Mistletoes of North

American Conifers

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-98

September 2002

Canadian Forest Service

Department of Natural Resources

Canada

Sanidad Forestal

SEMARNAT

Mexico
Technical Coordinators_______________________________

Brian W. Geils is a Research Plant Pathologist with the Rocky Mountain Research Station in

Flagstaff, AZ. Dr. Geils earned a Master of Science degree in forestry at the University of Idaho

and a Ph.D. degree in plant pathology at Colorado State University. His current research

focuses on the epidemiology and ecological effects of rusts and dwarf mistletoes in the interior

western United States.

Jose Cibrián Tovar is Director of Sanidad Forestal, SEMARNAT, Mexico D.F. He has

investigated the distribution and effects of dwarf mistletoes in Mexico and now leads the forest

health program for Mexico.

Benjamin Moody is Science Advisor on pest management for the Canadian Forest Service,

Department of Natural Resources Canada, Ottawa, Ontario. Dr. Moody obtained a Master of

Science degree in forestry from the University of New Brunswick and a Ph.D. degree in forestry

from the University of British Columbia. He serves as national advisor for the forest insect and

disease research programs and provides guidance on forest insect and disease issues for

federal policy and management development.

Abstract _________________________________________________________

Geils, Brian W.; Cibrián Tovar, Jose; Moody, Benjamin, tech. coords. 2002. Mistletoes of North American Conifers. Gen. Tech.

Rep. RMRS–GTR–98. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 123 p.

Mistletoes of the families Loranthaceae and Viscaceae are the most important vascular plant parasites of conifers in Canada, the

United States, and Mexico. Species of the genera Psittacanthus, Phoradendron, and Arceuthobium cause the greatest economic

and ecological impacts. These shrubby, aerial parasites produce either showy or cryptic flowers; they are dispersed by birds or

explosive fruits. Mistletoes are obligate parasites, dependent on their host for water, nutrients, and some or most of their

carbohydrates. Pathogenic effects on the host include deformation of the infected stem, growth loss, increased susceptibility to other

disease agents or insects, and reduced longevity. The presence of mistletoe plants, and the brooms and tree mortality caused by

them, have significant ecological and economic effects in heavily infested forest stands and recreation areas. These effects may be

either beneficial or detrimental depending on management objectives. Assessment concepts and procedures are available.

Biological, chemical, and cultural control methods exist and are being developed to better manage mistletoe populations for resource

protection and production.

Keywords: leafy mistletoe, true mistletoe, dwarf mistletoe, forest pathology, life history, silviculture, forest management

You may order additional copies of this publication by sending your

mailing information in label form through one of the following media.

Please specify the publication title and number.

Telephone (970) 498-1392

FAX (970) 498-1396

E-mail rschneider@fs.fed.us

Web site http://www.fs.fed.us/rm

Mailing Address Publications Distribution

Rocky Mountain Research Station

240 West Prospect Road

Fort Collins, CO 80526
Technical Coordinators:

Brian W. Geils

Jose Cibrián Tovar

Benjamin Moody

Mistletoes of North

American Conifers

Sponsors:

Forest Service, U.S. Department of Agriculture, U.S.A.

Sanidad Forestal, SEMARNAT, Mexico

Canadian Forest Service, Department of Natural Resources Canada

Preface _______________________

The mistletoes are serious disease agents affecting

forests in all three countries represented by the North

American Forestry Commission (NAFC 2002): Canada,

Mexico, and the United States of America. For this

reason, the Commission through its Insect and Disease

Study Group has asked us to prepare a practical guide for

field foresters. This publication provides basic information on the species of mistletoes, their hosts, distributions, effects, methods of evaluation, and management.

This work updates and expands the several pages dedicated to mistletoes in the previous guide produced for the

NACF (Davidson and Prentice 1967). This book covers

the mistletoes (Loranthaceae and Viscaceae) that parasitize conifer hosts and occur in North America (Canada,

United States, and Mexico). A similar management synopsis but from a different perspective is available for the

mistletoes of eucalypts in Australia (Reid and Yan 2000).

The literature on mistletoes is large (over 5,700 articles

on North American mistletoes) but widely scattered for

some topics and rare for other topics. We intend this

synoptic review as a guide to useful references for

addressing management concerns from biological conservation to wood fiber production. Because many references have been already cited by Hawksworth and

Wiens (1996) for the dwarf mistletoes, we give emphasis

to general reviews, recent publications, and information

on other mistletoes. An extensive, searchable, annotated bibliography (and other information) is available at

the Mistletoe Center (2002). To facilitate use of this

guide, we provide a glossary for specialized terms (see

appendix D) and post the text online.

Many chapters in this book were first drafted by F. G.

Hawksworth and D. Wiens as they were preparing botanical monographs for Phoradendron and for

Arceuthobium (published as Hawksworth and Wiens

1996). Although information in this book has been selected, revised, and updated, the present authors and

technical editors acknowledge Hawksworth and Wiens

as the original source for many passages of text.

The systematics of Psittacanthus, Phoradendron, and

Arceuthobium are the active subjects of current research; taxonomic revisions of these genera are expected in the near future. This book is not a systematics

and taxonomic review of these mistletoes; no new species names or combinations are proposed. The nomenclature used in this book for mistletoes is adapted from

several sources including taxonomic publications by

Hawksworth, Wiens, and others (see Hawksworth and

Wiens 1996), the International Plant Names Index (Plant

Names Project 1999), the VAST database (Missouri

Botanical Garden 2002), and the National PLANTS database (USDA, NRCS 2001).

The taxonomy of many host groups is also subject to

uncertainty, disagreement, and revision. The names of

many hosts cited in the literature are no longer accepted,

and the proper name for a population in question may not

be apparent. The host taxonomy accepted here follows

the Flora of North America (Flora of North America

Committee 1993) for Canada and the United States and

Perry (1991) for Mexico. These sources also provide

valuable host information including description, synonymy,

and distribution. Nomenclatures for taxa not represented

in these sources (and some authorities) are from the

International Plant Names Index (Plant Names Project

1999).

Because this book has international readership in

several languages (English and Spanish), we usually

refer to plants by their scientific name. To accommodate

those not familiar with these names, we provide appendix

B, relating scientific and vernacular names for trees

species; and we refer to common genera in the text as

“pines” for Pinusand “dwarf mistletoes” for Arceuthobium.

The formal nomenclature for mistletoes including authority, publication, and common synonyms is given for each

mistletoe before its description.

The technical editors are grateful to the many supporters, contributors, reviewers, and editors who helped

to prepare and publish this guide (see appendix C). We

thank C. G. Shaw and Jerome Beatty, USDA Forest

Service, Washington Office, for their helpful support to

this project. We also thank the members of the NAFC,

Insect and Disease Study Group for encouraging and

supporting this work. The NAFC (2002) is a commission

of the Food and Agriculture Organization of the United

Nations and consists of the national forest agencies of

Canada, the United States, and Mexico. Specialists

from the represented countries meet in study groups to

exchange technical assistance on insects and diseases, tree improvement, silviculture, fire management,

and other topics.

Technical Coordinators

Brian W. Geils

Jose Cibrián Tovar

Benjamin Moody

September 2002

Author Profiles ________________________________

Frank G. Hawksworth died in 1993 after starting this project and a monographic

review of the genus Arceuthobium. He had been Project Leader and Supervisory

Plant Pathologist with the Rocky Mountain Research Station in Fort Collins, CO.

Beginning with his doctoral studies at Yale University, Dr. Hawksworth dedicated his

professional life to the single question, “What is mistletoe?” He described numerous

species (most with Del Wiens), authored over 275 articles on dwarf mistletoes, and

compiled an extensive library on mistletoes of the world. He is commemorated by

Arceuthobium hawksworthii.

Delbert Wiens is retired Professor of biology, University of Utah, Salt Lake City. He

received a Ph.D. degree from the Claremont Graduate School, CA for his work on the

taxonomy of Phoradendron. Dr. Wiens has investigated numerous aspects of plant

biology including systematics, biogeography, cytology, reproductive biology, and

mimicry. He has a particular interest in the Loranthaceae and Viscaceae and the flora

of Africa and Southeast Asia. Dr. Wiens recently completed a popular book on the

mistletoes of Africa and continues exploring the world.

John A. Muir is a registered professional Forester by the Province of British

Columbia and serves as the Provincial Forest Pathologist for the Ministry of Forests,

Victoria, B.C. Dr. Muir earned a Bachelor and Master degrees in forest pathology from

the University of British Columbia; he has a Ph.D. degree in plant pathology from the

University of California, Berkeley. His current activities include development of forest

practices and regulations for diseases and development of models and decision aids

for dwarf mistletoes and other pathogens. His work supports innovative disease

management practices such as genetic resistance and intensive cultural techniques

to enhance growth and sustainability of young forests.

Simon F. Shamoun is a research scientist with the Canadian Forest Service, Pacific

Forestry Centre and adjunct professor at the University of British Columbia and at the

University of Victoria. Dr. Shamoun obtained a Bachelor of Science degree in forestry

from the University of Mosul, Nineveh, Mesopotamia (Iraq); Master of Science degree

in plant pathology from North Carolina State University; and a Ph.D. degree in plant

pathology from the University of Arkansas. His current research focuses on management of competing vegetation, forest weeds, and dwarf mistletoes using fungal

pathogens. He is investigating the population structure and genetic diversity of

several candidate biological control agents and their target plants.

Laura E. DeWald is an Associate Professor of forest genetics and conservation

biology in the School of Forestry, Northern Arizona University, Flagstaff, AZ. She

received a Bachelor of Science degree in forestry from Michigan Technological

University, Master of Science degree in forest genetics from Pennsylvania State

University, and a Ph.D. degree in forest genetics and physiology from Virginia

Polytechnic Institute and State University. In addition, she completed postdoctoral

studies in forest genetics and physiology at the University of Minnesota and the

University of Florida. Her current research focuses on application of ecological

genetics to forest management and restoration, insect and disease resistance of

Southwestern tree species, and riparian restoration.

Ignacio Vázquez Collazois a Research Investigator at the Centro de Investigaciones

del Pácifico Centro in Uruapan, Michoacán, Mexico. He has published numerous

articles on the biology, effects, and management of both dwarf mistletoes and leafy

mistletoes in central Mexico.
The use of trade or firm names in this publication is for reader information and does not

imply endorsement by the U.S. Department of Agriculture of any product or service

CAUTION:

PESTICIDES

Pesticide Precautionary Statement

This publication reports research involving pesticides. It does not

contain recommendations for their use, nor does it imply that the uses

discussed here have been registered. All uses of pesticides must be

registered by appropriate State and/or Federal agencies before they can

be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals,

desirable plants, and fish or other wildlife—if they are not handled or

applied properly. Use all pesticides selectively and carefully. Follow

recommended practices for the disposal of surplus pesticides and

pesticide containers.
B. W. Geils Chapter 1. Loranthaceae and Viscaceae in North America ........................... 1

I. Vázquez Collazo Mistletoes of Canada, Mexico, and the United States ......................................... 1

Key to Genera of Mistletoes in North America ..................................................... 4

Economic and Ecological Importance .................................................................. 4

Management Strategies ....................................................................................... 5

I. Vázquez Collazo Chapter 2. Psittacanthus in Mexico ................................................................. 9

B. W. Geils General Life Cycle ............................................................................................. 10

Description of Genus ......................................................................................... 12

Description of Species ....................................................................................... 13

1. Psittacanthus americanus ........................................................................ 13

2. Psittacanthus angustifolius ....................................................................... 13

3. Psittacanthus calyculatus ......................................................................... 14

4. Psittacanthus macrantherus ..................................................................... 14

5. Psittacanthus pinicola ............................................................................... 14

6. Psittacanthus schiedeanus ....................................................................... 15

Damage and Effects on Host ............................................................................. 15

Economic and Ecological Importance ................................................................ 15

Management Strategies ..................................................................................... 16

Biological Control.......................................................................................... 16

Chemical Control .......................................................................................... 16

Silviculture .................................................................................................... 16

B. W. Geils Chapter 3. Phoradendron in Mexico and the United States......................... 19

D. Wiens Life History and Biology ..................................................................................... 20

F. G. Hawksworth Life Cycle ...................................................................................................... 20

Host–Parasite Physiology............................................................................. 20

Host–Parasite Ecology ................................................................................. 20

Description of Genus ......................................................................................... 21

Description of Species ....................................................................................... 22

 1. Phoradendron bolleanum ....................................................................... 22

 2. Phoradendron capitellatum..................................................................... 23

 3. Phoradendron densum ........................................................................... 23

 4. Phoradendron juniperinum ..................................................................... 25

 5. Phoradendron libocedri .......................................................................... 25

 6. Phoradendron longifolium....................................................................... 26

 7. Phoradendron minutifolium..................................................................... 26

 8. Phoradendron pauciflorum ..................................................................... 26

 9. Phoradendron rhipsalinum ..................................................................... 27

10. Phoradendron saltillense ........................................................................ 27

Importance ......................................................................................................... 27

Management Strategies ..................................................................................... 28

F. G. Hawksworth Chapter 4. Arceuthobium in North America .................................................. 29

D. Wiens General Life Cycle ............................................................................................. 29

B. W. Geils Life History.................................................................................................... 30

Dispersal ................................................................................................ 30

Establishment ........................................................................................ 31

Incubation .............................................................................................. 31

Reproduction .......................................................................................... 31

Spread and Intensification ............................................................................ 32

Rating systems ...................................................................................... 32

Description of Genus ......................................................................................... 33

Key to North American Species of Arceuthobium .............................................. 35

Contents Page
Description of Species ....................................................................................... 38

 1. Arceuthobium abietinum ......................................................................... 38

1a. White Fir Dwarf Mistletoe ................................................................. 38

1b. Red Fir Dwarf Mistletoe ................................................................... 40

 2. Arceuthobium abietis-religiosae ............................................................. 40

 3. Arceuthobium americanum..................................................................... 40

 4. Arceuthobium apachecum ...................................................................... 41

 5. Arceuthobium aureum subsp. petersonii ................................................ 41

 6. Arceuthobium blumeri............................................................................. 41

 7. Arceuthobium californicum ..................................................................... 42

 8. Arceuthobium campylopodum ................................................................ 42

 9. Arceuthobium cyanocarpum ................................................................... 43

10. Arceuthobium divaricatum ...................................................................... 43

11. Arceuthobium douglasii .......................................................................... 44

12. Arceuthobium durangense ..................................................................... 45

13. Arceuthobium gillii .................................................................................. 45

14. Arceuthobium globosum ......................................................................... 46

14a. Arceuthobium globosum subsp. globosum .................................... 46

14b. Arceuthobium globosum subsp. grandicaule ................................. 46

15. Arceuthobium guatemalense .................................................................. 47

16. Arceuthobium hondurense ..................................................................... 47

17. Arceuthobium laricis ............................................................................... 47

18. Arceuthobium littorum............................................................................. 48

19. Arceuthobium microcarpum.................................................................... 48

20. Arceuthobium monticola ......................................................................... 49

21. Arceuthobium nigrum ............................................................................. 49

22. Arceuthobium oaxacanum ...................................................................... 50

23. Arceuthobium occidentale ...................................................................... 50

24. Arceuthobium pendens........................................................................... 50

25. Arceuthobium pusillum ........................................................................... 51

26. Arceuthobium rubrum ............................................................................. 51

27. Arceuthobium siskiyouense .................................................................... 52

28. Arceuthobium strictum ............................................................................ 52

29. Arceuthobium tsugense .......................................................................... 52

29a. Arceuthobium tsugense subsp. tsugense ...................................... 53

29b. Arceuthobium tsugense subsp. mertensianae ............................... 54

30. Arceuthobium vaginatum ........................................................................ 54

30a. A. vaginatum subsp. vaginatum ..................................................... 54

30b. Arceuthobium vaginatum subsp. cryptopodum .............................. 55

31. Arceuthobium verticilliflorum................................................................... 56

32. Arceuthobium yecorense ........................................................................ 56

B. W. Geils Chapter 5. Damage, Effects, and Importance of Dwarf Mistletoes.............. 57

F. G. Hawksworth Physiology of Dwarf Mistletoe Parasitism .......................................................... 58

Direct Effects to Host Trees.......................................................................... 58

Reproduction ................................................................................................ 58

Cones and seeds ................................................................................... 58

Seedlings ............................................................................................... 59

Growth .......................................................................................................... 59

Longevity ...................................................................................................... 60

Wood Quality, Decay, and Breakage ........................................................... 61

Witches’ Brooms........................................................................................... 61

Ecological and Evolutionary Effects ................................................................... 62

Interactions ................................................................................................... 62

Fungi ...................................................................................................... 62
Insects and spiders ................................................................................ 62

Fire and fuels ......................................................................................... 63

Forest Structure and Composition ................................................................ 63

Old-growth forests .................................................................................. 63

Wildlife habitat ........................................................................................ 63

Consequences to Resources and Other Values ................................................ 64

Importance.................................................................................................... 64

Species affected .................................................................................... 64

Area affected .......................................................................................... 64

Growth loss ............................................................................................ 64

Amenity Values............................................................................................. 65

Coevolution................................................................................................... 65

J.A. Muir Chapter 6. Dwarf Mistletoe Surveys ............................................................... 67

B. Moody General Requirements and Procedures ............................................................ 68

Dwarf Mistletoe Surveys .................................................................................... 69

Aerial Surveys .............................................................................................. 70

Aerial Photography ....................................................................................... 70

Forest Inventory Plots................................................................................... 70

Road and Plot Surveys ................................................................................. 70

Transects and Grids ..................................................................................... 71

Permanent Plots ........................................................................................... 71

Project Area Assessments ........................................................................... 71

Developing management prescriptions .................................................. 71

Assessments complex stands ................................................................ 72

Evaluations Using Dwarf Mistletoe Survey Data................................................ 72

Further Needs for Surveys and Evaluations ...................................................... 73

S. F. Shamoun Chapter 7. Management Strategies for Dwarf Mistletoes:

L .E. DeWald Biological, Chemical, and Genetic Approaches ...................................... 75

Biological Control ............................................................................................... 76

Integrating Biological Control with Silviculture .............................................. 76

Insects .......................................................................................................... 76

Fungi............................................................................................................. 76

Aerial shoot fungi ................................................................................... 77

Canker fungi associated with endophytic system .................................. 78

Resin disease syndrome ........................................................................ 79

Summary ................................................................................................ 79

Chemical Control ............................................................................................... 79

Summary ...................................................................................................... 80

Management Through Selection for Genetic Resistance .................................. 80

Summary ...................................................................................................... 81

J. A. Muir Chapter 8. Management Strategies for Dwarf Mistletoe: Silviculture ......... 83

B. W. Geils Designing Silvicultural Treatments..................................................................... 84

Management of Even-Aged Stands ................................................................... 87

Prevention of Spread Into Cut Blocks........................................................... 87

Silvicultural Treatments of Young Stands..................................................... 87

Recently harvested and regenerated stands ......................................... 88

Sanitation thinning ................................................................................. 88

Thinning precommercial stands ............................................................. 89

Commercial Thinning Treatments ................................................................ 89

Uneven-Aged Silviculture and Selection Cutting ............................................... 90

Prescribed Burning ............................................................................................ 91

Models to Assess Treatment Opportunities ....................................................... 91
Federal Recycling Program Printed on Recycled Paper

Management for Recreation, Wildlife Habitat, and Other Ecosystem Values .... 93

Treatments in Developed Recreation Sites .................................................. 93

Treatments for Wildlife Habitat and Other Ecosystem Values...................... 94

Appendices....................................................................................................... 95

Appendix A: References .................................................................................... 95

Appendix B: Scientific and Common Names of Trees .................................... 113

Appendix C: Acknowledgments ....................................................................... 115

Appendix D: Glossary ...................................................................................... 116

Appendix E: Index ............................................................................................ 120
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 1

B. W. Geils

I. Vázquez Collazo

1

Chapter

Mistletoes of Canada, Mexico, and the

United States ____________________________________

The mistletoes are a diverse group in the order Santales of

shrubby, usually aerial, parasitic plants with fruits possessing a

viscid layer (Kuijt 1968, 1969a). They are widely distributed

geographically and as a group have a broad host range on conifers

and other woody plants (Calder 1983). Many mistletoes are

specially adapted for avian pollination and dispersal, and several

avian species make extensive use of these resources (Kuijt 1969a,

Watson 2001). The mistletoes are damaging pathogens of trees;

and in many parts of the world are serious forest pests

(Hawksworth 1983, Knutson 1983). General information on

mistletoes is available at Calder and Berhhardt (1983), Cházaro

and others (1992), Geils (2001a, 2001b), Gill and Hawksworth

(1961), Kuijt (1969a), Mistletoe Center (2002), Nickrent (2002),

Sinclare and others (1987), and Vega (1976).

Loranthaceae and

Viscaceae in North

America
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Vázquez Loranthaceae and Viscaceae in North America

The principal families of mistletoe are the

Loranthaceae and Viscaceae (Calder 1983). The

Eremolepidaceae, Misodendraceae, and several genera of Santalaceae could also be included as “mistletoes,” but these interesting parasites do not occur on

North American conifers (Kuijt 1969a, 1988, Wiens

and Barlow 1971). The loranthaceous and viscaceous

mistletoes had been considered sub-families within

the Loranthaceae but are now recognized as distinct,

related families (Barlow 1964). There are several

anatomical, embryological, and chromosomal differences between the two families (Kuijt 1969a, Wiens

and Barlow 1971), but a practical difference is that the

flowers in the Viscaceae are small and inconspicuous,

whereas those in the Loranthaceae are large, colorful,

and possess a calyculus (see Venkata 1963). The

Viscaceae occur in tropical and temperate zones of the

Northern Hemisphere; the Loranthaceae are generally tropical (Barlow 1983). The two families overlap

in Mexico (Cházaro and Oliva 1987a, 1987b, 1988a).

The mistletoes of conifers in the New World are

Cladocolea, Struthanthus, Psittacanthus,

Dendropemon (Loranthaceae) plus Arceuthobium,

Phoradendron, and Viscum (Viscaceae) (table 1-1).

Scharpf and others (1997) review these genera and list

the other mistletoes that infect conifers elsewhere.

The most important genera to North American forestry are Arceuthobium, Phoradendron, and

Psittacanthus. Field guides or keys for the identification of these mistletoes include: Bello (1984), Bello and

Gutierrez (1985), Hawksworth and Scharpf (1981),

Scharpf and Hawksworth (1993), Standley (1920),

Tropical Agriculture Research and Training Center

(1992), and Unger (1992).

The genus Cladocolea Tiegh. includes at least 23

little-studied mistletoes mostly of Central and Southern Mexico (Cházaro 1990, Kuijt 1975a). Plants are

erect or vine-like shrubs (fig. 1-1); most species are

parasites of oaks or other broadleaf trees. These mistletoes cause little damage to their hosts; their greatest

importance is scientific, as rare species in a curious

genus. The species reported to infect conifers (table 1-2)

are:

• Cladocolea cupulata Kuijt [Journal Arnold Arboretum 56(3):285–286, 1975]

• C. microphylla (Kunth) Kuijt [Journal Arnold

Arboretum 56(3):313–317, 1975]

Mistletoes of the genus Struthanthus Mart. are

climbing vines to several meters long (fig. 1-2). These

mistletoes (“mata palo” or “tripa de pollo”) include 50

to 60 species from Mexico to Argentina (Bello 1984,

Cházaro and Oliva 1988a, Kuijt 1964, 1975b). The

Struthanthus mistletoes typically have broad host

ranges that occasionally include a few conifers. The

genus Struthanthus is a taxonomically chaotic and

difficult group (Kuijt 1969a); applied names should be

accepted with caution. The species reported to infect

conifers (table 1-2) are:

• Struthanthus deppeanus (Schldt. & Cham.)

Blume [Systema Vegetabilium 7:1731, 1830]

• S. interruptus (Kunth) Blume [Systema

Vegetabilium 7:1731, 1830]

• S. palmeri Kuijt [Canadian Journal Botany

53(3):252. 1975]

• S. quericola (Schltdl. & Cham.) Blume [Systema

Vegetabilium 7:1731, 1830]

Table 1-1—Mistletoes of North American conifers.

Family Genus Distribution in North America Conifer hosts in North America

Loranthaceae

Cladocolea Mexico Pinus

Struthanthus Mexico Pinus, Taxodium

Psittacanthus Mexico Abies, Pinus

Viscaceae

Arceuthobium Canada, Mexico, United States Abies, Larix, Picea, Pinus,

Pseudotsuga, Tsuga

Phoradendron Mexico, United States Abies, Calocedrus, Cupressus,

Juniperus, Taxodium

Viscum Canada, United States *

*In North America, Viscum occurs as an introduced species only on angiosperms; elsewhere Viscum infects Abies, Picea, Pinus,

Pseudotsuga, and Juniperus.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 3

Loranthaceae and Viscaceae in North America Geils and Vázquez

Figure 1-1—Cladocolea cupulata, A habit, pistillate plant

and B fruit and supporting structure (three fruits removed).

Illustration courtesy of Job Kuijt, edited from figure 9 in

Journal Arnold Arboretum. 56(3): 285.

Table 1-2—Cladocolea and Struthanthus occurrence on conifers in Mexico.

Mistletoe Distribution Hosts Reference

Cladocolea Jalisco Pinus jaliscana Cházaro and others (1992)

cupulata P. lumholtzii

Cladocolea Michoacán Pinus leiophylla, Bello Gonzalez (1984)

microphyllus P. montezumae,

P. pseudostrobus

Struthanthus Chiapas, Pinus patula Cházaro and Oliva (1988a)

deppeanus Oaxaca,

Puebla,

Veracruz

Struthanthus Michoacán Pinus lawsonii Bello Gonzalez (1984)

interruptus

Struthanthus Sonora Taxodium distichum Kuijt (1975b)

palmeri var. mexicanum c

Struthanthus Pinus sp. Cházaro and Oliva (1988a)

quericola

Note: These mistletoes are principally parasites of hardwoods over most of their distribution; this table

presents only reports of the mistletoe on a conifer host and their joint distribution. a

Reported as Struthanthus microphyllus; determination by Kuijt (personal communication) b

Reported as Struthanthus venetus; determination by Kuijt (personal communication). c

Reported as Taxodium mucronatum.

Figure 1-2—Struthanthus palmeri, A a habit, staminate plant and B fruit and supporting structure.

Illustration courtesy of Job Kuijt, edited from figures

6 and 7 in Canadian Journal of Botany 53:252.

B

A

mm

b

cm

mm

B

cm

A
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Vázquez Loranthaceae and Viscaceae in North America

Struthanthus palmeri (fig. 1-2) is found as far north

as 60 km south of Nogales, AZ, and may be the most

northern of the New World Loranthaceae (Kuijt 1975b).

Struthanthus mistletoes cause little economic damage

and are most important for scientific interest.

The genus Psittacanthus consists of 75 to 80 species,

distributed from Mexico to Argentina on a wide range

of angiosperm and gymnosperm hosts. In contrast to

most of the other mistletoes of Mexico, the flowers are

large and conspicuous-red, yellow, or orange (Cházaro

and Oliva 1988a). Although about 10 species occur in

Mexico, only four are parasites of fir or pine. These

mistletoes are widely distributed but seriously damaging in only a few locations. The biology and management of these mistletoes are discussed in chapter 2.

The genus Phoradendron includes about 250 species, exclusive to the New World in tropical or temperate zones. Hosts include several genera of conifers,

many broadleaf trees and shrubs, and other mistletoes. In some areas, Phoradendron mistletoes can be

quite common and cause serious damage to conifers

(Hawksworth and Scharpf 1981). The Phoradendron

species on conifers are described in chapter 3.

The dwarf mistletoes, genus Arceuthobium, consist

of 42 species of North and Central America, Europe,

Asia, and Africa (Hawksworth and Wiens 1996). These

mistletoes are restricted to conifers and usually quite

damaging to their host. Descriptions, hosts, and distributions of 40 taxa are presented in chapter 4; damage,

effects, and importance in chapter 5; survey methods

in chapter 6; and management in chapters 7 and 8.

Two other genera of mistletoes are worthy of mention here. The European mistletoe, Viscum album L,

was introduced to California by Luther Burbank about

1900 (Hawksworth and others 1991), and a recent

introduction was discovered in 1988 for British Columbia (Muir 1989). Although subspecies of Viscum

album are able to infect fir, spruce, pine, Douglas-fir,

and juniper native to North America, Viscum album in

Canada and the United States is only reported on

broadleaf trees (Barney and others 1998). This mistletoe is not considered a threat to natural conifer stands

in North America. Two additional species of mistletoes

on conifers have been collected from Hispaniola in the

Caribbean (Kuijt, personal communication).

Dendropemon constantiae Krug & Urban is an uncommon species usually found on Pinus occidentalis; and

D. pycnophyllys Krug & Urban is a common species,

apparently restricted to pine hosts.

Although many mistletoe genera share host species

in common, there are only a few examples of two

genera of mistletoe infecting the same tree

(Hawksworth and Wiens 1996). Abies concolor is

coinfected by Arceuthobium abietinum and Phoradendron pauciflorum in California. Pinus engelmannii

is coinfected by A. vaginatum subsp. vaginatum and

Psittacanthus macrantherus in Durango, Mexico. Pinus

pseudostrobus is coinfected by A. globosum subsp.

grandicaule and Psittacanthus macrantherus in

Michoacán, Mexico.

Economic and Ecological

Importance _____________________

Mistletoes have long been held by many peoples as

special. Their sacred, mythical role in numerous cultures

is documented by Frazer (1930) in his classic study of

magic and the golden bough. Mistletoes are an inspiration for art (Becker and Schmoll 1986); their haustoria

produce woodroses. In traditional, agricultural societies,

mistletoes provide fodder, dyes, and drugs. Mistletoes

are used for holiday decoration and models for new

pharmaceuticals. A few North American mistletoes are

narrow endemics threatened with extinction. Rolston

(1994) describes the values of such species and why they

ought to be preserved. Most mistletoes, however, have

Key to Genera of Mistletoes in North America

1. Flowers with a calyculus, usually large and showy (Loranthaceae) ................................................................2

2. Flowers less than 1 cm long, light green; leaves less than 5 cm long and 2 cm wide ................................3

3. Inflorescence a determinate spike of monads ........................................................................ Cladocolea

3. Inflorescence generally indeterminate, dioecious .............................................................. Struthanthus

2. Flowers 3–5 cm long, yellow or reddish; leaves 5–8 cm long and over 2 cm wide .................Psittacanthus

1. Flowers without a calyculus, less than 3 mm long, same color as the shoots; plants leafless or with

leaves less than 5 cm long or 2 cm wide (Viscaceae) .........................................................................................4

4. Fruit elongated and bicolored; seeds explosively dispersed (one exception); leafless; parasitic on pine,

Douglas-fir, spruce, larch, fir, or hemlock.............................................................................. Arceuthobium

4. Fruit round, uniformly colored pink, reddish, or white; seeds dispersed by birds; leafless or with

well-developed leaves, parasitic on juniper, cypress, incense-cedar, bald-cypress or fir

.................................................................................................................................................. Phoradendron
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 5

Loranthaceae and Viscaceae in North America Geils and Vázquez

wide distributions, are locally abundant, and significantly alter the environment (Watson 2001). These mistletoes are important agents of disease, disturbance, and

evolution. As pathogens, mistletoes affect host physiology (Knutson 1983, Kolb 2002). The results of tree

disease are brooming, dieback, reduced growth, survival,

and reproduction and increased susceptibility to other

diseases and injuries. The consequences of an infestation

are both economic and ecological. Mistletoes are forest

pests for the commercial losses they cause and are

influential symbionts for the many and complex interactions they affect. Assessing mistletoe importance revolves around two questions: how much (extent and

abundance) and what effects.

Because mistletoes have major resources impacts

(such as on timber yield), information is compiled

regionally to describe their incidence and severity.

North America consists of Canada, the continental

United States of America, and the Republic of Mexico

(fig. 1-3). North American mistletoes are found in most

of the major coniferous forests and parasitize pine, fir,

spruce, Douglas-fir, larch, hemlock, juniper, cypress,

incense-cedar, and bald-cypress (table 1-1). The significant conifers not parasitized are arborvitae (Thuja),

redwood (Sequoia), and giant sequoia

(Sequoiadendron). The only mistletoes in Canada are

dwarf mistletoes, but these occur across the country

from Newfoundland to British Columbia. The most

important are in eastern spruce bogs (Magasi 1984),

central jack and lodgepole pine forests (Brandt and

others 1998), and coastal hemlock forests (Alfaro 1985).

Both Phoradendron and Arceuthobium occur in the

United States. Although Phoradendron mistletoes are

widely distributed across the Southern and Western

States, the species that infect conifers are most common in the Western–Southwestern portion of the country (from western Texas to California, Colorado, and

Oregon). Phoradendron mistletoes are abundant and

damaging in some locations, but we know of no regional estimates of their incidence and severity. The

dwarf mistletoes occur in the Northeastern States,

Northern Lake States, Western States, and southeastern Alaska (see Forest Health Protection 2002).

Drummond (1982) reports the infested area as 14

percent for the black spruce type in the Northern Lake

States; 22 percent for the Rocky Mountain Douglas-fir

type; 34 percent of the Rocky Mountain ponderosa

pine type; 40 percent of the lodgepole pine type; and 22

percent of the commercial host type in Pacific states.

Mistletoes including Psittacanthus, Phoradendron,

and Arceuthobium are the principal cause of forest

disease across Mexico. Mistletoes are most abundant

in the cool or temperate coniferous forests and are

found on more than 10 percent of the forest area

(Hawksworth 1983). The forest area infected varies by

State—Durango 15 percent, Nayarit 10 percent, Sonora

9 percent, Chihuahua 8.5 percent, Baja California 7

percent, Zacatecas 24 percent, Sinaloa 10 percent, and

Jalisco 12 percent (Caballero 1968, 1970). Although

the actual extent of infested area on a regional basis

changes little from year to year, various definitions

and data sources are used. These generate somewhat

different estimates that are in broad agreement that

mistletoes are common in some areas.

From an economic perspective, the effects of mistletoe infestation are described by Hawksworth (1993).

Relevant to timber production, mistletoes reduce

growth, yield, and quality and increase operation and

protection costs for planning, harvesting, regeneration, and fuel management. Mistletoes are a concern

in recreation areas for increased hazard from broom

breakage (Hadfield 1999) and increased expense in

vegetation management (Lightle and Hawksworth

1973).

From an ecological perspective, the effects of mistletoe infestations are complex because there are numerous criteria and relationships that might be considered relevant in a given situation. Allen and Hoekstra

(1992) suggest describing ecological phenomena from

alternative viewpoints or “criteria” of the population,

species, community, landscape, and ecosystem. For a

diseased tree, mistletoe infection means reduced competitive status and reproduction fitness (but see van

Ommeren and Whitham 2002). The symbiotic relation

between host and mistletoe has numerous population

genetic and coevolutionary consequences that cannot

be properly categorized as positive or negative (see

Atsatt 1983, Norton and Carpenter 1998). Other species in addition to a host also are connected to the

mistletoe by herbivory, pollination, use of the witches’

broom, or other relations. Watson (2001) recognizes

mistletoes as keystone resources in many communities. Canopy effects are especially significant. Crown

deformation and tree death affect composition of trees

that compose the forest canopy and the structure of

that canopy (Reid and others 1995). Numerous species, landscape, and ecosystem processes are consequently influenced—there are winner and losers, increases and decreases. Many indirect and long-term

interactions involving mistletoes exhibit chaotic behaviors; a range of outcomes are likely rather than a

single one determined (see Gleick 1988). The relevant

fact is that mistletoes are often an important ecological and evolutionary agent driving that system (Holling

1992).

Management Strategies __________

The mistletoe literature indicates not only that

mistletoes have important effects but also that infestations can be affected by management intervention to

change their spread and intensification. Effective
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Vázquez Loranthaceae and Viscaceae in North America

AK

WA

ID

MT ND

SD

WY

CO UT

NV

OR

CA

B.C.

B.C.S.

Chih.

Coah.

N.L.

Tamps.

Qro.

Hgo.

Chis. Tab.

Q.Roo

Yuc.

Camp.

Oax.

Son.

Sin. Dgo.

Nay.

Ags.

Col.

D.F.

Tlax.

Mor.

Mich.

Gro.

Jal.

Zac.

AZ NM OK

NE IA

AK

MS AL

KS MO

OH PA

NY

NJ

DE

DC

TX

NC

SC

FL

LA

ME VT

MA

RI

CT

VA

WV

GA

MN

WI

IL IN

KY

TN

MI

Y.T.

N.W.T.

B.C. Alta.

Man.

Ont.

Que.

Lab.

Nfld.

N.B.

N.S.

P.E.I.

Sask.

Figure 1-3—North America, Canada, United States of America, and Mexico with political subdivisions.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 7

Loranthaceae and Viscaceae in North America Geils and Vázquez

Canadian Provinces, Territories and 

abbreviations

Province-Territory Abbreviation

Alberta Alta.

British Columbia B.C.

Manitoba Man.

New Brunswick N.B.

Newfoundland Nfld

Northwest Territories N.W.T.

Nova Scotia N.S.

Ontario Ont.

Prince Edward Island P.E.I

Quebec Que.

Saskatchewan Sask.

Yukon Territory Y.T.

U.S. states and abbreviations

Province-Territory Abbreviation

Alabama

Alaska

Arizona

Arkansas

California

Colorado

Connecticut

Delaware

Dist. of Columbia

Florida

Georgia

Idaho

Illinois

Indiana

Iowa

Kansas

Kentucky

Louisiana

Maine

Maryland

Massachusetts

Michigan

Minnesota

Mississippi

Missouri

Montana

Nebraska

Nevada

New Hampshire

New Mexico

New Jersey

New York

North Carolina

North Dakota

Ohio

Pennsylvania

Rhode Island

South Carolina

South Dakota

Tennessee

Texas

Utah

Vermont

Virginia

Washington

West Virginia

Wisconsin

Wyoming

AL

AK

AZ

AR

CA

CO

CT

DE

DC

FA

GA

ID

IL

IN

IA

KS

KY

LA

ME

MD

MA

MI

MN

MS

MO

MT

NE

NV

NH

NM

NJ

NY

NC

ND

OH

PA

RI

SC

SD

TN

TX

UT

VT

VA

WA

WV

WI

WY

Mexican States and abbreviations

Mexican state Abbreviation

Aguascalientes Ags.

Baja California B.C.

Baja California Sur B.C.S.

Colima Col.

Coahuila Coah.

Chiapas Chis.

Distrito Federal D.F.

Durango Dgo.

Guerrero Gro.

Guanajuato Gto.

Hidalgo Hgo.

Jalisco

Michoacan

Morelos

México

Nayarit

Nuevo León

Oaxaca

Puebla

Quintana Roo

Querétaro

Sinaloa

San Luis Potosí 

Sonora

Tabasco

Tlaxcala

Tamaulipas

Veracruz

Yucatán

Zacatecas

Jal.

Mich.

Mor.

Edo de Mex.

Nay.

N.L.

Oax.

Pue.

Q. Roo

Qro.

Sin.

S.L.P.

Son.

Tab.

Tlax.

Tamps.

Ver.

Yuc.

Zac.
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Vázquez Loranthaceae and Viscaceae in North America

intervention is both purposeful and persistent. Tkacz

(1989) describes an approach called Integrated Resource Management used in the Southwestern Region, USDA Forest Service, that incorporates forest

insect and disease considerations into a planning,

implementing, and monitoring process. Many other

organizations have comparable management systems.

Common elements of these systems include (1) formulation of objectives, (2) review of the expected performance of alternatives, (3) selection and implementation, and (4) monitoring and reaction. Holling and

Meffe (1996) warn of the dangers from attempting

rigid control in natural resource management; they

advocate an adaptive process for complex environments with changing objectives and management options. Although management in mistletoe-infested

stands has not always been successful for various

reasons (Conklin 2000), management processes and

techniques are available with the potential for producing desirable results.

A simplistic review of one management strategy

that once dominated conifer forestry is instructive. A

prevailing objective on public forests in the 20th century was sustained economic production of timber.

Foresters knew that dwarf mistletoes were obligate

parasites that died when the host tree was cut and had

limited capability of spread (Weir 1916b). The preferred control technique was clearcutting in large

blocks to remove the mistletoe and retard reinfestation

(Stewart 1978). Where employed, it worked. A challenge to forest pathologists arose when objectives were

expanded to include wildlife and aesthetic values, and

treatments required or produced infrequent, selective

removal that left infected trees. At least in the American Southwest, dwarf mistletoe infestations were not

fading away (Conklin 2000, Maffei and Beatty 1988).

Other control techniques based on biological, chemical, genetic, and silvicultural approaches were needed

(Scharpf and Parmeter 1978, Muir 1993).

Hawksworth (1978) and Parmeter (1978) describe

the epidemiological bases for control of dwarf mistletoes that can be extended with modification to other

mistletoes. For technical and management reasons,

silvicultural approaches have been used more commonly than chemical or biological control or genetic

selection. Although there has been some success with

chemical controls, phytotoxicity and need for reapplication have limited this approach (Adams and others

1993, Lichter and others 1991, Scharpf 1972). The

concepts of control with biological agents are well

developed (DeBach 1964), and use of insects and fungi

on mistletoes has been considered (Cházaro and others 1992, Julian 1982, Mushtaque and Balock 1979).

There is evidence for inherited variation in host resistance to infection by at least the dwarf mistletoes.

Genetic selection may provide regeneration alternatives (Ringnes and others 1996). Silvicultural approaches include pruning, sanitation, species replacement, and other techniques that rely on cutting trees.

As with chemical, biological, and genetic approaches,

cultural methods must be adapted to fit the mistletoe

and host combination in the context of specific management objectives and constraints.

Assessment and monitoring are essential elements

of a strategy for managing mistletoes. Mistletoe infestations initially develop slowly but accelerate rapidly

and cause significant departure from typical stand

development. These facts suggest that early intervention provides greater flexibility and that a good model

of stand response is useful for predicting what a

treatment might produce in 20 to 40 years. Although

the Dwarf Mistletoe Impact Model (Forest Health

Technology Enterprise Team 2002) is primarily intended for assessing silvicultural alternatives, it (and

other models) can be modified or developed for evaluating tactics of deploying biological agents or genetically selected stock (Robinson and others 2002).

The mistletoes of North American conifers range

from obscure species in remote locations to major

forest pests. Management varies from intensive timber production to biological conservation. Although

these mistletoes can have significant impacts on forest

conditions, they are also subject to management influence through various methods that alter rates of

spread and intensification. Knowing which methods

are appropriate and effective requires an understanding for each kind of mistletoe: its life history, hosts,

distribution, effects, and ecology.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 9

I. Vázquez Collazo

B. W. Geils Chapter

Psittacanthus in

Mexico

The Psittacanthus, parrot-flower, is the only genus of the

family Loranthaceae that is significant to conifer forestry in

North America. These mistletoes do not occur in Canada or

the United States; and in Mexico, they are only important in

central and southern portions. Psittacanthus also occurs in

Central America (rarely on conifers) and other regions of the

tropical New World where these mistletoes achieve their

greatest diversity and abundance on numerous hardwoods.

Plants are showy (fig. 2-1), become quite large, and are

locally abundant. They are damaging to conifers, but they

also provide special resource values. Because there are few

studies for Psittacanthus on conifers (for example, Vázquez

1993a) and the taxonomy is confused, information on these

mistletoes is sparse and difficult to interpret. This chapter

reviews Psittacanthus on conifers with regard to life cycle,

description, damage, importance, and management.

2
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Vázquez and Geils Psittacanthus in Mexico

General Life Cycle _______________

The life cycle of Psittacanthus is divided into the

fundamental processes of dispersal and development

separated by inoculation and germination. Although

some seeds are dispersed to the lower branches of an

infested host by gravity, Psittacanthus is typically

dispersed by birds feeding on fruits and defecating on

branches. Incubation and production of the first flowers require several years. Once established, however,

the infection is perennial, and the mistletoe produces

a large haustorium with many long branches. Although Psittacanthus does photosynthesize, it is a

A

cm

B mm

Figure 2-1—Psittacanthus angustifolius, A habit,

leaves and flowers and B tip of petal. Illustration

courtesy of Job Kuijt, edited from figure 10 in

Annals Missouri Botanical Garden. 74:524.

parasite, and when it becomes large, it seriously interferes with host growth and reproduction.

Some of the Psittacanthus features that enhance

bird-dispersal are time and duration of maturation,

fruit size and attractiveness, adaptations for passage

through the digestive tract, adhesion, and rapid germination after being voided (see Kuijt 1969a). Watson

(2001) reviews the literature on coevolution of mistletoes and associates. Salas (1988) reports a study of

Psittacanthus dispersal by birds at three sites in

Michoacán (table 2-1). He observes that only eight out

of 162 captured birds (4.9 percent, two crescent-chested

warblers, a single Audubon’s warbler, and two Bullock’s

orioles) carried mistletoe seeds in their feathers. Typical dispersal of Psittacanthus is for a passerine bird to

feed on the fruit, fly to another tree, and void the seed

to a suitable branch for infection. As with other mistletoes, those factors that influence bird abundance,

distribution, and feeding behavior also affect the

mistletoe’s dispersal, population dynamics, host relations, and evolution (Lopez and Ornelas 1999).

