year-old seedlings correlates well with field performance at these three sites. However, the field sites 

need to be monitored over the long term to evaluate the impacts of continued new exposures to 

infection by C. ribicola, as well as the subsequent impact from existing stem symptoms. Additional 

field trials have been established in Oregon and Washington, including some with control crossed 

progenies as well as seedlots from the Interior West and British Columbia blister rust resistance 

programs. This set of trials will provide baseline information on the utility of current levels of blister 

rust resistance under a wide range of site conditions and the adaptability of the species under 

changing climatic conditions. The data from these trials will provide key information to allow land 
GENERAL TECHNICAL REPORT PSW-GTR-240

366

managers to utilize resistant western white pine. Long-term monitoring of these sites will be essential 

to evaluate durability of the various resistance types.

Most of the families represented here are wind-pollinated progenies from original field selections.

The top parents or some of their progeny surviving rust testing at DGRC have been grafted into 

orchards managed by several groups, including the USDA FS; U.S. Department of the Interior, 

Bureau of Land Management; Washington Department of Natural Resources; Quinault Indian Nation;

and British Columbia Ministry of Forests. Many of these orchards for the breeding zones in the 

Pacific Northwest have reached reproductive age and bulked seedlots to use in reforestation and 

restoration have been collected and are in use for some of them. Some of these orchard seedlots are 

now in testing at DGRC and in field trials. The level of resistance expected from the first cycle of 

orchards developed will depend on several factors, but will be less than that of the top families in 

these trials. Control crossing between orchard selections is ongoing. It is anticipated that the breeding 

and subsequent selections from artificial inoculation trials will supply higher levels of resistance from 

advanced-generation orchards. These anticipated levels and combinations of resistances in future 

orchards should provide land managers with higher levels of success in restoration and reforestation 

plantings.

Acknowledgments

The authors thank the USDA Forest Service for funding and personnel for the many phases of this work. We 

thank Jim Smith and Plum Creek Corporation for establishment of Coast Plum site, and Jerry Hill for his work 

on all these trials; and Douglas Savin for providing some of the graphs. We also acknowledge the help of 

Deems Burton and Dean Davis with many aspects of the Happy Camp trial. We thank Holly Kearns, Amy 

Kroll, Michael Murray, and Tom DeSpain for their reviews of an earlier version of this paper.

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adaptability, and pest resistance; IUFRO Working Party 2.02.15. Proceedings RMRS-P-32. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 203–208.

http://www.fs.fed.us/rm/pubs/rmrs_p032/rmrs_p032_203_208.pdf. (15 January 2012).

Sniezko, R.A.; Hill, J.; Danchok, R.; Kegley, A.; Long, S.; Mayo, J.; Smith, J. 2010. White pine blister rust 

resistance in a seven year old field trial of 28 western white pine (Pinus monticola) families in the Coast 

Range of Oregon. [Poster]. In: American phytopathology society meeting; 2010 August, Charlotte, NC.

Sniezko, R.A.; Kegley, A. 2003a. Blister rust resistance experiences in Oregon and Washington: evolving 

perspectives. In: Stone J.; Maffei, H., comps. Proceedings of the 50th Western International Forest Disease 

Work Conference. Bend, OR: U.S. Department of Agriculture, Forest Service, Central Oregon Service 

Center: 111–117. http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5280665.pdf .(15 January 

2012).

Sniezko, R.A.;Kegley, A.J. 2003b. Blister rust resistance of five-needle pines in Oregon and Washington. In: 

Xu, M.; Walla, J.; Zhao, W., eds. Proceedings of the second IUFRO rusts of forest trees working party 

conference. Forest Research. 16 (Suppl.): 101–112.

http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5280720.pdf. (15 January 2012).