Vázquez (1989) summarizes a 5-year study of

Psittacanthus calyculatus on Pinus douglasiana in

Michoacán. Additional data from that study are reported here (fig. 2-2) with observations of annual

phenology (table 2-2). Bello (1984) provides photographs of an establishing seed, young plant, developing haustorium, and severely infested tree.

Psittacanthus fruits are large (2.0 by 2.5 cm), and

seeds have a sticky (viscous) layer that easily adheres

them to a branch. When the basal portion of a mature

seed makes contact, the seed germinates, opens its

large cotyledons, and establishes an infection. Then 5

months later, the first true leaves are produced. Vegetative growth with more leaves and branches continues throughout the first year. Although shoot growth

is determinate, the plant branches dichotomously expand its total length over the first 3 years at a rate of

30 cm per year. In May of the fourth year, shoot

terminals begin producing flower buds. Full flowering

is reached in 6 months; pollination occurs in November and December. The usual pollinators for most

species are thought to be hummingbirds; but Freeman

and others (1985) suggest passerine birds are the

principal pollinators of P. calyculatus in Sinaloa.

Senescing flowers are shed from November through

March of the fourth year. Fruit maturation requires

about 1 year and occurs from November to February of

the fifth year. A generation therefore requires on

average about 5 years to complete. Mature plants

continue to flower and grow each year with an annual

phenology that varies by host and elevation. On Pinus

douglasiana at 1,700 m above sea level, full flowering

occurs in November; on P. pseudostrobus at 2,400 m,

flowering is delayed 3 months. Although an infection

begins as a small plant growing on a host branch, it can
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 11

Psittacanthus in Mexico Vázquez and Geils

Table 2-1—Dispersal of Psittacanthus calyculatus at three sites in Michoacán, Mexico, for three

guilds of bird species.

Site

Guild Species Canoa alta Capácuaro Cicapien

Insectivore

a Flycatcher X - -

Audubon’s warbler X X X

Hermit warbler - - X

Common yellowthroat X - -

Black and white warbler - X -

Painted redstart X - X

Gray-sided chickadee - - X

Olive warbler - X X

Bushtit - X X

White-breasted nuthatch - - X

Warbling vireo X - -

Hutton’s vireo - - X

Crescent-chested warbler X - X

Wilson’s warbler - - X

Omnivore

Bullock’s oriole X X X

Gray silky-flycatcher X - X

American robin - X X

Granivore

Rufous-capped brush-finch - X X

Black-headed grosbeak - X X

Source: Salas (1988).

Figure 2-2—Life cycle of Psittacanthus calyculatus, from observations by Vázquez (1989) over a

5-year period. Year 1: A October,

fruit matures; B November, infection; C November, cotyledons appear,

D April, leaf buds appear; E October, leaves sprout; F October,

leaves develop. Year 2: G continued

vegetative growth. Year 3: H additional shoots develop. Year 4: I November, flowering. Year 5: J November,

fruits mature.

1

2

3

4

5

A

G

F

E

D

C

B

H

I

J
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Vázquez and Geils Psittacanthus in Mexico

ultimately replace the entire terminal portion of the

host branch. With the vegetative growth of a single

plant and sexual reproduction generating new plants,

an infestation can eventually take over most of a tree’s

crown. Vázquez (1986) suggests a four-class system

for rating infestation severity.

Kuijt (1967, 1970) describes the interesting anatomy

and morphology of seedlings, seedling establishment,

and the haustorium of Psittacanthus. Seedlings and

the haustorium have several particular features that

help construct a phylogeny for the genus (Kuijt 1983).

The haustorium in Psittacanthus becomes quite large

and is even harvested as a specialty product (see

below).

Description of Genus ____________

Psittacanthus

Mistletoe, parrot-flower, muérdago verdadero

Shrubby parasites of trees and other woody plants;

stems brittle, erect, cylindrical or square, frequently

ridged; epicortical root absent, primary haustorium

often large; phyllotaxy opposite or whorled, leaves

large (maximum 1 m in length), leathery or fleshy,

green, persistent, opposite; leaf blade cordate, obovate, oval or lanceolate; leaf apex pointed; floral bracts

short and stout; flower bisexual, six-partite, with a

tubular perianth, 3 to 8 cm long, yellow, red, or orange,

smooth; stamens dimorphic, as numerous as perianth

lobes; anthers rarely more than 6 mm; ovary inferior,

one-chambered; fruit berry, elliptical, green initially,

developing into black or dark brown seed; endosperm

lacking or apparently so (Standley 1920 but see Kuijt

1983).

Psittacanthus is endemic to the New World and

ranges from Mexico to Argentina (Cházaro and Oliva

1988b). According to Reséndiz and others (1989),

Table 2-2—Phenology of Psittacanthus calyculatus on Pinus douglasiana.

Month a

Stage b Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

V1 X X - - - - - - - - - -

V2 X X X - - - - - - - - -

V3 X X X X X X X X X - - -

FL1 X X X X X X X X ----

FL2 X X - - - - - - - - - -

FL3 X X - - - - - - - - - -

FR1 X X X X --------

FR2 X X X X X X X X X X X X

FR3 X X X X --------

a

 Month during which stages of life cycle are evident.

b

 V1 = early vegetative, V2 = full vegetative, V3 = final vegetative, FL1 = early flowering, FL2 = full flowering, FL3 = final flowering, FR1 = early

fruiting, FR2 = full fruiting, FR3 = final fruiting.

Psittacanthus is found in 25 Mexican States, absent

only from Aguascalientes, Coahuila, Chihuahua,

Hidalgo, Nuevo León, Tamaulipas, and Distrito Federal. Although Psittacanthus is distributed throughout Mexico, it is most common in the Central and

Southern regions. Most species of Psittacanthus have

broad host ranges on numerous woody hardwoods;

some include conifers among their hosts. On conifers,

Psittacanthus is limited to the cool temperate forests

where conifers occur. The elevational distribution in

Mexico ranges from 800 m on Pinus oocarpa to 3,300 m

on P. rudis.

The taxonomy of Psittacanthus is quite confused; a

comprehensive, monographic treatment would greatly

benefit our understanding of the genus. Because of

misidentification or subsequent taxonomic revision,

numerous reports and publications refer to mistletoes

using names that do not properly apply; sufficient

information to identify the subject mistletoe is seldom

given. These taxonomic difficulties are being overcome with projects such as Flora Mesoamericana

(2002). Managers and researchers can reduce the

confusion in the future by filing voucher specimens

with a recognized herbarium for identification and

future reference.

Standley (1920) initially describes only seven species of Psittacanthus for Mexico; Reséndiz and others

(1989) later recognize 14 species for the country. Host

data are frequently absent or sparse (genus only) on

herbarium labels, but Reséndiz and others (1989)

compile available data for Mexico from numerous

collections. They report as hosts of Psittacanthus more

than 50 genera of angiosperms and conifers, including

trees, shrubs, and cactus. The primary angiosperm

hosts are Quercus, Acacia, Juglans, Ficus, Populus,

Salix, Prunus, Prosopis, Annona, Bursera, Citrus,

Nerium, Olea, Crataegus, Bacharis, Fraxinus,

Eucaliptus, Persea, Cassuarina, Pseudosmondingium,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 13

Psittacanthus in Mexico Vázquez and Geils

Arbutus, Ulmus, Liquidambar, Psidium, Spondia,

Phitecellobium, Amphipterigium, Pyrus, Mimosa, and

Cydonia. No monocots are known to be parasitized.

Psittacanthus throughout its range is reported for

conifers mostly on pine (table 2-3). In Mexico, the most

common Psittacanthus species on conifers are P.

calyculatus and P. macrantherus (Bello and Gutierrez

1985). Mathiasen and others (2000c) first report P.

angustifolius on pine in Southern Mexico; this and

other species (for example, P. pinicola) may be more

widely distributed than apparent from the literature.

Description of Species ___________

Only several species of Psittacanthus are reported

as parasites of conifers in Mexico. Because of taxonomic uncertainty and the recent discovery of a new

species for Mexico, we include in addition to frequently

reported mistletoes several other species known or

suspected to infect conifers in Mexico.

1. Psittacanthus americanus

Psittacanthusamericanus (Mart.), Flora 13:108, 1830.

=Loranthus americanus L

Description. Shrub 1 m tall; shoots erect and spreading, square or more or less angular, smooth; leaves

fleshy, ovate, elliptical, rounded, 6 to 10.5 cm long by

3 to 6.5 cm wide, apex very obtuse; petiole short;

perianth 6 cm long, bright red or orange; fruit berry,

0.8 by 1.0 cm, green initially, developing into reddish

brown (Bello and Gutierrez 1985, Standley 1920).

Discussion. Vázquez and others (1986) refer to

Psittacanthus americanus as abundant on Pinus

leiophylla, P. teocote, and P. montezumae at sites in

Michoacán. Standley (1920) adds Guerrero, and Bello

and Gutierrez (1985) add Chiapas and Veracruz to the

distribution. Reséndiz and others (1989) describe the

species as having the smallest elevational range of the

Mexican species. Kuijt (personal communication),

however, reserves the name Psittacanthusamericanus

for a mistletoe of the Lesser Antilles that is not found

in North or Central America. The collections of

Psittacanthus americanus from Mexico should be reexamined.

2. Psittacanthus angustifolius

Psittacanthus angustifolius Kuijt, Ann. Missouri Bot.

Gard. 74:523–525, 1987.

Description. Stems sharply angular; leaves paired,

narrow, thin, 17 by 2.5 cm, base acute, apex attenuate;

petiole to 5 mm long; inflorescences terminal, of four to

six triads (groups of three); triad peduncles about 1 cm

long, lowest with bracts to 2 cm long; bud stout,

straight or somewhat curved; petals orange, 7.5 to 8

cm long, petal apices 4 mm wide, blunt, each with a

Table 2-3—Conifer hosts of Psittacanthus as reported in examined literature.

Host species Reference

Abies religiosa Bello (1984), Bello and Gutierrez (1985)

Cupressus sp. MartÌnez 1983

Pinus caribaea var. hondurensis Kuijt (1987), Mathiasen and Howell (2002)

Pinus douglasiana Bello (1984), Bello and Gutierrez (1985), Vázquez (1989)

Pinus lawsonii Bello (1984)

Pinus leiophylla Bello (1984), Bello and Gutierrez (1985), Gibson (1978),

Vázquez (1989), Vázquez and Pérez (1989),

Vázquez and others (1982, 1985, 1986)

Pinus maximinoi Mathiasen and others (2000b)

Pinus michoacana Bello (1984), Bello and Gutierrez (1985)

Pinus montezumae Vázquez (1989), Vázquez and Pérez (1989),

Vázquez and others (1982, 1985, 1986)

Pinus oocarpa Mathiasen and others (2000b)

Pinus oocarpa var. ochoterenia Mathiasen and others (2000c)

Pinus pseudostrobus Bello (1984), Gibson (1978)

Pinus tecunumanii Melgar and others (2001)

Pinus teocote Bello (1984), Bello and Gutierrez (1985), Vázquez and Pérez (1989),

Vázquez and others (1982, 1985, 1986)
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Vázquez and Geils Psittacanthus in Mexico

fleshy ligule-like median crest extending inwards (see

fig. 2-1); anthers 6 mm long (Kuijt 1987).

Discussion. Kuijt (1987) only reports the host as

pine, but the reported host range now includes Pinus

caribaea var. hondurensis, P. oocarpa, P. oocarpa var.

ochoterenia, P maximinoi, P. tecunumanii, and Psidium

guineese (Mathiasen and Howell 2002, Mathiasen and

others 2000b, Mathiasen and others 2000c, Melgar

and others 2001). The mistletoe is known from Nicaragua, Belize, Honduras, Guatemala, and Mexico

(Chiapas). Although the mistletoe appears to be more

common and damaging in Central America (Mathiasen

and others 2000c), it has only recently been described,

and new populations are being discovered. With its

wide host range, it may be more common in southern

Mexico than presently reported.

3. Psittacanthus calyculatus (sensu lato)

Psittacanthus calyculatus (DC.) G. Don, Gen. Syst.

3:415, 1834.

Psittacanthus rhynchanthus (Bentham) Kuijt, Ann.

Missouri Bot. Gard. 74:529, 1987

=P. chrismarii

Description. Shrub 1.0 by 1.5 m tall, herbaceous

initially but becoming woody; stems green, quadrangular or ridged when young; leaves dark green, 5 to 14

cm long by 1.4 by 6 cm wide, leathery, lanceolate or

elliptical to ovate, smooth; leaf blade asymmetric,

margin undulating, with long attenuate apex and

cuneate base, venation pinnate and prominent; inflorescence terminating the shoot; flower buds strongly

incurved, 4 cm long, tip acute, base dilated, on peduncles up to 2 cm long, bracts fused to cup-like

structure, in triads, perianth 3 to 5 cm long, red to

orange, smooth; stamens as numerous as perianth

lobes; fruit berry, 2.5 cm long by 2 cm wide, glabrous,

with flaring calyculus (Bello and Gutierrez 1985,

Hernandez 1991). Bello (1984) and Cházaro and Oliva

(1988a) provide brief descriptions and illustrations.

Kuijt (1987) recognizes two similar taxa of

Psittacanthus initially described as Loranthus

calyculatus and L. rhynchanthus. He applies the name

P. calyculatus to the Mexican species (Puebla and

Morelia) in which the mature, unopened bud is nearly

straight with a blunt tip, and the name P. rhynchanthus

to a lowland, Mesoamerican (to Venezuela) species in

which the bud is distinctively curved and beaked. He

describes a number of additional characteristics that

distinguish the two, such as symmetrical leaves 8 by 4

cm for P. calyculatus and asymmetrically curved,

larger leaves 12 by 4 cm for P. rhynchanthus. Kuijt

(1987) does not mention any host preference differences, but given the southern and lowland distribution of P. rhynchanthus, we suspect the more common parasite of conifers in Mexico is P. calyculatus.

These differences, however, can only be resolved by

examination of voucher specimens in light of Kuijt’s

interpretation of the type material.

Discussion. Bello (1984) lists the conifer hosts of

Psittacanthus calyculatus as Abies religiosa, Pinus

douglasiana, P. lawsonii, P. leiophylla, P. michoacana,

P. pseudostrobus, and P. teocote. Vázquez here adds

Pinus montezumae, P. herrerai, P. pringlei, and P.

rudis and describes this as the species with the most

number of conifer hosts, largest distribution, and most

importance. Bello and Gonzales (1985) locate the mistletoe (without host distinction) as from Tamaulipas to

Jalisco, Chiapas, Yucatan, Oaxaca, Valley of Mexico,

Guanajuato, Morelia, and Michoacán. Freeman and

others (1985) add Sinaloa, and Hernandez (1991) adds

Tlaxcala. The mistletoe in Michoacán is widespread,

mostly found in the subhumid temperate zones, from

1,300 to 2,750 m (Bello and Gonzales 1985). In natural

stands of P. leiophylla and P. pseudostrobus in

Michoacán, Gibson (1978) observes the mistletoe has

a patchy distribution and some sites are severely

infested. Vázquez (1989) describes the life cycle and

phenology of this mistletoe. Vázquez (1994b) and

Vázquez and others (1986) discuss control.

4. Psittacanthus macrantherus

Psittacanthus macrantherus Eichl., Mart. Flora

Brasilense. 5(2):26, 1868.

Description. Shrub 1.0 m tall; shoots stiff, brown,

cylindrical, glabrous; leaves 6 to 7.5 cm long, fleshy,

elliptical, obovate, margin entire, apex obtuse, base

attenuate; perianth 5.5 to 6.5 cm long, yellow or

orange, large; anthers 18 mm long, as numerous as

perianth lobes; fruit berry, green, glabrous (Bello and

Gutierrez 1985, Standley 1920).

Discussion. Bello and Gutierrez (1985) only identify the hosts as pine and fir; but Vázquez (here)

describes the pine hosts as Pinus engelmannii, P.

herrerai, P. lawsonii, P. lumholtzii, P. oocarpa, and P.

pseudostrobus. The mistletoe occurs locally in the

Sierra de San Pedro Nolasco, Jalisco (Cházaro 1989b),

Oaxaca and Michoacán (Bello and Gutierrez 1985),

and Sinaloa (Gentry 1946). It ranges in elevation from

1,300 to 2,200 m. It is the second most important

Psittacanthus on conifers in Mexico.

5. Psittacanthus pinicola

Psittacanthus pinicola Kuijt, Ann. Missouri Bot. Gard.

74:525–529, 1987.

Description. Stems terete, becoming fissured and

black with age; leaves symmetrical, in irregular whorls

of three 11 by 2.5 cm, elliptical to lanceolate; apex

rounded, base tapered; inflorescences lateral, axillary, often on older leafless stems, an umbel of two or

three dyads (groups of two); petals 4 cm long, red with

yellow-green tip, orange in middle, ligulate at base;

buds inflated at ovary to 5 mm, tapering to slender,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 15

Psittacanthus in Mexico Vázquez and Geils

curved tip at 1.5 mm; anther 3 to 4 mm long (Kuijt

1987, includes two illustrations).

Discussion. This attractive mistletoe is distinguished by the combination of parasitism on pine

(Pinus caribaea) and inflorescences composed of pairs

of flowers. The species is known from Central America

at elevations below 650 m; it appears not to cause

serious damage (Mathiasen and Howell 2002). Although we are aware of no collections from Mexico,

other mistletoes (namely Arceuthobium hondurense)

have recently been found to have widely disjunct

distributions from Honduras to Mexico.

6. Psittacanthus schiedeanus

Psittacanthus schiedeanus (Schltdl. & Cham.) Blume,

Sys. Veg. 7(2):1730, 1830.

Description. Shrub large, to 50 cm; stems, sharply

quadrangular and four-winged until large lenticels

develop; nodes flattened; haustorium very large; leaves

bluish-green, 20 cm long by 8 cm wide; leaf blade

asymmetric, ovate 6 to 16 by 1.4 by 4.5 cm; apex

attenuate; petiole distinct and stout; venation pinnate; inflorescence terminal, leafless, forked; flowers

6.5 to 8 cm long in bud, on peduncles 1.5 to 2 cm long,

perianth orange, 3 to 5 cm long, segments linear,

separated to base, recurved; stamens dimorphic, very

slender; fruit berry, 1.5 cm long by 1 cm wide (Bello

and Gonzales 1985, Standley 1920). Bello (1984),

Cházaro and Oliva (1988a), and Hernandez (1991)

provide illustrations. Kuijt (1967) describes seedling

structure and development in great and illustrated

detail.

Discussion: The hosts most commonly reported for

Psittacanthus schiedeanus are oaks and other hardwoods (Bello 1984, Lopez and Ornelas 1999). Vázquez

and others (1982) name Pinus leiophylla, P.

montezumae, and P. teocote as important, damaged

hosts in Michoacán. Collections from Honduras (EAP)

extend the hosts to include P. oocarpa. Standley (1920)

reports this mistletoe as occurring in Central America

and Mexico from Veracruz to Michoacán and Oaxaca.

Hernandez (1991) describes its distribution in Tlaxcala,

and it is collected from Chiapas (Flora Mesoamericana

2002).

Damage and Effects on Host ______

Damages produced by Psittacanthus to pine hosts

include reductions of diameter increment, cone production, and seed viability. Vázquez and others (1982,

1985) report a series of studies from Michoacán to

determine the effects of Psittacanthus to Pinus

leiophylla, P. montezumae, and Pinus teocote.

Vázquez and others (1982, 1985) observe that the

reduction in diameter increment for trees infected by

Psittacanthus varies by host species and size class.

The diameter increment of infected Pinus leiophylla

trees is only 10 percent of uninfected trees (0.7:7.0 mm

per year). The diameter increments of infected P.

montezumae and P. teocote are both 47 percent of

uninfected trees of the species, although the two species grow at different absolute rates (0.2:5.3 mm and

0.7:1.5 mm per year, respectively). Increment losses

are greatest in the 20-cm diameter class for P. leiophylla

and P. montezumae and in the 40-cm class for P.

teocote. Reduction in diameter increment can also be

expressed as loss in productivity or volume. Reduced

volume production by infected P. leiophylla corresponds to half the annual productivity of 127 trees per

ha or 0.0186 m3

 per tree per year. Infected P.

montezumae lose the equivalent of 0.0843 m3 per tree

per year; infected P. teocote lose 0.0150 m3 per tree per

year. In terms of growth, P. montezumae is the species

most severely impacted.

Vázquez (1986) uses a four-class rating system

(table 2-4) to stratify Psittacanthus-infected trees by

disease severity and to assess the effects on reproductive potential (Vázquez and Pérez 1989). They observe that severely infected trees of Pinus montezumae

and P. teocote fail to produce cones, and P. leiophylla

produces 23.8 percent fewer cones. Moderately infected trees of P. montezumae produce 37.5 percent

fewer cones, and moderately infected P. teocote produce 19.4 percent fewer cones. No reduction in cone

production is noted for moderately infected P.

leiophylla or lightly infected trees of any species.

They also note an effect on seed germination. Seeds

from severely infected P. leiophylla exhibit only a 67

percent germination rate. Seed germination from

moderately infected trees is reduced 25 percent for P.

montezumae and 5 percent for P. teocote. In terms of

reproductive loss, P. montezumae is the species most

severely impacted.

Economic and Ecological

Importance _____________________

Although Psittacanthus is established as a widespread and damaging parasite of conifers in Mexico, it

is also important for medicine, crafts, and wildlife

Table 2-4—Four-class rating system for evaluating severity of diseases caused by Psittacanthus.

Disease Percent of

index Infection class crown infected

0 uninfected 0

1 light 1–30

2 moderate 31–60

3 severe 61–100
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Vázquez and Geils Psittacanthus in Mexico

(Cházaro and others 1992). Vázquez and others (1982,

1985) and Vázquez and Pérez (1989) document the

impacts of Psittacanthus on conifer growth and reproduction. Martinez (1983) reports that 3,396 ha of

Pinus lumholtzii, P. montezumae, P. leiophylla,

Cupressus, Quercus, and Alnus in Jalisco, Mexico, and

Michoacán are infested by Psittacanthus. Over most of

its extensive range, however, Psittacanthus appears

to occur as small patches of a few infected trees.

Traditional medicines are produced from the mistletoe; Browner (1985) identifies some of these uses in

Oaxaca. The large haustorium of an old Psittacanthus

infection causes distorted growth of the host branch

into an interesting form resembling a rose or similar

flower after the mistletoe tissue is removed. Artisans

use these woodroses to produce lamp stands and other

decorative, craft items (Cházaro and others 1992).

These mistletoes are also used and are important to

numerous birds for nectar and fruit (Freeman and

others 1985, Lopez and Ornelas 1999, Salas 1988).

Management Strategies __________

The Psittacanthus mistletoes are easily detected,

obligate parasites, with long life cycles, and slow rates

of spread and intensification. Because of these attributes, economic control is generally feasible. Chemical and silvicultural methods are used for mistletoe

control; some biological control occurs naturally but

has not been developed as management tool (Cházaro

and others 1992, Hernandez 1991).

Biological Control

The principal insects that feed on Psittacanthus

belong to the order Homoptera, including scale insects

Coccus, Saccharicoccus, Gascardia, and Aenidomytilus,

and the aphid Macrosiphum (Vázquez and others

1986). These homopterans feed exclusively on plant

sap, infesting leaves, branches, flowers, and fruits. A

heavy infestation weakens and may eventually kill a

host mistletoe plant. Macrosiphum has the best potential as a biological control agent because aphids are

excellent vectors of viruses (Horst 2002), which are

themselves agents of biological control.

Vázquez and others (1986) report isolating the fungi

Alternaria, Ceratocystis, and Fumago from

Psittacanthus. The disease caused by Alternaria (see

García 1977, Horst 2002) in Psittacanthus produces

leaf spot of older leaves and blight of young branches.

The fungus spreads quickly during wet periods and

induces concentric dark lesions, which lead to extensive necrosis of mistletoe leaves and shoots. Because

Ceratocystis causes much damage and is readily cultured, it has a good potential as a biological control

agent. Fumago causes blights and sooty molds; these

fungi are very common in tropical and subtropical

agriculture around the world. The development of

Fumago is promoted by the secretions of some Homopterans (aphids and scales). The resulting disease

and infestation can produce reactions in the host plant

similar to symptoms caused by mistletoe itself; severe

leaf infestations reduce photosynthesis and therefore

growth (García 1977, Horst 2002).

Chemical Control

Few studies for chemical control of Psittacanthus on

conifers are published (for example, Vázquez 1994b).

Vázquez and others (1986) describe an experiment in

Michoacán on Psittacanthus calyculatus and P.

americanus infecting Pinus leiophylla. They report 1-

month and 6-month evaluations of commercial application of four herbicides: two 2,4-D derivatives (Esterón

and Fitoamina), one pyridine (Gramoxone), and one

urea derivative (Karmex). At neither observation time

did Karmex appear to damage the mistletoe. At 1

month, Gramoxone appears to provide excellent control with high mortality rate (80 percent) of fruits,

leaves, and branches and slight transient phytotoxicity in the pine. At 6 months, however, the mistletoe

treated with Gramoxone recovers and produces new

vigorous buds, flowers, and fruits. Therefore,

Gramoxone only causes a temporary delay in mistletoe development. At 1 month, Fitoamina causes severe

damage, 40 percent defoliation, to mistletoe leaves

and tender buds but has less effect on the mistletoe

fruits. At 6 months, Fitoamina affects 80 percent of the

mistletoe with defoliation, leaf spotting, and fruit

deformity. At 6 months, Esterón causes complete defoliation of the mistletoe and failure to set fruit. The 2,4-

D derivative herbicides are the more effective chemical control agents, but their use must be consistent

with local regulations.

Silviculture

Several silvicultural practices are useful for controlling Psittacanthus in severely infested stands. The

appropriateness of a given method depends on numerous factors, including stand type and location, infection intensity, management objectives, and constraints.

Sanitation, intermediate thinning, shelterwood, and

clearfelling are available techniques. Sanitation consists of removing severely infected trees and leaving

light and moderately infected trees. Periodic examinations are made to monitor disease intensification;

trees are removed as they become heavily infected.

Sanitation is usually conducted at the time of intermediate thinnings, but if intensification is rapid

relative to the thinning cycle, early removal may be

considered. During shelterwood regeneration cuts,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 17

Psittacanthus in Mexico Vázquez and Geils

mistletoe-infected trees are generally removed and

not used as seed trees. Lightly infected, genetically

superior trees are occasionally retained to provide

seed and then removed after 5 years. Where more than

75 percent of trees are infected and most are severely

infested, clearfelling is usually employed for stand

regeneration. Replacement with species less damaged

or resistant to mistletoe infection can be considered.

As with all mistletoes, the first decision on control is

whether it is appropriate given the management objectives of the stand, values produced in the stand, and

available options for treatment.

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 19

B. W. Geils

I

3

D. Wiens Chapter F. G. Hawksworth

The generally familiar mistletoes are the leafy Phoradendron

that typically infest hardwood trees and are placed at doorways for

winter celebrations. Several of these mistletoes, however, more

resemble the dwarf mistletoes by their apparently leafless stems

and presence on conifers; but their large, fleshy berries that are

attractive to birds clearly identify them as Phoradendron, “the

tree thief.” For several reasons, the conifer-infecting Phoradendron

(the group reviewed here) have not gotten the level of attention

from forest managers that the Arceuthobium have. Phoradendron

most typically cause slight damage to junipers in Southwestern

woodlands. But these mistletoes include a number of different

species, range from Oregon to Mexico, infect a variety of hosts, and

provide an interesting model of host–parasite interactions.

Along with the other mistletoes, the taxonomy, biology, physiology, and ecology of the Phoradendron are reviewed by Gill and

Hawksworth (1961) and Kuijt (1969a). Foresters have long been

interested in the Phoradendron on incense cedar (Meinecke 1912).

Phoradendron in

Mexico and the

United States
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils, Wiens, and Hawksworth Phoradendron in Mexico and the United States

Picture guidebooks are available by Walters (1978) for

the Southwest and by Scharpf and Hawksworth (1993)

for the Pacific States. General summaries of information including control are provided by Hawksworth

and Scharpf (1981), Hernandez (1991), and Vega

(1976). The taxonomy of the Phoradendron on conifers was first monographed by Trelease (1916) and

then revised by Wiens (1964). Confusion over the

position and ranking of several taxa (Hawksworth

1979) and recent evidence from molecular systematics (Ashworth 2000) suggest the group requires another taxonomic monograph.

Life History and Biology __________

The Phoradendron have a typical mistletoe life cycle

characterized by bird dispersal of sticky seeds, internal parasitism of a woody host, and aerial shoots for

flower and fruit production. Phoradendron biology,

reproduction, and parasitism have received intense

study from ecological and evolutionary perspectives.

Life Cycle

The most detailed but still comprehensive study and

review of a Phoradendron life cycle is that by Felix

(1970a) for the mistletoe on Abies concolor. A number

of bird species feed on the mistletoe fruits and disperse

seeds by excreting or regurgitating them. The most

important birds for effective dispersal include cedar

waxwings, euphonias, silky flycatcher, bluebirds,

thrushes, robins, and solitaires (see Sutton 1951, Gill

1990). Seeds pass quickly, and because birds perch in

trees, seeds are deposited at suitable sites for infection

(often the top or warm side of a potential host, commonly one already infected). Germinating seeds produce a radicle, holdfast, and penetrate the host branch

usually near a needle (Ruhland and Calvin 2001). The

endophytic system consists of longitudinal strands

and sinkers (Calvin and others 1991, Felix 1970a).

Once aerial shoots are produced, the Phoradendron

does photosynthesize, but it is a parasite (see below)

not a simple epiphyte. Plants are either male or

female. Although Dawson and others (1990a) report a

male-bias and provide hypotheses why there might be

such a bias, Daughtery and Mathiasen (1999) find the

sex ratio is one to one. Flower production and reproduction is typical, except that natural hybridization

occurs but rarely (Wiens and Dedecker 1972). Fruits

are produced several years after infection; older plants

produce more and larger fruits (Dawson and Ehleringer

1991, Dawson and others 1990a, 1990b). Although the

shapes of shoots and leaves of Phoradendron might

mimic their host, the biology behind the appearance is

unclear (Atsatt 1993b). Endophytic systems are

perennial, but plants of all ages are subject to mortality by extreme low temperatures (Wagener 1957).

Host–Parasite Physiology

The physiology of Phoradendron is generally reviewed by Fisher (1983), Knutson (1983), and Kolb

(2002) as typical for most mistletoes (but different

from that of the dwarf mistletoes). Many details of the

nitrogen, carbon, and water relations of the “xylemtapping” Phoradendronjuniperinum are elucidated in

a series of recent ecophysiology studies (Ehleringer

and others 1986, Marshall and Ehleringer 1990,

Marshall and others 1993, 1994). The mistletoes do fix

some carbon but get much from their host; mistletoes

transpire a lot of water (all from their host); and they

get a lot of nitrogen from the host, as the inevitable

consequence of the physiological relation (Marshall

and others 1994). Lei (1997) concludes that heavy

mistletoe infection increases host-plant water stress

and reduces vigor, viability, and reproductive success

of the host in favor of the mistletoe. McHenry (1934)

reports that Phoradendron at the Grand Canyon kills

juniper. Hawksworth and Wiens (1966) indicate that

junipers may also form witches’ broom in response to

infection.

Host–Parasite Ecology

Numerous authors have studied the effects of

Phoradendron parasitism on population dynamics and

interactions with other species. Hreha and Weber

(1979) compare pinyon dwarf mistletoe (Arceuthobium

divaricatum) and juniper mistletoe (Phoradendron

juniperinum) at the South Rim of the Grand Canyon,

reporting more infection in bigger (older) trees, a lack

of infestation in recently burned (young) areas, and

general stability in the populations. They consider the

pinyon dwarf mistletoe the more detrimental because

it more readily kills its host. Juniper mistletoe often

has a patchy distribution, with some trees heavily

infested. Gregg (1991) concludes the critical difference

between infested and uninfested sites is the need for a

dependable moisture supply to maintain the high

demand of infected trees. Moisture stress in firs infected by Phoradendron is associated with reduced

resistance and increased successful attack by the fir

engraver (Felix 1970a, Ferrell 1974). Gehring and

Whitham (1992) report that on droughty sites, juniper

severely infested by mistletoe have lower rates of

beneficial mycorrhiza infection, and female junipers

are more seriously affected than male junipers. Another three-way, conditional interaction is described

by van Ommeren and Whitham (2002) for juniper,

mistletoe, and their avian dispersers. Although mistletoe has a negative impact on the health of infected
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 21

Phoradendron in Mexico and the United States Geils, Wiens, and Hawksworth

trees, mistletoe provides a more dependable food source

than juniper for the shared avian dispersers of each

species’ seeds. By considering dispersal and reproductive success in the interaction model, they conclude

that the net effect of mistletoe parasitism, depending

on populations, shifts between favoring the mistletoe

to favoring the juniper.

The Phoradendron also exhibit a curious phenomenon whereby a mistletoe is parasitized by another

mistletoe of the same or different species (Hawksworth

and Wiens 1966, Wiens and Calvin 1987). Although

several of the conifer-infecting Phoradendron may be

infected by other Phoradendron mistletoes (Felix

1970b), the occurrence is too rare to effect a significant

biological control.

Description of Genus ____________

Phoradendron

Mistletoe, injerto

Shrubby parasite of trees and other woody plants

(fig. 3-1), woody at base, glabrous or hairy; shoots

cylindrical, green or less often reddish, 20 cm in

height, nodes constricted, lacking epicortical roots;

phyllotaxy opposite or decussate; inflorescence a spike;

perianth generally three-parted and persistent; anthers small, sessile and bilocular; fruit a berry, 3 to 6

mm, single color of white, pink, or reddish. A genus of

about 200 species in temperate and tropical America.

Type species: Phoradendron californicum Nuttall.

The genus Phoradendron, the largest genus of mistletoes, is in serious need of definitive taxonomic study.

Trelease (1916) is the first monographic study of the

genus, but Wiens (1964) revises many of Trelease’s

taxa that occur on conifers (see table 3-1). This taxonomy is further reconsidered by Hawksworth and

Wiens (1993a, 1993b) for the flora of California, Nevada, and Arizona. Other floristic treatments with

keys and illustrations include Hitchcock and others

(1964) for the Pacific Northwest, McMinn (1939) for

California, Standley (1920) for Mexico, and Wiggins

(1980) for Baja California. Kuijt (1996) questions the

significance and interpretation of many of the traditional morphological features used for the taxonomy of

Phoradendron. Ashworth (2000) provides much insight to the relations among these mistletoes using

techniques of molecular systematics.

Most of the taxa of Phoradendron that parasitize

conifers are in the section Pauciflorae as described by

Wiens (1964). Ashworth (2000) reports that these taxa

(see table 3-1), delimited by morphological features,

also form a single, related cluster based on molecular

features. Phoradendronrhipsalinum is also a parasite

of conifers but is apparently more closely related to a

number of Phoradendron that typically infect hardwoods. A few other Phoradendron reportedly occur on

conifers, but these are either rare cases or possibly

misidentifications. Usual conifer host genera are Abies,

Calocedrus, Cupressus, Juniperus, Taxodium (table 3-2).

Martínez (1948) illustrates Phoradendron velutinum

on Pinus leiophylla in Pueblo, but this is a rare host for

a mistletoe with a wide host range, and we know of no

other reports on pines.

In 1990, Wiens and Hawksworth drafted a further

revision of Phoradendron section Pauciflorae that has

never been completed and published (copy on file at

Flagstaff Laboratory, Rocky Mountain Research Station). In that draft, they proposed recognizing seven

new species on American conifers and raising to specific rank a previously submerged taxon not now

generally accepted at that level. Although we do not

describe any new species in this publication, we do

suggest here that there may be differences in some

populations that warrant recognition at the specific

level. We leave it to other taxonomists using morphological and molecular techniques to judge these opinions. Specimens for these “ined” taxa are assembled

for deposition at the U.S. National Herbarium.

Figure 3-1—Phoradendron (all scales represent 5 mm). A, P. juniperinum on host; B, P.

juniperinum with young fruit; C, P. densum on

host; D, young fruits of P. densum. Adapted from

Hitchcock and others (1964), p. 99, drawing by

J.R. Janish. Our C and D originally labeled as P.

bolleanum in figure, but accompanying text suggests figure represents P. bolleanum var. densum

(Torr.) Fosberg, which we recognize as P.

densum.

A

B C

D
22 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils, Wiens, and Hawksworth Phoradendron in Mexico and the United States

Description of Species ___________

1. Phoradendron bolleanum

Bollean Mistletoe

Phoradendron bolleanum (Seem.) Eichl., Mart. Fl.

Bras, v. II. 134.

Description. Plants 20 to 30 cm high, reddish;

internodes 10 to 20 mm long; leaves spatulately linearelliptical, 8 to 22 mm long and 1 to 4 mm wide, sessile,

apex acute reddish, densely hairy; mature fruit is

about 4 mm diameter, white to straw-colored (Trelease

Table 3-1—Taxonomic history of Phoradendron that parasitize conifers.

Trelease (1916) Wiens (1964) Accepted here

P. bolleanum Eichler P. bolleanum subsp. bolleanum P. bolleanum (Seem.) Eichl.

P. tequilense n. sp.

P. capitellatum Torrey n. sp. P. capitellatum Torr. ex Trel. P. capitellatum Torr. ex Trel.a

P. densum Torrey n. sp. P. bolleanum subsp. densum P. densum Torr. ex Trel.a,b

P. guadalupense n. sp.c (Torr.) Wiens, comb. nov.

P. saltillense n. sp. P. saltillense Trel.d

P. juniperinum Engelmann P. juniperinum subsp. juniperinum P. juniperinum Engelm. ex A. Graya,b

P. ligatum n. sp.

P. libocedri Howell P. juniperinum subsp. libocedri P. libocedri (Engelm.) T.J. Howellb

(Engelm.) Wiens, comb. nov.

P. longifolium Eichler P. longifolium Eichler ex Trel. P. longifolium Eichler ex Trel.

P. minutifolium Urban P. minutifolium Urban P. minutifolium Urban

P. pauciflorum Torrey P. bolleanum subsp. pauciflorum P. pauciflorum Torr.a,b

(Torr.) Wiens, comb. nov.

P. rhipsalinum Rzed.e

a

Name used in Hawksworth and Wiens (1993a).

b

Name used in Hawksworth and Wiens (1993b).

c

Wiens (1964) indicates this taxon probably now extinct.

d

Referred to by Hawksworth and Cibrián (1985) as P. densum subsp. saltillense (Trel.) Wiens. e

Taxon first recognized in 1972.

Table 3-2—Host range and distribution of Phoradendron on conifers.

Phoradendron Principal hosts Mexico United States

P. bolleanum Juniperus, Arbutus X -

P. capitellatum Juniperus X -

P. densum Juniperus, Cupressus X X

P. juniperinum Juniperus, Cupressus X X

P. libocedri Calocedrus X X

P. longifolium Quercus, Pinus X -

P. minutifolium Juniperus X -

P. pauciflorum Abies X X

P. rhipsalinum Taxodium, Quercus X -

P. saltillense Juniperus, Cupressus X -

Total number of species 10 4

1916, Cházaro and Oliva 1987b with illustration).

This mistletoe is usually characterized by its bright

reddish to brown color; however, some all-green populations have been found in central Chihuahua

(Hawksworth and Cibrián 1985).

Hosts. Usual hosts are Juniperus and Arbutus. This

is the only Phoradendron on Arbutus (A. arizonica and

A. xalapensis); the closely related Arctostaphylos (A.

pungens) is also rarely infected. Vega (1976) lists the

hosts of “Phoradendron bolleanum” as not only

Juniperus and Arbutus (A. unedo) but also Quercus,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 23

Phoradendron in Mexico and the United States Geils, Wiens, and Hawksworth

uncompleted draft. Ashworth (2000) identifies several

sister groups for taxa of the complex: P. bolleanum

subsp. bolleanum groups with P. minutifolium, P.

densum and P. pauciflorum (both previously considered as subspecies of P. bolleanum) group together.

2. Phoradendron capitellatum

Hairy Mistletoe

Phoradendron capitellatum Torr. ex Trel., Genus

Phoradendron 25, pl 17, 1916.

=Phoradendron bolleanum var. capitellatum (Torr.

Ex Trel.) Kerney & Peebles

Description. Plants 30 to 80 cm tall, yellow–green;

internodes 5 to 15 mm long, pendulous with age;

leaves 6 to 14 mm long and 2 to 3 mm wide, densely

hairy with stellate trichomes; flowers December to

February; mature fruit is about 3 mm in diameter,

pinkish–white (Hawksworth and Scharpf 1981with

color picture and 1993a).

Hosts. Known hosts are Juniperus deppeana, J.

pinchotii, J. monosperma, and J. osteosperma. It usually occurs at elevations below Phoradendron

juniperinum, but the two species are sometimes sympatric and rarely even on the same host tree.

Distribution. United States (Arizona, New Mexico)

and Mexico (Chihuahua, Sonora). This mistletoe has a

restricted distribution and is known only in central

and southeastern Arizona (nine counties), southwestern New Mexico (Grant, Hidalgo, Luna), northeastern

Sonora, and northwestern Chihuahua (Hawksworth

and Cibrián 1985). Hawksworth and Scharpf (1981)

map its distribution in the United States and into

Mexico. Known elevational range is 800 to 1,700 m.

Discussion. This highly distinctive parasite of junipers is characterized by its small, densely stellate–

pubescent leaves. It is the only leafy mistletoe known

to occur on junipers in Arizona and is poorly known.

Ashworth (2000) finds that Phoradendroncapitellatum

is the most divergent taxa within the Pauciflorae,

although there is some support for a sister group with

P. juniperinum.

3. Phoradendron densum

Dense mistletoe

Phoradendron densum Torr. ex Trel., Genus

Phoradendron 27, 1916.

=Phoradendron bolleanum var. densum (Torr. ex Trel.)

Fosberg,

=Phoradendron bolleanum subsp. densum (Torr. ex

Trel.) Wiens

Description. Plants 10 to 30 cm high, green (fig. 3-1);

internodes 6 to 17 mm long; leaves 10 to 20 mm long and

2 to 4 mm wide, glabrous, sessile, apex obtuse; mature

fruit is about 4 mm in diameter, white to straw-colored.

(Hawksworth and Scharpf 1981with color photo,

Hitchcock and others 1964 with illustration).

Pinus, and Abies. Given that he lists the distribution

for this mistletoe to include Baja California, we suspect he is including P. pauciflorum and other

Phoradendron under this name. Cupressus benthami

is an unusual host in the pine–cypress woodlands of

Veracruz (Cházaro 1989a).

Distribution. Mexico (Chihuahua, Durango,

Hidalgo, Jalisco, Nayarit, Querétaro, Sonora, Sinaloa,

Veracruz, and Zacatecas). Common in Sierra Madre

Occidental, and Zacatecas to central Chihuahua

(Hawksworth and Cibrián 1985). Cházaro (1989a)

records Phoradendron bolleanum on Cupressus (C.

benthami) in Veracruz. This is the most widespread

Phoradendron on conifers in Mexico. Known elevational

range is from 1,900 to 2,500 m. The mistletoe on

junipers in eastern New Mexico, western Texas, and

northern Coahuila are considered another taxa.

Discussion. Phoradendron bolleanum is a widespread and diverse mistletoe with a complicated taxonomic history. We are attempting here to include only

reports of P. bolleanum subsp. bolleanum and populations resembling the type on juniper from the Sierra

Madre Occidental and parasitizing both Juniperus

and Arbutus. We describe other taxa under P. densum

and P. pauciflorum.

Phoradendron bolleanum is unusual for mistletoes

of section Pauciflorae senus Wiens (1964) in that it

commonly parasitizes both gymnosperms and angiosperms (Hawksworth and Wiens 1966). Trelease

(1916) questions whether one species of mistletoe

would occur on such unrelated host genera; and without experimental evidence, the question is still open.

However, no morphological grounds for a taxonomic

separation of the populations on the two hosts have

been found. Although infections of Juniperus and

Arbutus frequently occur together, we have seen

many situations from Chihuahua to Jalisco where

only one host is infected, even though the other is

present. For example, P. bolleanum has been found in

Jalisco on Arbutus only, even though Juniperus occurs

in the same areas (M. Cházaro B., personal communication). This suggests that there may be two host races

of the mistletoe, one primarily parasitic on Juniperus

and one on Arbutus. The areas where both types of

hosts are infected may represent instances of sympatry of the two races. Carefully controlled, crossinfection

experiments are needed to determine the status of the

host populations of this mistletoe.

Various authors have attempted to sort out the

systematic relations among taxa of Phoradendron

bolleanum complex. Wiens (1964) revised the previous

work by Trelease and Fosberg but later revised himself in floral treatments for Texas (Wiens 1970), Arizona

(Hawksworth and Wiens 1993a), and California

(Hawksworth and Wiens 1993b). Further changes by

Hawksworth and Wiens are contemplated in an
24 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils, Wiens, and Hawksworth Phoradendron in Mexico and the United States

Hosts.Cupressus arizonica, C. bakeri, C. goveniana,

C. macnabiana, C. macrocarpa (rare), C. sargentii,

Juniperus californica, J. monosperma, J. occidentalis,

J. osteosperma, J. pinchotii. The mistletoe is reported

on Pinus monophylla at Mt. Pinos (Ventura, California)

by McMinn (1939), but this has not been confirmed.

Distribution. United States (Arizona, California,

Oregon) and Mexico (Baja California). The mistletoe

ranges from southern Oregon (Klamath and Jackson)

throughout California to the lower elevations of the

Sierra San Pedro Mártir in Baja California, Mexico,

and with outlying populations in central Arizona on

Cupressus (Coconino, Yavapai, Maricopa, and Gila).

Trelease (1916) reports Phoradendron densum from

Sonora, and the Forest Pathology Herbarium-Fort

Collins, CO, holds collections as well from Coahuila

and Nuevo Leon, although these collections may more

properly belong under Phoradendron saltillense.

The population in the Sierra San Pedro Mártir

infects Juniperus californica but not Cupressus, which

occurs at higher elevations (Hawksworth and Wiens

1966). The reciprocal situation occurs in Arizona; the

mistletoe infects cypress but not junipers, even though

junipers are widely distributed and abundant in the

State. Hawksworth and Scharpf (1981) provide a distribution for Oregon, California, and Arizona. Known

elevational range is 200 to 2,300 m.

Discussion. Ashworth (2000) illustrates the close

systematic relationships among the taxa of section

Pauciflorae senu Wiens. Her work, however, also brings

out the taxonomic difficulty among Phoradendron

bolleanum, P. densum, and the various taxa at one

time or other considered subspecies of these (see table

3-1). For the reviewer (such as here), the problem is

determining to which taxa various reports of hosts and

distribution refer. Based on their descriptions and

distributions, Hawksworth and Wiens (1993a, 1993b)

restrict their interpretation of P. densum to include

only certain populations from Oregon to Baja California on juniper and cypress and from central Arizona on

cypress. Many of the mistletoes on either cypress or

juniper from the Sierra Madre Oriental are now referred to Phoradendron saltillense. From eastern New

Mexico, western Texas, and northern Coahuila (Wiens

1964), it is now referred to Hawksworth’s mistletoe

(see below). Several other populations usually referred to either P. bolleanum or to P. densum also have

differences in morphology, life history, hosts, and

distribution, so that they may warrant recognition at

the specific level. Considering the difficulty of classifying these mistletoes using traditional criteria, molecular techniques (for example, Ashworth 2000) should be

considered.

The population of Phoradendron on various junipers in western Texas, southern New Mexico, and

Coahuila has been referred to as Phoradendron

bolleanum subsp. hawksworthii (Wiens 1970) and

Phoradendron hawksworthii (Hawksworth 1979,

Hawksworth and Cibrián 1985, Hawksworth and

Scharpf 1981). Although the name Phoradendron

hawksworthii Wiens is accepted by the PLANTS

database (USDA, NRCS 2001), it is not yet a validly

published name and remains “ined.” The

Hawksworth’s mistletoe plants are about 10 to 25 cm

tall, dark green, with internodes 6 to 12 mm long.

Leaves are 6 to 25 mm long, 2 to 2.5 mm wide and

slightly hairy. The mature fruit is white and about 4

mm in diameter. The hosts are Juniperus ashei, J.

deppeana, J. flaccida, J. monosperma, and J. pinchotii.

This mistletoe is common throughout western Texas

(Brewster, Culbertson, Edwards, Hudspeth, Presido,

Terrell, Val Verde) and occurs in southern New Mexico

(Dona Ana, Lincoln, Otero, Socorro?) and northern

Coahuila (Sierra del al Encantada). [Note: the population collected from northwest of Carrizozo, NM, in

1969 could not be relocated and may be extinct.] The

Hawksworth’s mistletoe resembles P. saltillense but

is distinguished by several features (table 3-3).

Hawksworth’s mistletoe is not sympatric with P.

capitellatum, which also occurs on junipers in southern New Mexico. Hawksworth’s mistletoe, however,

is sympatric with P. juniperinum and may even infect

Table 3-3—Comparison of Hawksworth’s mistletoe and Phoradendron saltillense.

Character Hawksworth’s mistletoea Phoradendron saltillense

leaf width narrow, 2 mm or less wide, more than 3 mm

leaf transection upper surface flattened, lower rounded dorsoventrally flattened

tip of mature leaf abrupt, with point 0.2–0.3 mm or with the never developing a point

 scar of such a point

internode lengthb short, 6–12 mm, mean 9 mm long, 6–17 mm, mean 11 mm

segments per staminate usually only one segment typically two segments

 inflorescence

a

Phoradendron densum populations in western Texas and southern New Mexico. b

Internode length is correlated with total plant size.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 25

Phoradendron in Mexico and the United States Geils, Wiens, and Hawksworth

the same tree; but Hawksworth’s mistletoe usually

extends to lower elevations than the distribution of P.

juniperinum. Both P. juniperinum and Hawksworth’s

mistletoe appear to induce a witches’ broom formation on its host (Hawksworth and Wiens 1966, D.

Conklin, personal communication).

4. Phoradendron juniperinum

Juniper mistletoe

Phoradendronjuniperinum Engelm. ex A. Gray, Mem.

Am. Acad. N. S. iv. 59, 1849.

Description. Plants 20 to 40 cm tall, globose, green

to yellow–green, glabrous; internodes 5 to 10 mm long;

leaves reduced to minute scales; mature fruit is about

4 mm in diameter, pinkish–white colored (Hawksworth

and Wiens 1993a, 1993b, Hichcock and others 1964

with illustration).

Hosts. Common hosts are Juniperus californica, J.

deppeana, J. pinchotii, J. flaccida, J. monosperma, J.

occidentalis, J. osteosperma, and J. scopulorum.

Cupressus arizonica is commonly parasitized in central Chihuahua, but this tree is rarely infected in

Texas (Hawksworth and Cibrián 1985), Arizona

(Hawksworth and Wiens 1966), and New Mexico

(Linnane 1987). Other rare hosts are C. bakeri in

California (Hawksworth and Wiens 1966),

Chamaebatiara millefolium (Rosaceae) in Arizona

(Hawksworth 1952), and Pinus monophylla

(Hawksworth 1979). Vega (1976) adds Juniperus

mexicanus as a host.

Distribution. United States (Oregon, California,

Nevada, Utah, western Colorado, Arizona, New Mexico,

and western Texas) and Mexico (Baja California, Chihuahua, Durango, Sonora). It occurs in the Chisos

Mountains in Big Bend National Park, Texas

(Brewster), but is not yet known in adjacent Coahuila,

Mexico. A collection in the Forest Pathology Herbarium-Fort Collins, CO, includes a single collection

by J.R. Weir (no date) from Oakley, ID (Cassia) with a

Juniperus osteosperma infected by Phoradendron

juniperinum. Bunderson and others (1986) attempt to

predict the distribution on the mistletoe from environmental site factors. Hawksworth and Scharpf (1981)

provide a distribution map. Known elevational range

is 1,000 to 2,600 m.

Discussion. The taxon described by Trelease (1916)

as Phoradendron ligatum for its constricted scales is

now included under P. juniperinum (Wiens 1964).

This distinctive leafless species is the most widespread Phoradendron on conifers. Phoradendron

juniperinum is geographically sympatric with P.

capitellatum in Arizona, with Hawksworth’s mistletoe in New Mexico, and with P. densum in California.

In fact, natural hybrids of P. juniperinum x P. densum

that appear to be sterile F1 plants have been found in

the Inyo and San Bernardino Mountains in California

(Wiens and DeDecker 1972). Vasek (1966) observes

that P. juniperinum usually does not parasitize

Juniperus californica, and P. densum usually does not

parasitize J. osteosperma.Phoradendronjuniperinum

forms a sister group with P. libocedri, but there is also

some evidence in support of another sister group with

P. capitellatum (Ashworth 2000).

Phoradendron juniperinum is a widespread and

common mistletoe on junipers in many of the Western

States. It is the subject of numerous studies on ecophysiology (see below). Several curious observations

(Hawksworth and Wiens 1966) on P. juniperinum

include rarely occurring massive witches’ brooms and

parasitism by Phoradendron villosum subsp. coryae

(normally on oak).

5. Phoradendron libocedri

Incense–cedar mistletoe

Phoradendron libocedri (Engelm.) T.J. Howell, Fl.

N.W. Amer. 1:608, 1902.

=Phoradendron juniperinum Engelm. ex Gray var.

libocedri Engelm.

=Phoradendron juniperinum Engelm. ex Gray subsp.

libocedri (Engelm.) Wiens

Description. Plants 20 to 80 cm tall, woody only at

base, older plants pendulous, green; internodes 10 to

29 mm long; leaves reduced to minute scales; pistillate

inflorescences usually with one segment and two flowers (occasionally more); mature fruit 4 mm in diameter, pinkish white to straw colored (Hawksworth and

Wiens 1993b).

Hosts. On Calocedrus decurrens. There is one collection by Platt and Felix (Dodge Ridge, Tuolumne,

CA, 1968) of Phoradendronlibocedri on Abies concolor.

They report only a single fir tree was infected in a

stand where the mistletoe was common on the associated incense cedar. Weir (5995 from Del Norte CA in

1917) is a Chamaecyparis lawsoniana with a single

infection of P. librocedri; he reports infected incense

cedar are nearby.

Distribution. United States (California, Oregon,

Nevada) and Mexico (Baja California). The main range

of the mistletoe is from southern Oregon (Jackson,

Josephine, Klamath) through the Cascade and Sierra

Nevada Ranges to southern California. The population in Nevada (Douglas) is near the California border.

Isolated known occurrences are in northern Oregon

(Jefferson, Warm Springs Indian Reservation) and on

San Benito Peak in the South Coast Range in Monterey

County, California. It also occurs in the Sierra Juarez

and Sierra San Pedro Mártir in Baja California

(Wiggins 1980). Although the mistletoe has a wide

range, it is not very common (McMinn 1939).

Hawksworth and Scharpf (1981) provide a picture of

an infested incense cedar and a distribution map.

Known elevational range is 400 to 2,500 m.
26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils, Wiens, and Hawksworth Phoradendron in Mexico and the United States

Discussion. Phoradendron libocedri had been considered a subspecies of P. juniperinum (Wiens 1964)

but is here recognized again at the specific level

(Trelease 1916). Phoradendron libocedri and P.

juniperinum are sympatric in the Sierra Nevada, but

each remains restricted to it own host; P. libocedri has

longer internodes and is more pendulous (mimicking

its host? see Ashworth 2000). The two taxa, however,

do form a very close sister group (Ashworth 2000).

Meinecke (1912) describes the swellings and burls

caused by Phoradendron libocedri and speculates on

the age and parasitism of the mistletoe.

6. Phoradendron longifolium

Phoradendron longifolium Eichler ex Trel. Genus

Phoradendron 53, 1916.

Description. Plants becoming generally woody;

internodes 15 to 30 mm long; leaves linear, 32 to 84

mm long by 6 to 9 mm wide, apex rounded to acute;

fruit 3 to 4 mm diameter, lightly puberulent (Wiens

1964, Bello 1984 with illustration, Bello and Gutierrez

1985).

Hosts. Usually Quercus but also Alnus, Pinus

pseudostrobus, P. michoacana (Bello Gutierrez 1985).

Distribution. Mexico (Durango, Tamaulipas,

Hidalgo, Mexico, Oaxaca).

Discussion. Wiens (1964) places Phoradendron

longifolium in section Calyculatae but admits little

material was available for morphological comparisons

to other Phoradendron. Ashworth (2000) reports P.

longifolium forms a sister group with P. galeottii with

which it shares several morphological features, but

not time of flowering, which Wiens uses as a taxonomic

character. Although P. longiflorum occurs in pine

woodlands of central Mexico, we have no information

on how common or damaging it is.

7. Phoradendron minutifolium

Injerto de páraro

Phoradendron minutifolium Urban Bot. Jahrb. Syst

23, Beibl. 57:2, 1897

Description. Plants 30 to 80 cm tall, dull green;

internodes 4 to 12 mm long; leaves 3 to 5 mm long and

1 to 1.5 mm wide, resembling scales, glabrous; spike

one-jointed; mature fruit is about 4 mm in diameter,

pinkish–white colored (Hernandez 1991 with illustration, Trelease 1916).

Hosts. Juniperus deppeana (Acosta and others 1992).

Other juniper species are probably hosts, but our host

collections are only identified as Juniperus sp. Depending on how populations in Colima and Jalisco are

classified, also on Cupressus.

Distribution. Mexico (Chihuahua, Coahuila,

Durango, Puebla, Tlaxcala, Veracruz). The mistletoe

is most common in Tlaxcala (Hernández 1991) and

Veracruz and in the Sierra Madre Occidental in southern Chihuahua and Durango. An apparently extreme

outlier occurs in northern Coahuila in the Sierra del

Carmen (across from Big Bend, TX). Depending on

how populations on Cupressus are classified, also in

Colima and Jalisco. Known elevational range is 2,000

to 2,750 m.

Discussion. Phoradendron minutifolium in

Coahuila is sympatric with Phoradendron saltillense

on juniper; the two mistletoes sometimes even coinfect

the same tree. Phoradendron minutifolium and P.

bolleanum subsp. bolleanum form a close sister group

(Ashworth 2000).

A population of Phoradendron that appears similar

to P. minutifolium occurs on Cupressus in the nearby

vicinities of El Sauz and Terrero, Sierra de Mammitlan

and Cerro Grande in Colima and Jalisco. Although

these mistletoe are referred by the name Phoradendron

olivae (Cházaro 1990, Cházaro and others 1992), we

are unable to confirm that this name has been validly

published, but the holotype is reported to be a collection by Wiens, Cházaro, Hawksworth, and Olivo (7051,

1 August 1989) deposited at IBUG (Universidad de

Guadalajara). The hosts are variously identified as

Cupressus benthami and C. lusitanica. The plants on

Cupressus are larger and more open-formed because of

longer internodes (15 to 25 mm) and smaller leaves

(only 1 to 2 mm long). The mistletoes of Cupresssus

have a distribution far to the west and south of those

on Juniperus. Additional study of these mistletoes is

needed.

8. Phoradendron pauciflorum

Fir mistletoe

Phoradendron pauciflorum Torr., Pacif. Rail. Rep. iv.

134.

=Phoradendron bolleanum (Seem.) Eichl. var.

pauciflorum (Torr.) Fosberg

=Phoradendron bolleanum (Seem.) Eichl. subsp.

pauciflorum (Torr.) Wiens

Description. Plants 20 to 40 cm high, green; internodes 10 to 21 mm long; leaves with short petiole, 5 to

30 mm long and 5 to 8 mm wide, glabrous; leaf apex

obtuse; mature fruit is about 4 mm wide, pinkish white

to straw colored (Hawksworth and Scharpf 1981 with

color picture, Hawksworth and Wiens 1993a, 1993b).

Hosts. Abies concolor is usually the only host, although autoparasitism has been reported (Felix 1970b).

It rarely parasitizes Cupressus arizonica var. montana

in the Sierra San Pedro Mártir, Baja California, Mexico,

where this tree is associated with infected Abies

(Hawksworth and Wiens 1966).

Distribution. United States (Arizona, California)

and Mexico (Baja California). This mistletoe is common in the central and southern Sierra Nevada

(Calaveras) south to the Sierra San Pedro Mártir in

Baja California, Mexico (Wiggins 1980). An extreme

disjunct occurs in southern Arizona (Pima) in the
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 27

Phoradendron in Mexico and the United States Geils, Wiens, and Hawksworth

Santa Catalina and Rincon Mountains. Hawksworth

and Scharpf (1981) provide a distribution map. Known

elevational range is 1,400 to 2,600 m.

Discussion. This species was previously submerged

under Phoradendron bolleanum (table 3-1) and confused with P. densum (Wiens 1964). Felix (1970a)

provides a detailed study of the biology of P. pauciflorum

including information on reproduction and dispersal,

shoot growth, endophytic system, branch mortality,

mistletoe mortality, effects on host, associated species, and epiparasitism.

A population of an unnamed Phoradendron occurs

on the rare Abies durangensis in Chihuahua, Durango,

and Jalisco. This mistletoe on fir is sometimes sympatric with P. bolleanum, but has yellow-green, linear

leaves with an acuminate apex. The populations include a few widely scattered localities in the Sierra

Madre Occidental; all the A. durangensis populations

that we have visited in Durango are parasitized by this

mistletoe.

9. Phoradendron rhipsalinum

Phoradendron rhipsalinum Rzed., Cact. Suc. Mex

17:102, 1972.

Description. Plants 2 to 4 m long, pendulous,

bright green; leaves 4 to 6 cm long and 4 to 5 mm wide,

glabrous; mature fruit yellow–green, 1.5 mm in diameter (Bello 1984 with illustration, and Bello and

Gutierrez 1985 with picture and detailed description).

Hosts. Taxodium distichum var. mexicanum.

Quercus castanea is also infected (Bello 1984, Bello

and Gutierrez 1985).

Distribution. Mexico (Guanajuato, Jalisco, Mexico,

Michoacán). Known elevational range is 1,600 to 2,300

m in pine-oak woodlands to subtropical matorral.

Discussion. This is one of the most distinctive

Mexican mistletoes. Its 4 to 6 cm long “strap–like”

leaves make it unusual. It forms huge pendulous

masses hanging from the bald-cypress trees that look

from a distance like masses of Spanish moss. Ashworth

(2000) concludes this mistletoe does not fit with the

other parasites of conifers in the section Pauciflorae

but appears more closely related to P. brachystachyum

rather than P. bolleanum as suggested by Kuijt (1996).

It causes severe mortality to bald-cypress, for example

in the vicinity of Zamora, Jalisco. Although approximately 120 species of mistletoe occur in Mexico, this is

the only species described by a Mexican botanist

(Cházaro and others 1992).

10. Phoradendron saltillense

Phoradendron saltillense Trel. Genus

Phoradendron 27, 1916.

=P. bolleanum subsp. densum (Torr.) Wiens, pro parte.

Description. Plants moderately long and stout;

internodes 10 to 20 mm, papillate-hispid; leaves

narrowly oblong, 20 to 30 mm long by 2 to 3 mm wide,

sessile, apex acute; spike 50 to 60 mm long, with single

joint and pistillate two-flowered (Trelease 1916,

Standley 1920).

Hosts.Cupressus arizonica, C. benthami, Juniperus

deppeana, J. flaccida, J. monosperma, and J. saltillense.

Distribution. Mexico (Coahuila, Nuevo Leon, San

Luis Potosi, Puebla). Known elevational range is 1,850

to 2,850 m. This mistletoe is common the Sierra del

Carmen, northern Coahuila, and it may occur in the

Chisos Mountains, Texas. Hawksworth and Cibrián

(1985) lists P. densum subsp. saltillense (Trel.) Wiens

on Cupressus arizonica in Coahuila and on Juniperus

in the Sierra Madre Oriental from Coahuila to

Zacatecas.

Discussion. Phoradendron saltillense is first described by Trelease (1916), accepted by Standley (1920),

submerged under Phoradendron bolleanum subsp.

densum by Wiens (1964), and recognized here as a

validly published name. It is separated from P. densum

populations in Arizona by more than 1,000 km, but

near to Hawksworth’s mistletoe in the Sierra del

Carmen. It is distinguished from Hawksworth’s mistletoe by several morphological features (table 3-3).

Phoradendron saltillense is sympatric in Nuevo Leon

and Coahuila with populations of an unnamed

Phoradendron that have longer, narrower, thinner

leaves and a more open branching habit; these two

contrasting mistletoes may even occur in the same

tree.

Importance _____________________

Although the Phoradendron mistletoes that infect

conifers are widely distributed in the Western United

States and in Mexico on a number of common and

valuable hosts, their importance is mostly on a local

basis and for special uses. In the United States,

Phoradendron are most important in California on

incense cedar and true fir in certain areas and important broadly across the Southwest (California to Texas)

on junipers. In Mexico, Phoradendron (all species) are

found throughout the Republic, but only recognized as

a forest plague on about 4,000 ha in Jalisco, Mexico,

and Michoacán (Martinez 1983). Hawksworth and

Cibrián (1985), however, add that Phoradendron are

damaging to junipers in the north (Sierra Madre

Occidental, Oriental, and del Carmen). Although

Phoradendron are a minor issue on a few forest species

(see Felix 1970a, Meinecke 1912), they can be a serious

concern in some recreation and other high-value sites

(Frankel and others 1989, Linnane 1987). These mistletoes have a high nutritional value as animal forage

(Urness 1969) and are utilized as such (Cházaro and

Oliva 1988b, Gallina 1988). Phoradendron mistletoes are also consumed by humans as a stimulating
28 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils, Wiens, and Hawksworth Phoradendron in Mexico and the United States

beverage (Whiting 1950) and as a traditional medicine

for childbirth and several ailments (Cházaro and Oliva

1988a, Moore 1979, Whiting 1950). A strong caution,

however, is appropriate. Various compounds from

Phoradendron and other mistletoes are being investigated for their pharmacological potential, but many of

these compounds are present in toxic dosages to humans (Turner and Szczawinski 1991). Although it is

the more leafy Phoradendron mistletoes on hardwoods that are usually harvested commercially for

winter-festival greenery, the conifer mistletoe can still

be enjoyed for attracting numerous, colorful birds

(Sutton 1951).

Management Strategies __________

Phoradendron mistletoes seldom cause sufficient

damage in an area that control is required; but where

management objectives indicate that mistletoe control is justified, cultural methods are available. Although there are insects that feed on these mistletoes

(Burke 1975) and fungi that caused disease (Horst

2002, Scharpf and Hawksworth 1966), there are no

biological control programs for the Phoradendron.

Chemical control has been tested using various herbicides (Quick 1963) and ethephon (Adams and others

1993), but none are recommended. We know of no

genetic improvement programs to develop conifer resistance to Phoradendron mistletoes. Cultural methods are briefly discussed by Frankel and others (1989),

Hawksworth and Scharpf (1981), and Hernandez

(1991). Operations include regenerating with a nonhost

tree, thinning trees to improve vigor and tolerance of

the infestation, and sanitation by removing infected

trees or branches or removing aerial shoots. Pruning

infected branches is often sufficient; where the loss of

infected branches cannot be accepted, the aerial shoots

can be just knocked off. Removing the shoots does not

eliminate the mistletoe infection but does reduce its

reproduction and damage. Shoots will reappear after

several years. Covering infected branches with

tarpaper or creosote has not proven either attractive

or effective. Perhaps the best way for discouraging

additional bird-dispersal of mistletoe seeds is with

branch pruning or shoot removal (since it is often the

mistletoe fruits that initially attract the birds). Given

the modest damage and slow rate of increase of these

mistletoes, these methods are usually sufficient.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 29

Chapter

Arceuthobium in

North America

F. G. Hawksworth

D. Wiens

B. W. Geils

4

The biology, pathology, and systematics of dwarf mistletoes

are recently and well reviewed in Hawksworth and Wiens

(1996). That monograph forms the basis for the text in this and

chapter 5 and should be consulted for more information (for

example, references, photographs, and distribution maps). In

addition to extracting the information that would be most relevant to forest managers and arborists, we here include new

references on hosts, distributions, and ecology. The synonymy in

this chapter is neither formal nor complete; rather, we provide

additional names used in previous, significant literature (such as

Gill 1935, Hawksworth and Wiens 1972, Kuijt 1955).

General Life Cycle ________________________________

The life cycle of dwarf mistletoe is distinctive because of two

features—obligate parasitism (shared with all mistletoes) and

hydrostatically controlled, explosive dispersal (with one exception). The details of cytology, anatomy, embryology, genetics, and

evolution that underlie these features are described by

Hawksworth and Wiens (1996) and Kuijt (1960a, 1960b, 1969a).
30 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Especially for dwarf mistletoes with their reduced

morphologies, differences in reproductive phenology

and host specificity are taxonomically decisive

(Hawksworth and Wiens 1996). The life histories of

several dwarf mistletoes are well studied (Gilbert

1984, 1988, Hawksworth 1961, 1965, Scharpf and

Parmeter 1982, Strand and Roth 1976).

Life History

Dwarf mistletoe life history comprises four stages:

dispersal, establishment, incubation, and reproduction (fig. 4-1). Dispersal begins when a mature fruit

discharges its seed into ballistic flight. Establishment

includes the time from the seed lodging at a safe-site

until the parasitic relationship is initiated. Several

years of incubation pass while an extensive, endophytic system develops under the host’s bark. The

reproductive stage continues with repeated, intermittent production of aerial shoots and flowers and continued expansion of the endophytic system. Reproduction ends with the death the mistletoe plant; this

usually does not occur until the host itself dies. Various physical and biological factors affect the temporal

and spatial unfolding of these processes into population consequences and afford an opportunity for management intervention.

Dispersal—Mistletoe dispersal is effected by the

hydrostatic contraction of a mature fruit that propels

a single, small seed upon ballistic flight to either a

location where a host may be inoculated (safe-site) or

elsewhere. Unlike other mistletoes that are primarily

dispersed by birds consuming mature fruits and defecating viable seeds, the dwarf mistletoes rely almost

exclusively on this ballistic mechanism. Birds and

mammals are important, however, for the rare, longdistance dissemination of seeds to new infection centers (Nicholls and others 1984). The exception is

Arceuthobium verticilliflorum, which is found in widely

spaced pine forests of Mexico. This species has nonexplosive fruits twice the size of other dwarf mistletoes

and is predominately dispersed by birds.

The special morphological and anatomical features

that facilitate dispersal include the supporting structure for the fruit (pedicel) and characteristic, sticky,

viscin cells (Wilson and Calvin 1996, Hawksworth and

Wiens 1996). When the fruit matures, the pedicel

elongates and water pressure increases. With separation of the fruit from the pedicel, the seed is ejected at

nearly 24 m per second (Hinds and Hawksworth 1965)

and tumbles in a short ballistic flight until it lands

upon and sticks to a surface. The shape of the ballistic

trajectory is influenced by height above the ground,

pedicel–fruit orientation, seed shape and weight, discharge velocity, and gravity (Hawksworth 1961). Dwarf

mistletoe seeds have a mass of 2 to 3 mg; wind affects

the flight, but seeds fall to their destination within

seconds. Although maximum horizontal displacement

may reach 16 m, 10 m is a more typical, free-flight

distance (see Escudero and Cibrián 1985). Most seeds

are displaced horizontally only 2 to 4 m and deposited

lower in the crown; some seeds, however, are shot

Figure 4-1—Generalized life

cycle of a typical dwarf mistletoe.

Illustration courtesy of W. R.

Jacobi.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 31

Arceuthobium in North America Hawksworth, Wiens, and Geils

higher into the crown to effect vertical spread

(Hawksworth and Geils 1985, Richardson and van der

Kamp 1972, Shaw and Hennon 1991, Wicker and

Hawksworth 1991). Because of variation in crown

density, foliage display, and mistletoe position, the

rate of seed interception within any tree crown is

highly variable. Only about 40 percent of seeds are

intercepted by any tree crown; 60 to 80 percent of seeds

are retained in the crown from which they originated

(reinfection); of those that escape, 90 percent may be

intercepted by an adjoining tree (contagion)

(Hawksworth 1965, Smith 1985).

After their ballistic flight, seeds continue to move by

gravity or rarely by birds and mammals (Nicholls and

others 1984). The viscin coating helps the seed adhere

to any surface it strikes, including host foliage. After

the initial interception, this viscin imbibes water,

swells, loosens, and permits the seed to slide down the

needle (see Hawksworth and Wiens 1996 for illustrations). If the needle points upward, the seed lodges on

a twig at the base of a needle (a good safesite); otherwise, the seed slides off and relocates in the crown or

falls to the ground. Most infections, but not all, occur

on young branch wood (Sproule 1996a). Although only

a few seeds reach safesites (less then 10 percent,

according to Hawksworth 1965), large numbers of

seeds are produced on heavily infected trees

(Hawksworth 1965, Smith 1977, Wicker 1967a). Although it seems inefficient, for short range spread and

intensification this dispersal mechanism is effective

enough for dwarf mistletoes to have persisted since

the Miocene, adapted to nearly a hundred host species,

and spread throughout the conifer forests of North

America.

Establishment—The physical process of dispersal

brings the mistletoe seed within millimeters of establishing a new infection; biological growth completes

the establishment phase. Although the embryo of

some tropical species begins growth soon after dispersal, most temperate mistletoes do not resume growth

(germinate) until the following spring when light,

moisture, and temperature are suitable (Gill and

Hawksworth 1961, Lamont 1983, Scharpf 1970, Wicker

1974).

Genetic factors, predation, and environmental conditions reduce the number of viable seeds; field germination varies from 7 to 90 percent (Hawksworth and

Wiens 1996). The chlorophyllous endosperm helps

maintain the embryo and permits growth of the hypocotyl (see Deeks and others 2001). If the germinating

seed rests on a host shoot with thin bark and its growth

encounters a needle base, it then develops an external

holdfast structure and penetration wedge that grows

into the host cortex (Scharpf and Parmeter 1967).

From the penetration wedge, fine strings of mistletoe

tissue — the endophytic system — ramifies throughout

the host cortex and eventually becomes embedded in

xylem as “sinkers” (Cibrián and others 1980, Calvin

and Wilson 1996, Hunt and others 1996). With the

establishment of the endophytic system, the parasitic

nutritional relation is initiated. Although little is

know about the mechanisms of host resistance (see

chapter 7), a high degree of host specificity and inherited variation in susceptibility suggest that physiological compatibility is required for an infection to

become established (Kolb 2002).

Incubation—The endophytic system expands

within the cortex and becomes embedded in the xylem

for a number of years before aerial shoots are produced

(incubation period). The endophytic system both encircles the infected branch and grows along it. The

nature of distal–proximal growth depends upon the

dwarf mistletoe species and point of origin. When a

species such as Arceuthobium douglasii infects the

host’s apical meristem, a systemic infection is established whereby the growth of the endophytic system

keeps up with the growth of the host shoot. In other

cases, growth of the endophytic system is limited, and

a localized (nonsystemic) infection establishes. In

nonsystemic infections, the infected branch develops a

distinct fusiform swelling (except by a few host species). The incubation period extends from 2 to 12 years

depending on mistletoe species and environmental

conditions (Hawksworth and Wiens 1996). Typically,

incubation periods range from 3 to 4 years.

Even after aerial shoots are produced, the endophytic system continues to grow (Calvin and Wilson

1996). Pathological effects of the mistletoe infection

become evident as infected branches develop persistent witches’ brooms, and the upper crown thins and

dies. Although a single, systemic infection can eventually develop into a large witches’ broom, most severe

pathological effects result from multiple infections.

Rarely, the endophytic system grows into the bole and

establishes a main stem infection that persists as long

as the host lives. Branch infections usually occur in the

lower crown. These parasitized branches do not readily

self-prune but are subject to breakage (especially

large brooms in brittle hosts) and consumption by fire

(brooms tend to be low and are highly flammable).

Infections in the upper crown are lost as crown-dieback in severely diseased trees progresses.

Reproduction—Dwarf mistletoes are dioecious

plants that only reproduce from seeds borne on shoots

(see Gilbert 1988). Although dwarf mistletoe shoots

have chlorophyll, they have no photosynthetic significance. Their function is primarily reproductive and

secondarily in water regulation and synthesis of growth

compounds (Wilson and Calvin 1996). Shoots range in

size from several millimeters to 0.5 m, but most species are 2 to 10 cm tall. Generally, 1 to 2 years elapse
32 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

from shoot appearance to the initial flowering. Several

flower crops (range one to five) are usually produced,

and shoots many be retained for 2 to 7 years. Meiosis

may occur either immediately before flower production (direct flowering) or approximately 5 to 8 months

before anthesis (indirect flowering). Most species exhibit definite annual flowering periods, but a few

tropical species appear to flower continuously throughout the year. The sex ratio for most species is about

50:50 (Mathiasen and others 1998, Mathiasen and

Shaw 1998, Wiens and others 1996). Pollen is either

dispersed by wind or insects, and because of the

clustered distribution of mistletoes, pollen is seldom

limiting. Although fruit maturation in some tropical

species occurs in as little as 4 to 5 months, most species

require about 1 year (to 19 months) from flowering to

seed dispersal. The number of fruits per infection is

controlled by variation in the size of the endophytic

system, host–parasite physiology, activity by pathogens and insects, and weather. Strand and Roth (1976)

observe that the number of seeds produced by

Arceuthobium campylopodum is related to plant age,

but the coefficient of variation usually exceeds 100

percent (even greater than 200 percent). Wicker (1967a)

estimates the number of mistletoe seeds produced on

trees infected by A. campylopodum range from 800 to

2.2 million per year. Escudero and Cibrián (1985)

report that Arceuthobium globosum produces more

than 7.3 million seeds per hectare.

As parasites, dwarf mistletoes inhabit a relatively

safe and constant environment and live for many

decades. Because they rely upon a host for nutrition

and because reproductive success does not require

annual seed production, dwarf mistletoes can persist

for years without producing aerial shoots (latent infections). Although little is known of the physiological

mechanisms that regulate flowering, shoot production

is apparently suppressed in the low light (Shaw and

Weiss 2000) and in the nutrition environment of shaded

lower crowns (Kolb 2002). Opening the canopy (removing trees) commonly results in a proliferation of mistletoe shoots on the residual trees (see chapter 8).

Spread and Intensification

Because ballistic dispersal and parasitism are important attributes of life history, these features are

critical factors in determining population characteristics and dynamics (Bloomberg and Smith 1982,

Hawksworth and Scharpf 1984, Parmeter 1978, Smith

1977). Ballistic dispersal is effective for short- range

dissemination only, and parasitism requires a living

host. Consequently, mistletoe plants are clustered

within trees, and infected trees occur in patches

(Robinson and others 2002). The spatial dynamics of

mistletoe populations operate across a range of scales—

the tree, neighborhood, stand, and landscape. Because

mistletoes are clustered, infestations are usually described on the bases of incidence (percent of trees

infected), severity (relative abundance), area distribution (extent), and spatial patterns (contagion). Successful reproduction leads to spread (Dixon and

Hawksworth 1979) and intensification (Geils and

Mathiasen 1990). In this context, spread refers to an

increase in number of infected trees and the extent of

an infestation (including the special case of vertical

spread); intensification is increase in the abundance of

mistletoe in an infested population. Stand development and management often generate grouping of

trees whereby mistletoe disperses readily within groups

but infrequently between groups. Even in stands with

random or uniform patterns of tree distribution, the

abundance of dwarf mistletoe plants often displays

spatial autocorrelation. Spread and intensification, of

course, are limited (Trummer and others 1998). Infected trees and the dwarf mistletoes they sustain

eventually die from fire, insects, disease, or cutting,

leading to fragmentation or local extinction of the

dwarf mistletoe population.

Rating systems—There are numerous dwarf mistletoe rating systems for describing host susceptibility,

mistletoe abundance, and witches’ broom abundance

(Hawksworth and Wiens 1972, Hawksworth 1977,

Tinnin 1998). Each rating system provides a quantitative reference scale for indicating the population status of a mistletoe infestation and its potential for

spread and intensification. New systems focus on

potential use by wildlife (Parker 2001), fire ecology

(Maffei 2002), and adaptations for woodland trees.

The host susceptibility system developed by

Hawksworth and Wiens (1972) classifies candidate

host species by the percentage expected to become

infected where suitably exposed to an inoculum source.

The classification is based on either direct field observations or general field experience. The system is

meant to reflect the potential physiological susceptibility to infection and parasite development, not the

distributional commonness or rarity of the host–pathogen combination. Species with greater than 90 percent

infection where exposed to a mistletoe seed source are

described as principal hosts; infestations on a principal host population are self-sustaining. Secondary,

occasional, and rare hosts exhibit infection levels of 90

to 50 percent, 50 to 5 percent, or less than 5 percent,

respectively. Infestations in populations of occasional

or rare hosts usually occur where an infected principal

host is present. Some species are recognized as hosts

either by artificial inoculation or by natural infection

of individuals planted beyond their normal range

(extralimital hosts). Incompatible hosts are those species in which the dwarf mistletoe is able to establish a

parasitic, nutritional relation but not to form aerial

shoots. The physiological requirements necessary for
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 33

Arceuthobium in North America Hawksworth, Wiens, and Geils

parasitism are satisfied for only a few host and mistletoe combinations; most species are immune.

Although mistletoe abundance could be quantified

by number of plants, biomass, or other indicators,

mistletoe severity is usually described by a relative

index for the amount of host crown affected, the dwarf

mistletoe rating, DMR (Hawksworth 1977). By this

system (fig. 6-1), the live host crown is divided into

thirds; each third is rated as 0 if no live branches are

apparently infected, 1 if not more than half of the

branches are infected, or 2 if more than half of the

branches are infected. The system allows a description

of mistletoe distribution within crown thirds, or by

summing values for crown thirds, abundance for the

tree as a whole (DMR), or by averaging tree ratings,

severity for a group or stand of trees. If tree ratings (0

to 6) are averaged over all susceptible trees in a stand,

the result is stand-DMR; if tree ratings are averaged

over infected trees only (1 to 6), the result is stand-DMI

(Geils and Mathiasen 1990). The distinction is useful

because of the computational identity among DMR,

DMI, and the fraction of trees infected (incidence):

DMR=DMI x (incidence).

DMR is a good single index of mistletoe severity; but

DMI and incidence may be preferred to illustrate

separately the severity of infection upon infected trees

and relative abundance of infected trees in the population.

Although the DMR system applies well to many

important hosts such as spruce, larch, and yellow and

white pines, it is less practical for other hosts (Dooling

1978, Shaw and others 2000). In many hemlock and fir

stands, the upper crown where much of the mistletoe

would be found is obscured by height and foliage. The

low, round, compact form of pinyons and general

distribution of mistletoe throughout the crown make

division into crown thirds impractical. In Douglas-fir,

individual branches are difficult to count, but systemic

witches’ brooms are obvious. Tinnin (1998) suggests a

variation to the DMR system, BVR for broom-forming

hosts; in his system broom volume substitutes (in

part) for number of infected branches in rating a crown

third. Other variations are possible, but to avoid

confusion, these other variations should not be referred to as DMR.

Spread and intensification are both strongly influenced by the same factors and are really just alternative views of the same basic life history processes—

dispersal, establishment, incubation, and reproduction.

Intensification of an infected host can occur from autoinfection, allo-infection, or both. The initial infection

of a previously uninfected host (both spread and intensification) can only result from allo-infection. Dispersal is primarily affected by the physical configuration of the seed’s environment—tree and crown density,

vertical crown distribution (structure), and stand species composition. Establishment, incubation, and reproduction are determined by weather, genetic, and

other biological factors, some of which are nearly fixed

such as host susceptibility. Other factors such as host

height growth and predation are extremely variable

and difficult to predict. In most cases, the most valuable piece of information for predicting dwarf mistletoe behavior and response to management is knowledge of the mistletoe species. Although all dwarf

mistletoes share a common genus morphology, most

taxon are readily identifiable when size, branching

pattern, color, and brooming response are considered

together. Furthermore, most species can be determined based on host and distribution.

Description of Genus ____________

Arceuthobium

Dwarf mistletoe

Arceuthobium M. Bieb. Flora Taurico-Caucasica 3(IV)

Supplement, p. 629, 1819. Nom. Cons. 2091

= Razoumofskya Hoffman.

Herbs or shrubs from 0.5 cm to approximately 70 cm

high (see fig. 4-2 and 4-3); parasitic on Pinaceae and

Cupressaceae; plants glabrous, variously colored from

greenish yellow to orange, reddish, or black; dioecious;

stems with variant (anomalous) patterns of secondary

growth; leaves reduced to minute, opposed, connate

scales; internodes angled (at least when young); flowers generally decussate or rarely whorled on young

shoots, 2 to 4 mm across; staminate flowers with a

central nectary, perianth segments usually three to

four (rarely two and up to seven) bearing a sessile, onechambered, circular anther on each perianth segment;

pollen spherical with six alternating spiny and smooth

sections; pistillate flower manifestly epigynous with

one style, perianth segments persistent, adnate to

ovary, two-merous; ovary one-chambered; fruit an

ovoid berry, one-seeded, mucilaginous and bicolored

(distal and basal portions of different shades), explosive at maturity (one exception); seeds without true

integuments, usually 3 to 5 mm long, ovate-lanceolate,

containing one (rarely two) distal, cylindrical embryo,

with copious endosperm.

A genus of 42 species in two subgenera. Subgenus

Arceuthobium is characterized by verticillate (whorled)

branching and occurring mostly in the Old World

represented in North America by three species (A.

abietis-religiosae, A. americanum, and A. verticilliflorum). Subgenus Vaginata occurs only in the New

World and characterized by flabellate (fan-like) branching. Thirty-six taxa are described for North America

(table 4-1). Type species: Arceuthobium oxycedri (DC.)

M. Bieb.
34 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Table 4-1—Dwarf mistletoes of Canada, Mexico, and the United States.

Arceuthobium taxon Canada United States Mexico

A. abietinum f. sp. concoloris - XX

A. abietinum f. sp. magnificae - XA. abietis-religiosae - -X

A. americanum X XA. apachecum - XX

A. aureum subsp. petersonii - -X

A. blumeri - XX

A. californicum - XA. campylopodum - XX

A. cyanocarpum - XA. divaricatum - XX

A. douglasii X XX

A. durangense - -X

A. gillii - XX

A. globosum subsp. globosum - -X

A. globosum subsp. grandicaule - -X

A. guatemalense - -X

A. hondurense - -X

A. laricis X XA. littorum - XA. microcarpum - XA. monticola - XA. nigrum - -X

A. oaxacanum - -X

A. occidentale - XA. pendens - -X

A. pusillum X XA. rubrum - -X

A. siskiyouense - XA. strictum - -X

A. tsugense subsp. tsugense X XA. tsugense subsp. mertensianae X XA. vaginatum subsp. vaginatum - -X

A. vaginatum subsp. cryptopodum - XX

A. verticilliflorum - -X

A. yecorense - -X

Total number of taxa 6 21 23
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 35

Arceuthobium in North America Hawksworth, Wiens, and Geils

Figure 4-3—Arceuthobium douglasii in spring, A

pistillate plant (left) and staminate plant (right), B

detail of mature fruit, C staminate shoot, D staminate shoot with open mature buds (left) and detail of

open flower (right), E staminate shoots with closed

buds. Illustration from Hawksworth and Wiens

Figure 4-2—Arceuthobium americanum in spring, A staminate (1972).

plant with verticillate (whorled) branching, B pistillate plant, C

staminate flower, D pistillate flower. Illustration from Hawksworth

and Wiens (1972).

C

D

2 mm

B

1 cm

A

C

D

2 mm

B

1 mm

E

2 mm

1 mm 2 mm

A 2 mm

Key to North American Species of Arceuthobium

1. Distributed in Mexico ..........................................................................................................................................2

2. Parasites of fir or Douglas-fir ........................................................................................................................3

3. Shoots 1–3 cm high; parasites of Douglas-fir ................................................................11. A. douglasii

3. Shoots more than 5 cm high; parasites of fir .........................................................................................4

4. Shoots less than 10 cm high, not verticillate, greenish; Chihuahua ..................... 1. A. abietinum

4. Shoots 10–20 cm high, some verticillate, yellow; Central Mexico .............. 2. A. abietis-religiosae

2. Parasites of pine .............................................................................................................................................5

5. Baja California ........................................................................................................................................6

6. Shoots olive–green, about 1–2 mm diameter; parasites of pinyon ...................10. A. divaricatum

6. Shoots yellowish, about 2–4 mm diameter; parasites of Pinus jeffreyi or P. coulteri .....................

............................................................................................................................ 8. A. campylopodum
36 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

5. Mainland Mexico ............................................................................................................................................7

7. Parasites of pinyon or white pine...........................................................................................................8

8. Parasites of pinyon .....................................................................................................24. A. pendens

8. Parasites of white pine.....................................................................................................................9

9. Shoots greenish purple to purple; parasites of Pinus ayacahuite var. ayacahuite; southern

Mexico...........................................................................................................15. A. guatemalense

9. Shoots yellow or gray; parasites of Pinus strobiformis or P. ayacahuite var. brachyptera;

northern Mexico ........................................................................................................................10

10. Shoots yellowish, usually less than 4 cm high; northern Coahuila ....................................

.................................................................................................................... 4. A. apachecum

10. Shoots gray, usually more than 6 cm high; Chihuahua, Durango, or Nuevo León ...........

.......................................................................................................................... 6. A. blumeri

7. Parasites of yellow pine ........................................................................................................................11

11. Shoots dark, usually some shade of black, reddish (or dull brown when dried).......................12

12. Male and female plants similarly branched (little sexual dimorphism); fruits not glaucous

...................................................................................................................................................13

13. Shoots usually more than 10 cm high and more than 1 cm diameter at base; fruits 4–5 mm

long, not shiny ........................................................... 30a. A. vaginatum subsp. vaginatum

13. Shoots usually less than 10 cm high and less than 1 cm diameter at base; fruits about

3 mm long, shiny ........................................................................................... 26. A. rubrum

12. Male and female plants dissimilarly branched (sexually dimorphic); fruits markedly

glaucous .................................................................................................................. 21. A. nigrum

11. Shoots yellow, brown, gray, or red ..............................................................................................14

14. Staminate flowers verticillate on deciduous spikes; mature fruits more than 10 mm long.....

................................................................................................................... 31. A. verticilliflorum

14. Staminate flowers not verticillate on deciduous spikes; mature fruits less than 6 mm long

...................................................................................................................................................15

15. Plants of northern Mexico ..................................................................................................16

16. Male and female plants dissimilarly branched (sexually dimorphic) ...................... 17

17. Male plants essentially non-branched and female plants densely branched .......

..........................................................................................................28. A. strictum

17. Male plants with very open branches and female plants densely branched ........

................................................................................................................13. A. gillii

16. Male and female plants similarly branched (little sexual dimorphism) .................. 18

18. Shoots yellow or yellow–brown ........................................................................... 19

19. Shoots bright yellow, in globose clusters, usually more than 10 cm high

.................................................................... 14a. A. globosum subsp. globosum

19. Shoots yellow or brown, not in globose clusters, usually less than 10 cm high

.................................................................................................. 32. A. yecorense

18. Shoots some shade of orange ............................................................................... 20

20. Shoots dark–orange, usually more than 20 cm high; mature fruit 7 mm long;

Durango or southward .........................................................12. A. durangense

20. Shoots yellow–orange, usually less than 20 cm high; mature fruit 5 mm long;

Chihuahua, Sonora, or Coahuila ......................................................................