Sniezko, R.A.; Kinloch, B.B., Jr.; Bower, A.D.; Danchok, R.S.; Linn, J.M.; Kegley, A.J. 2004b. Field 

resistance to Cronartium ribicola in full-sib families of Pinus monticola in Oregon. In: Sniezko, R.A.; 

Samman, S.; Schlarbaum, S.E.; Kriebel, H.B., eds. Breeding and genetic resources of five-needle pines: 

growth, adaptability, and pest resistance; IUFRO Working Party 2.02.15. Proceedings RMRS-P-32. Fort 

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 243–249.

http://www.fs.fed.us/rm/pubs/rmrs_p032/rmrs_p032_243_249.pdf. (15 January 2012).
GENERAL TECHNICAL REPORT PSW-GTR-240

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Are Needle Reactions in Resistance to 

Cronartium ribicola a Hypersensitivity Response?

Katarina Sweeney,

1 Jeffrey Stone,1 Kathy Cook,1 Richard A. Sniezko,2 

Angelia Kegley,

2 and Anna W. Schoettle3

White pine blister rust (WPBR) is caused by the fungal pathogen Cronartium ribicola. The pathogen 

is native to Eurasia and was introduced to North America early in the 20th century and is still 

spreading destructively throughout the range of native western white pines (Douglas ex D. Don) 

(McDonald and Hoff 2001). All of the North American five-needle (white) pines are susceptible to 

the pathogen and the disease is currently present in native populations of each species, except 

Pinus longaeva D.K. Bailey (Schwandt et al. 2010, Sniezko et al. 2011). 

Naturally occurring, heritable resistance to blister rust has been detected in populations of North 

American five-needle pine species and has provided the basis for operational resistance breeding 

programs for P. monticola Douglas ex D. Don and P. lambertiana Douglas for several decades 

(Sniezko et al. 2011) and more recently restoration and proactive management programs for the other 

species (Keane and Schoettle 2011, Schoettle and Sniezko 2007). One type of resistance to C. 

ribicola has been attributed to major gene resistance (MGR) and has been described as a classical 

hypersensitivity response (HR) in infected needles and is conferred through a dominant allele at loci 

designated Cr1, Cr2, and Cr3 in P. lambertiana, P. monticola, and P. strobiformis Engelm.,

respectively (Kinloch and Dupper 2002; Kinloch and Littlefield 1977; Kinloch et al. 1970, 1999).

Basidiospores germinate on the needle surface and hyphae penetrate into the needle via the stomata. 

In Cr1, Cr2, and Cr3 genotypes, it has been presumed that HR-mediated cell death near the point of 

C. ribicola entry confines fungal growth and prevents colonization of the mesophyll and ultimately 

the vascular cylinder (see fig. 1).

Localized cell death, or HR, is one of the major components of defense responses in plants against 

pathogen attack (Dangl and Jones 2001). It is generally characterized by rapid death of cells 

surrounding the infected tissues via vacuolar lysis, degrading of cellular machinery and complete 

cellular collapse. Generally, HR phenotypes are visible as yellow or brown necrotic lesions at the site 

of pathogen entry (Heath 2000, Morel and Dangl 1997). The HR constitutes coordinated plant 

responses to pathogen colonization, which involve oxidative burst with accumulation of reactive 

oxygen species (ROS), calcium ion fluxes, changes to the host cell wall, expression of pathogenesisrelated (PR) proteins and localized host cell death (Innes 1998, Liu and Ekramoddoullah 2003, van 

Doorn 2011). These are key components of HR, which usually occur within hours of attack by 

pathogens (Lamb and Dixon 1997). This autolytic cell death is a means to eliminate infected cells, 

which will simultaneously restrict flow of nutrients to the invading pathogen hyphae and delimit 

fungal spread.

The HR is triggered by the interaction of disease resistance (R) genes of host plants and 

corresponding avirulence (AVR) genes in pathogens (Hammond-Kosack and Jones 1997, 

Staskawiecz et al. 2001). The AVR genes in the pathogen code for elicitor molecules that activate R

genes in the host. Hence, host and pathogen recognize each other by their gene products and activate 

signal transduction cascades which orchestrate defense responses (Gilchrist 1998, Innes 1998). The 

complementary relationship between host and pathogen was first described as the gene-for-gene 

 1 Dept. of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331. 2 USDA Forest Service, Dorena Genetic Resource Center, Cottage Grove, OR 97424. 3 USDA Forest Service, Rocky Mountain Research Center, Fort Collins, CO 80526.