............................................................30b. A. vaginatum subsp. cryptopodum

15. Plants of southern Mexico (Chiapas and Oaxaca) ...........................................................21

21. Shoots glaucous, olive-brown to gray green; parasite of P. oocarpa or P. maximinoi

............................................................................................................16. A. hondurense

21. Shoots not glaucous; reddish, dark green, yellow, or orange ................................... 22

22. Shoots reddish; Oaxaca.............................................................. 22. A. oaxacanum

22. Shoots dark greenish yellow or orange; Oaxaca or Chiapas.............................. 23

23. Shoots yellow, often over 2 cm in diameter at base; elevations above 2,700 m

................................................................14b. A. globosum subsp. grandicaule

23. Shoots yellow–orange, usually less than 2 cm in diameter; elevations below

2,400 m ............................................................ 5. A. aureum subsp. petersonii
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 37

Arceuthobium in North America Hawksworth, Wiens, and Geils

1. Distributed in the United States or Canada ...................................................................................................24

24. Parasites principally of pine ......................................................................................................................25

25. Parasites of pinyon or white pine.......................................................................................................26

26. Parasites of pinyon ............................................................................................10. A. divaricatum

26. Parasites of white pine ................................................................................................................27

27. Parasites of Pinus strobiformis ................................................................................................28

28. Shoots usually less than 4 cm high, yellow; southern Arizona or southern New Mexico

.................................................................................................................... 4. A. apachecum

28. Shoots usually more than 6 cm high, gray; Huachuca Mountains of Arizona ...................

.......................................................................................................................... 6. A. blumeri

27. Parasites of white pines other than Pinus strobiformis .........................................................29

29. Parasites of Pinus aristata; Arizona .................................................... 19. A. microcarpum

29. Parasites of pines other than Pinus aristata or if parasite of Pinus aristata then not

Arizona ...............................................................................................................................30

30. Shoots usually less than 6 cm high, densely clustered around the host branch;

parasites of Pinus flexilis, P. albicaulis, P. aristata, or P. longaeva ...........................

...........................................................................................................9. A. cyanocarpum

30. Shoots usually more than 6 cm high, not densely clustered around the host branch;

parasites of Pinus monticola or P. lambertiana ....................................................... 31

31. Shoots dark brown; parasites of Pinus monticola; southwestern Oregon or

northeastern California ................................................................ 20. A. monticola

31. Shoots yellow to green; parasites of Pinus lambertiana; California .....................

.................................................................................................... 8. A. californicum

25. Parasites of yellow pine .............................................................................................................................32

32. Shoot branches verticillate; parasites principally of Pinus contorta or P. banksiana ........................

................................................................................................................................... 3. A. americanum

32. Shoot branches flabellate; parasites principally of pines other than Pinus contorta and P. banksiana

.............................................................................................................................................................33

33. Arizona, Utah, or eastward .........................................................................................................34

34. Fruits glaucous; male plants much more openly branched than female plants; parasites of

Pinus leiophylla var. chihuahuana........................................................................... 13. A. gillii

34. Fruits glabrous; male and female plants branch in similar manner; parasites of Pinus

ponderosa var. scopulorum, P. arizonica, and P. engelmannii; north to Colorado ...................

...................................................................................... 30b. A. vaginatum subsp. cryptopodum

33. Pacific Coast States, Nevada, Idaho, or British Columbia ........................................................35

35. Coastal areas (within 10 km of the Pacific Ocean) .................................................................36

36. Shoots usually less than 10 cm high; staminate flowers mostly 3-merous; parasites of

Pinus contorta var. contorta; Orcas Island, Washington or British Columbia ..................

.......................................................................................... 29a. A. tsugense subsp. tsugense

36. Shoots usually more than 10 cm high; staminate flowers mostly 4-merous; parasites of

Pinus radiata or P. muricata; California .................................................... 18. A. littorum

35. Inland areas ..............................................................................................................................37

37. Plants consistently forming witches’ brooms; mature fruits about 6 mm long; shoots more

than 3 mm diameter at base; parasites principally of Pinus ponderosa var. ponderosa, P.

jeffreyi, or P. coulteri; California, Oregon, Washington, Idaho, or Nevada ........................

.............................................................................................................. 8. A. campylopodum

37. Plants not forming witches’ brooms; mature fruits about 4 mm long; shoots less than 3 mm

diameter at base; parasites principally of Pinus sabiniana or P. attenuata; California or

southwestern Oregon ........................................................................................................38

38. Anthesis from late September to November; parasites principally of Pinus sabiniana;

foothills surrounding Central Valley of California .......................................................

.............................................................................................................23. A. occidentale

38. Anthesis in August; parasites of Pinus attenuata; southwestern Oregon or

northwestern California.................................................................. 27. A. siskiyouense
38 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Several other keys and floristic treatments of the

dwarf mistletoes are available. Scharpf and

Hawksworth (1993) provide photographs and field

descriptions for the mistletoe from Washington, Oregon, and California. Unger (1992) provides a similar,

general coverage for Canada. Numerous but brief and

partial descriptions describe the dwarf mistletoes of

Mexico (Cházaro and Olivae 1987a, Hawksworth 1987,

Hawksworth and Cibrián 1985, Najera and others

1987, Rodriguez 1983). Recent taxonomic notes include Hawksworth and Wiens (1965, 1977, 1989) and

Hawksworth and others (1992b). Because the taxonomy of dwarf mistletoes has changed considerably

in the past few decades, especially for some regions

(Mexico) and some groups (campylopodum, globosum,

vaginatum), care is required when reading the literature to relate the information presented to the proper

taxa. Host and distribution information is ultimately

derived from specimen collections and field observations. When the name applied to a specimen changes,

the information also refers to a different taxa;

24. Parasites principally of hemlock, larch, Douglas-fir, fir, or spruce..................................................39

39 Parasites of hemlock, larch, or Douglas-fir..................................................................................40

40. Shoots usually less than 4 cm high; parasites of Douglas-fir .......................... 11. A. douglasii

40. Shoots usually more than 5 cm high; parasites of larch or hemlock .....................................41

41. Parasites principally of larch ...........................................................................17. A. laricis

41. Parasites principally of hemlock .......................................................................................42

42. Parasites of Tsuga heterophylla; California to Alaska .................................................

.................................................................................... 29a. A. tsugense subsp. tsugense

42. Parasites of Tsuga mertensiana; California to Idaho and British Columbia .......... 43

43. Host associated with infected Larix occidentalis; northern Idaho ........................

............................................................................................................. 17. A. laricis

43. Host not associated with infected Larix occidentalis; central Sierra Nevada of

California to southern British Columbia ................................................................

.....................................................................29b. A. tsugense subsp. mertensianae

39. Parasites of fir or spruce ..............................................................................................................44

44. Parasites of fir...........................................................................................................................45

45. Shoots usually more than 10 cm high, yellowish; staminate buds same color as the

subtending bracts; host not associated with infected hemlock; Arizona, southern Utah,

Nevada, California, Oregon, or Washington east of the Cascade Crest .............................

...................................................................................................................... 1. A. abietinum

45. Shoots usually less than 6 cm high, green to purplish; staminate buds conspicuously

lighter than the subtending purplish bracts; host associated with infected hemlock;

Oregon west of Cascade Crest to Alaska along Pacific Coast .........................................46

46. Host associated with infected Tsuga heterophylla ........................................................

.................................................................................... 29a. A. tsugense subsp. tsugense

46. Host associated with infected Tsuga mertensianae ......................................................

............................................................................ 29b. A. tsugense subsp. mertensianae

44. Parasites of spruce....................................................................................................................47

47. Shoots less than 2 cm high; parasites of Picea mariana, P. glauca, or P. rubens;

Saskatchewan and Great Lake region eastward to New Jersey and Newfoundland

...................................................................................................................... 25. A. pusillum

47. Shoots usually more than 5 cm high; parasites on Picea engelmannii or P. pungens;

Arizona or southern New Mexico......................................................... 19. A. microcarpum

published information goes out of date and may be

associated with the wrong mistletoe.

Description of Species ___________

1. Arceuthobium abietinum

Fir Dwarf Mistletoe

Arceuthobium abietinum Engelm. ex Munz, Manual

Southern California Botany: 114, 1935.

=A. campylopodum f. abietinum.

Description. Mean shoot height 8 (maximum 22)

cm. Shoots yellow green to yellow, branches flabellate.

Basal diameter of dominant shoots 1.5 to 6.0 (mean 2)

mm. Third internode 4 to 23 (mean 14) mm long, 1.5 to

4.0 mm (mean 2) mm wide; length/width ratio is 7:1 to

9:1. Staminate flowers 2.5 mm across; perianth threemerous, sometimes four-merous, apex acute; same

color as shoots; segments 1.2 mm long, 1.0 mm wide.

Mature fruit 4 by 2 mm; proximal portion 2.5 mm long.

Seeds 2.8 by 1.2 mm.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 39

Arceuthobium in North America Hawksworth, Wiens, and Geils

Phenology. Meiosis in July. Anthesis usually August and September. Fruits mature in September or

October of the year following pollination; maturation

period averages 13 to 14 months. Seeds germinate

from February through June.

Hosts. Fir.

Discussion. Parmeter and Scharpf (1963) first report that the dwarf mistletoe on Abies concolor does

not infect associated A. magnifica, and conversely, the

parasite of A. magnifica does not parasitize associated

A. concolor. We are unable, however, to find any

morphological, phenological, or chemical differences

useful to distinguish between the two mistletoes. Because the host affinities of these two dwarf mistletoes

are distinct and they are of considerable importance in

forestry, we treat them as formae speciales. Branch

death or “flagging” by the fungus Cytospora abietis is

one of the most conspicuous field symptoms for infection by this dwarf mistletoe (Scharpf 1969a). The

biology, pathology, and management of fir dwarf mistletoe are discussed by Filip and others (2000), Scharpf

(1969b), and Scharpf and Parmeter (1967, 1982).

Hunt (1993) reorganizes the taxonomy of Abies and

recognizes several combinations not previously used

in the dwarf mistletoe literature. In his treatment,

Abies lasiocarpa refers to west-side populations in the

Pacific Northwest and British Columbia and A. bifolia

to east-side and Rocky Mountain populations. Status

of the corkbark fir (=Abies lasiocarpa var. arizonica )

in the Southwest is left as uncertain. Abies lowiana is

recognized as species rather than subspecies.

1a. Arceuthobium abietinum

White Fir Dwarf Mistletoe

Arceuthobium abietinum Engelm. ex Munz f. sp. concoloris

Hawksw. & Wiens, Brittonia 22:267, 1970.

Hosts. The principal hosts of this dwarf mistletoe

are Abies concolor, A. grandis, A. durangensis, and A.

lowiana. Abies concolor (Nevada, Utah, and Arizona)

and A. lowiana (California) are about equally susceptible, although the dwarf mistletoe is more widely

distributed on the latter. The rare Picea breweriana in

Oregon is associated with infected Abies concolor and

is heavy infected by Arceuthobium abietinum. On the

Key to the Formae Speciales

1. Parasitic principally on Abies concolor or A. grandis; known in two areas in Chihuahua on A. durangensis. The

primary distribution is from southern Washington southward through the Cascade and southern Coast

Ranges in Oregon, and the North Coast and Cascade Ranges, Sierra Nevada to southern California. Isolated

populations occur in southern Utah, northern and southern Arizona, and Chihuahua ...................................

................................................................................................................... 1a. A. abietinum f. sp. concoloris.

1. Parasitic principally on Abies magnifica from southwestern Oregon (Josephine) to the southern Sierra Nevada,

California ...................................................................................................... 1b. A. abietinum f. sp. magnificae.

North Rim of Grand Canyon, Arizona, Abies bifolia

(usually referred to Abies lasiocarpa var. arizonica) is

occasionally parasitized where this tree grows in association with infected A. concolor. Abies amabilis is a

rare host of this dwarf mistletoe at Crater Lake,

Oregon. Pinus ayacahuite var. brachyptera, P. contorta

var. murrayana, P. lambertiana, and P. monticola are

rare hosts.

Distribution. United States (Washington, Oregon,

California, Nevada, Utah, and Arizona), and Mexico

(Chihuahua). Arceuthobium abietinum f. sp. concoloris

is widely distributed from southern Washington

(Skamania, Wenatchee, and Klickitat) south through

the Cascade Range and Sierra Nevada to the San

Bernardino Mountains, California. A single, relict

population is known in the Willamette Valley, Oregon.

It also occurs along the coast ranges from Mendocino,

California, to Curry, Oregon. Isolated populations are

known in Nevada (Spring, Sheep, and Groom Mountains) and Utah (Kane). The parasite is known in

Arizona from the Grand Canyon, the Chiricahua Mountains (Cochise), and the Santa Catalina Mountains

(Pima). This dwarf mistletoe is reported on Abies

durangensis from in two localities in Chihuahua 1,000

km south of Arizona. Arceuthobium abietinum f. sp.

concoloris occurs from near sea level along the coast of

northern California and southern Oregon to over 2,650

m in the Spring Mountains of southern Nevada.

Discussion. In the Northwest, two other species

of Arceuthobium occur on fir: (1) Arceuthobium

tsugense on Abies amabilis, A. grandis, and A.

lasiocarpa and (2) Arceuthobium laricis on Abies

grandis and A. lasiocarpa. However, insofar as we

are aware, neither of these dwarf mistletoes is sympatric with Arceuthobium abietinum. Arceuthobium

tsugense and A. laricis rarely infect pure stands of

fir, but they may parasitize fir secondarily in stands

where the principal hosts of these dwarf mistletoes

are parasitized (for example, hemlock by A. tsugense

and larch by A. laricis). Arceuthobium tsugense

differs from A. abietinum by shorter (7 cm), green to

purple shoots compared with the longer (10 cm),

yellowish shoots of A. abietinum. Arceuthobium
40 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

laricis is readily distinguished from A. abietinum by

shorter, darker shoots (4 cm versus 10 cm) and

shorter (in summer) staminate spikes (2 to 3 mm

versus 5 to 7 mm). 1b.

Arceuthobium abietinum

Red Fir Dwarf Mistletoe

Arceuthobium abietinum Engelm. ex Munz f. sp.

magnificae Hawksw. & Wiens, Brittonia 22:268, 1970.

Hosts. Abies magnifica.

Distribution. United States (Oregon and California). Arceuthobium abietinum f. sp. magnificae is

distributed from Josephine, Oregon, to Kern, California, in the southern Sierra Nevada. Guyon and Munson

(1991) record it within 3 km of the Nevada border.

Elevational range is 1,500 to 2,400 m.

Discussion. Arceuthobium abietinum f. sp.

magnificae is a common and serious disease agent of

the Abies magnifica forests of the Sierra Nevada

(Scharpf 1969b).

2. Arceuthobium abietis-religiosae

Mexican Fir Dwarf Mistletoe

Arceuthobium abietis-religiosae Heil, Zentralblatt f ̧r

Bakteriologie Abteilung 2:28, 1923 [and see

Hawksworth and Wiens, Brittonia 17:231, 1965].

Description. Mean shoot height 10 (maximum 16)

cm. Shoots olive green, older shoots typically with

black variegations, occasionally with verticillate

branching. Basal diameter of dominant shoots 2 to 10

(mean 4) mm. Third internode 8 to 24 (mean 15.4 ±5.3)

mm long, 1 to 4 (mean 2.8) mm wide, length/width

ratio 5.5:1. Staminate buds two to four per node.

Staminate flowers 2 mm long, 2.4 mm across; perianth

mostly three-merous, sometimes four-merous; apex

obtuse-acute; same color as shoots on outer surface,

reddish on inner surface distal to anther; segments 1.2

mm long, 0.9 mm wide. Pistillate flowers 1.0 mm long,

0.5 mm across. Mature fruit 3.5 by 2 mm; proximal

portion 2.5 mm long. Seeds 2.2 by 1.0 mm.

Phenology. Meiosis in September. Anthesis poorly

known but apparently flowering in March to April and

September to October. Fruits probably mature in

October or November.

Hosts. Known only on fir. Abies religiosa (including

var. emarginata) is by far the most common host, but

also this dwarf mistletoe also parasitizes A. vejarii and

probably other Mexican firs.

Distribution. Mexico (Distrito Federal, Hidalgo,

Jalisco, Mexico, Michoacán, Nuevo León, Puebla,

Tamaulipas, Tlaxcala). This dwarf mistletoe is common in the Abies religiosa forests of Central Mexico

and Sierra Madre Oriental (Hernandez and others

1992, Madrigal 1967). Elevational range is 2,500 to

3,350 m.

Discussion. This distinctive Mexican dwarf mistletoe is characterized by its large shoots, occasional

verticillate branching, and exclusive parasitism of fir.

With the exception of the rare occurrence of

Arceuthobium abietinum in Chihuahua, this is the

only dwarf mistletoe that parasitizes fir in Mexico.

3. Arceuthobium americanum

Lodgepole Pine Dwarf Mistletoe

Arceuthobium americanum Nutt. ex Engelm. in Gray,

Boston Journal Natural History 6:214, 1850.

Description. Mean shoot height 5 to 9 (maximum

30) cm. Shoots yellowish to olive green, with verticillate branching (fig. 4-2). Basal diameter of dominant

shoots 1 to 3 (mean 1.5) mm. Third internode 6 to 23

(mean 12 ±3.0) mm long, 1 to 2 (mean 1.2) mm wide

(20 collections), length/width ratio 10.1:1. Staminate

flowers borne on pedicel-like segments, 2 mm long,

2.2 mm across; perianth mostly three-merous, sometimes four-merous; same color as the shoots; segments 1.1 mm long, 1.0 mm wide. Pistillate flowers

verticillate; 1.5 mm long, 1.0 mm across; two-merous.

Mature fruit 3.5 to 4.5 (mean 4) mm long, 1.5 to 2.5

(mean 2) mm wide; proximal portion about 2.5 mm

long. Seeds 2.4 by 1.1 mm.

Phenology. Meiosis in August. Anthesis usually

from early April to early June, with extremes from late

March to late June. Fruits mature in late August or

September of the year following pollination; maturation period averages 16 months. Germination begins

in May in Colorado.

Hosts. The principal hosts are Pinus contorta var.

latifolia, var. murrayana, and P. banksiana; all are

about equally susceptible. Pinus contorta var. contorta

is infected in southern coastal British Columbia (Smith

and Wass 1979). Pinus ponderosa var. scopulorum is

frequently parasitized in Colorado, Utah, and Wyoming,

usually where this tree is associated with infected P.

contorta but also in pure stands of Pinus ponderosa.

Pinus ponderosa var. ponderosa, however, is less susceptible and only occasionally infected. Other occasional hosts include P. albicaulis, P. flexilis, and P.

jeffreyi. Rare, artificially inoculated, or extra-limital

hosts are Abies lasiocarpa (Mathiasen and others

1996a), Picea engelmannii, P. glauca (incompatible),

P. pungens, P. mariana (incompatible), Pinus aristata,

P. mugo, P. sylvestris, and Pseudotsuga menziesii

(incompatible).

Distribution. Canada (British Columbia, Alberta,

Saskatchewan, Manitoba, and Ontario) and the United

States (Washington, Idaho, Montana, Oregon, California, Utah, Wyoming, Colorado, and possibly Nevada).

Arceuthobium americanum has the most extensive

distribution of any North American dwarf mistletoe.

The distribution of Arceuthobium americanum is centered on the range of its principal host, Pinus contorta,

and rarely occurs within the distribution of Pinus

contorta var. contorta (shore pine). Arceuthobium

americanum occurs in outlying populations of Pinus
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 41

Arceuthobium in North America Hawksworth, Wiens, and Geils

contorta var. latifolia in Southeastern Alberta and in

north central Montana (Phillips, Hill, and Liberty).

Arceuthobium americanum distribution maps include

Alberta, British Columbia, Manitoba, Saskatchewan,

Montana, Utah, Colorado, and California (see Brandt

and others 1998, Hawksworth and Wiens 1996, Muir

2002). This dwarf mistletoe varies in elevation from

200 m near Lake Athabasca in northern Alberta and

Saskatchewan to 3,350 m in central Colorado.

Discussion. Arceuthobium americanum induces

characteristic systemic witches’ brooms on Pinus

contorta and produces the same type of broom on P.

ponderosa. The witches’ brooms formed on Picea

engelmannii, however, are nonsystemic (Hawksworth

and Graham 1963a). Kuijt (1960a) notes that A.

americanum cannot perpetuate itself over time on

Pinus jeffreyi or P. ponderosa var. ponderosa in California. In northern Colorado and southern Wyoming,

however, the parasite is aggressive in pure stands of

P. ponderosa var. scopulorum outside the range of

A. vaginatum subsp. cryptopodum, which is the typical parasite on P. ponderosa in the Rocky Mountains.

Hawksworth and Johnson (1989a) provide a synopsis

of the biology and management of this mistletoe in the

Rocky Mountains. Other general and silvicultural

information is given by Baranyay (1970), Hawksworth

and Dooling (1984), van der Kamp and Hawksworth

(1985), and Van Sickle and Wegwitz (1978).

4. Arceuthobium apachecum

Apache Dwarf Mistletoe

Arceuthobium apachecum Hawksw. & Wiens, Brittonia

22:266, 1970.

=A. campylopodum f. blumeri

Description. Mean shoot height 3 to 4 (maximum

9) cm. Shoots yellow, green, or reddish, branches

flabellate and densely clustered. Basal diameter of

dominant shoots 1 to 2 (mean 1.8) mm. Third internode 5 to 10 (mean 7.2 ± 2.0) mm long, 1 to 2 (mean 1.5)

mm wide, length/width ratio 4.8:1. Flowers axillary.

Staminate flowers 2.7 mm across; perianth three- to

four-merous; same color as shoots; segments 1.3 mm

long, 0.9 mm wide. Mature fruit 4 by 2.5 mm; proximal

portion 2.5 mm long. Seeds 2.8 by 1.2 mm.

Phenology. Meiosis in July. Anthesis from late

July to mid-September, peak in mid-August. Fruits

mature from mid-August to mid-October, peak in

September; maturation period averages about 13

months.

Host. Known only naturally on Pinus strobiformis,

but successfully inoculated by Mathiasen (1978) on

Pinus flexilis.

Distribution. United States (Arizona, New Mexico)

and Mexico (Coahuila). This dwarf mistletoe has a

limited distribution in southern Arizona and central

New Mexico, with an outlier in the Sierra del Carmen

in northern Coahuila. In Arizona, it occurs in the

White, Pinaleno, Santa Catalina, Santa Rita, and

Chiricahua Mountains and in New Mexico in the

Mangas, San Mateo, Magdalena, and Capitan Mountains. Elevational range is 2,000 to 3,000 m.

Discussion. The exclusive occurrence of two dwarf

mistletoes species, Arceuthobium apachecum and A.

blumeri, on a single host species, Pinus strobiformis, is

unique in Arceuthobium. Geographically consistent

morphological and broom differences indicate that

separate taxonomic status is warranted (Mathiasen

1982). Although they are not sympatric, they approach 60 km of each other in southern Arizona.

Arceuthobium apachecum, but not A. blumeri, frequently induces witches’ broom formation.

5. Arceuthobium aureum subsp. petersonii

Peterson’s Dwarf Mistletoe

Arceuthobium aureum Hawksw. & Wiens subsp.

petersonii Hawksw. & Wiens, Brittonia 29:415, 1977.

=A. globosum

Description. Shoots 14 to 40 (mean 24) cm tall,

golden to yellow-brown, branches flabellate. Basal

diameter of dominant shoots 14 to 35 (mean 23) mm.

Third internode 14 to 35 (mean 23) mm long and 2.5 to

8 (mean 5) mm wide.

Phenology. Anthesis in September. Fruits mature

June and July; maturation period of 9 to 10 months,

which is several months less than is common for many

dwarf mistletoes.

Hosts. Pinus michoacana, P. montezumae, P.

oaxacana, P. oocarpa, P. patula, and P. pseudostrobus

are the principal and only hosts. Pinus michoacana is

somewhat less susceptible and is infected only when it

grows in association with the other principal hosts.

Distribution. Mexico (Oaxaca, Chiapas). This dwarf

mistletoe is common between San Cristóbal de las

Casas and Teopisabout (Chiapas). Its distribution in

Oaxaca is poorly known by a few collections from

Miahuátlan to Suchixtepec. Elevational range is 2,200

to 2,450 m.

Discussion. The taxon recognized here as

Arceuthobium aureum had been in the Arceuthobium

globosum complex (Hawksworth and Wiens 1972,

1977). Arceuthobium aureum includes two subspecies,

but only subspecies petersonii is found in Mexico. This

subspecies is characterized by tall, slender, brown to

golden shoots, long fruits (5 mm), long pedicels (4 mm),

and tendency to form witches’ brooms.

6. Arceuthobium blumeri

Blumer’s Dwarf Mistletoe

Arceuthobium blumeri A. Nels., Botanical Gazette

56:65, 1913.

=A. campylopodum var. cryptopodum

=A. campylopodum f. blumeri.

Description. Mean shoot height 6 to 7 (maximum

18) cm, gray to straw or light green, branches flabellate.
42 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Basal diameter of dominant shoots 1 to 3 (mean 2.1)

mm. Third internode 5 to 14 (mean 9.1 ±2.5) mm long,

1 to 2 (mean 1.6) mm wide, length/width ratio 5.5:1.

Staminate flowers 2.5 mm long, 2.5 to 3.0 mm across;

perianth three- to six-merous (mostly three- or fourmerous), segments 1.3 mm long, 1.0 mm wide, apex

acute. Mature fruit 4 by 2.5 mm, proximal portion 2.5

mm long. Seeds 2.7 by 1.0 mm.

Phenology. Meiosis in July. Anthesis from midJuly to late-August, with a peak in early August

(Mathiasen 1982). Fruits mature from late August to

early October, with a peak in mid-September; maturation period averages 13 to 14 months.

Hosts. Pinus strobiformis and P. ayacahuite var.

brachyptera. The host affinities of Arceuthobium

blumeri are not clear because of the taxonomic confusion surrounding the white pine complex of Pinus

flexilis-strobiformis-ayacahuite (Equiluz 1991,

Hawksworth 1991, Perry 1991). Most host populations of this dwarf mistletoe are best referred to P.

ayacahuite var. brachyptera in the Sierra Madre Occidental and P. strobiformis var. potosiensis on Cerro

Potosí (Nuevo León). Pinus flexilis can be infected by

inoculation (Mathiasen 1978).

Distribution. United States (Arizona) and Mexico

(Sonora, Chihuahua, Durango, Nuevo León, and

Coahuila). This dwarf mistletoe extends southward

from the Huachuca Mountains in southern Arizona

through the Sierra Madre Occidental in Chihuahua

and Sonora to southern Durango. In the Sierra Madre

Oriental, it is known only from Cerro Potosi (Nuevo

León) and San Antonio de las Alazanas (Coahuila), but

it probably occurs elsewhere over this extensive distribution (Cibrián and others 1980). Elevational range is

2,150 to 3,250 m.

Discussion. The parasitism of Arceuthobium

blumeri and A. apachecum on Pinus strobiformis is

discussed under A. apachecum. Distinctive features of

Arceuthobium blumeri include its gray-colored shoots,

four- to six-merous staminate flowers, and rare formation of witches’ brooms.

7. Arceuthobium californicum

Sugar Pine Dwarf Mistletoe

Arceuthobium californicum Hawksw. & Wiens,

Brittonia 22:266, 1970.

=A. campylopodum f. cryptopodum

=A. campylopodum f. blumeri.

Description. Mean shoot height 8 cm (maximum

12) cm, greenish to bright yellow, turning brown at

base of older shoots, branches flabellate. Basal diameter of dominant shoots 1.5 to 4.0 (mean 2) mm. Third

internode 6 to 16 (mean 10.5 ±2.9) mm long, 1 to 2

(mean 1.5) mm wide, length/width ratio 7.0:1. Flowers

axillary. Staminate flowers 3.3 mm across; perianth

three- or four-merous, segments 1.5 mm long, 1.1 mm

wide. Mature fruit 4 by 2.5 mm; proximal portion 2.0

mm long. Seeds 3.2 by 1.2 mm.

Phenology. Meiosis in July. Anthesis usually in

mid-July to mid-August, with extremes from early

July to late August. Fruits mature from mid-September to mid-October, with extremes from late August to

early November; maturation period averages 13 to 14

months.

Hosts. The only principal host is Pinus lambertiana.

In association with infected P. lambertiana, P.

monticola is secondarily parasitized (Mathiasen and

Hawksworth 1988). Infected P. lambertiana produce

large, compact witches’ brooms.

Distribution. United States (California). This species is distributed from Mount Shasta southward

through the North Coast Range, and through the

Cascade Range south to Lake County and the west

side of the Sierra Nevada to the Cuayamaca Mountains (San Diego). Elevational range is 600 to 2,000 m.

Discussion. Arceuthobium californicum is common in many areas and a serious pathogen of Pinus

lambertiana (Scharpf and Hawksworth 1968).

8. Arceuthobium campylopodum

Western Dwarf Mistletoe

Arceuthobium campylopodum Engelm. in Gray, Boston Journal Natural History 6:214, 1850.

=A. campylopodum f. typicum.

Description. Mean shoot height 8 (maximum 13)

cm, olive green to yellow, branches flabellate. Staminate plants brownish, and pistillate plants greenish.

Basal diameter of dominant shoots 1.5 to 5.0 (mean 3)

mm. Third internode 7 to 22 (mean 11.3 ±3.8) mm long,

1.5 to 2.5 (mean 2.0) mm wide, length/width ratio

5.6:1. Staminate flowers 3.0 mm across; perianth

three-merous (occasionally four-merous), segments

1.4 mm long, 1.0 mm wide. Mature fruit 5.0 by 3.0 mm.

Phenology. Meiosis in July. Peak anthesis usually

from mid-August to early October, with extremes from

early August to late October. Fruits usually mature

from early September to mid-November, with extremes from late August to late November; maturation period averages 13 months.

Hosts. The principal and most commonly infected

hosts are Pinus ponderosa var. ponderosa and P.

jeffreyi. Pinus jeffreyi is somewhat more susceptible

than P. ponderosa, but both species incur considerable

damage. Other trees frequently infected, particularly

when associated with the above hosts, are Pinus

attenuata and P. coulteri. In the Spring Mountains,

Nevada, P. ponderosa var. scopulorum is a common

and seriously damaged host, but this is the only known

area where Arceuthobium campylopodum occurs naturally within the range of scopulorum. Occasional hosts

for A. campylopodum are P. contorta var. latifolia, var.

murrayana, and P. sabiniana. Pinus lambertiana is a
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 43

Arceuthobium in North America Hawksworth, Wiens, and Geils

rare host. Hosts by artificial inoculation are Abies

concolor, A. grandis, Picea abies, Pinus sylvestris, P.

mugo, P. resinosa, and Larix occidentalis. Although

Abies concolor, A. grandis, and Larix occidentalis are

commonly associated with Pinus ponderosa infected

by A. campylopodum, they are not known to be naturally infected. Pinus washoensis is expected to be

susceptible, but we know of no collections or reports on

this species.

Distribution. United States (Washington, Idaho,

Oregon, California, and Nevada) and Mexico (Baja

California Norte). Arceuthobium campylopodum occurs from northern Washington and eastern Idaho,

south through Oregon and California (but not the

southern Coast Range) to the Sierra Juárez and Sierra

de San Pedro Mártir (Baja California Norte). The distribution of this and other taxa in California is discussed

by Kuijt (1960a). In Nevada, it occurs near Lake Tahoe

and in the Spring Mountains (Clark). Arceuthobium

campylopodum is distributed by elevation from 30 m

along the Columbia River, near Hood River, Oregon, to

2,500 m in the Spring Mountains, Nevada.

Discussion. Arceuthobium campylopodum is a serious pathogen of Pinus jeffreyi and P. ponderosa. Our

observations suggest that host damage is more severe

in the southern or drier parts of the distribution. The

most severely infested stands are in the California

Laguna Mountains and on the east-side of the Sierra–

Cascade forests. The biology, ecology, and management of this mistletoe are discussed by Kimmey and

Mielke (1959), Schmitt (1996), and Stand and Roth

(1976). The serious mortality caused by this mistletoe

to pine in Oregon is described by Roth (2001).

9. Arceuthobium cyanocarpum

Limber Pine Dwarf Mistletoe

Arceuthobium cyanocarpum (A. Nels. ex Rydb.) A.

Nels., New Manual of Botany of the Central Rocky

Mountains, p. 146, 1909.

=A. campylopodum f. cyanocarpum.

Description. Mean shoot height 3 (maximum 7)

cm, yellow–green, branches flabellate, densely clustered. Basal diameter of dominant shoots 1 to 2 (mean

1.4) mm. Third internode 2 to 14 (mean 5.2 ±2.0) mm

long, 1.0 to 1.5 (mean 1.1) mm wide; length/width ratio

4.7:1. Staminate flowers 3.0 mm across; perianth

three-merous (rarely four-merous), same color as

shoots; segments 1.4 mm long, 1.0 mm wide, apex

acute. Mature fruit 3.5 by 2.0 mm; proximal portion

2.0 mm long. Seeds 2.0 b 0.9 mm.

Phenology. Meiosis in July. Peak anthesis from

mid-July to early September, with extremes from

early July to mid-September. Fruits mature from midAugust to late September; maturation averages 12

months. Seed germination mostly in June.

Hosts. Pinus flexilis is the most common host of this

dwarf mistletoe throughout its extensive geographical

range. Pinus albicaulis, P. aristata, and P. longaeva

are also principal hosts even though they are not

common within the range of Arceuthobium

cyanocarpum. Pinus albicaulis is infected in western

Wyoming, northern Nevada, central Oregon, and northern California. Infection of P. aristata is known from

La Veta Pass, Colorado, in association with infected P.

flexilis. Pinus longaeva is parasitized in many areas of

Utah and Nevada. In northern California, Pinus

monticola is a secondary host; and Pinus balfouriana

is an occasional host (Mathiasen and Daughtery 2001).

Tsuga mertensiana in central Oregon is another secondary host; and other occasional or rare hosts include

Picea engelmannii (doubtful), P. contorta var. latifolia,

and P. ponderosa var. scopulorum. Pinus strobus and

P. strobiformis are susceptible to infection by artificial

inoculation (Hawksworth and Wiens 1972).

Distribution. United States (Idaho, Montana,

Oregon, California, Nevada, Utah, Wyoming, and

Colorado). This dwarf mistletoe occurs from southern

Montana and northern Wyoming south to southern

Colorado and west to Oregon and California where it

occurs on the east side of the Sierra Nevada, in the

Panamint Mountains (Death Valley National Monument), and in the San Bernardino to San Jacinto

Mountains (southern California). Distribution maps

for Arceuthobium cyanocarpum are available for Colorado and Nevada (see Hawksworth and Wiens 1996).

Elevational range is 1,600 m in southern Montana to

nearly 3,050 m in central Colorado.

Discussion. This dwarf mistletoe, which characteristically infects Pinus flexilis and associated highaltitude white pines, is easily recognized by small,

densely clustered shoots and common branch flagging. Witches’ brooms are typically small and compact, and infection is usually throughout the entire

crown. Arceuthobium cyanocarpum causes heavy

mortality in Pinus flexilis in the Rocky Mountains and

in P. albicaulis on Mount Shasta, California (Mathiasen

and Hawksworth 1988).

10. Arceuthobium divaricatum

Pinyon Dwarf Mistletoe

Arceuthobium divaricatum Engelm. in U.S. Geographical Survey West of 100th Meridian (Wheeler Report)

6:253, 1878.

=A. campylopodum f. divaricatum.

Description. Mean shoot height 8 (maximum 3)

cm, olive green to brown, branches flabellate. Basal

diameter of dominant shoots 1.5 to 4.0 (mean 2) mm.

Third internode 6 to 15 (mean 9.8 ±2.4) mm long, 1 to

2 (mean 1.6) mm wide, length/width ratio 6.1:1. Staminate flowers 2.5 mm across; perianth three-merous;

segments 1.1 mm long, 0.9 mm wide. Mature fruit 3.5

by 2.0 mm; proximal portion 2.0 mm long. Seeds 2.0 by

0.9 mm.
44 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Phenology. Meiosis in July. Peak anthesis usually

from early August to late September. Fruits usually

mature from early September to late October in the

year following pollination; maturation period averages 13 months.

Hosts. Arceuthobium divaricatum is restricted to

pinyon. The most common principal hosts are Pinus

edulis (Arizona, Colorado, New Mexico, Texas, and

Utah) and P. monophylla (California, Nevada). A second set of pinyons including P. californiarum, P.

cembroides, P. discolor, and P. quadrifolia are also

classed as principal hosts even though the mistletoe is

not common in their distributions. Infestations occur

locally on P. californiarum in the Mojave Desert Ranges

of New York Mountains, Providence Mountains, Joshua

Tree National Monument (subsp. californiarum), and

Southwest mountains of Zion National Park, Black

Hole, and central Arizona (subsp. fallax). Pinus

cembroides is parasitized only in the Davis Mountains, Texas. Pinus discolor is parasitized only at Fort

Bayard and the Mule Mountains, New Mexico. Pinus

quadrifolia is parasitized in the Sierra Juárez and

Sierra San Pedro Mártir of Baja California and Laguna Mountains, California.

Distribution. United States (California, Nevada,

Utah, Colorado, Arizona, New Mexico, and Texas) and

Mexico (Baja California Norte). Arceuthobium

divaricatum occurs in eastern and southern California (the White and Inyo Mountains, the Mount Pinos

area, the San Bernardino Mountains, and the Mojave

Desert Ranges), the southern three-fourths of Nevada

and Utah, western Colorado, Arizona (except far southwest), New Mexico (except far northeast), and south to

the Davis Mountains (western Texas). In Mexico, it is

known only in northern Baja California. The northernmost population of which we are aware is in the

Pilot Range (Box Elder, Utah). Kuijt (1960a) identifies

several the scattered populations of this parasite in

California; its distribution is probably more common

than indicated by collections. Arceuthobium

divaricatum and Phoradendron juniperinum commonly infest the two dominant species respectively of

pinyon–juniper woodlands of the Southwestern United

States, especially at the Grand Canyon (Hreha and

Weber 1979). Distribution maps are published for

Colorado, Utah, and New Mexico (see Hawksworth

and Wiens 1996). Elevational range is from 1,200 m

near Sedona, Arizona to 3,000 m in the San Mateo

Mountains of New Mexico.

Discussion. The witches’ brooms induced by this

dwarf mistletoe are often poorly developed and not

conspicuous because of the stunted habit of even

healthy trees. Our observations suggest that witches’

brooms are more consistent in Pinus edulis than in P.

monophylla. Shoots of the mistletoe are often long,

slender, and spreading, especially the staminate plants

that also tend to have relatively few flowers per shoot.

Arceuthobium divaricatum is the only dwarf mistletoe

of pinyon in the United States. Mathiasen and others

(2002a) summarize information on this mistletoe.

11. Arceuthobium douglasii

Douglas-fir Dwarf Mistletoe

Arceuthobium douglasii Engelm. in U.S. Geographical Survey West of 100th Meridian (Wheeler Report)

6:253, 1878.

Description. Mean shoot height 2 (maximum 8)

cm, olive green, branches flabellate (fig. 4-3). Basal

diameter of dominant shoots 1.0 to 1.5 (mean 1) mm.

Third internode 2 to 6 (mean 3.6 ±1.2) mm long, 1.0

mm wide, length/width ratio 3.6:1. Flowers usually

axillary in pairs, occasionally borne on pedicel-like

segments. Staminate flowers 2.0 mm long, 2.3 mm

across; perianth mostly three-merous (occasionally

four- or two-merous); segments rounded at the apex,

without a keel, inner surface reddish to purple, lower

surface same color as shoots, about 1.0 mm long, 1.0

mm wide. Pistillate flowers 1.5 mm long, 1.5 mm

across. Mature fruit olive–green 3.5 to 4.5 (mean 4)

mm long, 1.5 to 2.0 mm wide, obovate; proximal

portion 2.5 mm long. Seeds 2.4 by 1.1 mm.

Phenology. Staminate meiosis in September, pistillate meiosis in April. Peak anthesis is usually in

April or May, but with marked latitudinal variation—

March in Mexico, late April to early May in Arizona

and New Mexico, late May in Colorado, Utah, and

Oregon, and early to mid-June in Washington, northern Idaho, and Montana. Fruit maturity is more uniform throughout the distribution, however, usually

from late August to late September; maturation period averages 17 to 18 months. The seeds germinate in

March.

Hosts. The principal and only commonly infected

host is Pseudotsuga menziesii. Both var. menziesii

(Washington, Oregon, and California) and var. glauca

(from British Columbia through the Rocky Mountains

to Central Mexico) are parasitized, although it is much

more common on var. glauca. Where associated with

infected Pseudotsuga menziesii, Abies amabilis is occasionally infected. Rare hosts are Abies concolor, A.

grandis, Picea pungens, and P. engelmannii. Mathiasen

(1999) reports that the two taxa Abies lasiocarpa, a

secondary host (66 percent infected), and Abies bifolia,

an occasional host (15 percent infected) differed significantly in susceptibility to Arceuthobium douglasii

on plots where the principal host was over 90 percent

infected.

Distribution. Canada (British Columbia), United

States (Washington, Idaho, Montana, Oregon, California, Nevada, Utah, Colorado, Arizona, New Mexico,

and Texas) and Mexico (Chihuahua, Durango,

Coahuila, and Nuevo León). Arceuthobium douglasii
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 45

Arceuthobium in North America Hawksworth, Wiens, and Geils

has the greatest latitudinal range (3,000 km) of any

species in the genus. This dwarf mistletoe is common

in eastern Washington, eastern Oregon, Idaho, western Montana, Idaho, Utah, Colorado, and New Mexico.

It is rare in Nevada (Wheeler Peak), Wyoming (Teton),

and Texas (Guadalupe Mountains). Marshall and Filip

(1999) relate the occurrence of this mistletoe to stand

and ecological relations in Oregon. The distribution of

the dwarf mistletoe in Mexico is poorly known, and it

is probably more widespread than suggested by a few

available records from Chihuahua, Coahuila, Durango,

and Nuevo León. Distribution maps of Arceuthobium

douglasii are published for British Columbia, Montana, Utah, Colorado, New Mexico, and California (see

Hawksworth and Wiens 1996). The altitudinal range

of this dwarf mistletoe is correlated with latitude; it

occurs as low as 300 m near Lytton (British Columbia)

and as high as 3,250 m on Cerro Potosí (Nuevo León).

Discussion. This dwarf mistletoe is the smallest in

Western North America, but its typically systemic

mode of infection produces large witches’ brooms and

causes severe growth loss and mortality in Pseudotsuga

menziesii (Tinnin and others 1999). Brooms provide

special wildlife habitat for foraging, resting, and nesting (see chapter 5). Hadfield and others (2000) and

Schmitt (1997) discuss the biology, ecology, and management of this mistletoe.

12. Arceuthobium durangense

Durangan Dwarf Mistletoe

Arceuthobium durangense (Hawksw. & Wiens)

Hawksw. & Wiens, Phytologia 66:7, 1989.

=A. vaginatum subsp. durangense.

Description. Mean shoot height 20 to 30 (50) cm,

bright orange; older shoots becoming pendulous. Basal

diameter of dominant shoots 4 to 8 (mean 6) mm. Third

internode 9 to 22 (mean 17.9 ±4.1) mm long, 3.5 to 6.0

(mean 4.5) mm wide, length/width ratio 3.3:1. Internodes often slightly swollen at base. Staminate flowers

2.5 mm long, 2.5 mm across, segments 1 mm long, 1

mm wide. Mature fruit 7 by 3.5 mm; bluish; proximal

portion 4 mm long. Seeds 4 by 1.5 mm.

Phenology. Time of meiosis unknown (probably

February). Anthesis usually in April. Fruits mature

from mid-July to September of the year following

pollination; maturation period averages 15 to 18

months.

Hosts. Pinus douglasiana, P. durangensis, P.

michoacana, P. montezumae, and P. pseudostrobus

are the principal hosts. Pinus herrerai is occasionally

parasitized when it occurs near infected principal

hosts. The host status of Pinus oocarpa needs confirmation.

Distribution. Mexico (Durango, Sinaloa, and Jalisco).

This rather local dwarf mistletoe occurs on the western

escarpment of the Sierra Madre Occidental (Durango,

Sinaloa, and perhaps Nayarit) and in the Sierra de

Quilla (Jalisco). Elevational range is 1,450 to 2,750 m.

Discussion. Although previously referred to as a

subspecies of Arceuthobium vaginatum, we now recognize this dwarf mistletoe as a distinct species.

Arceuthobium durangense is not sympatric with A.

vaginatum and differs by its larger, bright orange

shoots, distinct branching pattern, and larger fruit.

13. Arceuthobium gillii

Chihuahua Pine Dwarf Mistletoe

Arceuthobium gillii Hawksw, & Wiens, Brittonia 16:22,

1964.

=A. vaginatum subsp. cryptopodum.

Description. Mean shoot height 8 to 15 (maximum

25) cm, greenish–brown, branches flabellate. Basal

diameter of dominant shoots 2.5 to 8.0 (mean 4) mm.

Third internode 5 to 18 (mean 10.7 ±3.4) mm long, 2.0

to 4.5 (mean 2.8) mm wide, length/width ratio 3.8:1.

Staminate flowers 3.5 mm long, 2.5 to 4.0 (mean 3.2)

mm across. Pistillate flowers 1.5 mm long, 1 mm

across. Mature fruit 4 to 5 mm long, 2 to 3 mm wide, the

proximal portion of fruit conspicuously glaucous. Seeds

3.1 by 1.4 mm.

Phenology. Meiosis in September. Anthesis usually in March and April. Fruits mature in October of

the year following pollination; maturation period averages 19 months, the longest in the genus. Seed

germination begins in April.

Hosts. The principal and only commonly infected

hosts are Pinus leiophylla var. chihuahuana, P.

lumholtzii, and P. herrerai. Although Pinus. leiophylla

var. leiophylla is a principal host, it is not common

within the range of Arceuthobium gillii. In western

Chihuahua, this dwarf mistletoe rarely parasitizes

Pinus arizonica var. arizonica and P. cooperi.

Distribution. United States (Arizona and New

Mexico) and Mexico (Chihuahua, Durango, Sinaloa,

and Sonora). This dwarf mistletoe occurs in southeastern Arizona (Santa Catalina, Rincon, Santa Rita,

Huachuca, and Chiricahua Mountains) and the Animas

Mountains in southwestern New Mexico. It is most

common in western Chihuahua, but it is also distributed in adjacent northern and eastern Sonora, northern Durango, and northeastern Sinaloa. Elevational

range is from 1,700 m in southern Arizona to 2,650 m

in southern Chihuahua.

Discussion. This dwarf mistletoe has long been

confused with Arceuthobium vaginatum subsp.

cryptopodum, but it differs in host preference, phenology, and its conspicuously glaucous fruits. These two

dwarf mistletoes are usually separated by at least 300

m of elevation in Arizona and New Mexico. Where they

co-occur in central Chihuahua, there is no evidence of

hybridization. A characteristic feature of Arceuthobium

gillii is its strong sexual dimorphism—staminate plants
46 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

tall and openly branched and pistillate plants small

and densely branched. This dwarf mistletoe causes

open, nonsystemic witches’ brooms and serious mortality in Pinus leiophylla var. chihuahuana and P.

lumholtzii.

14. Arceuthobium globosum

Arceuthobium globosum Hawksw. & Wiens, Brittonia

17:223, 1965.

Description. Shoot height 20 to 50 (maximum 70)

cm, yellow to greenish, branches flabellate. Basal

diameter of dominant shoots 3 to 48 mm. Third internode 4 to 37 mm long, 2 to 24 mm wide. Staminate

flowers about 3.5 to 5.0 mm long, 3.0 to 3.5 mm across;

perianth three- or four-merous; same color as shoots;

segments 1.3 mm long, 1.0 mm wide. Pistillate flowers

1.5 mm long, 1.5 mm across. Mature fruit 5 to 7 mm

long, 3 to 4 mm wide; proximal portion 3.5 mm long,

with pedicels 4.0 to 5.0 mm long. Seeds 5 by 2 mm.

Hosts. Common on yellow pine.

Discussion. Hawksworth and Wiens (1972) note

considerable variation within collections determined

as Arceuthobium globosum. Subsequent studies by

Hawksworth and Wiens (1977) and Wiens and Shaw

(1994) have resulted in the segregation of Arceuthobium

globosum (sensu lato) into five taxa:

• Arceuthobium aureum subsp. aureum (Guatemala)

• Arceuthobium aureum subsp. petersonii (Southern Mexico)

• Arceuthobium globosum subsp. globosum (Northwestern Mexico)

• Arceuthobium globosum subsp. grandicaule (Central Mexico and Central America)

• Arceuthobium hawksworthii (Central America)

14a. Arceuthobium globosum subsp. globosum

Rounded Dwarf Mistletoe

Description. Shoots 15 to 20 (maximum 50) cm

high, bright yellow, branches flabellate. Basal diameter of dominant shoots 3 to 10 (mean 7) mm. Third

internode 19 mm long, 4 mm wide. Staminate flowers

4 mm wide. Mature fruit 5 by 2.5 mm. Seeds 4 by 2 mm.

Phenology. Anthesis usually March and April.

Fruits mature June and July; maturation period averages 15 to 16 months.

Hosts. The principal hosts are Pinus cooperi, P.

durangensis, and P. engelmannii. Pinus arizonica is

occasionally parasitized; Pinus teocote is a rare host.

Distribution. Mexico (Sonora, Chihuahua,

Durango, and Jalisco). This subspecies is widely distributed in the pine forests of the Sierra Madre Occidental from northwestern Chihuahua and adjacent

Sonora, through Durango to northern Jalisco.

Elevational range is 2,300 to 2,800 m.

Discussion. This dwarf mistletoe is characterized

by its bright yellow, globose clusters, and absence of

witches’ broom formation.

14b. Arceuthobium globosum subsp. grandicaule

Large-Stemmed Dwarf Mistletoe

Arceuthobium globosum Hawksw. & Wiens subsp.

grandicaule Hawksw. & Wiens, Brittonia 29:413, 1977.

Description. Shoots 18 to 50 (maximum 70, mean

25) cm tall, yellow green, typically dark at the base of

older shoots, branches flabellate. Basal diameter of

dominant shoots 10 to 48 (mean 17) mm. Third internode 14 to 37 (mean 27) mm long, 3 to 20 (mean 7) mm

wide. Staminate flowers 5 mm wide, four-merous.

Mature fruits 6 to 7 mm long, 3.5 mm wide. Seeds 5 by

3 mm.

Phenology. Meiosis in December. Anthesis from

January through May, with peak March and April.

Fruits maturing July through October; maturation

period averages 16 to 18 months. Seed dispersal from

early July to early November, with a peak from midAugust to mid-September (Escudero and Cibrián 1985).

Hosts. This subspecies has one of the broadest host

ranges of any dwarf mistletoe. It infects at least 12

species of Mexican pines, all of which appear to be

about equally susceptible (but see Valdivia 1964):

Pinus douglasiana, P. durangensis, P. hartwegii, P.

Key to the Subspecies of Arceuthobium globosum

1. Plants yellowish; shoots usually 15 to 20 (maximum 50) cm tall, less than 1 cm diameter at the base; witches’

brooms not formed; shoots usually only on the host branches; northern Mexico ..............................................

................................................................................................................. 14a. A. globosum subsp. globosum

1. Plants greenish to yellow–green, typically dark at the base of older shoots; shoots usually 25 to 40 (maximum

70) cm tall, greater than 1 cm diameter at the base; witches’ brooms usually induced; shoots on branches, but

also frequently on the lower main trunks; central and southern Mexico to the highlands of western Guatemala

............................................................................................................ 14b. A. globosum subsp. grandicaule.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 47

Arceuthobium in North America Hawksworth, Wiens, and Geils

lawsonii, P. maximinoi, P. michoacana, P. montezumae,

P. patula, P. pringlei, P. pseudostrobus, P. rudis, and

P. teocote.

Distribution. Mexico (Jalisco, Michoacán, Mexico,

Hidalgo, Distrito Federal, Guerrero, Puebla, Tlaxcala,

Veracruz, and Oaxaca), Guatemala, and Honduras.

Arceuthobium globosum subsp. grandicaule is the

most abundant dwarf mistletoe in Central Mexico,

common in western Guatemala, and recently reported

in Honduras (Melgar and others 2001). Elevational

range is 2,450 to 4,000 m (Hernandez and others

1992).

Discussion. This subspecies has large shoots reaching a height of 70 cm and with a basal diameter of 5 cm.

Valdivia (1964) reports Arceuthobium globosum is

present on nearly 40 percent of 400,000 ha of pine

forest in northeastern Michoacán. Vázquez (1994a)

discusses the importance and sampling method for

this mistletoe.

15. Arceuthobium guatemalense

Guatemalan Dwarf Mistletoe

Arceuthobium guatemalense Hawksw. & Wiens,

Brittonia 22:267, 1970.

Description. Mean shoot height 1 to 3 cm on systemic witches’ brooms, but shoots on nonsystemic

infections up to 7 cm high; living shoots greenish to

purple, yellow to brown when dried, branches flabellate. Basal diameter of dominant shoots 2.0 to 2.5 mm.

Third internode 8 to 15 (mean 11.4 ±2.8) mm long, 1.5

to 2.0 (mean 1.7) mm wide; length/width ratio 6.7:1.

Staminate flowers 2 mm across; perianth two- or

three-merous, segments 0.9 mm long, 0.7 mm wide.

Mature fruit 3.5 to 4.0 mm long, 1.5 to 2.0 mm wide;

distal portion 1.2 mm long; dark green, glabrous, with

a slightly swollen ring at the base of the fruit where it

joins the pedicel. Seeds 2.0 by 0.8 mm.

Phenology. Time of meiosis unknown. Anthesis

apparently in August and early September. Fruits

mature in September; maturation period about 12 to

13 months. Seed germination in September.

Hosts. Known only on Pinus ayacahuite var.

ayacahuite.

Distribution. Mexico (Oaxaca and Chiapas) and

Guatemala. This distinctive species is known only

from the high mountains of Western Guatemala and

Southern Mexico. Elevational range is poorly known;

our collections are from 2,450 to 3,100 m.

Discussion. The consistent formation of systemic

witches’ brooms is a distinctive characteristic of this

species; brooms sometimes measure 3 to 5 m across.

An unusual feature of these witches’ brooms is that the

shoots of the dwarf mistletoe are consistently formed

on 1-year-old host shoots and, in some cases, on the

current year’s growth. This species causes extensive

damage and considerable mortality to Pinus

ayacahuite.

16. Arceuthobium hondurense

Honduran Dwarf Mistletoe

Arceuthobium hondurense Hawksworth & Wiens,

Brittonia 22: 267, 1970.

=Arceuthobium nigrum

Description. Mean shoot height ca. 14 (max. 21)

cm, olive brown to grayish green, markedly glaucous;

branches flabellate. Basal diameter of dominant shoots

3 to 9 (mean 5) mm; nodes of older shoots swollen;

lateral branches of staminate plants at nearly right

angles to the axis of the main shoot; third internode 7

to 12 (mean 9.1 ± 1.5) mm long, 2.5 to 4.0 (mean 3.2)

mm wide; length/width ratio 6.1:1. Staminate flowers

approximately 2.5 mm across; inner surface reddish,

lower surface the same color as the shoots; perianth

usually three-merous (sometimes two- or four-merous),

segments approximately 1.2 mm long, 0.8 mm wide;

nectary with two large and one small lobe. Pistillate

flowers with stigmas exserted approximately 0.5 mm,

with copious stigmatic exudate at anthesis. Mature

fruit 5.5 by 3.0 mm, greenish glaucous; proximal

portion approximately 4.0 mm long. Seeds approximately 3.1 by 1.5 mm. n = 14.

Phenology. Meiosis in August or early September.

Anthesis and fruit maturity in September; maturation period averages ca. 12 months.

Hosts. The only known hosts are Pinus oocarpa var.

oocarpa, var. ochoterenia, and P. tecunumanii

(Mathiasen and others 1998, 2000a).

Distribution. Honduras, Mexico (Chiapas, Oaxaca),

and possibly El Salvador. The distribution of this

species is poorly known; only four collections are

known from Honduras and three from Mexico

(Mathiasen and others 2001, 2002b). Elevational range

is poorly known, probably between 1,200 and 1,650 m.

Discussion. Collections from Mexico had been previously confused with Arceuthobium nigrum

(Mathiasen and others 2001, 2002b). Arceuthobium

hondurense and A. bicarinatum, a species endemic to

Hispaniola, are both rare species threatened by deforestation and are distributed at the southern limits of

dwarf mistletoes and pines in the New World.

17. Arceuthobium laricis

Larch Dwarf Mistletoe

Arceuthobium laricis (Piper) St. John, Flora of Southeast Washington and Adjacent Idaho: 115, 1937.

=A. campylopodum f. laricis.

Description. Mean shoot height 4 (maximum 6)

cm, mostly dark purple, branches flabellate. Basal

diameter of dominant shoots 1.5 to 3.0 (mean 2) mm.

Third internode 5 to 14 (mean 8.0 to 2.0) mm long, 1.0

to 2.5 (mean 1.3) mm wide, length/width ratio 6.1:1.
48 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Staminate flowers 2.7 mm across; perianth mostly

three-merous (sometimes four-merous); segments 1.4

mm long, 1.1 mm wide. Pistillate flowers 1 mm long, 1

mm across. Mature fruit 4.5 by 2.5 mm; proximal

portion 2.5 mm long.

Phenology. Meiosis in June. Peak anthesis from

mid-July to late August, with extremes from early

July to early September. Fruits usually mature in

September, with extremes from early August to early

October; maturation period averages 13 to 14 months.

Hosts. Mathiasen (1998a) revises the host relations of Arceuthobium laricis based on field studies

and previous reports (Mathiasen and others 1995a).

The principal and commonly infected host is Larix

occidentalis. Tsuga mertensiana and Pinus contorta

var. latifolia are secondary hosts. Occasional hosts

are Abies lasiocarpa and P. ponderosa var. ponderosa; but Abies amabilis and Pinus albicaulis are

tentatively classified occasional as well. Abies

grandis, Picea engelmannii, Pinus monticola, and

Tsuga heterophylla are rare hosts. Extra-limital

hosts and artificially inoculated hosts include Larix

decidua, L. leptolepis, Picea abies, P. glauca, Pinus

banksiana, P. resinosa, and P. sylvestris.

(Hawksworth and Wiens 1996). Although natural

population of the high-elevation Larix lyallii appear

not to be infested, this species may become infected

if planted in a suitable environment (Mathiasen and

others 1995b).

Distribution. Canada (British Columbia) and the

United States (Washington, Oregon, Idaho, and Montana). Arceuthobium laricis occurs generally throughout the range of its principal host, Larix occidentalis,

in southern British Columbia, east of the Cascade

Mountains in Washington and northern Oregon, northern and central Idaho, and western Montana. Distribution maps of Arceuthobium laricis are published for

British Columbia and Montana (see Hawksworth and

Wiens 1996). Elevational range is 650 to 2,250 m.

Discussion. Arceuthobium laricis has long been

recognized as a serious pathogen of Larix occidentalis

(Weir 1916a). Infection usually results in the formation of heavy but compact brooms. Because larch

branches are brittle, larger brooms are readily broken

off. Surveys in the Inland Empire (eastern Washington, northern Idaho, and western Montana) show that

most larch stands are infested and infection rates are

commonly high (Hawksworth and Wiens 1996).

Mathiasen (1998b) reports that initial infection of

Larix occidentalis can be when the plants are quite

young; Mathiasen recommends that, to avoid spread,

removal of the infected overstory should be done before regeneration is 7 years old or 1 m tall. Other

publications of interest to managers include those by

Beatty and others (1997), Taylor (1995), Wicker and

Hawksworth (1991), and Weir (1961a).

18. Arceuthobium littorum

Coastal Dwarf Mistletoe

Arceuthobium littorum Hawksw., Wiens & Nickrent,

Novon 2:206, 1992.

=A. campylopodum f. typicum

=A. occidentale

Description. Shoots 8 to 20 (mean 12) cm, brown to

yellow-brown, branches flabellate. Basal diameter of

dominant shoots 2 to 5 (mean 3.5) mm. Third internode 10 to 20 (mean 15) mm long, 2 to 2.5 (mean 2.2) mm

wide, mature fruits 4 to 5 mm long; staminate flowers

mostly four-merous.

Phenology. Meiosis occurs in July, flowering begins in August, with peak anthesis probably occurring

in September. Seed dispersal probably peaks in September or October.

Hosts. Pinus radiata and P. muricata are the primary hosts. It occasionally infects Bolander pine (Pinus

contorta) where this tree is associated with infected P.

muricata.

Distribution. United States (California: Mendocino,

Sonoma, Marin, Monterey, and San Luis Obispo).

Arceuthobium littorum is restricted to a region within

10 km of the Pacific Ocean from Fort Bragg south to

Point Reyes on Pinus muricata and along the central

coast at Monterey and Cambria on P. radiata. It also

parasitizes the small population of P. muricata associated with infected P. radiata at Huckleberry Hill,

Monterey, and is established at three locations by

transplanting infected Pinus radiata—Stanford Arboretum, North Berkeley, and Hillsborough.

Elevational range is from sea level to 250 m.

Discussion. Previously, Hawksworth and Wiens

(1972) include Arceuthobium littorum in A. occidentale.

A primary feature for distinguishing A. littorum from

A. occidentale is the production of large, nonsystemic

witches’ brooms.

19. Arceuthobium microcarpum

Western Spruce Dwarf Mistletoe

Arceuthobium microcarpum (Engelm.) Hawksw. &

Wiens, Brittonia 22:268, 1970.

=A. campylopodum f. microcarpum.

Description. Mean shoot height 5 (maximum 11)

cm, green to purple, branches flabellate. Basal diameter of dominant shoots 1.5 to 3.0 (mean 2) mm. Third

internode 5 to 16 (mean 9.3 ±2.2) mm long, 1 to 2 (mean

1.5) mm wide, length/width ratio 6.2:1. Staminate

flowers 2.3 mm across; perianth mostly three-merous

(rarely four-merous); segments 1.2 mm long, 1.0 mm

wide. Pistillate flowers 1 mm long, 1 mm across.

Mature fruit 3.5 by 2.0 mm; proximal portion 2.5 mm

long. Seeds 2.4 by 1.0 mm.

Phenology. Meiosis in July. Anthesis in mid-August to early September, with extremes from late July

to late September. Fruits mature in September, with
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 49

Arceuthobium in North America Hawksworth, Wiens, and Geils

extremes from late August to early October; maturation period averages 12 to 13 months.

Hosts. This dwarf mistletoe is a common and serious pathogen only on Picea engelmannii and P. pungens.

On the San Francisco Peaks of northern Arizona,

however, it also parasitizes Pinus aristata. Pinus

strobiformis and Abies lasiocarpa var. arizonica are

rarely infected even where they are associated with

infected principal hosts. If populations of a spruce in

southern Arizona were determined to be Picea mexicana

rather than P. engelmannii (Taylor and others 1994),

this species would be an additional host.

Distribution. United States (Arizona and New

Mexico). Arceuthobium microcarpum has one of the

more restricted distributions in the genus. In Arizona,

the parasite occurs on the North Rim of the Grand

Canyon, the San Francisco Peaks and nearby Kendrick

Peak, White Mountains, and Pinaleno Mountains. In

New Mexico, this dwarf mistletoe is present at several

locations in the Mogollon Mountains and in the Sacramento Mountains. Elevational range is 2,400 to 3,150 m.

Discussion. This localized species in Arizona and

New Mexico is characterized by its near exclusive

occurrence on spruce (Hawksworth and Graham

1963a). This species induces small, dense witches’

brooms. Heavily infected trees bear hundreds of such

witches’ brooms. This dwarf mistletoe causes heavy

mortality in stands of Picea pungens and, to a lesser

extent, of P. engelmannii.

20. Arceuthobium monticola

Western White Pine Dwarf Mistletoe

Arceuthobium monticola Hawksw., Wiens & Nickrent,

Novon 2:205, 1992.

=A. campylopodum f. blumeri

=A. californicum

Description. Shoots 5 to 10 (mean 7) cm tall, dark

brown, branches flabellate. Basal diameter of dominant shoots 2 to 4 (mean 3) mm; third internode 8 to 15

(mean 12) mm long, 1.5 to 2.0 mm wide. Staminate

flowers mostly three-merous. Mature fruits 4.0 to 4.5

mm long, 2.0 to 2.5 mm wide.

Phenology. The period of anthesis is poorly known

but apparently occurs late July through August. Fruits

mature October and November; maturation period

averages 15 months.

Hosts. The principal and only commonly infected

host is Pinus monticola. Pinus lambertiana is a secondary host; Picea breweriana an occasional host; and

Pinus jeffreyi a rare host.

Distribution. United States (Oregon, California).

Arceuthobium monticola is a local endemic in the

Klamath Mountains of southwestern Oregon (Coos,

Curry, and Josephine) and the Siskiyou Mountains of

adjacent northwestern California (Del Norte and possibly Siskiyou). Elevational range is 700 to 1,900 m.

Discussion. Hawksworth and Wiens (1972) include

this taxon under Arceuthobium californicum; but subsequent field and laboratory studies demonstrate that it is

a distinct species and not apparently sympatric with A.

californicum. Arceuthobium monticola differs from A.

californicum in its much darker shoot color, later flowering and seed dispersal periods, and host preference for

Pinus monticola rather than P. lambertiana.

21. Arceuthobium nigrum

Black Dwarf Mistletoe

Arceuthobium nigrum (Hawksw & Wiens) Hawksw. &

Wiens, Phytologia 66:9, 1989.

=A. gillii subsp. nigrum.

Description. Mean shoot height 15 to 35 (45) cm,

dark brown to black. Basal diameter of dominant

shoots 3 to 8 (mean 5) mm. Third internode 5 to 19

(mean 10.8 ±3.8) mm long, 2.5 to 6.0 (mean 3.7) mm

wide (six collections), length/width ratio 2.9:1. Staminate flowers 3 mm long, 3.5 mm across. Mature fruit

6 to 9 (mean 7) mm long, 3.5 mm wide, proximal

portion 2 to 3 mm. Seeds 3.5 by 1.3 mm.

Phenology. This dwarf mistletoe is unusual in

Arceuthobium by having flowering periods in March

and April and September and October. Seed dispersal

occurs in September, presumably from flowers pollinated the previous year; when the seeds from the

March through April pollinations mature is unknown.

Hosts. This dwarf mistletoe is most common on the

principal hosts Pinus leiophylla vars. leiophylla, var.

chihuahuana, and P. lumholtzii. Pinus lawsonii, P.

oaxacana, P. patula, P. teocote are also highly susceptible and rated as principal hosts. Pinus montezumae

and P. pseudostrobus are occasional hosts. Pinus

arizonica var. arizonica and P. cooperi are rare hosts.

Distribution. Mexico (Durango, Zacatecas,

Guanajuato, Querétaro, Hidalgo, Michoacán, Mexico,

Tlaxcala, Puebla, Veracruz, Oaxaca, Chiapas) and

possibly Western Guatemala. This mistletoe is reported from the northeastern slope of Volcán la Malintzi

(Malinche), Tlaxcala (Hernandez and others 1992),

and is common on pines in Central and Eastern Mexico.

Elevational range is 1,800 to 2,800 m.

Discussion. Arceuthobium nigrum resembles A.

gillii. Both species possess markedly glaucous fruits,

strong sexual dimorphism (open, divaricate branching in staminate plants versus densely branched in

pistillate plants), and parasitize similar hosts. Although A. nigrum was previously classed as a subspecies of A. gillii, specific status is warranted

(Hawksworth and Wiens 1989). Arceuthobium nigrum

is a larger plant than A. gillii and has dark green to

black shoots 15 to 35 (maximum 45) cm high, whereas

those of A. gillii are only 8 to 15 (maximum 25) cm tall,

and colored greenish brown. Arceuthobium nigrum

also has the two flowering periods (March and April,

and September and October) compared to only one for
50 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

A. gillii (March and April). To our knowledge, these

species are not sympatric.

22. Arceuthobium oaxacanum

Oaxacan Dwarf Mistletoe

Arceuthobium oaxacanum Hawksw. & Wiens,

Phytologia 66:7, 1989.

=A. rubrum

Description. Shoots 8 to 20 (mean 12) cm tall, pale

brown to reddish, branches flabellate. Basal diameter

of dominant shoots 2 to 4 (mean 3) mm. Third internode 10 to 17 (mean 12) mm long and 2 to 3 mm wide.

Phenology. Anthesis in July. Fruits mature in

August of the following year; maturation period averages 13 months.

Hosts. Pinus lawsonii, P. michoacana, and P.

pseudostrobus are principal hosts; all are about equally

susceptible. Pinus oaxacana is an occasional host.

Distribution. Mexico (Oaxaca). This species is known

from only three localities (two south of Miahuatlán and

one near Ixtlán). Elevational range is 2,000 to 2,200 m.

Discussion. Hawksworth and Wiens (1989) recognize Arceuthobium oaxacanum as a distinct species

previously considered an extreme disjunct (about 1,200

km) of A. rubrum. In general, A. oaxacanum is a larger,

lighter colored, more openly branched plant and causes

larger witches’ brooms than A. rubrum. Furthermore,

Arceuthobium oaxacanum principally parasitizes

Pinus lawsonii, P. michoacana, P. pseudostrobus, and

occasionally P. oaxacana; none of these pines occurs

within the range of A. rubrum.

23. Arceuthobium occidentale

Digger Pine Dwarf Mistletoe

Arceuthobium occidentale Engelm., U.S. Geographical Survey West of 100th Meridian (Wheeler Report)

6:375, 1878.

=A. campylopodum f. typicum.

Description. Mean shoot height 8 (maximum 17) cm,

yellowish, glaucous, branches flabellate. Basal diameter

of dominant shoots 1.5 to 5.0 (mean 2) mm. Third internode 7 to 18 (mean 12.7 ±2.0) mm long, 1.5 to 3.5 (mean 1.8)

mm wide, length/width ratio 7.1:1. Staminate flowers 3.0

mm across; perianth three- or four-merous; segments 1.5

mm long, 1.0 mm wide. Mature fruit 4.5 by 3.0 mm;

proximal portion 3.0 mm long. Seeds 2.6 by 1.0 mm.

Phenology. Meiosis in August. Peak anthesis from

late September to late November, extremes from early

September to early December. Most fruits mature

from mid-October to mid-January, with extremes from

late September to early February; maturation period

averages 13 months.

Hosts. Pinus sabiniana is the most common and only

principal host. Pinus coulteri and P. attenuata are secondary hosts where they occur with infected P. sabiniana.

Whether the occasional hosts Pinus ponderosa and P.

jeffreyi are infected, however, varies by location and

association with their principal dwarf mistletoe, A.

campylopodum. In the California Coast Range (outside the distribution of Arceuthobiumcampylopodum),

these pines are commonly infected where they are

occur with infected P. sabiniana. In the Sierra Nevada

(where A. campylopodum occurs), they are seldom

infected even under infected P. sabiniana. Extralimital and hosts by artificial inoculation are Pinus

banksiana, P. bungeana, P. caribaea, P. halepensis, P.

palustris, P. pinea, P. radiata, P. sylvestris, P.

thunbergii, P. torreyana, and P. virginiana.

Distribution. United States (California). This dwarf

mistletoe is a California endemic and commonly occurs on Pinus sabiniana throughout the foothills and

low mountains surrounding the Central Valley of

California and along the Coast Ranges from Mount

Pinos (Ventura) north to Mendocino. Elevational range

is about 30 to 1,200 m in the southern Sierra Nevada.

Discussion. Even though Pinus sabiniana typically

occurs in open, savanna-like forests, Arceuthobium

occidentale is widely distributed in these stands. Isolated,

infected trees more than 100 m away from the closest

infected trees are sometimes found, which suggests that

bird vectors (possibly phainopepla) are involved in longdistance seed transport of this dwarf mistletoe.

24. Arceuthobium pendens

Pendent Dwarf Mistletoe

Arceuthobium pendens Hawksw. & Wiens, Brittonia

32:348, 1980.

Description. Mean shoot height 15 (maximum 22)

cm, light green, branches flabellate. Basal diameter of

dominant shoots 1.5 to 3.5 (mean 2) mm. Third internode 12 to 20 (mean 16) mm long, 1 to 2 (mean 1.5) mm

wide. Staminate flowers 2.5 mm across, three-merous.

Phenology. Peak anthesis possibly in September.

Fruit maturity from June to September (Cházaro and

Oliva 1987a).

Hosts. Known only on Pinus discolor (San Luis

Potosí) and P. cembroides subsp. orizabensis (Veracruz

and Puebla).

Distribution. Mexico (San Luis Potosí, Veracruz,

and Puebla). The distribution of this dwarf mistletoe

is poorly known (Cházaro and Oliva 1987a) from only

Sierra San Miguelito (San Luis Potosí) and Frijol

Colorado (Veracruz), and on the nearby Cerro Pizzaro

(Puebla). Elevational range is 2,250 to 2,700 m.

Discussion. The species exhibits striking sexual

dimorphism—pistillate plants are densely branched

and typically less than 8 cm tall, and mature staminate plants are openly branched and pendant, 15 to 20

cm long. A unique feature of this species, at least in one

population, is that only the staminate plants appear to

induce systemic witches’ brooms. This dwarf mistletoe

likely occurs in other areas and on other pinyons.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 51

Arceuthobium in North America Hawksworth, Wiens, and Geils

25. Arceuthobium pusillum

Eastern Dwarf Mistletoe

Arceuthobium pusillum Peck, Transactions Albany

Institute 7:191, 1872.

=A. minutum

=A. abigenium.

Description. Mean shoot height 1 (maximum 3)

cm, green to brown, usually without secondary branching, but flabellate when occurs. Basal diameter of

dominant shoots 1.0 mm. Third internode 1 to 4 (mean

1.9 ±0.8) mm long, 0.5 to 1.5 (mean 1.0) mm wide,

length/width ratio 1.9:1, often markedly wider at top

than at base. Pistillate shoots often longer than the

staminate. Staminate flowers 1.7 to 2.2 (mean 1.8 mm)

across; perianth mostly three-merous (sometimes twoor four-merous); segments 0.8 mm long, 0.7 mm wide.

Mature fruit 3.0 mm long, 1.25 to 1.75 mm wide (mean

1.5 mm); proximal portion 2.0 mm long. Seeds 2.0 by

0.9 mm.

Phenology. Staminate meiosis in September, pistillate meiosis in May. Anthesis usually in April or

May, with extremes from late March to June. Fruits

mature in September or early October of the same year

as pollination; maturation period averages 5 months,

perhaps the shortest in the genus. Seed germination

mostly in May and June.

Hosts. Arceuthobium pusillum is most common and

widely distributed on Picea mariana. Picea glauca and

P. rubens appear to be about as susceptible as P.

mariana, so are also principal hosts, although the

dwarf mistletoe is not as common (except in some oldgrowth stands). Larix laricina is an occasional host.

Abies balsamea, Pinus banksiana, P resinosa, and P.

strobus are rare hosts. Picea pungens is an extralimital host.

Distribution. Canada (Saskatchewan, Manitoba,

Ontario, Québec, New Brunswick, Nova Scotia, Prince

Edward Island, and Newfoundland) and the United

States (Minnesota, Wisconsin, Michigan, New York,

Pennsylvania, New Jersey, Connecticut, Massachusetts, Vermont, New Hampshire, Rhode Island, and

Maine). Arceuthobium pusillum occurs in Canada

from Hudson Bay, the Cumberland areas in eastern

Saskatchewan to southern Manitoba, southern

Ontario, Québec, and the Maritime Provinces including Newfoundland. Its northern limits in Ontario and

Québec are poorly known. In the United States, it

occurs in the northern parts of Minnesota, Wisconsin,

and Michigan, northeastern Pennsylvania, extreme

northwestern New Jersey, and all of New England. It

is rare near the limits of its southern distribution.

Arceuthobium pusillum occurs on several islands isolated 30 km or more from the nearest known populations of the dwarf mistletoe—Mantinicus and Monhegan

Islands off the Maine Coast and Beaver Island in Lake

Michigan. The numerous distribution maps published

for Arceuthobium pusillum are identified by

Hawksworth and Wiens (1996). Elevational ranges

from sea level in Maine and the Maritime Provinces to

800 m on Mount Katahdin, Maine.

Discussion. In spite of having the smallest shoots

of any North American mistletoe, Arceuthobium

pusillum is a damaging pathogen of spruce in many

parts of its distribution (Singh and Carew 1989).

Mortality is severe in Picea glauca along the Maine

Coast, and the parasite is considered the most serious

disease agent of P. mariana in the Great Lakes region.

Its biology and management are discussed by Ostry

and Nicholls (1979). Witches’ brooms appear to be

mostly of the systemic type. Shoots usually first appear in late summer or autumn as small eruptions in

the bark of host branches 2 years old and mature

during the third season. They flower the following

spring; fruits mature by autumn. Shoots usually fall

after flowering (staminate) or fruiting (pistillate), and

only rarely do shoots produce a second crop of flowers.

This pattern of reproduction is unique among northern temperate species of the genus. Large swellings on

the main trunk are commonly induced by Arceuthobium

pusillum in old-growth P. rubens in New England and

New York, but such swellings have not been reported

on other spruces. Arceuthobium pusillum has an interesting literary and historic past in Thoreau’s (1858)

description of the witches’ broom in spruce at Walden

Pond and Lucy Millington’s later discovery of the

mistletoe itself (Smith 1992). Arceuthobium pusillum

is common in spruce bogs and generally absent from

drier upland sites. In Québec, A. pusillum is apparently

restricted to within 2 km of lakes or rivers. In Maine,

it occurs on P. glauca only within 300 to 400 m of the

coast. A. pusillum may require an uninterrupted period of high atmospheric humidity in the spring for

normal growth.

26. Arceuthobium rubrum

Ruby Dwarf Mistletoe

Arceuthobium rubrum Hawksw. & Wiens, Brittonia

17:233, 1965.

Description. Mean shoot height 10 (maximum 18)

cm, dark red, brown to blackish, branches flabellate.

Staminate plants usually taller than pistillate plants.

Basal diameter of dominant shoots 2 to 3 (mean 2.4)

mm. Third internode 4 to 12 (mean 6.9 ±2.7) mm long,

2 to 3 (mean 2.3) mm wide, length/width ratio 3.4:1.

Staminate flowers 1.0 to 1.5 mm across; mostly threemerous; segments 0.6 mm long, 0.6 mm wide. Mature

fruit 3.5 by 2.0 mm. Seeds 2.0 by 1.0 mm.

Phenology. Meiosis probably in June. Anthesis

usually in July. Fruits mature from mid-July to August of the year following pollination; maturation

period averages 12 to 13 months.

Hosts. Principal hosts are P. cooperi, P. durangensis,

P. engelmannii, P. herrerai, and Pinus teocote, all of

which appear to be highly susceptible.
52 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Distribution. Mexico (Durango and Sinaloa). This

species has a localized distribution in the Sierra Madre

Occidental of Western Mexico. Elevational range is

2,250 to 2,900 m.

Discussion. This distinctive, slender, reddish dwarf

mistletoe is apparently widespread in the mountains

of Durango. As the plants dry, the red color turns to

dull brown, and the apical portion of each segment

becomes golden yellow. This gives dried specimens a

characteristic banded appearance. The shiny fruits, a

character shared only with Arceuthobium oaxacanum,

readily distinguish this species. Another distinctive

characteristic of A. rubrum is the exceptionally small,

scarcely opened staminate flowers. The populations in

the Pueblo Altares area in northern Durango, about

150 km north of the populations around El Salto, have

taller, darker, and stouter shoots that superficially

resemble Arceuthobium vaginatum subsp. vaginatum.

27. Arceuthobium siskiyouense

Knobcone Pine Dwarf Mistletoe

Arceuthobium siskiyouense Hawksw., Wiens &

Nickrent, Novon 2:204, 1992.

=A. campylopodum

Description. Mean shoot height 8 (maximum 10)

cm, dark brown, branches flabellate. Basal diameter

of dominant shoots 2.0 to 2.5 mm across. Third internode 8 to 15 (mean 9) mm long, 2 mm wide. Mature

fruits 3.6 by 2.1 mm.

Phenology. Peak anthesis in August. Fruits at

maturation not observed.

Hosts. Pinus attenuata is the principal and only

common host of Arceuthobium siskiyouense. This dwarf

mistletoe rarely parasitizes P. contorta, P. jeffreyi, and

P. ponderosa where these trees grow in association

with infected P. attenuata.

Distribution. United States (California and Oregon). The distribution of Arceuthobium siskiyouense

is restricted to the Klamath Mountains of southwestern Oregon (Curry and Josephine) and the Siskiyou

Mountains in adjacent northwestern California (Del

Norte and Siskiyou). Elevational range is 400 to

1,200 m.

Discussion. Hawksworth and Wiens (1972) include

this taxon in Arceuthobium campylopodum, but subsequent studies demonstrate it a distinct species related to A. campylopodum. The two species are sympatric in several areas, and their flowering periods

partially overlap; but each maintains its own host

preferences and distinctive morphologies (for example,

A. siskiyouense does not induce witches’ brooms).

28. Arceuthobium strictum

Unbranched Dwarf Mistletoe

Arceuthobium strictum Hawksw. & Wiens, Brittonia

17:234, 1965.

Description. Mean shoot height 7 (maximum 13)

cm, pistillate shoots generally greenish yellow brown,

branches flabellate. Staminate plants brownish, rarely

branching. Staminate plants usually taller than pistillate plants. Basal diameter of dominant shoots 2.5

to 4.0 (mean 3.1) mm. Third internode 1 to 8 (mean 3.6

±2.0) mm long, 1.5 to 3.5 (mean 2.3) mm wide; length/

width ratio 1.6:1. Staminate flowers 3 mm across,

perianth three-, four-, or five-merous (rarely six or

seven-merous), segments 1.5 mm long, 1 mm wide.

Mature fruit 4 by 2.5 mm. Seeds 2.5 by 1.0 mm.

Phenology. Meiosis in July. Anthesis late July

through October, with peak in September. Fruits

mature from mid-September to October of the year

following pollination; maturation period averages 13

months.

Hosts. Pinus leiophylla var. chihuahuana is the

principal host. Pinus teocote is an occasional host, and

P. engelmannii is a rare host.

Distribution. Mexico (Durango). This species is

known only in the Sierra Madre Occidental south and

west of the city of Durango. Elevational range is 2,200

to 2,500 m.

Discussion. The most distinctive feature of this

dwarf mistletoe is the lack of branching by staminate

plants. The staminate shoots at anthesis become single

spikes 6 to 13 cm long with numerous perianth segments (up to seven, more than any other dwarf mistletoe). The pistillate plants, in contrast, exhibit abundant secondary branching. This dwarf mistletoe causes

heavy mortality in its principal host, Pinus leiophylla

var. chihuahuana.

29. Arceuthobium tsugense

Hemlock Dwarf Mistletoe

Arceuthobium tsugense (Rosendahl) G.N. Jones, University of Washington Publications in Biology 5:139,

1936 (as A. tsugensis).

=A. campylopodum f. tsugensis.

Description. Mean shoot height 5 to 7 (13) (cm),

greenish to reddish, darker in winter, branches

Key to the Subspecies

1. Parasitic primarily on Tsuga heterophylla or Pinus contorta var. contorta; shoots 3–13 (mean 7) cm

high .................................................................................................................. 29a. A. tsugense subsp. tsugense

1. Parasitic primarily on Tsuga mertensiana; shoots 3–9 (mean 5) cm high .........................................................

............................................................................................................ 29b. A. tsugense subsp. mertensianae
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 53

Arceuthobium in North America Hawksworth, Wiens, and Geils

flabellate. Basal diameter of dominant shoots 1.5 to

4.0 (mean 2.0) mm. Third internode 4 to 16 (mean 9.2

±2.5) mm long, 1 to 2 (mean 1.5) mm wide, length/

width ratio 6.1:1. Staminate flowers 2.8 mm across;

perianth three- or four-merous, segments 1.2 mm

long, 1.0 mm wide. Pistillate flowers 1 mm long, 1 mm

across. Mature fruit 3 by 2 mm; proximal portion 2.0

mm long.

Hosts. Mathiasen (1994) reviews the host range of

the several taxa included here under Arceuthobium

tsugense based on natural infection and artificial inoculation. His report provides the basis for hosts and

susceptibility used here.

Discussion. Hawksworth and Wiens (1972) comment on the unusually broad host range of Arceuthobium

tsugense, which encompasses not only both western

species of hemlock but also several species of fir,

spruce, and pine. Arceuthobium tsugense is segregated into subspecies tsugense and mertensianae and

subspecies tsugense into two physiologically differentiated host races as western hemlock and shore pine

(Hawksworth and others 1992b). Additional field studies on distribution, host preference, and phenology are

being conducted to resolve continuing taxonomic uncertainty (see Mathiasen 1994). At this time, however,

we retain the taxonomy and host relations presented

by Hawksworth and Wiens (1996). Hennon and others

(2001) provide a general review and management

guide for hemlock dwarf mistletoe. The subspecies are

similar morphologically, but the shoots are about 30

percent taller in subsp. tsugense than in subsp.

mertensianae (differences statistically significant at

P<0.01).

Phenology. Meiosis in July for both subspecies, but

the subsequent phenologies of flowering for the subspecies differ. Flowering averages about 1 to 2 weeks

earlier in subsp. tsugense (peak anthesis in August,

extremes from late July to late September) than for

subsp. mertensianae (peak anthesis from mid-August

to mid-September). In contrast to flowering, the seed

dispersal for subsp. tsugense averages about 2 to 4

weeks later (late September to early November) than

for subsp. mertensianae (mid-August, rarely to late

October).

29a. Arceuthobium tsugense subsp. tsugense

Western Hemlock Dwarf Mistletoe

Description. As the species, but shoots vary from 3

to 13 cm high, mean 7 cm.

Hosts. Tsuga heterophylla is the principal and common host; but A. lasiocarpa var. lasiocarpa is also

considered a principal host, as are (tentatively) Abies

amabilis and A. procera. Abies grandis and Pinus

contorta var. latifolia are occasional hosts. Rare hosts

are Picea engelmannii, P. sitchensis, Pinus monticola,

Pseudotsuga menziesii, and Tsuga mertensiana. The

status of Pinus contorta var. contorta (shore pine) as a

host is discussed below. Extra-limital and hosts by

inoculation (for the western hemlock race) are Larix

decidua (incompatible),L. occidentalis (incompatible),

Picea abies, P. glauca, Pinus contorta var. latifolia,

Pinus ponderosa, P. radiata, P. sylvestris, Pseudotsuga

menziesii, and Tsuga canadensis.

Distribution. Canada (British Columbia) and the

United States (Alaska, Washington, Oregon, and California). Arceuthobium tsugense subsp. tsugense is distributed from Haines, Alaska, to Mendocino, California. Arceuthobium tsugense subsp. tsugense is common

in the Tsuga heterophylla forests of coastal Alaska,

British Columbia, Washington, and Oregon; but rare

in northwestern California; and unlikely in northern

Idaho. Elevational range is from sea level in Alaska,

British Columbia, and Washington to about 1,250 m in

Oregon.

Discussion. Many of the lower infections in Tsuga

heterophylla produce few shoots of the dwarf mistletoe

(Shaw and Weiss 2000). Because dwarf mistletoes are

sensitive to light, the absence of dwarf mistletoe shoots

from the lower infections may be explained by the

dense shade in the lower portions of coastal hemlock

forests (Smith 1969). In such situations, vigorous

shoots are often found only along margins of stands, on

young trees in openings, or in higher branches of older

trees. Information on the epidemiology of this mistletoe and management of hemlock is available for Alaska

(Shaw and Hennon 1991, Trummer and others 1998,

and Wittwer 2002) and Canada (Bloomberg and Smith

1982, Edwards 2001, Muir 1993, Smith 1977).