Corresponding author: sweeneka@science.oregonstate.edu.
Proceedings of the 4th International Workshop on Genetics of Host-Parasite Interactions in Forestry

369

hypothesis by Flor (1942, 1955). This system is highly specific, as only pathogens with AVR genes 

are detected by the host and subsequently induce the HR response in host tissues. Accordingly, 

genotypes of C. ribicola virulent to Cr1 P. lambertiana and Cr2 P. monticola have been termed vcr1 

and vcr2, respectively (Kinloch and Dupper 2002). Complementary virulence, avirulence, and 

resistance genes in C. ribicola and white pine host species appears to conform to the Flor gene-forgene model (Kinloch and Dupper 2002).

In the C. ribicola pathosystem, the HR paradigm was described for the Cr1 phenotype in P.

lambertiana and the Cr2 phenotype in P. monticola. Individuals resistant to C. ribicola were reported

as having cell degeneration and necrosis leading to localized cell death at the site of fungal infection 

sufficient to stop pathogen growth. Kinloch and Littlefield (1977) noted that the onset of HR in 

resistant pines differs from that in classical HR; the onset of needle lesions in P. lambertiana is not 

observed until several weeks or even months after initial entry of C. ribicola. This observation 

suggests that the resistance mechanisms expressed by Cr1 or Cr2 may differ from the pattern of HR 

responses seen in most incompatible host reactions. 

To further evaluate the apparent HR response in this pathosystem, we undertook structural 

analyses of early C. ribicola colonization in susceptible and resistant P. monticola and P. lambertiana. 

Resistant and susceptible P. albicaulis Engelm. and P. flexilis James phenotypes are also being 

compared to the reactions seen in susceptible and Cr1 and Cr2 seedlings. Secondary needles were 

harvested from 2-year-old seedlings grown and inoculated at the U.S. Department of Agriculture, 

Forest Service, Dorena Genetic Resource Center (DGRC) in Cottage Grove, Oregon. The 

histopathology of infection in resistant P. monticola and P. lambertiana suggests that the symptoms 

associated with HR phenotypes and their underlying physiology are fundamentally different from the 

typical HR described in other host- pathogen systems (fig. 2). We observed extensive proliferation of 

fungal hyphae in the host and penetration of the needle endodermis and vascular tissue by the 

pathogen prior to the onset of cell necrosis. Furthermore, preliminary results suggest the amount of 

fungal tissue present and its growth may be similar for both resistant and susceptible white pines (fig. 

2).

Our results suggest that although resistance to WPBR in P. monticola and P. lambertiana

conforms to the genetic model of HR, the mechanism of resistance may differ from that of typical HR.

Histological examination of infected needles from Cr1 and Cr2 seedlings suggest that the blister rust 

pathogen proliferates within the host rapidly after inoculation and several weeks before the onset of 

typical macroscopic needle lesion symptoms or microscopic signs of autolytic cell death or tissue 

necrosis. Analyses of infection in resistant and susceptible P. albicaulis and P. flexilis phenotypes 

will provide additional insights. Resulting information will be used to help interpret resistance 

mechanisms in the species and provide baseline information useful for resistance breeding and 

disease management. 
GENERAL TECHNICAL REPORT PSW-GTR-240

370

Figure 1—Cross section from a non-inoculated Pinus monticola secondary needle embedded with 

methacrylate and stained with toluidine blue. Arrows indicate the infection pathway. Pathogen enters 

through the stoma and grows within the mesophyll. The endodermis forms a barrier between 

mesophyll and vascular tissue. Crossing the endodermal layer and entrance into the vascular tissues 

allows the pathogen to spread into stem. Scale bar = 100μm. (Image: K. Cook)

Figure 2—Cross section from resistant (Cr1) Pinus lambertiana secondary needle embedded with 

methacrylate and stained with toluidine blue. Sample taken from 2-year-old seedling, 6 weeks after 

inoculation with wild-type rust. Arrows point to Cronartium ribicola invading vascular tissues and 

endodermal cells. Scale bar = 100um. (Image: K. Cook)

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M.P.; Smith, C.M., eds. The future of high-elevation, five-needle white pines in western North America: 

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GENERAL TECHNICAL REPORT PSW-GTR-240

372

The Potential of Breeding for Enhanced Inducibility 

in Pinus pinaster and Pinus radiata

Rafael Zas,1 Alejandro Solla,2 Xoaquin Moreira,3 and Luis Sampedro3

Abstract

Most resistance mechanisms against pests and pathogens in pine trees involve the production of chemical 

defenses. These defenses are not cost free and the production of secondary metabolisms is generally inversely 

related with other plant fitness correlates, such as growth. The existence of these negative genetic correlations 

imposes an important obstacle for breeding for resistance and productivity simultaneously. Increased 

susceptibility to pests and diseases is, indeed, a common side-effect of enhancing productivity in many tree 

breeding programs. In a companion communication (Sampedro et al., these proceedings), we have reported 

negative correlation between constitutive resistance and growth potential of pine trees.

Pines are able to respond to insect and fungus attacks eliciting a wide array of responses. Induced defenses, i.e.,

those that are activated after biotic damage, are assumed to have evolved as a cost-saving strategy, as costs of 

induced resistance materialize only when strictly necessary. Despite the fact that breeding for resistance is 

emerging as an important tool to diminish the impact of forest pests and diseases on managed forests, to date no 

attention has been paid to the possibility of improving forest resistance through breeding for improving 

inducibility, i.e., the ability to respond to biotic damage eliciting induced defenses. Such a breeding strategy 

could likely integrate the benefits of both resistance and productivity, as inducible defensive strategies are cost 

saving. The very first step to explore whether inducibility can be increased through breeding is to determine 

whether these phenotypically plastic responses are under genetic control. Here we show that inducibility of 

chemical defenses in Pinus pinaster Aiton and P. radiata D. Don, two main forest tree species in southwestern

Europe and other temperate regions, is genetically variable within populations. 

We performed two greenhouse experiments with half-sib families of the two pine species. Plant material was a 

random selection of the actual breeding populations of these species in northwest Spain. We simulated biotic 

damage by spraying a solution of methyl jasmonate (MJ), a plant phytohormone that is known to be involved in 

the signalling and triggering of induced defenses, and when exogenously applied, elicit similar responses as real 

herbivory. Fifteen days after MJ application, we measured the concentration of diterpenes in the stem, total 

polyphenolics, and condensed tannins in the needles as measures of quantitative chemical defenses.

We found large genetic variation in all the defensive traits studied, but more interestingly, we also found 

additive genetic variation in the inducibility of the stem diterpenes in both species, as revealed by the significant 

family × MJ interaction. Although family × MJ interaction was not significant for phenolics, the different levels 

of genetic variation observed in control and MJ-induced plants, with significant differences among families 

found only in the MJ-induced treatment, does show the existence of genetic variation in the inducibility in this 

trait, too. Results indicate the existence of additive genetic variation for the inducibility of the three studied 

defensive traits in the two pine species, and the possibility of improving resistance of managed forests by 

artificially selecting for high inducibility potential, thus minimizing the possible undesired side-effects of 

enhancing productivity.

 1 Misión Biológica de Galicia, CSIC, Apdo. 28, 36080 Pontevedra, Spain.

2 Universidad de Extremadura, Virgen del Puerto 2, 10600 Plasencia, Spain.

3 Centro de Investigación Forestal de Lourizán, Apdo 127, 36080 Pontevedra, Spain.

Corresponding author: lsampe@uvigo.es.
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