Discussion on Western Hemlock Compared to

Shore Pine. The taxonomic status of the dwarf mistletoe populations on Pinus contorta var. contorta is the

subject of continued debate (Hawksworth and Wiens

1972, 1996, Hunt and Smith 1978, Smith and Wass

1976, 1979). Dwarf mistletoe population on western

hemlock (Tsuga heterophylla) and shore pine (P.

contorta var. contorta) are similar morphologically,

phenologically, and chemically but appear to have

consistent differences in host compatibility. Comparing dwarf mistletoe populations on western hemlock

to those on shore pine, respectively, maximum shoot

height is about 30 percent greater; fruits are slightly

but statistically smaller; anthesis and peak dispersal

are later. Flavonoid composition and isozyme patterns

are similar. Inoculation of shore pine with dwarf

mistletoe seeds from western hemlock produce few

infections, but those infections that are successful

produce abundant aerial shoots. In contrast, inoculations of western hemlock with dwarf mistletoe seeds

from shore pine result in more infections but few

produce any shoots. Tsuga heterophylla and Pinus

monticola are considered rare hosts. Other species

infected by the shore pine race by artificial inoculation

include Abies amabilis, A. grandis, Larix occidentalis,
54 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

Picea glauca, P. engelmannii, Pinus contorta var.

latifolia, P. ponderosa, and Pseudotsuga menziesii.

Arceuthobium tsugense subsp. tsugense parasitizes

Pinus contorta var. contorta in southwestern British

Columbia and the San Juan Islands, Washington.

Populations of this dwarf mistletoe are distributed on

isolated rocky outcrops along the east coast of

Vancouver Island, on the Channel Islands, and the

mainland of British Columbia north of Vancouver.

Two outlying populations occur 250 km north at Port

Clements (Queen Charlotte Islands) and at Terrace

(British Columbia mainland). In the United States,

populations are known from Orcas and San Juan

Islands (Washington). The elevational range is from

sea level to 800 m.

29b. Arceuthobium tsugense subsp. mertensianae

Mountain Hemlock Dwarf Mistletoe

Arceuthobium tsugense (Rosendahl) G.N. Jones subsp.

mertensianae Hawksw & Nickrent, Novon 2:209, 1992.

Description. Shoots are typically shorter (5 cm)

than in subsp. tsugense (7 cm).

Hosts. The common principal host of Arceuthobium

tsugense subsp. mertensianae is Tsuga mertensiana;

T. heterophylla is only rarely infected, even where this

species is closely associated with infected T.

mertensiana. Other principal hosts are A. amabilis

and Abies lasiocarpa. Pinus albicaulis is a secondary

host, and Pinus monticola is an occasional host. Picea

breweriana and Pinus contorta var. latifolia, are rarely

infected.

Distribution. Western Canada (southern British

Columbia) and Western United States (Washington,

Oregon, and California). The distribution of Arceuthobium

tsugense subsp. mertensianae is still poorly known,

but it extends from near Vancouver (British Columbia), in the Olympic Mountains, through the Cascade

Mountains (Washington and Oregon), and to the central Sierra Nevada (Alpine, California). Hildebrand

and others (1997) report on a distribution survey in

Washington. Elevational range is 1,200 to 2,500 m.

Discussion. Some populations of Tsuga mertensiana

such as on Mount Baker and in the Olympic Mountains are exceptionally heavily infected by this dwarf

mistletoe.

30. Arceuthobium vaginatum

Arceuthobium vaginatum (Willd.) Presl in Berchtold,

O PUirozenosti Rostlin aneb Rostinár 2:28, 1825.

Description. Mean shoot height from 20 to 30

(maximum 55 or greater) cm, orange to dark brown,

reddish, or black, usually densely branched and erect,

but large older plants sometimes become pendulous;

branches flabellate; basal diameter of dominant shoots

1 to 3 cm long, 0.2 to 0.4 cm wide. Staminate flower up

to 3.5 mm long and up to 3.5 mm across, mostly thremerous (sometimes four-merous), segments up to 2.0

mm long and up to 1.5 mm wide, apex acute to obtuse.

Pistillate flower up to 2.5 mm long, up to 1.5 mm

across. Fruit 4 to 6 mm long, 2 to 3 mm wide, elliptical

to obovate.

Phenology. Anthesis from approximately late

March through May.

Hosts. Parasitic on yellow pine.

Discussion. The distributions of the two subspecies

overlap in the mountains of central Chihuahua (between latitudes 28° 00' and 28° 30' N) where intermediate characteristics are shown in some populations.

Even here, however, there is a tendency to segregate

by elevation with subsp. vaginatum at lower elevations and subsp. cryptopodum at higher elevations.

Although the characteristics distinguishing subspecies are greater than those in other species (such as

Arceuthobium tsugense), this is the only case where we

find intermediate populations, therefore we use subspecific rank rather than species rank for this taxon.

30a. A. vaginatum subsp. vaginatum

Mexican Dwarf Mistletoe

Description. Mean shoot height 20 (maximum 55)

cm, dark brown to black, rarely reddish. Basal diameter of dominant shoots 4 to 20 (mean 7) mm. Third

internode 5 to 30 (mean 17.4 ±6.0) mm long, 2.5 to 8.5

(mean 5.0) mm wide, length/width ratio 2.9: 1. Staminate flower segments 1.6 mm long, 1.1 mm wide.

Mature fruit 5.5 by 3.5 mm.

Phenology. Meiosis in February. Anthesis usually

March and April. Fruits mature in August of the year

following pollination; maturation period averages 16

to 17 months.

Key to the Subspecies

1. Plants dark brown to black, usually over 20 cm tall; staminate flowers usually greater than 3 mm long and wide;

anthesis March–April; Sierra Madre Occidental from central Chihuahua southward to the Central Cordillera,

and in the Sierra Madre Oriental .......................................................... 30a. A. vaginatum subsp. vaginatum

1. Plants orange, usually less than 20 cm tall; staminate flowers usually less than 3 mm long and wide; anthesis

May–June; Sierra Madre Occidental of central Chihuahua and Sonora and mountains of central Coahuila

northward to central Utah and northern Colorado ............................ 30b. A. vaginatum subsp. cryptopodum
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 55

Arceuthobium in North America Hawksworth, Wiens, and Geils

Hosts. Arceuthobium vaginatum subsp. vaginatum

has the broadest known host range of any species in

the genus. It is collected on 13 species of Mexican pines

and undoubtedly occurs on others. It is common on the

principal hosts Pinus arizonica vars. arizonica, var.

stormiae, P. cooperi, P. durangensis, P. engelmannii,

P. hartwegii, P. herrerai, P. lawsonii, P. montezumae,

P. patula, and P. rudis. Pinus teocote is a secondary

host because it was parasitized only when it was

associated with an infected principal hosts. It rarely

infects Pinus culminicola under infested P. rudis on

Cerro Potosí (Nuevo León).

Distribution. Mexico (Chihuahua, Coahuila,

Distrito Federal, Durango, Hidalgo, Jalisco, Mexico,

Nayarit, Nuevo León, Oaxaca, Puebla, Querétaro,

Sinaloa, Tamaulipas, Tlaxcala, Veracruz, and

Zacatecas). This is the most widely distributed dwarf

mistletoe in Mexico, extending from the Sierra Madre

Occidental in western Chihuahua south through

Durango, Jalisco; into the Central Cordillera of Mexico

and Puebla; and occurring in the Sierra Madre Oriental from Coahuila and Nuevo León to Oaxaca.

Elevational range is from 2,100 m in Nuevo León to

3,900 m on Nevado de Toluca near Mexico City.

Discussion. The shoots of Arceuthobium vaginatum

subsp. vaginatum exceed 55 cm in height in Central

Mexico. The plants exhibit considerable sexual dimorphism and variation. The staminate plants tend to be

taller than the pistillate plants, but Vázquez (1991)

reports on a population near Texcoco, Mexico, where

the pistillate plants were short, erect, and dark, and

staminate plants were long, pendulous, and reddish.

Plants in some areas of the northern Sierra Madre

Oriental are often reddish, but plants just 40 km to the

south are again typically black (Hawksworth and

Cibrián 1985). The hosts and ecological requirements

of Arceuthobium vaginatum subsp. vaginatum and A.

globosum are similar; and they frequently sympatric

and even occur on the same tree.

30b. Arceuthobium vaginatum subsp. cryptopodum

Southwestern Dwarf Mistletoe

Arceuthobium vaginatum (Willd.) Presl subsp.

cryptopodum (Engelm.) Hawksw. & Wiens, Brittonia

17:230, 1965.

=A. vaginatum f. cryptopodum.

Description. Mean shoot height 10 cm (maximum

27) cm, usually orange to reddish brown, sometimes

dark to near black. Basal diameter of dominant shoots

2 to 10 (mean 4) mm. Third internode 4 to 16 (mean

7.8 ±3.2) mm long, 2.0 to 4.5 (mean 3.1) mm wide,

length/width ratio 2.5:1. Staminate flowers 2.5 to 3.0

(mean 2.7) mm across; perianth segments 1.3 mm

long, 1.0 mm wide. Mature fruit 4.5 to 5.5 (mean 5.0)

mm long, 2.0 to 3.0 (mean 2.5) mm wide; proximal

portion 3.5 mm long. Seeds 2.7 by 1.1 mm.

Phenology. Meiosis in March or April. Anthesis

usually in May and June, with extremes from late

April to early July. Fruits mature in late July or early

August, with extremes from early July to early September. Both anthesis and seed dispersal in Colorado

occur 1 to 2 weeks later than in Arizona and New

Mexico; maturation period averages 14 to 15 months.

Seed germination from August to September, immediately following dispersal.

Hosts. Pinus ponderosa var. scopulorum is the most

common host in Arizona, New Mexico, Colorado, Utah,

and Texas. The two races of var. scopulorum recognized by Conkle and Critchfield (1988) (Rocky Mountain and Southwestern) appear to be about equally

susceptible, but most of the distribution of the Rocky

Mountain race is primarily north of that of

Arceuthobium vaginatum subsp. cryptopodum. Other

principal hosts include P. arizonica var. arizonica

(Arizona, New Mexico, Chihuahua, and Sonora) and

var. stormiae (Coahuila), P. engelmannii (Arizona,

Chihuahua, and Sonora), and P. durangensis (Chihuahua and Jalisco). Pinus cooperi is a secondary host.

Occasional hosts are Pinus aristata and P. contorta

var. latifolia. Rare hosts are Pinus flexilis and P.

strobiformis. Pinus sylvestris is an extra-limital host.

Distribution. Northern Mexico (Sonora, Chihuahua, and Coahuila) and United States (Utah, Arizona,

Colorado, New Mexico, and Texas). Arceuthobium

vaginatum subsp. cryptopodum is widely distributed

on Pinus ponderosa var. scopulorum from central

Utah (Sevier and Emery) and northern Colorado

(Larimer) to Arizona, New Mexico, western Texas

(Guadalupe and Davis Mountains), at least as far

south as the Sierra de la Madera (Coahuila) and the

Sierra Madre Occidental (Sonora and Chihuahua).

Arceuthobium vaginatum subsp. cryptopodum occurs

in nearly every mountain range where P. ponderosa

var. scopulorum grows, including isolated ranges such

as the Virgin, Trumbull, and Hualapai Mountains

(Arizona), the Ladron, Organ, Guadalupe, and San

Andreas Mountains (New Mexico), Navajo Mountain

(Utah), and Mesa de Maya (Colorado). Arceuthobium

vaginatum subsp. cryptopodum distribution maps have

been published for Colorado, Utah, and New Mexico

(see Hawksworth and Wiens 1996). Elevational range

is 1,700 to 3,000 m, although it is found primarily

between 2,000 and 2,600 m in Arizona and New

Mexico.

Discussion. Arceuthobium vaginatum subsp.

cryptopodum is characterized by thick, orange-colored

shoots. Populations, however, show various color gradations commonly from yellow to red, greenish in deep

shade or on Pinus contorta var. latifolia or rarely from

dark purple such as in the Black Forest of Colorado.

This dwarf mistletoe is particularly damaging to

Pinus ponderosa in the Sacramento Mountains in
56 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hawksworth, Wiens, and Geils Arceuthobium in North America

south-central New Mexico (Lincoln National Forest

and adjacent Mescalero Apache Indian Reservation;

Hawksworth and Lusher 1956), central Arizona, and

along the Front Range in Colorado. For reasons yet to

be explained, the parasite is common but less damaging in southwestern Colorado and southeastern Utah.

The witches’ brooms induced by Arceuthobium

vaginatum subsp. cryptopodum are similar on all

hosts except for Pinus contorta var. latifolia with

small witches’ brooms and large branch swellings.

Because of the severe damage caused by this mistletoe

and the importance of its principal host, there are

numerous reports on its biology and management; the

most comprehensive study is by Hawksworth (1961)

and a general leaflet by Lightle and Weiss (1974).

31. Arceuthobium verticilliflorum

Big-Fruited Dwarf Mistletoe

Arceuthobium verticilliflorum Engelm., Botany of

California 2:107, 1880.

Description. Mean shoot height 7 (maximum 11) cm,

mostly yellow to yellow-green to purplish, without

secondary branching, lightly glaucous when young.

Basal diameter of dominant shoots 2.5 to 5.0 (mean

3.6) mm. Third internode 2 to 7 (mean 3.0 ±1.2) mm

long, 2.5 to 4.5 (mean 3.2) mm wide, length/width ratio

0.9:1. Staminate flowers 3.5 to 4.5 (mean 4.0) mm

across; perianth mostly four-merous (sometimes threemerous); verticillate, with five to 10 flowers per whorl;

segments 1.8 mm long, 1.2 mm wide. Mature fruit 15

by 10 mm. Seeds about 11 by 6 mm; embryos 4 by 1 mm.

Phenology. Meiosis September to October. Anthesis usually March and April. Fruits mature in September and October of the year following pollination;

maturation period averages 18 to 19 months.

Hosts. This dwarf mistletoe principally parasitizes

Pinus arizonica, P. cooperi, P. durangensis, and P.

engelmannii.

Distribution. Mexico (Durango). Populations occur east of El Salto on the Durango–Mazatlán Highway, in the Sierra Candella, Sierra Huacol, Sierra

Guanacevi, and along the road from Santiago

Papasquiaro to Altares (Cibrián Tovar and others

1980). Elevational range is 2,000 to 2,750 m.

Discussion. Hawksworth and Wiens (1965) report

the rediscovery of this unusual species, first described

by Engelmann in 1880, from El Salto, Durango. The

species is perhaps the most distinctive and primitive

in the genus. The staminate shoots are characterized

by thick spikes (4 to 6 mm) with verticillate, fourmerous flowers; the entire spikes are deciduous after

flowering. This is the only dwarf mistletoe in which the

pedicels do not elongate and curve downward when

the fruits mature. Typically, the pericarp oozes off

the top of the seed, and seeds are released but not

explosively discharged. Compared to other dwarf

mistletoes, mature fruits are more than twice as large

and seeds weight 100 times more (200 to 270 mg). In

further contrast, most dwarf mistletoes are found in

closed canopy stands as pockets of infested trees with

severe infections in the lower crowns; but Arceuthobium

verticilliflorum is found in open stands with random

distributions in trees and crowns. Fruit and seed

morphology, ecological distribution, and observation

of birds feeding in infected crowns suggest this dwarf

mistletoe is dispersed by birds. This dwarf mistletoe

causes massive witches’ brooms, and the diameter of

infected branches is sometimes greater than that of

the trunk where the infected branch emerges. Infections on the main trunks of pines sometimes extend up

to 3 m in length.

32. Arceuthobium yecorense

Yecoran Dwarf Mistletoe

Arceuthobium yecorense Hawksw. & Wiens, Phytologia

66:6, 1989.

Description. Mean shoot height 12 (maximum 17)

cm, yellow-green to brown, branches flabellate. Basal

diameter of dominant shoots 2 to 5 (mean 3) mm. Third

internode 10 to 21 (mean 15) mm long, 2 to 4 (mean 2.4)

mm wide.

Phenology. Time of anthesis is unknown but suspected to be June. Time of fruit maturity is unknown,

but presumed to be September and October.

Hosts. The principal hosts in the Yecora region are

Pinus leiophylla var. chihuahuana and P. herrerai. In

the Sierra Madre Occidental, it occurs principally on

Pinus durangensis, P. herrerai, P. lumholtzii, and P.

leiophylla var. chihuahuana. Pinus engelmannii is a

secondary host.

Distribution. Mexico (Sonora, Chihuahua, and

Durango). The distribution of this dwarf mistletoe is

poorly known and only collected from the Yecora

region (Sonora and Chihuahua) and about 100 km

west of Santiago Papasquiaro (Durango). Because it is

abundant at two locations separated by more than 400

km, the dwarf mistletoe should be expected in the

intervening forest areas as well. Elevational range is

1,600 to 2,500 m.

Discussion. Arceuthobium yecorense is characterized by its slender, greenish–yellow to brownish shoots

and early summer flowering period. The plants are

morphologically most similar to A. aureum subsp.

aureum of the lowlands of Guatemala. The two populations are similar except that shoots from western

Durango have more yellowish and slightly taller

shoots. Yecora is the primary pine-producing area of

Sonora; the dwarf mistletoe there is both common

and damaging.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 57

B. W. Geils

F. G. Hawksworth

5

Chapter

Damage, Effects,

and Importance

of Dwarf Mistletoes

All dwarf mistletoes are parasites that extract water, nutrients, and carbohydrates from the infected host; they are also

pathogens that alter host physiology and morphology (Gill

and Hawksworth 1961, Hawksworth and Wiens 1996). Disease or direct effects are reductions in diameter and height

increment, survival, reproduction, and quality; witches’

brooms are formed in many pathosystems (Knutson and

Tinnin 1980). Where dwarf mistletoe populations develop

significant, long-term infestations, cumulative tree damages

have various ecological and evolutionary effects. Depending

on management objectives and priorities, these effects are

interpreted as positive, negative, or usually of mixed consequences. In chapter 4, we discuss in general how the mistletoe’s

environment affects its growth and development and relate

how mistletoe abundance is described by a relative severity

index, DMR. In following chapters, the authors present

information for quantifying host and mistletoe populations;
58 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Hawksworth Damage, Effects, and Importance of Dwarf Mistletoes

they also indicate numerous ways in which managers

can influence mistletoe through manipulation of biotic

agents, host genetics, and forest stands. In this chapter, we review the physiology of mistletoe parasitism,

describe disease effects on infected host trees, identify

some of the complex ecological and evolutionary interactions of which mistletoes play a role, and relate

these effects and interactions to their consequences for

resource management.

Physiology of Dwarf Mistletoe

Parasitism _____________________

Dwarf mistletoes cause tree disease by affecting

host water relations and growth (Knutson 1983, Kolb

2002). The shoot and leaf surface of dwarf mistletoes

is small compared to other mistletoes, but they have

significant effects on host water relations (Fisher

1983, Sala and others 2001, Wilson and Calvin 1996).

Although dwarf mistletoe shoots do transpire, significant transpiration loss is by host foliage, especially

those with large witches’ brooms. Dwarf mistletoes

affect host growth through the interaction of the host

with the mistletoe endophytic system (Alosi and Calvin

1985, Calvin and Wilson 1996). The physiological

processes involved include: (1) production of growth

regulating compounds and (2) expropriation and reallocation of water, minerals, and carbohydrates

(Livingston and others 1984, Rey and others 1991,

1992, Snyder and others 1996). The pathological symptoms are retention of infected branches, abnormal

growth of infected branches (witches’ brooms), crown

dieback, and death (Anderson and Kaufert 1959,

Broshot and Tinnin 1986, Hawksworth 1961).

Dwarf mistletoe infection affects host foliage, phenology, and respiration. Numerous authors report

that needles of severely infected trees are smaller,

fewer, and yellowish (Andrade and Cibrián 1981,

Hawksworth 1961, Hawksworth and Johnson 1989a,

Korstian and Long 1922, Weir 1916b). Pseudotsuga

menziesii with Arceuthobium douglasii initiate bud

break earlier and form longer shoots on brooms (Briede

and others 1991). Dwarf mistletoe infected trees have

lower respiration rates (Ryan 1990, Wanner and Tinnin

1986), perhaps the result of carbohydrate deficiency.

Tree vigor as a single “health” index is evaluated in

numerous ways; Schaffer and others (1983a) relate

mistletoe infection with vigor and electrical resistance

of bark tissues. Srivastava and Esau (1961) and Cibrián

and others (1980) examine the effects of infection for

distorting the host wood anatomy. One difficulty in

researching dwarf mistletoe–host physiology is detecting and quantifying the endophytic system especially during incubation and latency. Marler and others (1999) demonstrate a polymerase chain reaction

(PCR) method for identifying infected branches based

on presence of mistletoe DNA.

Direct Effects to Host Trees

The pathological results of dwarf mistletoe infection

are seen as reductions in reproduction, growth, longevity, and quality. The nature and magnitude of

these effects are determined by the mistletoe and host

species involved, infestation severity (usually measured as DMR, see chapter 4), and vigor of the host.

These factors are in turn affected by age, history, and

the influences of insects, other disease agents, competition, site quality, and climate (Hawksworth and

Scharpf 1978, Hawksworth and Shaw 1984,

Hawksworth and others 1992a). From a management

perspective of mitigating these effects, the important

considerations are time and opportunity. Both mistletoe intensification and damage are progressive and

cumulative; they begin at a slow rate, with little effect,

but increase exponentially, accumulating to a large

effect. Damage first becomes evident when the crown

of the host tree is about half infected (moderately

infected, DMR class 3) and becomes increasingly severe as the infection intensifies to its culmination

when the entire crown is infected and the tree dies.

Mistletoe intensification and damage are also interactive with each other and responsive to numerous

external factors. Models such as documented by

Hawksworth and others (1995) integrate these numerous interactions and factors and portray the development of an infestation with useful management

indicators such as numbers of trees, basal area, volume, and ingrowth.

Reproduction

Dwarf mistletoe affects host reproduction through

cone production, seed quantity and quality, and seedling survival. Mature trees are large and usually have

numerous reserves; a severe mistletoe infection, however, can reduce cone and seed production. Seedlings

are especially vulnerable; a single mistletoe infection

on the seedling is either lethal or so damaging the host

sapling appears more like a bush than a tree.

Cones and seeds—Few studies are available on

cone and seed production of dwarf mistletoe-infected

trees. Cone production on witches’ brooms as measured by numbers and size is usually reduced, but

some viable seed may still be produced (Bonga 1964,

Kuijt 1960b, Sproule 1996b, Weir 1916b). The reproductive output of infected trees appears to vary by

species and severity of infection. Seed germination

from parent Pinus ponderosa trees infected by mistletoe is reduced (Pearson 1912); germination is only 60

percent for seeds from moderately infected trees and
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 59

Damage, Effects, and Importance of Dwarf Mistletoes Geils and Hawksworth

75 percent for severely infected trees (Korstian and

Long 1922). Seed from infected P. jeffreyi are smaller,

germinate less (80 percent), and produce poor seedlings (Munns 1919). For P. contorta, Schaffer and

others (1983b) report cone size, seed size, and seed

germination are negatively correlated with infection

severity. Although Wanner (1986) has similar results

for cones and seeds, he observes an initial increase in

seedling stocking (at age 1 year) for some in heavily

infested stands and attributes this increase to better

seedbed conditions, which in these cases offset reduced numbers of seeds produced. Infected Picea

mariana produce fewer cones, fewer seeds, lighter

seeds, and lower germination rates than uninfected

trees (Singh 1981, Singh and Carew 1989). In contrast

to these reports, Reid and others (1987) did not observe an effect on cone production for infected Pinus

rudis.

Seedlings—Disease incidence in young stands can

be high (see for example Roth 1971, Scharpf and

Vogler 1986, Weir 1916b). This is especially serious

because seedlings and saplings are severely damaged

by infection with even a few mistletoe plants. Reduced

height of infected seedlings compared to uninfected

seedlings is reported by Knutson and Toevs (1972) and

Roth (1971). Seedlings are usually infected on the

main stem and quickly killed by the mistletoe. Because of high turnover rates and rapid deterioration

after death, mortality rates among seedlings are difficult to determine. Studies such as Roth (1971) in

which he observed 50 percent loss of infected seedlings

after 12 years, however, support the claim that early

mistletoe infection is usually lethal. Those that survive for a few years at least, often develop into little

more than a single broom and resemble a bush or

bonsai.

Growth

An obvious and important fact about conifer trees is

that they grow; they accumulate stem wood on a bole

that increases in width, length, and volume. The

annual increment for accretion in width varies along

the bole and is measured for convenience at a given

reference height. The variation in width along the bole

is described as form; measures of width, length, and

form are used to compute volume. By diverting the

tree’s resources to other outputs, a mistletoe infestation in a tree affects diameter growth and height

growth, and so consequently affects form and volume.

Fundamental to forest management is the ability to

project expected tree growth under various treatment

options. These projections are now often made with

simulation models such as the Forest Vegetation

Simulator (FVS, Forest Management Service Center

2001) and PrognosisBC (British Columbia Ministry

of Forests 2000). Numerous intrinsic and extrinsic

factors determine tree growth; these can be categorized as species, site, history, competition, and for

infected trees, mistletoe severity. Site covers those

long-term, generally fixed factors related to the potential productivity of the area such as soil fertility, water

holding capacity, and climatic suitability. History reflects past events (droughts) and conditions (stagnation) that affect a tree’s crown, its photosynthetic

engine. Competition encompasses factors measured

by stand density as basal area. Mistletoe severity is

usually quantified as DMR (Hawksworth 1977, chapter 4). Quantitative studies reveal that these factors

are usually confounded; that is, they interact so the

effect of one factor varies as the level of another factor

is changed.

Several techniques exist for study of tree growth.

Stem analysis (for example, Baranyay and Safranyik

1970) is the most intensive but provides detailed

information on diameter and height increment as well

as form and volume. Individual trees can be identified

and reexamined after a period of time to obtain information on each tree’s change in diameter and height

(for example, Hawksworth 1961). Alternatively, trees

can be examined once and past diameter growth determined from an increment core (for example, Tinnin

and others 1999). Some studies compare the diameters (or heights) for trees of different mistletoe classes;

but unless all the trees were the same size and infected

at the same time, this method introduces several

complications and does not really measure growth

response to infection.

Although numerous studies relate mistletoe severity to tree growth, few generalities can be made

(Hawksworth and Wiens 1996). Some recent studies

use a stem analysis technique (Andrade and Cibrián

1980, Baranyay and Safranyik 1970, Pousette 1991,

and Smith 1969). Other studies that examined trees

and increment cores include those by Barrett and Roth

(1985), Filip and others (1993), Knutson and Tinnin

(1986), Mathiasen and others (1990), Tinnin (2001),

Tinnin and others (1999), and Vera (1984). Reduction

in diameter increment is related to infection severity

in nonlinear fashion: with little or no significant reduction for the DMR classes 1 to 3, some reduction for

DMR class 4, more for DMR class 5, and much for DMR

class 6. The magnitude of these reductions depends on

numerous factors (Hawksworth and others 1995,

Hawksworth and Shaw 1984, Thomson and others

1997, Wicker and Hawksworth 1988). Reduction in

height increment is also related to infection severity;

height effects usually appear at a lower severity and

are proportionally greater with increase by DMR class.

The combined effects of diameter reduction and height

reduction on form and volume can vary by species and

age (Pousette 1991, Tinnin 2001). Volume reductions,
60 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Hawksworth Damage, Effects, and Importance of Dwarf Mistletoes

either accounting for stem form or not, are proportionately greater than reductions for diameter or height

alone. Because mistletoe infection often occurs earliest on some of the larger trees of a stand, size comparisons of trees in different severity classes do not well

reflect effects on growth increment.

Longevity

Mistletoe not only kills small trees but in time, a

severe infection can even kill a mature, large tree

(Roth 2001). A severe infestation (for example, Wood

and others 1979) with many seriously infected trees

can generate a high mortality rate. Mortality rates

(see Hawksworth and Wiens 1996) are determined

from either reexamining a plot after a known period of

time (dependable) or estimating which trees had died

within the reference period (undependable). The effect

of mistletoe on tree survival can also be expressed in

terms of tree longevity, the period of time over which

a fraction (usually 50 percent) of trees are expected

die. Because tree mortality is infrequent and then

occasionally synchronous with events such as droughts

(Childs 1960, Page 1981, Smith 1983), longevity studies over a long period with frequent observations

(Hawksworth and Geils 1990) are especially useful.

Like growth effects, mortality is related to a number of

interacting factors; the most important are species,

size, infection severity, and other mortality agents.

Hawksworth and Wiens (1996) identify 17 mistletoe

species that are especially lethal for certain hosts and

locations (table 5-1). These hosts include many important forest species such as Abies magnifica (Parmeter

and Scharpf 1982), Larix occidentalis (Weir 1916a),

Picea mariana (Baker and French 1991), Pinus contorta

(Baranyay and Safranyik 1970, Hawksworth and

Johnson 1989a), P. ponderosa (Hawksworth 1961,

Roth 2001), and Pseudotsuga menziesii (Filip and

others 1993, Mathiasen and others 1990). A study

reported by Hawksworth and Geils (1990) and Geils

and others (1991) demonstrates the interacting (and

nonlinear) effects of tree size (diameter) and infection

severity (DMR) on the longevity of mistletoe-infected

pine. The expected longevity for 50 percent of trees

with a severe infection (DMR 6) is less than 10 years

for smaller trees (less than 9 inches diameter) and

more than 10 years for larger trees. Over 40 years,

however, many of the larger, severely infected trees

died. During this time, some of the originally moderately infected trees became severely infected and died

at a rate greater than that for uninfected trees. Elevated mortality rates due to mistletoe infection are

built into the Dwarf Mistletoe Model Impact Model

(Forest Health Technology Enterprise Team 2002).

Extremes in temperature and moisture can affect

mortality rates of dwarf mistletoe-infected trees. Mortality rates are often highest following periods of

drought, but there are few quantitative data. The most

comprehensive studies of the interaction of drought

and mistletoe are by Page (1981) and Smith (1983) for

the California drought of 1975 through 1977. Drought

may increase mortality of mistletoe-infected trees

more than four times that of uninfected trees. Smith

and McMahan (2002) describe an eco-physiology extension for the Forest Vegetation Simulator (Forest

Management Service Center 2001). The method they

present could be modified and developed for adjusting

mistletoe-caused mortality rates to account for climatic variation.

Table 5-1—Combinations of North American taxa of Arceuthobium and their hosts in which host mortality rates are particularly high.

Arceuthobium Host Location

A. abietinum f. sp. magnificae Abies magnifica CA

A. americanum Pinus contorta; Pinus banksiana Western US and Canada; MB, SK, and AB, Canada

A. blumeri Pinus spp. Chihuahua and Durango, Mexico

A. campylopodum Pinus ponderosa Southern CA

A. cyanocarpum Pinus flexilis; Pinus albicaulis ID, UT, WY, CO; northern CA

A. douglasii Pseudotsuga menziesii Western North America

A. durangense Pinus spp. Jalisco, Mexico

A. gillii Pinus spp. Chihuahua and Durango, Mexico

A. guatemalense Pinus ayacahuite Southern Mexico and Guatemala

A. laricis Larix occidentalis Northwestern US and BC, Canada

A. microcarpum Picea pungens AZ, NM

A. nigrum Pinus spp. Durango and Puebla, Mexico

A. occidentale Pinus sabiniana CA

A. pusillum Picea mariana; Picea glauca Eastern North America

A. strictum Pinus leiophylla Durango, Mexico
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 61

Damage, Effects, and Importance of Dwarf Mistletoes Geils and Hawksworth

Wood Quality, Decay, and Breakage

Although mistletoe infections usually occur on

branches, the endophytic system can invade the bole

and potentially affect wood quality. Infections and

broken branches caused by heavy witches’ brooms

provide an entry court for decay fungi. Infected

branches and brooms are resinous and dense with

other flammable materials. These effects of mistletoe

infection are important in some situations.

Wood quality of mistletoe-infected trees is affected

by production of larger knots, development of abnormal grain, reduced strength, and other altered physical and chemical properties. Infected wood such as

found in mistletoe-burls is characterized by shorter,

distorted tracheids, increased ray volume, included

pitch, frass, and decay (Cibrián and others 1980, Piirto

and others 1974, Weir 1916a). The effects on sapwood

moisture content and specific gravity are variable:

higher, lower, or not different from uninfected wood

(Hawksworth 1961, Knutson 1970, Wellwood 1956).

Piirto and others (1974) report, however, that infected

wood and wood from other parts of infected trees is

weaker in strength for modulus of elasticity, modulus

of rupture, and work to proportional limit. The effects

on pulp quality, however, are negligible (Dobie and

Britneff 1975, Hunt 1971, Wilcox and others 1973).

The association of decay and mistletoe varies by

species and tree age. In fir, larch, or hemlock trees,

mistletoe infections often provide an infection court

for decay fungi, especially if the wood is exposed (Aho

1982). Englerth (1942) reports that nearly a third of

the decay in hemlock entered through dwarf mistletoe

stem infections and adjacent swollen limbs. Several

decay fungi are associated; the most frequent is the

common brown cubical slash decay fungus Fomitopsis

pinicola (Etheridge 1973). Decay is usually limited to

the area of the swollen bole canker (Aho 1982). Decay

is rarely associated with mistletoe infection in the

more resinous pines, spruce, and Douglas-fir. Wellmanaged, young-growth stands of true fir in California should also have little loss from mistletoe-associated decay (Parmeter and Scharpf 1982).

Witches’ Brooms

Most dwarf mistletoes and several other disease

agents induce abnormal development of host branches

into witches’ brooms. Mistletoe brooms are infected

host branches with excessive branching and shortened (or lengthened) internodes that develop in response to elevated levels of plant growth compounds

(Schaffer and others 1983c). Broom form is determined by the mistletoe and may even be a useful

taxonomic character. There are a variety of broom

forms and classification schemes based on the distribution of the endophytic system, on the host branching

pattern, and on the boom position relative to the bole.

Systemic or isophasic brooms are those in which the

endophytic system of the mistletoe grows with the

apical and cambial tissues of the host and produces

mistletoe shoots either along the branch or at branch

girdles (Hawksworth 1961, Kuijt 1960b). Nonsystemic

or anisophasic brooms are those in which the endophytic system remains localized near the original site

of infection and only grows with the host cambium

(fig. 5-1). Arceuthobium globosum subsp. globosum

and A. occidentale do not induce typical broom formation. Most North American mistletoes usually develop

nonsystemic brooms and rarely systemic brooms.

Arceuthobium americanum, A. douglasii, A.

guatemalense, and A. pusillum consistently produce

systemic brooms (Hawksworth and Wiens 1996).

Hawksworth (1961) classifies brooms of Arceuthobium

vaginatum as typical for nonsystemic brooms where

the localized infection is far from the bole, and although branching is prolific, segments are short (fig. 5-

1A). He describes the uncommon, volunteer leader

Figure 5-1—Witches’ brooms on Pinus ponderosa

induced by Arceuthobium vaginatum; stipple areas

indicate region invaded by the mistletoe. A, typical

broom; B, volunteer leader broom; C, weeping broom.

Adapted from Hawksworth (1961), figure 45.

A

B

C
62 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Hawksworth Damage, Effects, and Importance of Dwarf Mistletoes

brooms (fig. 5-1B) as localized infections, near the base

of the bole in which one or several leaders develop as

long, erect, normally branched forks. Weeping brooms

are very rare systemic brooms of numerous, long,

pendulous infected branches (fig 5-1C). Tinnin and

Knutson (1985) classify the systemic brooms of A.

douglasii by position. Type I brooms originate away

from the bole on long branches and thereby form large

brooms on drooping branches. Type II brooms form on

branches but near the bole and with a sturdy, upright

supporting branch; the supporting branch makes a

horizontal platform between the bole and mass of the

broom. Type III brooms form at or close to the bole and

lack the dominant, platform branch. Witches’ brooms

can also be produced by rust fungi (broom rusts and

gall rusts), other fungi (Elytroderma deformens), chromosomal condition (trisomatic cells), and physiological reaction to canopy opening or age (stimulation

brooms). Mistletoe brooms can be distinguished by the

presence of aerial shoots or their remnant basal cups.

Witches’ broom formation by dwarf mistletoes can

have a major impact on host growth and crown form.

Brooms become quite large on Douglas-fir or numerous on pine and larch. Brooms are a preferred growthsink; host resources that would have gone to the bole

and roots are diverted into broom growth. The importance of this effect is evident from the improved vigor,

growth, and survival of broom pruned trees (Lightle

and Hawksworth 1973, Scharpf and others 1988).

Large brooms, especially on trees with brittle wood,

may break off (Hadfield 1999). Brooms differ from

normal crown branches for numerous features:

needles, twigs, and accumulated detritus (Bonga

1964, Broshot and others 1986, Tinnin and Knutson

1980). These differences are important for their consequences on canopy structure, wildlife habitat, and

fuel loading.

Ecological and Evolutionary Effects

The effects of mistletoe infection on trees have numerous consequences for associated species and various natural processes. Mistletoes, especially in significant infestations, act as both keystone species (Watson

2001) and controlling disturbance agents (Holling

1992). From this perspective, we view how dwarf

mistletoes affect community dynamics by their interactions with fungi, insects, and fire, effects on vegetation, and use by wildlife. Hawksworth and Wiens

(1996) also discuss these topics and providing examples of pathogenic and biotic associates.

Interactions

The forest communities to which dwarf mistletoes

belong include large numbers of species of various

taxonomic groups and ecological roles. For consideration of the most obvious ecological effects, we focus

here on the interactions of mistletoes with other disease, injury, or disturbance agents.

Fungi—Forest fungi are important in nutrient recycling (decay and mycorrhiza) and as pathogens of

mistletoes and their hosts. The relation of mistletoes

and decay fungi is discussed above, and the pathogens

of mistletoes are described in chapter 7 as biological

control agents. In many forests, mistletoes are only

one of many tree pathogens; the most important are

canker fungi (Filip 1984), root disease fungi (Marsden

and others 1993), and stem rusts (Hawksworth and

others 1983).

Insects and Spiders—Insects and spiders that

react to mistletoe infestations can be categorized as

those associated with shoots, with brooms and infected branches, and with infested trees (Stevens and

Hawksworth 1970, 1984). Insects associated with

shoots include pollinators, herbivores, and their predators, parasites, and associates. Some of the important

shoot insects are potential biocontrol agents (see chapter 7), others include lepidopterians such as Mitoura

spinetorum (Grimble and Beckworth 1993) and aphids

with their attending ants. Numerous insects and spiders use mistletoe brooms with their accumulation of

needles and other detritus as a special habitat for

foraging and hunting. The significant insects associated with infected trees are tree defoliators and bark

beetles. Defoliators may feed upon mistletoe-infected

trees and contribute to tree damage and mortality

(Filip and others 1993, Wagner and Mathiasen 1985).

Mistletoe may affect tree phenology and shoot development, which has a consequence to defoliator development (Briede and others 1991). The attraction of

bark beetles to mistletoe-infected trees depends on the

species combination (mistletoe-tree-insect) and severity of infection. Hawksworth and Wiens (1996) review

the combinations for which mistletoe infection appears to increase, decrease, or be unrelated to bark

beetle attack. For example, Johnson and others (1976),

McCambridge and others (1982), and McGregor (1978)

discuss mistletoe as a predisposing factor for mountain pine beetle; Wilson and Tkacz (1992) for an

outbreak of Ips in pinyon. Nebeker and others (1995)

and Linhart and others (1994) consider the possible

chemical bases for insect attraction to infected trees.

An intermediate hypothesis to explain aggressive bark

beetle (for example, mountain pine beetle) attraction

to infected trees suggests that there would be no

difference in beetle attack between similar sized trees

that are uninfected or lightly infected (DMR 1 or 2),

greater attack for moderately infected trees (DMR 3 or

4), and reduced attack for severely infected trees

(DMR5 or 6). This hypothesis requires testing in

various situations.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 63

Damage, Effects, and Importance of Dwarf Mistletoes Geils and Hawksworth

Fire and Fuels—The fire ecology of dwarf mistletoes is reviewed by Alexander and Hawksworth (1975)

and updated by Zimmerman and Laven (1984). Several features of mistletoe infection increase the tree’s

flammability. Infection induces excess resin deposition and increases litter accumulation (including detached brooms). Retained brooms and infected branches

form a fuel ladder from the ground into the canopy.

Mistletoe severity (DMR) is related to scorching, mortality, and sanitation (Conklin and Armstrong 2001).

Although an extreme, stand-replacing fire kills most

trees, a few isolated mistletoe-infected trees can escape to not only reseed the stand but also reinfest it.

Disturbance regimes and stand structure resulting

from mistletoe and fire interactions are discussed by

Bradley and others (1992) and Kipfmueller and Baker

(1998).

Forest Structure and Composition

Forest insects and pathogens are increasingly being

recognized as important agents in shaping the structure and composition of forests (Hessburg and others

1994, Holling 1992, Monning and Byler 1992). Besides

their interaction with fire described above, mistletoes

affect the forest canopy, landscape pattern, and tree

species mix (Baker and French 1991, Mathiasen 1996,

Parker and Parker 1994, Reich and others 1991, Wanner and Tinnin 1989). The ecological importance of

witches’ brooms on community dynamics is examined

by Tinnin and others (1982); and the role of mistletoes

in forest canopies is reviewed by Mathiasen (1996).

The paper on canopy light and mistletoe distribution

by Shaw and Weiss (2000) is an example of detailed,

canopy ecology studies at the Wind River Canopy

Crane. Mistletoe and forest vegetation studies include

examinations of plant association (Marshall and Filip

1999) and biotic diversity (Mathiasen and Marshall

1999). Two additional topics that have received special

attention are effects and dynamics of mistletoe in oldgrowth stands and on wildlife habitat.

Old-Growth Forests—Numerous studies have

examined mistletoe effects on immature and mature

trees in managed stands, but there are few studies for

old trees (over 200 years) and old-growth stands.

Hawksworth and others (1992a) described a 300-yearold Pinus contorta stand infested with Arceuthobium

americanum. Although infected trees occurred on over

half the area, there were no isolated infection centers

as were found in nearby 70-year-old stands. Tree

mortality was higher among infected trees, but diameter growth was significantly reduced only among the

most severely infected trees (DMR 6). These older

trees grew slower, and on a percentage basis mistletoe

had less effect than seen in younger, faster growing

trees. Parker and Parker (1994) examined the spatial

pattern of tree density in seven P. contorta stands

about 120 to 140 years old. They observed dense,

closed-canopy stands that appeared to have developed

and closed rapidly after initiation (fire) and low density, open-canopy stands with recruitment that is

more continuous. They speculated that the open stand

might have resulted from low initial stocking and high

mortality from mistletoe. Kipfmueller and Baker (1998)

describe another set of 43 P. contorta stands also in the

Central Rocky Mountains and also representative of

unmanaged, older stands (some to 500 years). They

found that half of the stands were infested, and the

average disease severity (DMR) increased with time

since stand establishment. At the landscape scale,

mistletoe often occurred as severe infestation patches

but was absent from other areas of similar age. They

concluded that a healthy forest would include a mosaic

of infection centers and uninfested stands with periodic stand-replacing fires that vary in intensity.

Wildlife Habitat—Although dwarf mistletoes do

not provide large incentives for birds or mammals to

visit for pollination or seed dispersal as do other

mistletoes, dwarf mistletoes provide forage, foraging

sites, protected and special sites, and desirable stand

structures for numerous wildlife species. (Bird dispersal is important for Arceuthobium verticilliflorum

and possibly A. occidentale.) Hawksworth and Geils

(1996) review the use of mistletoe by birds and mammals for food, nesting, and cover. Numerous studies

have since been reported. Allred and Gaud (1994)

describe tree selection and bark grazing by Abert

squirrels and their high use of mistletoe-infected trees.

Brooms in Douglas-fir are frequently used for cover

and nesting (Hedwall 2000, Parks and others 1999a,

Parks and Bull 1997, Tinnin and Forbes 1999). Brooms

in ponderosa pine are also used (Garnett 2002). Brooms

and associated mistletoe-infested sites are important

for nesting by the northern spotted owl (Everett and

others 1997, Marshall and others 2000). Steeger and

Hitchcock (1998) describe the effects of several tree

diseases, including mistletoe, on stand structure preference for nuthatches. Reich and others (2000) examine the relationship of canopy opening in a mistletoeinfested strand on bird usage. Although Bennetts and

others (1996) found a positive association between the

stand severity of mistletoe and bird usage in Colorado,

Parker (2001) for a similar study in Arizona found a

mixture of responses depending on bird species. Mistletoe presence, incidence, and severity may not be good

indicators themselves of wildlife habitat value. Wildlife species are probably responding in a complex way

to special features such as brooms and snags, to

vertical crown structure, to canopy gap pattern, and

other factors affected by mistletoes (Reynolds and

others 1992).
64 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Geils and Hawksworth Damage, Effects, and Importance of Dwarf Mistletoes

Consequences to Resources and

Other Values ___________________

Dwarf mistletoes are important because they are

serious pathogens of valuable conifers in many forests

of North America. These conifers are valuable economically, primarily for their timber yields and ecologically for their role in forest ecosystems (Hawksworth

and Shaw 1984). Importance and worth, however, are

only meaningful and relevant within a given value

system that is selected by the forest manager, owner,

policymaker, stakeholder, or society.

Importance

Species Affected—Conifers that are hosts to dwarf

mistletoes can be divided between major species that

occur in great numbers over large areas, and rare

species with few, sparse populations. In Canada, the

major host species are Larix occidentalis, Picea

mariana, Pinus contorta, P. banksiana, and Tsuga

heterophylla. In the Eastern United States, the major

species are Picea mariana, P. glauca, and P. rubens;

and in the Western United States, they are Abies

magnificae, A. concolor, Larix occidentalis, Pinus ponderosa, P. contorta, Pseudotsuga menziesii, and Tsuga

heterophylla. Mexico has a great abundance and diversity of conifers (over 30 taxa) that are mistletoe hosts

(Hawksworth 1980, Hawksworth and Cibrián 1985).

Abies religiosa and Pseudotsuga menziesii are infected; but the most common hosts are pines, including

yellow pines, white pines, and pinyon pines. One of the

more rare conifers that are hosts for Arceuthobium

abietinum is Picea breweriana in Oregon; it is severely

infected (Hawksworth and Wiens 1996).

Area Affected—In Canada, Magasi (1984) reports

Arceuthobium pusillum is common in the Maritime

Provinces. Overall, 20 percent of sites are infested and

6 percent of trees infected, but nearly all of the infested

sites, infected trees, and mistletoe-caused mortality

are in wet areas. Brandt and others (1998) map and

summarize the distribution of severe mistletoe infestation (by A. americanum) in Manitoba, Saskatchewan,

and Alberta. About 5 percent of the forest area or

500,000 ha are specifically identified as severely infested sites, where mistletoe-caused mortality and

brooming are visually obvious (also see Baker and

others 1992). Moody and Amirault (1992) estimate

mistletoe incidence in individual, severely infested

stands ranges from 73 to 100 percent. In British

Columbia, A. americanum and A. tsugense are widespread, common, and damaging at many sites (Moody

1992, Thomson and others 1997). Hodge and others

(1994) report only 2 percent of managed stands infested and only 3 percent of trees infected (except in a

few stands, however, infection reaches 34 percent).

In the United States, Arceuthobium pusillum ranges

widely across the Northeastern and Lake States; but

its occurrence varies from locally common to rare in

some States (Hawksworth and Wiens 1996).

Drummond (1982) estimates 14 percent of the spruce

area in the Lake States is infested. Numbers for dwarf

mistletoe-infested area for each of the Western United

States, however, are available (Forest Health Protection 2002). Westwide, about 25 percent (Drummond

1982, Bolsinger 1978) or 28.7 million acres of Western

forests are infested (Forest Health Protection 2002).

In contrast to the report (and accompanying compact

disk) by Brandt and others, the United States’ summaries (Drummond 1982, Forest Health Protection 2002)

and the data on which they are based provide only

statistical estimates of area infested and cannot map

the specific, infested sites. A number of regional summaries are available. Andrews and Daniels (1960)

report on the distribution of dwarf mistletoe in Arizona and Mexico in terms of administrative area

(forest), harvest status, and various ecological factors.

The most important forest types are ponderosa pine

and Douglas-fir; Andrews and Daniels classify 36

percent of the ponderosa pine type as infested and 47

percent of the Douglas-fir type as infested. Maffei and

Beatty (1988) report on a resurvey of the ponderosa

pine area examined 30 years previously by Andrews

and Daniels (1960). Maffei and Beatty (1988) attribute

the apparent increase of infested area (8 percent more

of the type) to ineffective mistletoe control. Other

regional summaries are prepared by Bolsinger (1978)

for the Pacific Northwest, Byler (1978) for California,

DeNitto (2002) and Stipe and others (1992) for the

Northern Region (Montana), and Johnson and others

(1981) for the Rocky Mountain Region (Colorado and

Wyoming).

Dwarf mistletoe occurs throughout the conifer forests of Mexico. Vázquez (1994a) states that an estimated 1.8 million ha in Mexico are infested. Most the

information related to mistletoe damage in Mexico

occurs as reports of infested area and infection incidence at various localities (see Hawksworth 1980).

Caballero (1968, 1970) indicates the percentage of

inventoried forest sites infested for several States:

Zacatecas 24 percent, Durango 15 percent, Jalisco 12

percent, Nayarit 10 percent, Sinaloa 10 percent, Sonora

9 percent, Chihuahua 8.5 percent, and Baja California

7 percent. Within stands, the extent of the area infested and the percent of infected trees can be as high

as 85 percent (Acosta and Rodriguez 1989, Gutierrez

and Salinas 1989).

Growth Loss—Information on reduction of volume

increment, mortality, and area infested can be used to

estimate mistletoe impact to stand yield on an area

basis. The difference between realized volume in an

infested stand (reduced by loss of increment and
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 65

Damage, Effects, and Importance of Dwarf Mistletoes Geils and Hawksworth

mortality) and potential yield for the site (if mistletoe

were not present) is described as growth loss. The

growth loss concept can be applied nationally, regionally, forestwide, and to individual stands (Baker and

Durham 1997 describe a method for computing growth

loss). Drummond (1982) estimates a total annual

growth loss from mistletoe in the United States at 418

million cubic feet per year; Vázquez (1994a) for Mexico

reports a loss of 2 million cubic m per year. Estimates

for Canada are available for Newfoundland at 1 cubic

m per year per ha (Singh and Carew (1989); for

Manitoba, Saskatchewan, and Alberta at 2.4 cubic m

per year (Brandt and others 1998); and for British

Columbia at 1.8 million cubic m per year (Forest Insect

and Disease Survey 1994). DeNitto (2002) provides a

growth loss estimate of 33 million cubic feet per for

Montana (broken down by host species). Johnson and

others (1981) describe growth loss for forests of Colorado and Wyoming. Marsden and others (1993) illustrate use of a forest growth and yield program to

compare expected yield for a mistletoe-infested stand

to what might be had were the stand not infested.

Growth loss can be so significant in severely infested

stands (especially immature and infected at an early

age) that commercial yield cannot be obtained

(Hawksworth and Hinds 1964).

Amenity Values

Dwarf mistletoes are sufficiently unusual and influential that they are important to a number of

resource and amenity values besides commercial

timber yield. Other forest products have traditionally included watershed protection, recreation opportunity, and wildlife habitat. Over a half century ago,

concern over the effects of dwarf mistletoe to oldgrowth ponderosa pine at the Grand Canyon National Park lead managers to an intensive control

project (Lightle and Hawksworth 1973). The effects

of dwarf mistletoe on fuel loading and fire behavior

are still a serious interest to managers (Zimmerman

and Leven 1984). We have already discussed how

witches’ brooms and forest structure affect wildlife

abundance and diversity (Bennetts and others 1996,

Parker 2001, Reich and others 2000). Mistletoes are

also valuable as mistletoes themselves and as members of a biotic community.

Mistletoes possess aesthetic, scientific, and intrinsic

values. Although the mistletoe plant and diseased

trees are not usually considered attractive (exceptive

by some forest pathologists), a distinctively broomed,

dead tree against the backdrop of the Grand Canyon

does make a strong and interesting picture. Mistletoes

can provide chemical analogs that may be developed

into useful drugs. They serve as models for understanding the evolution of parasitism (Atsatt 1983a)

and phylogeny of their hosts (Hawksworth 1991).

Rolston (1994) describes the value of living entities

beyond their worth to humans as achievement and

part of the system of life.

Forests are not only managed for the resources they

produce but also to sustain and protect forest health

(Monning and Byler 1992) and ecosystem values (Taylor 1995). Dwarf mistletoes are important disturbance

agents (Holling 1992) with distinct ecological functions (Hessburg and others 1994). They contribute to

natural diversity structurally (Mathiasen 1996) and

biologically (Mathiasen and Marshall 1999). Some

mistletoes are considered species of special concern

(Hildebrand 1995), and truly rare species such as

Arceuthobium hondurense probably deserve protection. A balanced view of mistletoes as the cause of

losses of valuable resources, but also as natural agents

that shape forests, is emerging (Wittwer 2002).

Coevolution

Information from biogeography, paleobotany, host

relations, and molecular systematics indicates the

dwarf mistletoes have a long evolutionary history of

parasitism with their conifer hosts (Hawksworth and

Wiens 1996). Mistletoes are physiologically dependent on their hosts but cause symptoms that eventually result in death for both. What really matters,

however, from an evolutionary perspective is their

success at leaving descendents. To the present, host

and parasite have lived, reproduced, and died in natural ecosystems. These natural ecosystems, even wildlands, are increasingly controlled and affected by

managers and human society. We have a fair understanding of the physiology of mistletoe–host parasitism and a good ability to predict the effects of infection

on tree growth and survival. We are beginning to

appreciate the complex ecological interactions in which

mistletoes participate. By management with biological agents, chemicals, genetic manipulation, and silviculture, we attempt to influence how mistletoe affects

resources and our environment. For that management

to have a beneficial outcome, which is sustaining to the

biotic system on which we depend, it is advisable to

consider not only immediate results but also ecological

and eventually evolutionary consequences.

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 67

J.A. Muir

B. Moody

6

Chapter

Dwarf Mistletoe

Surveys

Dwarf mistletoe surveys are conducted for a variety of

vegetation management objectives. Various survey and sampling techniques are used either at a broad, landscape scale in

forest planning or program review, or at an individual, stand,

site level for specific project implementation. Standard and

special surveys provide data to map mistletoe distributions

and quantify disease severity. At a landscape scale, extensive

surveys assess regional impacts, estimate mistletoe occurrence, intensity, and effects, and estimate future growth and

yield. Intensive surveys evaluate stands, campgrounds, and

other sites to design projects and monitor treatments.

Numerous variations and combinations of techniques such

as aerial survey and photography, forest inventory, road and

plot survey, transects and grid survey, and permanent plots

are used to obtain dwarf mistletoe information for program

and project management (table 6-1). Only a few studies

compare alternative survey and sampling methods

(Drummond 1978, Hildebrand and others 1997, Mathiasen
68 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Moody Dwarf Mistletoe Surveys

and others 1996b, Vázquez 1993b, 1994a). Effective

and efficient sampling benefits from use of explicit

objectives recognizing the resources of interest (such

as timber, recreation, wildlife), specification of statistical standards, and consideration of cost and safety

issues (Tkacz 1989, and see assistance provided at

Forest Service 2002). In this chapter, we identify the

major types of dwarf mistletoe surveys, uses of that

data, and subjects for research and development.

General Requirements and

Procedures ____________________

Before selecting or adapting one or more methods, a

user should carefully consider and articulate the purpose and scope of the proposed survey. Almost all the

available methods for estimating mistletoe occurrence

and effects can be adapted to a variety of purposes

including timber management, vegetation inventory,

recreation, and wildlife management. Many techniques

can be adapted to a range of scales from regional or

forest landscapes to individual stands or sites. For any

management decision, a wide variety of information

from numerous sources on various subjects is needed.

In areas with significant dwarf mistletoe infestations,

data on mistletoe extent, severity, and potential make

an important contribution to the decision process

(Stage and others 1986, Tkacz 1989). Given the variety

of objectives and constraints encountered by managers, only general guidelines can be stated here. The

benefits of proposed treatment in each particular case

should be evaluated for expected costs and benefits,

impacts to other resources, and conflicts with other

objectives. Assessments for landscape-scale management usually require only extensive, relatively broad

information on dwarf mistletoe occurrence and effects.

A general strategy for dwarf mistletoe management in

areas of significant occurrence may be sufficient. At

the level of particular stands and sites, however,

management prescription may require detailed information on resource and ecological conditions and specific data for mistletoe distribution and abundance.

Site prescriptions also require consideration of the

general principles for managing infested stands (see

chapters 7 and 8) as well as local issues and forestlevel management objectives.

Several Provinces and States require a professionally certified evaluation of young forests to ensure that

damages from insects and diseases are less than

specified levels. This is especially important where the

previous stand had been infested by dwarf mistletoe

because the regeneration process may have left an

infected, residual overstory that allowed spread to the

seedlings (Alfaro 1985). In British Columbia, evaluations assess whether the young stand is free-growing

and contractor obligations satisfied (British Columbia

Ministry of Forests 1995); in other regions, this standard is described as producing adequate stocking of

healthy seedlings.

Several existing data sources are available for general information on regional occurrence and potential

impacts of dwarf mistletoes. These include Forest

Inventory and Analysis (2002), Forest Health Monitoring Program (1994), Current Vegetation Survey

(2002, Gregg and Michaels Goheen 1997), and the

Canadian Forest Insect and Disease Survey (Cerezke

and Emond 1989, Moody 1992, Myren and Gross 1977,

Wood 1986). Use of these kinds of data is reviewed by

Bolsinger (1978), Drummond (1982); limitations are

described by Drummond (1978), Hildebrand and others (1997), and Marsden and others (1990).

If archive, large-scale data are either lacking or not

sufficient, other established forest management plots

are available from standard resource inventories. Data

for dwarf mistletoes are relatively easily determined,

Table 6-1—Surveys for mapping the distribution and quantifying the effects of dwarf mistletoes.

Technique Objective(s) Reference Example

Aerial survey Landscape assessments Brandt and others 1998

Aerial photography Landscape assessments Baker and French 1991

Forest inventory plots Landscape assessments Hildebrand and others 1997

Road and plot surveys Landscape assessments Thomson and others 1997

Transects and grids Landscape assessments Maffei and Arena 1993

Permanent plots Detailed assessments Hawksworth and Marsden 1990

Project monitoring Lightle and Hawksworth 1973

Project area assessment Management prescription Tkacz 1989

Stand or Land Unit examination Vásquez 1994

Recreation management Scharpf and others 1988

Wildlife habitat Parks and others 1999
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 69

Dwarf Mistletoe Surveys Muir and Moody

as described below (see Forest Inventory Plots). For

landscape-level surveys, several techniques such as

road surveys or aerial photography can be used separately or in conjunction with inventory plots.

Surveys of dwarf mistletoes on a landscape or large

geographic area are undertaken to determine the

overall returns from a national or regional program or

to compare returns from standard operational treatments applied to a large forest area. More frequently,

landscape surveys are used to substantiate the general extent and effects of mistletoes and rank areas for

treatment priority. Exercises range from botanical

surveys for detection and identification of dwarf mistletoe species (for example, Hernandez 1991) to largescale, annual programs for tracking broad forest health

conditions and trends (Forest Health Monitoring Program 1994). Large-scale surveys have been conducted

for Canada (Moody 1992, Myren and Gross 1977), the

Southwestern States (Andrews and Daniels 1960,

Maffei and Beatty 1988), and California (Byler 1978).

Occasionally, there is a need for a detailed dwarf

mistletoe survey on a land unit for a specific purpose

such as determining disease impacts to forest ecology,

stand structure, productivity, or treatment effectiveness. Generally, detailed analyses are feasible only if

foresters or specialists have access to agency-endorsed

methods or models for analysis of forest growth and

dwarf mistletoe effects (for example, British Columbia

Ministry of Forests 2000, Forest Management Service

Center 2001). These models are useful in developing

detailed prescriptions for harvesting, silvicultural

treatment, and other management objectives such as

visual quality, wildlife, and recreation management.

Use of models to predict the effects of dwarf mistletoe

damage and analyze benefits from silvicultural treatments is described in chapter 8.

For intensive surveys such as silvicultural inspections and timber cruising, several tree stand attributes

related to dwarf mistletoe status are useful. These

include dwarf mistletoe species (see chapter 4), extent

of infested area, mistletoe incidence as percent of host

trees that are infected, and a measure or rating of

disease severity such as DMR (fig. 6-1) or other rating

for intensity of infection within trees.

Dwarf Mistletoe Surveys__________

Dooling (1978) summarizes several methods suitable for dwarf mistletoe surveys. The most commonly

used methods and recent examples (table 6-1) are

briefly described in the following sections.

Figure 6-1—Dwarf mistletoe rating system (DMR).
70 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Moody Dwarf Mistletoe Surveys

Aerial Surveys

A major technique or approach used for extensive

examinations is aerial survey. This technique is described for dwarf mistletoe (A. americanum) on jack

pine (Robins 1972) and lodgepole pine (Brandt and

others 1998), and Eastern spruce dwarf mistletoe (A.

pusillum) on black spruce (Baker and others 1992).

Aerial survey observers can best detect severe infestations by the distinct infection centers associated with

heavy mortality and brooming. Total extent and incidence are under estimated. Aerial survey is most

practical over low-relief terrain, but rotary-wing aircraft have been used successfully over mountainous

areas (Brandt and others 1998, Schwandt and Page

1978). Flight lines are run parallel at intervals of 5 to

10 km (or wider) through areas of susceptible forest

types. Fixed, overhead-wing aircraft can be used on

clear, bright days at ground speeds of 150 km per hour.

One observer alone or one observer on each side of the

aircraft draws or sketches the extent of each mistletoeinfested area on an appropriate map (which may be a

topographic, forest type, or inventory base map). In

some surveys, a lap-top computer with a digital map

display of topographic and forest-type features is used

with a global positioning system (GPS) to sketch-map

directly on the computer screen; this method is in

development (L. Rankin, 1999 personal communication). Geographic positions of infested stands can also

be located with a GPS-receiver (Brandt and others

1998, Zavala and Zavala 1993). Information is eventually incorporated into a GIS-database. Brandt and

others (1998) demonstrate the capability of this technique for mapping the severe damage caused by dwarf

mistletoe to jack pine and lodgepole pine for an area of

over 28 million ha in Central Canada (Alberta,

Saskatchewan, Manitoba).

Aerial Photography

Aerial photography is routinely used for vegetation

and forest inventory typing, classification, and sampling. Aerial photography is feasible, however, for

landscape dwarf mistletoe surveys only where there

are highly distinctive features such as mortality or

infection centers. These occur in Central to Eastern,

North American forests of black spruce (Baker and

French 1991) and jack pine (Muir and Robins 1973).

Especially in young infestations, mistletoe infections

are often most common in the lower crown and obscure

to aerial view. Large-scale aerial photography can be

used for individual stands to detect infected trees with

large witches’ brooms or to identify susceptible tree

species and suspect individuals based on various crown

attributes.

Forest Inventory Plots

Vegetation or forest inventory sampling is the commonly accepted means of describing and quantifying

the forest resource, and it is often the only acceptable

data source for projecting dwarf mistletoe impacts on

forest growth. Established inventory plots ensure an

acceptable sampling scheme and avoid the expense of

recollecting associated tree data. Dwarf mistletoe data

from routine inventories, however, may have low or

uncertain reliability because inventory crews may

lack the experience to recognize mistletoe presence

and damage (Drummond 1978, Marsden and others

1990). Where inventory plots are reexamined to obtain

mistletoe specific data, trained personnel and check

cruises are appropriate (Hildebrand and others 1997).

Forest inventory plots are often situated by a stratification scheme that if not properly accounted may lead

to biased estimates. Nonetheless, this is described and

used by several authors including Caballero (1970),

Filip and others (1993), DeMars (1980), Gregg and

Michaels Goheen (1997), Hildebrand and others (1997).

Surveys for dwarf mistletoes using an existing inventory plot system can be relatively simple and easy

to conduct. Each sample tree on the inventory plot is

examined and a rating (such as DMR) is recorded.

Wide-field, high-quality binoculars (for example, 8x40)

are useful; yellow-tinted eyeglasses are not used because they obscure dwarf mistletoe shoots (B. Geils

1999, personal communication). Training and quality

checks are appropriate to maintain quality and consistency (Shaw and others 2000).

Growth impacts are calculated from tree data and

information for mistletoe severity using the sample

design of the inventory (Marsden and others 1990). In

many situations, covariate factors are useful. For

example, Thomson and others (1997) point out that for

lodgepole pine, stand density (number of stems per

hectare) has a major effect on tree volume and should

be used to adjust estimates. Supplemental data can

determine the correlations between radial growth,

DMR, tree age, and stand density.

Road and Plot Surveys

Roadside surveys for dwarf mistletoes are popular

and suitable for many forests in Western North America

with reasonably extensive road access (Andrews and

Daniels 1960, Johnson and others 1981, Maffei and

Beatty 1988, Merrill and others 1985, Mathiasen and

others 1996b, Thomson and others 1997). Good results

are obtained in forests of almost pure, even-aged

stands of trees at least 20 years old. A vehicle driver

and an observer traverse roads through susceptible

forest stands at a low speed (say, 30 to 40 km per hour)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 71

Dwarf Mistletoe Surveys Muir and Moody

and rate levels of infestation in segments or fixed

intervals (such as 100 to 200 m) along the roads. Data

are recorded on inventory maps with cover type

information.

Tree infection along roads is usually rated using

percentage incidence categories: nil or 0, no infection

visible; low, 1 to 33 percent of trees infected; medium,

34 to 66 percent infected; and severe, 67 to 100 percent

infected. These data are converted to DMR by correlating roadside incidence ratings with plot DMR. A more

direct and probably more efficient method is to directly

estimate average DMR along roadsides rather than

estimating percentage incidence.

To obtain inventory tree data, fixed-radius or prism

plots are established at intervals of 5 to 10 km along

roads at a distance 50 m back from the road. On each

plot the usual inventory tree measurements are taken

for each tree, including DMR (fig. 6-1). Although it is

suspected that road development or some forest practices might have increased incidence of dwarf mistletoe at roadside units, Merrill and others (1985) found

that incidence at roadsides was similar to that in the

adjacent stand (also see Maffei and Beatty 1988).

Although a roadside survey is relatively easy to

conduct and produces estimates of growth impacts, it

is often difficult to reconcile these results with forest

inventory data. Locating plots along roads introduces

a sampling bias that may result in substantial discrepancies of tree species volumes and growth estimates

between roadside plots and standard inventory plots.

Data from a roadside survey as area infested or percent of trees infected cannot be readily incorporated

into forest inventory record systems. A potentially

more accurate and cost-effective alternative is to move

directly to resampling already established forest inventory plots that also have a history of tree growth

and mortality.

Transects and Grids

Sample plots systematically distributed as either

strip transects or on a gird are another approach for

surveying dwarf mistletoe. Transects and grid sampling have been used for landscape-scale surveys

(Hawksworth and Lusher 1956, Maffei and Arena

1993), but they are more frequently applied to individual stands. Their use has been largely superseded

by vegetation or inventory plots (for example, Current

Vegetation Survey 2002).

Permanent Plots

Another type of dwarf mistletoe survey technique is

the permanent sample plot system. Hawksworth and

Marsden (1990) catalogue a number of these installations. Permanent sample plots are also established to

monitor efficacy of management projects (Lightle and

Hawksworth 1973). These plots typically are much

larger than routine forest inventory plots, have fixed

boundaries, and include a map of the position of all plot

trees. Given the relatively high costs of establishment

and remeasurements, relatively few permanent sample

plots have been established recently or are currently

being maintained. They are, however, extremely valuable for measuring spatial aspects of dwarf mistletoe

spread and intensification and as benchmark stands

used to validate simulation models (Taylor and

Marsden 1997).

Project Area Assessments

Several methods or techniques for dwarf mistletoe

surveys are used primarily to assess stands or sites for

a variety of objectives including developing management prescriptions and management of recreation

areas and wildlife habitat.

Developing Management Prescriptions—Detailed surveys of stands are used to develop management prescriptions (Tkacz 1989) and for general stand

examinations (Mathiasen 1984, Vázquez 1994a).

For dwarf mistletoe infested sites, distribution and

severity data are used to assess management options

(Hawksworth 1978a, Parmeter 1978, van der Kamp

and Hawksworth 1985). The expected effects of a

treatment, for example, such as leaving dwarf mistletoe-infected trees on partially or selectively cutover

areas, can be evaluated in the specific context in which

it is to be applied. These effects can be projected with

data representing the actual stand of interest and

using the agency-supported growth model or simulation program (for example, PrognosisBC or Forest

Vegetation Simulator). The data required from the

survey are determined by the requirements of the

selected model. Generally these include tree data,

DMR (see fig. 6-1), and some ecological classification

describing site productivity. Data are usually collected using a grid or series of prism plots or fixedradius plots according to prescribe methods of the

agency (DeMars 1980). Baker and Durham (1997)

describe a transect survey for mistletoe in young jack

pine and a model to simulate expansion of infection or

mortality centers. Marsden and others (1993) evaluate management options for Southwestern ponderosa

pine stands with Armillaria root disease and dwarf

mistletoe with data from a systematic grid of inventory plots and the Forest Vegetation Simulator. Chapter 8 provides further information on use of models to

evaluate silvicultural treatments.

When dwarf mistletoe sanitation practices are

planned or have been undertaken, an important consideration is to determine both the potential and

realized benefits. In many regions of Western North

America, sanitation treatments after harvesting have
72 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Moody Dwarf Mistletoe Surveys

been a common practice for several years or even

decades. Postcontrol or postsuppression surveys and

evaluations have been undertaken in several regions

(Hawksworth and Johnson 1989b, Knutson and Tinnin

1986, Van Sickle and Wegwitz 1978).

The spatial pattern of infected trees and spatial

autocorrelation of mistletoe are important in some

situations (Robinson and others 2002). Infected young

trees may be clustered around residual infected trees

left as blocks, strips, or groups trees. Patterns that

deviate greatly from random or uniform toward clusters have significant consequences for sampling design and model projections. The spatial pattern of

infected trees and the spatial autocorrelation of mistletoe can be computed from a stem map of the stand of

interest or selected from another stand with a similar

appearance.

Assessing complex stands—that is, those consisting of two or more tree species, age classes, and

height classes — often involves making a compromise

between number of locations visited and the detail

recorded at each location. Because the dynamics and

effects of dwarf mistletoes vary by tree size, it is

important that surveys provide data on incidence and

severity by tree size class. This can be accomplished by

recording DMR and either tree diameter or height for

each sample tree and later computing class averages.

Alternatively, trees can be grouped into classes and

assigned average incidence and severity ratings while

the observer is at the plot. With training and experience, observers are able to retain data quality and

increase productivity. Vegetation structural stages

such as described in table 6-2 or other classification

schemes can be used to group trees into size classes.

The criteria for determining classes vary by situation

but represent canopy structure classes meaningful to

the manager. Where there are several mistletoe species with different host tree species, mistletoe incidence and severity should be estimated by structure

class for each susceptible tree species.

Assessments of Recreation Areas and Wildlife

Habitat—Dwarf mistletoe effects on trees (chapter

5)—including suppression of tree growth, formation of

large witches’ brooms, and increased mortality—can

be important considerations for management of recreation sites and for wildlife habitat. Occasionally, dwarf

mistletoe surveys are required for evaluating the need

for, or efficacy of, silvicultural treatments (see chapter

8) for these types of management.

Trees in recreation sites are regularly inspected for

defects and evaluated for potential hazard to users

and facilities (Hadfield 1999, Lightle and Hawksworth

1973, Scharpf and others 1988). Dwarf mistletoe infection is usually included in the inspections. In some

areas, as described in chapter 8, infected trees are

replaced with other less susceptible tree species. Severely infected trees are pruned to maintain tree

vigor. Tree data generally recorded are DMR and an

estimate of broom size if these are to be pruned.

Infected trees are usually inspected annually or more

frequently.

Surveys for dwarf mistletoe in conjunction with

wildlife habitat are used for management (Marshall

and others 2000) and research (Bennetts and others

1996, Parker 2001, Parks and others 1999a, Reich and

others 2000). Information collected about mistletoe

includes DMR and usually additional information on

broom type, size, and location (Garnett 2002, Hedwall

2000, Tinnin 1998, Tinnin and Knutson 1985).

Evaluations Using Dwarf Mistletoe

Survey Data ____________________

After a survey is conducted to determine forest-level

damage caused by dwarf mistletoe, one or more of

several methods are used to project forest growth

under different management regimes and evaluate

impacts and potential benefits of management programs for dwarf mistletoes (see Power and D’Eon

1991).

One example of this type of evaluation is use of a

whole-forest model such as FORPLAN or MUSYC.

These models predict timber supplies and possibly

other outputs such as wildlife habitat in infested

stands under various management regimes. They determine potential returns and benefits of dwarf mistletoe control programs. Landscape or forest-level yield

models require both extensive data on dwarf mistletoe

occurrence and severity, and response curves based on

individual land units or stands, similar to those proposed by Stage and others (1986) for root diseases.

To our knowledge, forest-level evaluations of dwarf

mistletoe effects have not yet been reported but they

should, however, be relatively simple to develop. Average curves can be developed for average stand conditions, using stand-level models with dwarf mistletoe

effects. Growth curves for lodgepole pine infected by

dwarf mistletoe are reported by Hawksworth and

Table 6-2—Vegetation Structural Stages (VSS), an example of

a classification system for describing dwarf mistletoe incidence and severity. (Table excludes VSS

class 1, nonforested.)

Size class

VSS class (cm of d.b.h.) Description

2 2–12.5 Seedlings/saplings

3 12.5–30 Young trees

4 30–45 Mid-age trees

5 45–60 Mature trees

6 60+ Old trees
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 73

Dwarf Mistletoe Surveys Muir and Moody

Johnson (1989a) and van der Kamp and Hawksworth

(1985) and are included in a review of forest growth

models by Eav and Marsden (1988).

In evaluating effects of dwarf mistletoe, data used to

construct the baseline or “healthy” stand growth curves

should be examined. If temporary plot data were used,

and plots were located without bias, then empirical

growth curves may already include mistletoe effects.

Stands that have been treated for dwarf mistletoe,

therefore, should grow more than the baseline stands.

Growth and yield data for landscape-level analyses,

however, are often derived from remeasured plots

selected to avoid dwarf mistletoe and other disturbances. If growth curves from these stands were used

to represent operational conditions, they represent

the growth of “healthy” stands or what is expected

when dwarf mistletoe infestations are suppressed. In

most analyses, these types of growth curves are usually reduced using one or more “operational adjustment factors” to account for unstocked or unproductive areas (such as swamps or rocky knolls). Tree

volumes are also reduced using factors for waste and

breakage during harvesting and internal wood decay

in live trees. All of these factors and assumptions

should be checked and verified as to the manner by

which mistletoe effects were incorporated.

Another potentially important use of dwarf mistletoe survey data is to evaluate potential benefits of

controlling or preventing effects of dwarf mistletoe on

site productivity. Site productivity is one of the major

factors affecting sustainability of the forest resource.

For an example with lodgepole pine, mistletoe infection at moderate to severe intensities generally reduces growth to such an extent that a forest inventory

based on mature trees would underestimate the site

index or productivity. Foresters might not be particularly interested in dwarf mistletoe as such. If it were

shown, however, that the productivity of the forest

land base were substantially underestimated and

underutilized and that it could be increased with

sanitation, interest may rise.

Further Needs for Surveys and

Evaluations ____________________

Large-scale, forest-growth projection methods need

to be used and modified to accommodate analyses of

the actual or potential benefits of dwarf mistletoe

control programs. In many regions, more or supplementary data will have to be collected by well-trained

personnel in conjunction with forest inventory sampling to provide a more credible basis for determining

dwarf mistletoe effects and defining treatment opportunities.

On an individual stand basis, information on spatial

patterns of trees and autocorrelation of mistletoe need

to be employed in more assessments (Robinson and

others 2002). For many stands with complex structures and heterogeneity (see Reich and others 2000),

an average DMR does not properly represent conditions where wildfire, windthrow, bark beetles, and

mistletoe infestation have created a mosaic of canopy

and gaps. Infected trees often occur in or at the edges

of residual stands, strips, or patches, or as scattered

individual trees; and spread of dwarf mistletoe from

these sources is unlike that across a uniform stand

(Muir 2002, Edwards 2002).

Detailed dwarf mistletoe surveys of land units are

essential for determining effects on forest ecology,

stand structure, and productivity or analyzing effectiveness and benefits of silvicultural treatment. These

surveys and evaluations, however, are feasible only if

foresters or specialists have access to methods or

models endorsed by their agencies. Given the increasing complexity of forest management issues, comprehensive and detailed stand-level models are now essential to develop detailed prescriptions for harvesting

and silvicultural treatments. These models are needed

to ensure that forest ecosystems are managed

sustainably and that these treatments do not detrimentally affect other management objectives such as

visual quality, wildlife habitat, and recreation management. Although there have been considerable improvements in models in recent years, there is a

continuing need for model development for new management scenarios. Access to several models is available from the British Columbia Ministry of Forests

(2000) and Forest Health Technology Enterprise Team

(2002); other vegetation management tools are at

Forest Service (2002). Access to and support for various models are still needed for field foresters to conduct surveys and analyze potential benefits of treatment programs. This is particularly urgent with the

increasing need to consider a wide array of effects and

objectives such as wildlife and fuel reduction.

Finally, the increasing imperative to manage uneven-aged forest stands infested by dwarf mistletoe

necessitates development of indices or measures of

tree-to-tree variation of incidence and infection severity of dwarf mistletoe. New or drastically improved

models are required to analyze the effects of dwarf

mistletoes on trees and the efficacy of silvicultural

treatments (including deployment of biological control

agents) in these complex situations. Measurements of

dwarf mistletoe occurrences and quantitative projections of effects of various forest and stand-level management regimes are essential to guide and help

resolve the various, often-conflicting views of desirable forest-resource management strategies.

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 75

7

Chapter

Management

Strategies for

Dwarf Mistletoes:

Biological, Chemical,

and Genetic Approaches

The opportunity and need for management of mistletoe populations with biological, chemical, and genetic approaches are

greatest for application to the dwarf mistletoes. Although much

information is available on these management strategies (see

reviews by Hawksworth 1972, Knutson 1978), significant research and development are still required for these to become

operational tools. In this chapter, we describe the potential for

these tools and status of their research and development. Resource managers and practitioners interested in using these

approaches can consult with forest pathologists and geneticists

for specific applications.

S. F. Shamoun

L .E. DeWald
76 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Shamoun and DeWald Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches

Biological Control _______________

Many fungi and insects are pathogens or herbivores,

respectively, of dwarf mistletoes (Hawksworth and

Geils 1996, Hawksworth and others 1977b, Kuijt 1963,

Stevens and Hawksworth 1970, 1984). None, however, are sufficiently studied and developed for operational use as biological control agents (Anonymous

1982, Hawksworth 1972). Some fungal pathogens and

insect herbivores (particularly lepidopteran larvae)

are highly destructive to dwarf mistletoes in some

areas and years. The factors that induce or regulate

these outbreaks result from complex and often indirect interactions of weather and a multitrophic community of organisms. Dwarf mistletoe pathogens and

herbivores are indigenous organisms that have coevolved with their hosts into relationships that are not

readily amenable to human control. Nonetheless, given

the potential number of agents and the advantages of

the approach, development of biological control as a

management option appears promising for the near

future (Hawksworth 1972, Shamoun 1998).

Integrating Biological Control with

Silviculture

Development of an effective biocontrol program requires technologies for mass production of the agent,

an efficient delivery system, and a deployment strategy. The biocontrol agent does not have to eradicate all

the dwarf mistletoe from the entire stand. A satisfactory strategy is to reduce mistletoe spread from residual trees in a regeneration area by timely introduction of biocontrol agents that kill or deflower the

parasite. The selection of a treatment area and schedule is a silvicultural decision based an understanding

of the epidemiology of the agent, the population dynamics of the mistletoe, and silvics of the host. The

spatial-statistical model described by Robinson and

others (2002) simulates mistletoe life cycles under

various treatments and schedules and aids the selection of a preferred strategy. The objective is to protect

new plantations from early mistletoe infestation where

a significant number of infected residual trees are to

be retained for various legacy values.

Insects

Initial research identifies several destructive insect

predators that are apparently endemic to Pakistan

(Baloch and Ghani 1980, Mushtaque and Baloch 1979),

but no steps have been taken to test their applicability

for introduction into North America. Other Asian

dwarf mistletoes also harbor candidates for biological

control of New World dwarf mistletoes (Tong and Ren

1980).

Fungi

The extensive literature on biological control of

unwanted higher plants (weeds) is reviewed by DeBach

(1964), TeBeest and Templeton (1985), Shamoun

(2000), Wall and others (1992), and Wilson (1969).

Mycoherbicides are developed practical tools in agriculture. Example mycoherbicides include:

Phytophthora palmivora (DeVine‚) for control of strangler vine in citrus (Ridings 1986), Colletotrichum

gloeosporioides f. sp. aeschynomene (Collego‚) for control of northern jointvetch in rice and soybean (Daniel

and others 1973), and Colletotrichum gloeosporioides

f. sp. malvae (BioMal‚ and Mallet WPTM) for roundleaved mallow in field crops (Jensen 2000, Makowski

and Mortensen 1992). Chondrostereum purpureum, a

well-known primary wood invader, is being developed

for biological control of woody vegetation in forests and

rights-of-way (de Jong and others 1990, Shamoun and

others 1996, Wall 1994). Chondrostereum purpureum

(ChontrolTM) may become the first biological control

agent in North America used for integrated forest

vegetation management (Shamoun and Hintz 1998).

In South Africa, Cylindrobasidium laeve (Stumpout‚)

is used to clear Australian wattle tree (Morris and

others 1998). Mortensen (1998) reviews a number of

other products in development.

A particular challenge for application of

mycoherbicides in controlling mistletoes is that death

of the plant is not assured by destruction of the aerial

shoots. The endophytic system of mistletoes within

the host survives even when the shoots are killed back

repeatedly; the endophytic system may persist for a

century (Gill and Hawksworth 1961).

For a fungal parasite to be an effective biological

control agent, it must possess a number of attributes

(Mark and others 1976, Wicker and Shaw 1968):

1. It parasitizes only the target mistletoe, not the

host or other vegetation.

2. Its activity seriously interferes with the life cycle

of the mistletoe.

3. It produces abundant inoculum and significant

infestations on the target mistletoe.

4. It has sufficient ecological amplitude to persistence throughout the range of the target mistletoe.

5. Its distribution coincides with that of the target

mistletoe.

6. It exhibits high infectivity.

7. It shows high virulence.

8. It has an efficient mode of action for curtailing

development of the target mistletoe.

Fungal parasites of dwarf mistletoe are of two general groups—those that attack aerial shoots and those

that attack the endophytic system (canker fungi).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 77

Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches Shamoun and DeWald

Although a large number of fungal parasites are associated with dwarf mistletoes (see Hawksworth and

Geils 1996), there are no complete and comprehensive

evaluations for most of these fungi, their hosts, and

their interactions (Hawksworth and others 1977b).

Aerial Shoot Fungi—These fungi usually parasitize pistillate flowers, shoots, and fruits of certain

spring-flowering species of mistletoe. Three of these

fungi—Colletotrichum gloeosporioides, Cylindrocarpon

(Septogloeum) gillii, and Caliciopsis (Wallrothiella)

arceuthobii—are common and widespread in Western

North America (Hawksworth and Geils 1996).

Colletotrichum gloeosporioides is commonly isolated

from dwarf mistletoes in the United States and the

Western Provinces of Canada (Kope and others 1997,

Muir 1967, Wicker and Shaw 1968). Although different isolates of the fungus are distinct in mycelial

growth, colony color, and sporulation, cross-inoculation experiments demonstrate that isolates are not

host-specific (Scharpf 1964). C. gloeosporioides infections first appear as small, brown to black, necrotic

lesions on the nodes of fruits and shoots (fig. 7-1 and

7-2). Lesions enlarge, coalesce, and cause dieback of

the shoots (Parmeter and others 1959, Wicker and

Shaw 1968). Parmeter and others (1959) observe that

the fungus invades the endophytic system of

Arceuthobium abietinum. Ramsfield (2002) did not

detect the presence of the fungus in the endophytic

system of A. americanum. Wicker (1967b) states that

both sexes of A. campylopodum are attacked, and that

from 35 to 67 percent of the plants or 24 percent of the

shoots may be destroyed. Although the fungus may

persist for years (Wicker and Shaw 1968), its occurrence is generally sporadic (Hawksworth and others

1977b). It can be destructive to A. americanum and

A. tsugense subsp. tsugense in Western Canada (Muir

1967, 1977, Ramsfield 2002, Kope and others 1997).

Muir (1977) concludes that it can exert significant

natural control of A. americanum.

Colletotrichum gloeosporioides is being developed as

biocontrol agent of Arceuthobium tsugense and A.

americanum. Successful projects to date include an in

vitro bioassay system (Deeks and others 2001, 2002)

and several laboratory and greenhouse experiments

and field trials (Ramsfield 2002). The fungus is easily,

inexpensively cultured and germinates over a wide

temperature range (Parmeter and others 1959,

Shamoun 1998). Its mode of action disrupts development of mistletoe shoots, thereby preventing reproduction. Because it attacks anytime after shoot emergence (Parmeter and others 1959), there is a broad

window when the agent can be applied.

Cylindrocarpon gillii (formerly Septogloeum gillii)

is a fungal parasite that causes anthracnose to staminate and pistillate shoots of dwarf mistletoes (Ellis

1946, Gill 1935, Muir 1973). The fungus and disease is

recognized (fig. 7-3) by white eruptions at shoot nodes

and conspicuous masses of hyaline, cylindrical to fusiform spores. The fungus parasitizes most dwarf mistletoes of Western North America (Hawksworth and

others 1977b), including A. americanum, A. douglasii,

and A. tsugense subsp. tsugense in Western Canada

(Kope and Shamoun 2000, Shamoun 1998, Wood 1986).

Mielke’s (1959) inconclusive results from inoculating

an isolate of a warm, dry climate to a cool, moist one

suggest the need for proper climate matching when

evaluating or using this fungus (Hawksworth and

others 1977b).

Caliciopsis arceuthobii (formerly Wallrothiella

arceuthobii) is the oldest known, fungal parasite of

dwarf mistletoes. It attacks the spring-flowering

mistletoes Arceuthobium pusillum, A. americanum,

Figure 7-1—Colletotrichum gloeosporioides infecting

shoots and fruits of Arceuthobium tsugense.

Figure 7-2—Colletotrichum gloeosporioides infecting

shoots of Arceuthobium americanum.
78 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Shamoun and DeWald Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches

A. douglasii, and A. vaginatum (Dowding 1931, Kuijt

1969b, Knutson and Hutchins 1979). Infection occurs

at anthesis; stigmas are inoculated with ascospores

carried by insects, wind, or rain. Within 2 months,

hyphae penetrate the fruits to the ovary wall. Host

cells deteriorate and are replaced by a black stromatic

mass of hyphae (fig. 7-4). Normal fruit development

and seed production are destroyed (Wicker and Shaw

1968). The fungus is widely distributed from Western

Canada, United States, and Mexico (Hawksworth and

others 1977b, Kuijt 1963). Parker (1970) demonstrates

the fungus germinates and grows on artificial media.

Its potential as a biocontrol agent, however, is limited

by large, annual variations of infection. In a given

location, natural infection will be high one year (80

percent of flowers infected) and fail (almost no

infection) the next (Dowding 1931, Hawksworth and

others 1977b, Weir 1915, Wicker and Shaw 1968).

Other fungal parasites associated with aerial shoots

of dwarf mistletoes are: Alternaria alternata,

Aureobasidium pullulans, Coniothyrium sp.,

Metasphaeria wheeleri, Pestalotia maculiformans,

Pestalotia heteroercornis, and Phoma sp. (Gilbert 1984,

Hawksworth and others 1977b, Hawksworth and

Wiens 1996, Kope and Shamoun 2000, Shamoun 1998).

The potential use of these species as biocontrol agents

requires additional evaluation.

Canker Fungi Associated with Endophytic

System—The canker fungi of dwarf mistletoe attack

both the cortex and endophytic system (Hawksworth

and Geils 1996). More than 20 species of canker fungi

are identified for Arceuthobium tsugense in British

Columbia (Baranyay 1966, Funk and Baranyay 1973,

Funk and others 1973, Funk and Smith 1981, Kope

and Shamoun 2000, Shamoun 1998). Their potential

as biological control agents includes both advantages

and disadvantages. Because they attack the endophytic system, effects are immediate, pronounced, and

likely to kill the mistletoe. Because the host tree may

be damaged as well, additional laboratory study is

required before field inoculations are attempted. Three

canker fungi are good candidates for biological control.

Neonectria neomacrospora (formerly Nectria

macrospora, Nectria neomacrospora) is characterized

by a stroma with dark red perithecia containing eightspored asci (Booth and Samuels 1981, Mantiri and

others 2001). The conidial sporodochia

(Cylindrocarpon) appear white and are found most

commonly on freshly cankered swellings (fig. 7-5 and

7-6) caused by Arceuthobium tsugense (Funk and others 1973, Kope and Shamoun 2000, Shamoun 1998).

Byler and Cobb (1972) report N. neomacrospora (as N.

fuckeliana) is a virulent pathogen of A. occidentalis on

Pinus muricata. The fungus is only weakly parasitic

on pine and is secondarily parasitic on western gall

rust cankers caused by Peridermium harknessii.

Cylindrocarpon cylindroides is more virulent than

Colletotrichum gloeosporioides on germinating seeds

and callus of Arceuthobium tsugense (Deeks and others 2002).

The characteristics that recommend Neonectria

neomacrospora as a biocontrol agent are its selectivity

for dwarf mistletoe-infected host tissue, pathogenicity, ability to invade, rapid canker production, abundant spore production, reduction of shoot growth,

girdling, and branch mortality. Further development

involves improvements of formulation and delivery

technologies (Funk and others 1973, Shamoun 1998,

Smith and Funk 1980).

Cytospora abietis is the best known fungus associated with dwarf mistletoe cankers and is common (20

percent) on Abies magnifica and A. concolor parasitFigure 7-3—Cylindrocarpon gillii infecting shoots

and fruits of Arceuthobium tsugense.

Figure 7-4—Caliciopsis arceuthobii infecting the

pistillate flowers of Arceuthobium americanum.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 79

Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches Shamoun and DeWald

ized by Arceuthobium abietinum (Scharpf 1969a,

Scharpf and Bynum 1975, Wright 1942). The fungus

occasionally parasitizes nonmistletoe-infected

branches. The overall interactions of the fungus, the

mistletoe, and the host tree need to be evaluated.

Although the fungus kills mistletoe-infected branches,

it is not known how much the mistletoe population is

reduced (Hawksworth 1972).

Resin Disease Syndrome—Resin disease syndrome is common on Arceuthobium americanum infecting Pinus contorta in the Rocky Mountains (Mark

and others 1976). The symptoms include excessive

resinosis of the mistletoe canker, necrotic lesions and

discoloration of the host bark, and retention of dead

needles, necrophylactic periderms, and dead mistletoe shoots. Numerous fungi are isolated from resin

disease cankers. Alternaria alternata is the most consistent (recovered from 89 percent of cankers), but the

syndrome appears to be a disease complex caused by

Alternaria alternata, Aureobasidium pullulans, and

Epicoccum nigrum (Mark and others 1976). However,

Gilbert (1984) isolated these fungi from nonsymptomic

mistletoe cankers and host wood; these fungi alone

may not be the sole cause of the syndrome. Additional

studies needed include: effects on reproductive potential of the mistletoe, comparisons for systematic and

nonsytematic mistletoe infections, and assessments of

environmental factors and each fungal component in

disease development (Mark and others 1976).

Summary—Numerous studies of the mycobiotic

associates of dwarf mistletoes are complete. The fungal parasites Colletotrichum gloeosporioides,

Cylindrocarpon gillii, Caliciopsis arceuthobii, and

Neonectria neomacrospora are effective in destroying

aerial shoots or the endophytic system. They can

disrupt the mistletoe life cycle and reduce dwarf mistletoe spread, intensification, and damage. Canker fungi

are attractive biological control agents because they

attack the mistletoe over a long period and infect the

endophytic system. These canker fungi have the potential of killing the mistletoe in addition to reducing

reproduction. The most promising biocontrol agents

are Colletotrichum gloeosporioides and Neonectria

neomacrospora.

Chemical Control _______________

The development of a selective herbicide to control

dwarf mistletoes has been a primary but elusive goal

for decades. The fundamental challenge is to find a

chemical that is easy to apply and kills the mistletoe

without toxic effects to the host or other nontarget

species. If the mistletoe cannot be killed, a second

strategy is to cause abscission of shoots, thereby reducing and delaying spread and intensification.

Numerous lethal herbicides have been tested for

control of dwarf mistletoes (Gill 1956, Quick 1963,

1964, Scharpf 1972). The most common chemicals

investigated in early studies are various formulations of 2,4-D and 2,4,5-T. Typically, these chemicals

are not effective at killing the mistletoe without also

injuring the host. At low rates that do not damage the

host, the endophytic system is not killed, and

resprouting occurs. The most promising herbicide

from a large study by Quick (1964) is an isooctyl ester

of 2,4,5-T; but it is now banned for concerns over

adverse, nontarget effects.

Figure 7-6—Neonectria neomacrospora

(anamorph: Cylindrocarpon cylindroides) infecting

the basal cup and the swelling (endophytic system)

of Arceuthobium tsugense.

Figure 7-5—Neonectria canker of Arceuthobium tsugense.

Note: symptoms of the disease are resinosis and necrosis of

mistletoe shoots.
80 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Shamoun and DeWald Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches

Hawksworth and Wiens (1996) review a series of

tests from 1970 to the early 1990s with a number of

herbicides and growth regulators including Dacamine,

MCPA, Butyrac, Goal, Thistrol, D-40, Weedone,

Emulsamine, DPX, Prime, and Florel. Although these

chemicals cause high shoot mortality with minimal

host injury, they also do not kill the endophytic system. Experiments to date with systemic chemical are

inconclusive.

The most promising chemical for inducing shoot

abscission is ethephon (Florel‚ active ingredient 2-

chloroethyl phosphoric acid). The mode of action of

ethephon releases ethylene, a natural growth-regulating chemical that causes early abscission of flowers,

fruits, and shoots (Hawksworth and Johnson 1989b).

Ethylene is a natural substance that dissipates quickly

and shows few nontarget effects. It has been evaluated

for numerous mistletoe–host combinations (Frankel

and Adams 1989, Hawksworth and Johnson 1989b,

Livingston and Brenner 1983, Livingston and others

1985):

• Arceuthobium americanum on Pinus banksiana

in Manitoba

• Arceuthobium americanum on Pinus contorta in

Colorado and California

• Arceuthobium campylopodum on Pinus ponderosa in California and Idaho

• Arceuthobium campylopodum on Pinus jeffreyi

in California

• Arceuthobium divaricatum on Pinus edulis in

New Mexico

• Arceuthobium douglasii on Pseudotsuga menziesii

in Oregon

• Arceuthobium laricis on Larix occidentalis in

Oregon

• Arceuthobium pusillum on Picea mariana in

Minnesota

• Arceuthobium vaginatum on Pinus ponderosa in

Colorado and New Mexico

An important consideration is achieving adequate

coverage. Ground application can be effective (Johnson

1992, Johnson and others 1989, Nicholls and others

1987a, 1987b). Robbins and others (1989) and Baker

and others (1989), however, report aerial applications

by helicopter are not effective due to poor penetration.

Most mistletoe infections are in the lower crown and

protected from the spray by overhead host foliage.

With good coverage, shoot abscission rates of 90 to

100 percent are achieved (Johnson 1992). Limited,

premature browning of older host needles may occur,

but serious side effects on the nontarget host are few

(Nicholls and others 1987a). Resprouting from the

endophytic system, however, limits effectiveness (Parks

and Hoffman 1991). When resprouting is rapid and

extensive, long-term protection from mistletoe spread

and intensification is not provided. With good control,

mistletoe seed production is delayed 2 to 4 years; but

it is not a permanent cure. Ethephon may be used to

protect understory trees beneath an infested overstory in high-value areas (Adams and others 1993).

Summary

Investigations for chemical control of dwarf mistletoes have considered numerous herbicides intended to

selectively kill the parasite or cause shoot abscission.

No material tested warrants widespread application.

Although the growth-regulating chemical ethephon is

approved by the U.S. Environmental Protection Agency

for control of dwarf mistletoes, it has limited use.

Because the chemical does not affect the endophytic

system, new shoots and fruits develop 3 to 5 years after

application (or sooner). This chemical is most useful

for high value trees at homes, offices, and parks, where

frequent applications are possible, but pruning is not

acceptable. A chemical treatment regime can be supplemented with various other cultural techniques such as

underplanting immune species.

Management Through Selection for

Genetic Resistance ______________

Hanover (1966) describes the need for identification

of heritable resistance and development of a controlled

breeding program for genetic resistance to mistletoes.

In the past, the relative low cost and ease with which

mistletoes were controlled silviculturally delayed the

development of these programs (Hawksworth and

Wiens 1996). In general, trees suspected to be resistant to mistletoe are identified in the process of other

management activities rather than through a deliberate search (Roth 1974a). A few scientists such as

Frank G. Hawksworth, Lewis F. Roth, and Robert F.

Scharpf have made consistent efforts to identify genetically resistance trees.

Native mistletoes can be relatively destructive in

natural forests, and because tree species have been

coevolving with mistletoes for 25 million years

(Hawksworth 1978a), we can expect trees to have

developed genetic resistance (Roth 1978). The existence of host-specific mistletoes and variation in host

preference suggests a genetic basis of resistance, at

least at the species level. Arceuthobium douglasii does

not infect Pinus ponderosa (Hawksworth and Wiens

1996). Scharpf (1984) notes that two-thirds of dwarf

mistletoes parasitize hosts in addition to a principal

species; the levels of infection in these hosts are highly

variable from secondary to rare for factors other than

escape. Arceuthobium pusillum exhibits variation

in ability to infect Larix laricina, Picea glauca,

Picea rubens, Pinus resinosa, and Pinus strobus — all
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 81

Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches Shamoun and DeWald

species naturally exposed to the mistletoe (Tainter

and French 1971).

In contrast to our knowledge of species-specific susceptibility, within-species susceptibility to mistletoe

has been less rigorously examined. Field observations

of mistletoe-free trees in areas with high levels of

mistletoe infection are reported for several host–mistletoe combinations. In Western North America, these

reports include healthy P. ponderosa in areas heavily

infected with A. vaginatum subsp. cryptopodum (Arizona and New Mexico, Hawksworth 1961); P. ponderosa and P. jeffreyi free of A. campylopodum (Oregon,

Roth 1953; California, Scharpf 1984, 1987, Scharpf

and Parameter 1967, Wagener 1965); Pseudotsuga

menziesii var. glauca free of A. douglasii (Arizona,

Nowicki and others, unpublished research); A.

americanum-free P. contorta (Colorado, Hawksworth

and Wiens 1996); healthy Tsuga heterophylla in areas heavily infected with A. tsugense (Vancouver

Island, Smith and others 1993); and Pinus hartwegii

free of mistletoe in heavily infected areas of Mexico

(De la Puente 1966). Although the progeny of these

“resistant” trees have not generally been tested for

resistance, these field observations suggest variation

of genetic resistance within host populations exists.

When trees suspected to be resistant to mistletoe are

identified in the field, they may be tested by artificially

inoculating grafts and out-planting grafted scions in a

mistletoe-infested area. Scharpf and Roth (1992) report high correlation between resistant Pinus ponderosa parents and their scions grafted and out-planted

in areas with heavy mistletoe infection. Artificially

inoculated grafted Tsuga heterophylla trees from resistant and susceptible parents also show resistance

correlations (Smith and others 1993). Although results of these studies using grafted material do not

prove heritable resistance, they do verify resistance is

being controlled genetically rather than environmentally. These sources represent good candidate trees for

progeny tests of heritable resistance.

Progeny tests for inherited genetic resistance to

mistletoes show mixed results. Some cases of field

resistance represent escapes or other nonheritable

mechanisms of resistance. The work of Roeser (1926)

and Bates (1927) represents one of the first attempts

to breed forest trees for disease resistance in the

United States. Regrettably, there are no differences

after 50 years in the incidence of infection between

slow-growing, susceptible and fast-growing, resistant

selections (Hawksworth and Edminster 1981). These

results suggest that growth rate is not a reliable

indicator of mistletoe resistance. Hawksworth and

Wiens (1996) discuss the early results of an unpublished study by G. Fechner examining putative resistance of selected P. contorta seedlings. The infection

rates 10 years after inoculation for putatively resistant families and susceptible families are similar

(Geils, personal communication). Other progeny tests

for mistletoe resistance show more positive results.

Roth (1971, 1974a, 1974b) demonstrates that Pinus

ponderosa seedlings from resistant parents have fewer

infections and faster growth rates than those from

more susceptible parents. Examination of these progeny tests 20 years later shows the same result (Scharpf

and Roth 1992). Scharpf (1987) identifies P. jeffreyi

trees with variation in field resistance; artificial inoculations on 7-year-old progeny from these parents

indicate the resistance is heritable (Scharpf and others 1992). Finally, Pseudotsuga menziesii var. glauca

progeny from healthy parents in heavily infested areas had fewer infections than progeny from infected

parents. Subsequent genetic laboratory analysis using allozymes support a heritable basis for this apparent resistance (Nowicki and others, unpublished

research).

Attempts to identify inherited mechanisms controlling resistance to mistletoe are few. Genetic resistance

to pathogens and insects in general is classified as

“vertical,” where specific resistant genes have developed, or as “horizontal,” where aggregate combinations of genes have developed to create a phenotypic

response. Roth (1974a and 1974b) suggests that horizontal resistance is more likely controlling resistance

to mistletoe in Western conifers. Age-related changes

may confer some resistance to mistletoe in pines (Roth

1974b, Scharpf and Roth 1992), but younger true fir

trees appear to be more resistant to mistletoe than

older individuals (Scharpf 1984). Anatomical changes

associated with age are under a high degree of genetic

control and may serve as a clue for locating genetically

controlled resistance mechanisms. In Larix laricina,

the formation of a wound periderm that restricts

endophytic growth of Arceuthobium pusillum is identified as a resistance mechanism (Tainter and French

1971); however, inheritance of the wound periderm

response has not been demonstrated. Atsatt (1983a)

suggests resistant hosts may produce chemicals that

inhibit mistletoe or lack chemicals needed by the

mistletoe to initiate and/or develop haustoria formation. In general, production of secondary chemicals is

a common, genetically controlled, defense strategy

used by plants; secondary chemicals may play a role in

genetic resistance to mistletoe.

Summary

Despite the relatively limited investigation, there

are field observations, progeny tests, and graft studies

that all point to the presence of some degree of resistance to mistletoe in North American conifers. The

recent need to develop options to traditional, evenaged silvicultural treatments has led to the renewed

interest in developing genetic and breeding programs

for resistance to dwarf mistletoe. Field identification
82 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Shamoun and DeWald Management Strategies for Dwarf Mistletoes: Biological, Chemical, and Genetic Approaches

of resistant sources, progeny testing to confirm heritably, plus screening and breeding programs such as

exists for blister rust (Cronartium ribicola) are critically needed for a genetic strategy to become a viable.

A screening program is being developed by Ringnes

and others (1996). The objectives of this program are

to identify trees exhibiting resistance to dwarf mistletoe, to evaluate testing methods for screening candidates, to identify resistance levels of candidates and

their progeny, and to determine the mode and strength

of inheritance for resistance mechanisms. Additional

mistletoe resistance screening programs for

Pseudotsuga menziesii var. glauca (DeWald and

Nowicki Northern Arizona University, Flagstaff, AZ),

and Tsuga heterophylla (Shamoun, Canadian Forest

Service, Pacific Forestry Centre, Victoria BC, Canada)

have been initiated. Finally, biotechnology approaches

(including tissue culture, see Deeks and others 2001,

Marler and others 1999) can be used to supplement

traditional resistance screening and breeding programs. Trees whose resistance to mistletoe has been

confirmed can be searched for molecular DNA markers. These markers can then be used in marker-aided

selection for mistletoe resistance to eliminate the long

generation times currently needed to confirm genetic

resistance.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 83

J. A. Muir

B. W. Geils

8

Chapter

Management

Strategies for

Dwarf Mistletoe:

Silviculture

Although there are numerous sources for information

on the practice of silviculture (Forest Service 2002),

special considerations are required for control of dwarf

mistletoe (Scharpf and Parmeter 1978). Mistletoe-infested forests, stands, and trees develop and respond to

treatment differently than their uninfested counterparts (chapter 5). The spread, intensification, damage,

and impacts of dwarf mistletoe can be reduced, maintained, or enhanced by silvicultural treatments alone or

in combination with other control techniques (chapter 7).

Silvicultural treatments discussed here include:

• Harvest, retention, and regeneration by clear-felling (even-aged silviculture), or selection harvesting

to establish and maintain uneven or all-aged stand

structures.
84 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Geils Management Strategies for Dwarf Mistletoe: Silviculture

• Design and layout of harvest and treatment

blocks.

• Site preparation and vegetation management by

brushing, prescribed burning, and other methods.

• Planting or retaining residual and advanced regeneration.

• Thinning and sanitation.

• Pruning brooms and infected branches.

General guidelines for silvicultural treatment that

integrate dwarf mistletoe information are presented

in symposium proceedings (Scharpf and Parmeter

1978), regional directives (British Columbia Ministry

of Forests 1995), and compendia (Alexander 1986).

New strategies may be suggested and examined with

simulation models (Robinson and others 2002), then

tested and evaluated in practice at demonstration

forests (Besse and others 2001, Edwards 2002, Nevill

and Wood 1995).

The choice to initiate a silvicultural action, and the

subsequent selection of techniques, timing, and location, are dictated by considerations in three major

areas. First, each dwarf mistletoe species, forest type,

and region present different situations. Some mistletoes have a wide distribution and cause serious damage; others are rare curiosities, spread slowly, cause

little damage, or even enhance some aspect of the

environment (chapter 5). Second, management objectives and constraints for individual stands (or sites),

compartments (planning units), and whole forests

determine the intended purpose of the treatment.

Different objectives require different approaches. Objectives may be to produce timber and fiber (British

Columbia Ministry of Forests 1995), enhance wildlife

habitat (Reynolds and others 1992), or even promote

wild mushroom production (Amaranthus and others

1998). Finally, any action must be consistent with an

overall plan of forest regulation and a silvicultural

system for regeneration. With even-aged silviculture,

clear-felling, shelterwood, and seed tree harvests,

planting, sanitation, and intermediate thinning all

provide opportunities to direct stand and mistletoe

development. With uneven-aged silviculture, tree and

group selection determine forest character. Fuel management and prescribed burning may be used in both

systems. Aesthetic values and economics may allow

special cultural practices such as pruning to be used on

high value trees such as found in recreation areas.

In this chapter, we describe silvicultural treatments

that have been recommended or are used to prevent,

mitigate, or encourage dwarf mistletoe development

and effects. We provide examples of frequently encountered management situations. The discussion is

organized into six topics. In Designing Silvicultural

Treatments, we describe biological and ecological factors that apply to silvicultural decisions, especially the

features that make mistletoes amenable to treatment.

We also identify sources for species-specific guidelines. For Management of Even-aged Stands, we describe the strategies used primarily to prevent or

reduce detrimental effects of dwarf mistletoes on timber and fiber production. The first and best opportunity is to prevent mistletoe spread into a clean, regenerated stand. Established stands with mistletoe present

can still be treated with sanitation, thinning, harvesting, or be reestablished. In the discussion of Unevenaged Silviculture and Selection Cutting, we recognize

a shift in forestry to management for ecosystem structure and functions, retention of old-growth forest character, wildlife habitat, recreation, and other amenity

values. Although we have less research and management experience for this kind of management, mistletoe can play a large role in determining whether those

objectives are met. Techniques and tools are available

for influencing the patterns and rates of mistletoe

spread and intensification. Prescribed Burning is an

especially useful tool for either even-aged or unevenaged silvicultural systems. Regardless of the treatment considered, a manager needs to be aware of the

likely responses to a proposed action. Because mistletoes add complexity and because the consequences of

specific decisions may not be apparent for decades,

managers can use Models to Assess Treatment Opportunities. Finally, in Management for Recreation, Wildlife, and Ecosystem Values, we describe some of the

special requirements and techniques applicable to infested stands and trees managed with these objectives.

Designing Silvicultural

Treatments _____________________

Dwarf mistletoes markedly affect the growth, form,

and survival of infected trees and therefore how these

trees and their stands develop and respond to silvicultural treatment (chapter 5). Effects to trees include:

distorted growth from branch and stem infections,

changes in wood quality, reduced overall tree growth,

increased susceptibility to attack by secondary insects

and fungi, and increased mortality. These damages

aggregate over time, affecting forest health,

sustainability, and productivity (DeNitto 2002,

Hawksworth and Shaw 1984, Monning and Byler

1992). Consequently, mistletoes affect the basic ecological processes of primary productivity, biomass

allocation, mortality, mineral recycling, and succession (Kipfmueller and Baker 1998, Mathiasen 1996,

Tinnin and others 1982, Wanner and Tinnin 1989,

Zimmermann and Laven 1984). Because significant

infestations of dwarf mistletoe have profound, fundamental, and particular effects on stands, mistletoes

need to be specifically considered in designing silvicultural treatments on infested sites (fig. 8-1).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 85

Management Strategies for Dwarf Mistletoe: Silviculture Muir and Geils

Dwarf mistletoes and the forest stands at risk of

infestation develop at a pace that appears slow from a

human perspective but not from that of the host trees.

For example, a rule of thumb for spread of a mistletoe

infestation is 10 m per decade (Dixon and Hawksworth

1979); intensification in trees is one DMR class per

decade (Geils and Mathiasen 1990); half-life of DMR

class-6 trees is one decade (Hawksworth and Geils

1990). [Note: many factors influence rates of spread,

intensification, and mortality. These rules of thumb

are meant only to suggest the magnitude of the rate of

change and are not intended as specific management

guidelines.] With stand replacement times of one to

two centuries, mistletoes are able to produce tens of

generations and increase unchecked at a low exponential rate. Noticeable effects are delayed until infection

reaches a moderate level, but damage accumulates at

an increasing rate after that point (Hawksworth 1961,

Tinnin and others 1999).

The potential impacts of dwarf mistletoe infestation

and their dynamics have several implications for designing silviculture treatments. First, over time a

treated stand that remains infested will develop differently than an uninfested stand. Second, early and

frequent interventions provide greater opportunities

to affect stand and infestation dynamics and impacts

than later or infrequent entries. The timing and number of entries are, of course, determined by other

factors as well. Therefore, an early treatment assessment (such as immediately after completion) may not

provide a satisfactory indication of its long-term consequences without an adequate model.

Several biological and ecological features make dwarf

mistletoes especially amenable to silvicultural treatment (Hawksworth 1978a, Parmeter 1978). The epidemiological bases of these features are discussed in

chapters 4 and 5; here we suggest their silvicultural

implications:

• Obligate parasitism. Dwarf mistletoes require a

living host to survive and reproduce. When an

infested tree or branch dies (or is cut), the attached mistletoe plants die as well. There is no

need to burn or destroy slash or pruned branches

to kill and sterilize the pathogen.

• Host specificity. Dwarf mistletoes generally infect only a single, susceptible host species or

group of related species. Retained immune and

less susceptible hosts reduce spread and severity

of damage.

• Extended life cycles. Life cycles of dwarf mistletoes are relatively long compared to other tree

disease agents; a generation ranges from 2 to 10

or more years. Dwarf mistletoe spread from tree

to tree, and increase within tree crowns is relatively slow. Because numerous infections are

required to cause serious damage, the effects

accumulate slowly. Time is available to plan and

implement a treatment regime.

• Limited seed dispersal. Dwarf mistletoe seeds

are dispersed a maximum horizontal distance of

only 10 to 15 m; gravity and foliage limit effective

spread in the vertical and horizontal planes;

animal vectoring of dwarf mistletoe (with one or

two exceptions) is rare enough to be ignored

other than from ecological and evolutionary perspectives. Consequently, mistletoe tends to occur

as pockets of infestation. Spatial variation in

mistletoe abundance provides numerous patches

in which different, appropriate treatments can

be applied. Even with severe infestations, the

amount of mistletoe seed produced is limited;

small, young understory trees present a minimum target. There is an opportunity to regenerate a stand under an infected overstory before

the young trees are infected.

• Slow intensification within tree crowns. Dwarf

mistletoe infection typically begins in the lower

tree crown, and vertical spread is slow enough

that trees with rapid height growth can outgrow

or at least keep pace with mistletoe intensificaFigure 8-1—A portion of lodgepole pine stand in

the Bighorn Mountains, Wyoming. As evidenced

by the numerous, large witches’ brooms, most

trees are severely infected with Arceuthobium

americanum. If the management objective were

timber-oriented, this stand is a good candidate for

regeneration and a poor candidate for commercial

thinning. Fuel distribution and canopy structure

depart greatly from what would be expected in an

uninfested stand with significant consequences to

fire and wildlife objectives.
86 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Geils Management Strategies for Dwarf Mistletoe: Silviculture

tion (Hawksworth and Geils 1985, Roth 1971).

Good sites for tree growth allow rapid height

growth at higher stand densities, which has

several effects on mistletoe. Greater crown closure reduces light within the canopy, reducing

mistletoe reproduction and increasing the rate of

crown lift; the distance of seed dispersal in a

dense stand is also reduced (Shaw and Weiss

2000). At some point, however, for each stand,

competitive effects impact tree growth, and eventually trees reach their height limit. Density

management and pathological rotation allow

silviculturalists to influence the balance between

growth of the host and the pathogen (Alexander

1986, Barrett and Roth 1985, Muir 1970,

Safranyik and others 1998).

The silvicultural guidelines and treatments we discuss here can only be of a general nature. The literature on damage and control is already summarized by

Hawksworth and Scharpf (1978) and suggests that

different mistletoes in different regions require different approaches. Recent silvicultural guides with recommendations for mistletoe-infected trees and infested stands are available for some of the principal

conifers of North America (table 8-1). There are also

regional guides: British Columbia Ministry of Forests

(1995), Conklin (2000), Hadfield and Russell (1978),

Knutson and Tinnin (1980), and Wicker and

Hawksworth (1988). Numerous older publications

emphasize methods for reducing dwarf mistletoe populations and damage including: Buckland and Marples

(1953), French and others (1968), Gill and Hawksworth

(1954), Hawksworth and Lusher (1956), Kimmey and

Mielke (1959), Korstian and Long (1922), Wagener

(1965), and Weir (1916b). Although dwarf mistletoes

cause significant growth losses and mortality in Mexico,

we know of only a few publications that discuss silvicultural treatment of Mexican conifers in general

terms (Hernandez and others 1992, Reid and others

1987).

Where silviculture dwarf mistletoe management is

conducted, treatments to mitigate mistletoe impacts

can be integrated with other activities to reduce susceptibility to forest insects, other diseases, and fire.

The complex interactions between mistletoes and bark

beetles are reviewed by Stevens and Hawksworth

(1970, 1984) and include situations where reduction of

mistletoe also results in reduction of hazard to bark

beetles. Thinning stands to reduce bark beetle hazard

presents an opportunity for mistletoe sanitation. Although the effect on the mistletoe infestation was

minimal, Vandygriff and others (2000) describe an

attempt to relocate bark beetle attacks with aggregant

baits to mistletoe-infected trees. Marsden and others

(1993) explore the options for management in a stand

infested with both root disease and mistletoe. This is

Table 8-1—Silviculture guides for management of North American conifers with dwarf mistletoe.

Forest type Host species Arceuthobium sp. References

Black spruce Picea mariana A. pusillum Johnson (1977)

Ostry and Nicholls (1979)

California true fir Abies concolor A. abietinum Filip and others (2000)

Abies magnifica Scharpf (1969b)

Douglas-fir Pseudotsuga menziesii A. douglasii Hadfield and others (2000)

Schmitt (1997)

Lodgepole pine Pinus contorta A. americanum Hawksworth and Johnson (1989a)

var. latifolia van der Kamp and Hawksworth (1985)

Pinyon pine Pinus edulis A. divaricatum Mathiasen and others (2002a)

P. monophylla

Sugar pine Pinus lambertiana A. californicum Scharpf and Hawksworth (1968)

Western hemlock Tsuga heterophylla A. tsugense Hennon and others (2001)

Muir (1993)

Western larch Larix occidentalis A. laricis Beatty and others (1997)

Taylor (1995)

Western pines Pinus jeffreyi A. campylopodum Schmitt (1996)

Pinus ponderosa Smith (1983)

Rocky Mountain Pinus ponderosa A. vaginatum Conklin (2000)

ponderosa pine var. scopulorum subsp. cryptopodum Lightle and Weiss (1974)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 87

Management Strategies for Dwarf Mistletoe: Silviculture Muir and Geils

an especially complex situation because trees killed by

mistletoe or cut for mistletoe reduction provide stumps,

which are the food base for root disease; simulation

models are especially useful in such cases. We later

discuss prescribed burning as a tool for mistletoe

management, but it can be noted here that fuel reduction by cutting or burning can also reduce mistletoe.

Applications of direct mistletoe control by chemical

and biological means and genetic selection (see chapter 7) can be considered an adjunct to traditional

silviculture treatments with the chain saw, planting

bar, and drip torch.

An important consideration in the design of a silvicultural entry is whether dwarf mistletoe treatment is

necessary. In many cases the presence of dwarf mistletoe poses no threat to stand objectives. The mistletoe

may be infrequent and have a low potential for increase and damage. Mistletoes may not be a concern at

their altitudinal or geographic range limits (Trummer

and others 1998) or where conditions permit rapid tree

growth (such as with ponderosa pine in southern

Colorado). Where wildlife objectives take precedence,

retention of some dwarf mistletoe may even be desired

to generate snags (Bennetts and others 1996) or mistletoe brooms (Parks and others 1999a).

Management of Even-Aged

Stands ________________________

Even-aged, single-storied stands composed of one or

two tree species are the simplest to treat for mistletoe.

Prevention of dwarf mistletoe infestation in a regenerated stand is essentially guaranteed where all susceptible host trees are harvested or killed soon after

harvest.

Most early guidelines assume the objective of management is timber production, and the purpose of

treatment is the timely and economical eradication of

dwarf mistletoe (Korstian and Long 1922, Weir 1916a).

The traditional recommendation for dwarf mistletoe

has been clear-cut harvesting with relatively large

blocks, followed, if necessary, by intermediate thinning and sanitation to create even-aged stands free of

mistletoe (Wicker and Hawksworth 1988). This method

has been used extensively and successfully for many

Western and Northern species (but see Johnson 1994,

Stewart 1978). Treatment before or after harvest

removes or kills infected and suspect trees to prevent

the young stand becoming infested.

Prevention of Spread Into Cut Blocks

One of the primary issues of dwarf mistletoe treatment in even-aged silviculture is the design and layout

of cut blocks (treatment units) to prevent or reduce

invasion of dwarf mistletoe from adjacent infested

areas. Preventative measures recommended by previous authors and some agencies include:

• Wherever possible, locate cutting boundaries in

noninfested stands, nonsusceptible timber types,

and natural or created openings, and take advantage of natural or constructed barriers such as

roads, streams, openings, or meadows.

• Design cut blocks within infested stands to create large ratios of area to perimeter and minimize the length of infested border; avoid long,

narrow blocks and units of less than 8 ha, but

compromise where required for natural regeneration of heavy-seeded trees (Alexander 1986).

• Unless local, long-term, successful plantings have

been demonstrated, do not plant barriers of

nonsusceptible tree species around the cut block

perimeter. In the majority of cases, this strategy

fails because of rapid natural regeneration and

fast growth of the susceptible tree species; however, in a few exceptions, a mixture of

nonsusceptible tree species has retarded mistletoe spread.

• If infected trees are to be left on the boundary,

avoid leaving fringes or narrow strips but rather

maintain dense blocks of trees and leave a relatively uniform, abrupt (nonfeathered) margin. In

British Columbia and Alberta, mistletoe spread

into an adjacent young stand appears retarded

from dense stands with abrupt edges (Muir 1970).

Where spread and infection of young trees occurs, remove or kill infected trees at the next

treatment entry.

• Avoid leaving single trees or small clumps of

residual infected trees scattered throughout the

harvested area. Scattered overstory trees are a

significant inoculum source for young, understory regeneration, because improved light or

growing conditions favor production and dispersal of dwarf mistletoe seeds (Muir 1970, 2002).

Remove or destroy these trees.

• When regenerating stands with seed tree or

shelterwood systems, select residual trees that

are mistletoe-free or only lightly infected (DMR

2 or less). If infected trees are left, remove them

before regeneration reaches 1 m in height or

about 10 years of age, or prune residual seed

trees to remove infected branches. Because of its

deciduous habit and ability to produce epicormic,

adventitious branches, larch can be severely

pruned.

Silvicultural Treatments of Young Stands

When an even-aged, immature stand is already

infested by dwarf mistletoe, management options are

available to reduce mistletoe at one or more stages of
88 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Geils Management Strategies for Dwarf Mistletoe: Silviculture

early stand development. Factors involved in evaluating the need, kind, and timing of treatment include

stocking level, growth rate, and disease level. Although mistletoe may kill some small trees in young

stands, infections are usually too recent and too light

to cause much growth loss; damage is a poor management

indicator. More important is the potential for future,

unavoidable damage as indicated by the areal extent of

the infestation, the percent of trees infected, and the rate

of spread. In general, treatment options for mistletoe

control are to remove infected overstories, favor

nonsusceptible tree species, sanitation, and thinning.

Recently Harvested and Regenerated Stands—

The best opportunity for preventing reinfestation of

an area by dwarf mistletoe is through complete harvest, removal, or killing of infected trees of the previous stand. This opportunity may be exercised during

or soon after the harvest and regeneration period.

Although the length of time and size of seedlings

before which they are at serious risk of infection vary

by species and site, few are infected before they are 5

to 15 years old or about 1 m tall (Wicker 1967a). The

decisions to be made on the basis of management

objectives and specific situation are the number of

infected residual trees to be retained and the length of

time they remain.

The most important means by which a regenerated

stand becomes infested is through infected residual

trees left on site. In decreasing order of importance,

infected advanced regeneration, spread from adjacent

stands (see above), and long-distance animal vectoring play lesser roles. Trees are intentionally retained

for a number of reasons, even though some of these

trees may happen to be infected. For example, visual

quality, screening, and wildlife objectives may call for

the retention of “legacy” trees. The potential for these

trees to survive and fill their role must be weighted

against their possible contribution to the infestation of

the new stand. Total eradication of mistletoe-infected

trees is neither realistic nor necessary; a sufficient

goal of sanitation can be to allow for effective mistletoe

management. A new stand with some infested legacy

trees can still be treated with periodic sanitation

thinning (see below) to selectively remove more severely infected trees and by pruning infected branches.

Some residual trees are left not for legacy objectives

but because they have no merchantable value. Many

timber contracts and silviculture prescriptions stipulate the felling of diseased, nonmerchantable trees for

safety and forest health reasons. If undesired residual

trees remain after harvest, remedial work may be

appropriate. For mistletoe control purposes alone,

only residuals over 3 m in height with branch infections need to be felled; shorter trees and those with

only bole infections have limited potential for spreading the pathogen (Mark and Hawksworth 1974).

Another option for controlling mistletoe infestation

in a new stand is to regenerate with a mixture of

species including trees less susceptible to mistletoe.

Robinson and others (2002) report on simulations of

stands infested by Arceuthobium tsugense and regenerated under three different scenarios including a 20

percent mixture of an immune species (cedar). Their

simulations suggest that over time, mistletoe incidence (percent infected) and severity (DMR) are less

for the 20 percent mixture compared to the other

scenarios. Different mixtures may be better in other

situations.

Sanitation Thinning—The purpose of sanitation

thinning is to reduce mistletoe incidence. As trees

increase in size, stands can benefit from silvicultural

thinning to select crop trees and distribute growth to

those individuals. Sanitation is conducted in young

stands; silviculture thinning with sanitation is practiced in precommercial and commercial stands. Sanitation is most practical in young stands after initial

infection appears but before subsequent spread occurs. A postregeneration survey is useful to determine

stocking and the distribution and incidence of infection (see chapter 6). A decision is required as to

whether there is sufficient stocking of noninfected,

potential crop trees. The options are for sanitation or

for destruction and reestablishment of the stand. A

third option is, of course, to redefine management

objectives that reset the decision criteria for selecting

a treatment. Each situation requires appropriate assessment because of the ecological and economic constraints of different management objectives, different

hosts, and mistletoes with different potentials for

growth and damage. Numerous sanitation and thinning studies and computer simulations suggest a few

general guidelines where the manager wishes to minimize mistletoe damage and maximize tree growth.

Sanitation is most effective in lightly infested stands

younger than 15 to 30 years old. At early ages, infection percentages are less; at later ages potential crop

trees can be selected. In the past, most timber stands

less than 30 years old were sufficiently stocked (over

1,200 stems per ha) and infested at a low enough

percentage (10 to 20 percent) that sanitation was

feasible. A sanitation treatment that removes all visibly infected trees can significantly reduce an infestation (Hawksworth and Graham 1963b); but due to

latent infections, missed trees, and spacing requirements, complete elimination of mistletoe is unlikely

(Conklin 2002). A sanitation treatment usually retains the best, apparently mistletoe-free trees and

whatever additional lightly infected trees are required

to meet stocking and spacing standards. Mistletoe is

sometimes found as a light or moderate infection

(DMR 2 or 3) in the larger of the young trees. Given the

potential for future spread and growth loss, these
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 89

Management Strategies for Dwarf Mistletoe: Silviculture Muir and Geils

initially larger trees may not be as desirable for retention as smaller healthy trees. For stands about 40

years old and with few patches of infected trees,

approximately 1,200 healthy stems per hectare on

good sites are sufficient to retard mistletoe spread.

The effectiveness of sanitation is doubtful in heavily

infested young stands. Although stands about 20 years

old with half or more of the stems infected may sometimes be encountered, they are poor candidates for

sanitation (Scharpf and Vogler 1986). These stands

generally do not have a sufficient number of healthy

trees to stock the site. Severely infected trees (DMR 3

to 6) do not sufficiently respond to spacing, and reducing stand density may increase mistletoe spread and

intensification. Generally, the degree of infestation in

the stand, not strictly stand age, is the best criterion to

decide whether sanitation is practical. For example, a

general rule for lodgepole pine is that stands with

more than 40 percent of the trees infected (average

stand rating greater than DMR 0.5) are too heavily

infested for sanitation. In these stands, removing all

infected trees reduces stocking below minimal standards and depresses yields (Hawksworth 1978b). An

alternative is stand replacement by clear-cutting, roller

chopping, or prescribed burning.

Thinning Precommercial Stands—Whether or

not an early sanitation treatment was conducted, the

standard practice of precommercial thinning conducted

in some forests—even for healthy stands—provides an

opportunity to promote tree growth and reduce mistletoe spread and intensification. For infested stands, the

usual criteria for scheduling and marking thinning

treatments are supplemented with several mistletoerelated considerations. The silvicultural evaluation

that precedes the drafting of a prescription can include

an assessment of the size and location of patches of

infected trees within a stand, approximate number

and location of infected residual trees, and number of

potential crop trees. An intensive, systematic survey

can provide these data (see chapter 6).

Silviculturalists need to balance two results of thinning that work in opposition to one another. First,

spacing reduces tree-to-tree competition and over a

density range stimulates height growth and crown lift.

Second, opening a canopy also stimulates mistletoe

shoot growth, seed production, spread, and intensification (Hodge and others 1994). In practice, thinning

is most likely to favor the host where trees are no more

than moderately infected (less than DMR 3) and growing in height faster than the vertical spread of the

mistletoes (Barrett and Roth 1985, Parmeter 1978,

Roth and Barrett 1985). In a similar finding,

Hawksworth (1978b) found that thinning in stands

less than moderately infested (40 percent incidence)

and on better quality sites can produce satisfactory

volumes, but not on more severely infested stands or

on poor quality sites. As with sanitation, replacement

and acceptance are options for stands that cannot be

satisfactorily thinned. The sale of merchantable timber may be available to help offset cleaning and reforestation of immature, severely infested stands. Simulation models are useful for particular situations

(Hawksworth 1978b, Strand and Roth 1976) and help

managers to better understand the range of outcomes

that are likely to follow from specific activities.

Sanitation—removing as many infected trees as

practical—is usually an integral part of precommercial

thinning. For stands where average tree diameter

exceeds 5 cm, the prethinning evaluation can include

an assessment of potential crop trees. The priority for

crop trees depends on species but is often set as:

1. Noninfected dominant and codominant trees.

2. Dominant and codominant trees with mistletoe

confined to branches in the lower one-third of live

crown (DMR 2 or less).

3. Dominant and codominant trees with mistletoe

confined to less than one-half of the branches in

the lower two-thirds of the live crown (DMR 3 or

less).

4. Intermediate trees with no visible infection.

In mixed species stands where immune or lesssusceptible species are available, their priority for

retention can be determined by their intrinsic value

plus their disease-mitigation value. If acceptable stocking cannot be obtained, alternative objectives and

treatments can be considered. Thinning crews must be

able to recognize mistletoe infections if a sanitation

objective is to be realized. Economics may permit a

single precommercial treatment but are unlikely to

support additional entries until there is a commercial

opportunity. Although usually considered in the context of uneven-aged management, forest health and

fuel reduction treatments may be justified as well in

young or old even-aged stands.

Commercial Thinning Treatments

As trees reach commercial size and the stand approaches harvest (rotation) age, a different set of

concerns and opportunities are presented to the manager. As before, information on mistletoe distribution

is useful, but as the infestation develops, disease level

as average DMR becomes more relevant than percent

of trees infected. Trees rated with a DMR of 3 or

greater exhibit growth loss, greater mortality, reduced reproductive capability, and increased potential for mistletoe spread. Trees may be harvested at

intermediate thinnings, shelterwood cuts, or at rotation. Simulation programs that project final, cumulative yields can be used to assess the number, timing,

and severity of thinnings, to select the kinds of trees to

harvest at various entries, and to set the regeneration
90 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Geils Management Strategies for Dwarf Mistletoe: Silviculture

schedule. Mistletoe factors can be integrated into

these simulations to address specific situations.

Thinning trials and simulations suggest three general guidelines for management at this stage (Filip

and others 1989, Hawksworth 1978b, Hawksworth

and others 1977a, Knutson and Tinnin 1986, Tinnin

and others 1999). Intermediate thinning in stands

with an average DMR rating of 3 or greater is not

practical. As most trees are infected, stocking requirements cannot be met with healthy trees; many trees

are so severely infected that growth responses are

poor. These stands can be considered for early harvest

and regeneration. Because severely infected trees of

DMR 5 or 6 show little growth and have a high risk of

mortality, they can be removed at any opportunity.

Within 10 to 20 years of harvest, however, other

sanitation and thinning treatments may be deferred.

An important consideration is the early selection of

potential seed trees for regeneration; uninfected host

trees and nonsusceptible species are usually preferred.

Uneven-Aged Silviculture and

Selection Cutting________________

Because the spread and intensification of dwarf

mistletoe in uneven-aged, multistory strands can be

quite rapid, management of these stands is a serious

challenge. But they also present opportunities. Dwarf

mistletoe spread is greatest when seeds rain down

from an infested overstory to a susceptible understory.

With greater crown closure and competition, understory trees do not increase rapidly in height and are

less likely to outgrow the mistletoe. Managers, however, do have several factors to work with. Unevenaged, multistory stands are usually a mosaic of different size and density of trees and mistletoes. These

patches can be used to isolate pockets of mistletoe.

Such stands are also often composed of several tree

species with a range of susceptibility to the prevalent

mistletoe. Nonhost species provide not only immune

stocking but also screening, which reduces mistletoe

spread. Selection for greater species diversity has

numerous, ecological benefits.

Management in uneven-aged stands consists of frequent entries for harvest or improvement thinning. If

these entries are timely and removals sufficient, sanitation can check mistletoe spread, intensification, and

damage. Several cautions are warranted, however.

Mistletoe spread can be several times faster than

managers expect from their experience in even-aged

strands. Overtopped or severely infected trees (DMR 3

or greater) grow at reduced rates and do not outgrow

mistletoe. Periodic entries at 10- to 20-year intervals

with modest sanitation may be adequate to check

mistletoe; but in 30 to 40 years without control, it can

spread throughout the stand. Writing a prescription

and marking trees in these stands requires a high skill

level to detect mistletoe, recognize its potential, and

select the proper action.

Guidelines for uneven-aged management are available (Mathiasen 1989, Conklin 2000). In principle,

many of the suggestions described in previous sections

for even-aged stands are applicable here also. The

goals are to maintain individual tree ratings at DMR

3 or less and prevent infection in the top of the crown.

Diligence and thoroughness can be major obstacles in

applying treatments, and monitoring is important

(Merrill and others 1998). One of the key considerations in uneven-aged management is whether silvicultural treatment (cutting trees) maintains the height

growth of remaining trees at a rate that exceeds

mistletoe vertical spread. Where trees outgrow the

mistletoe and infections remain in the lower crown,

impacts on tree growth are generally insignificant

(Hawksworth 1978b, Parmeter 1978). For coastal hemlock, Richardson and van der Kamp (1972) suggest

that trees growing 36 cm per year outgrow the mistletoe. Parmeter (1978) suggests a rate of 20 cm per year

for lodgepole pine. For ponderosa pine in the Pacific

Northwest, Barrett and Roth (1985) and Roth and

Barrett (1985) report that infected ponderosa pine

saplings outgrew the effects of dwarf mistletoe for 20

years at 25 cm annual height growth. Similarly, Wicker

and Hawksworth (1991) state that after thinning,

western larch grew 37 cm per year, while the larch

dwarf mistletoe spread upward only 9 cm per year.

Because mistletoe spread and effects vary with stand

density, site quality, and other factors, these are only

approximate rates (Bloomberg and Smith 1982).

Management of mistletoe-infested uneven-aged

stands is discussed in detail by Mathiasen (1989) and

Conklin (2000). At each entry they recommend that:

more severely infected trees (DMR 5 and 6) are cut;

healthy trees and those with a DMR of 1 and 2 are

retained; moderately infected (DMR 3 and 4) trees are

retained only where height growth is expected to

exceed 30 cm per year or where the next cutting entry

is scheduled within 20 years. Pruning infected branches

or large witches’ brooms from moderately to severely

infected trees reduces spread, intensification, and

damage. Pruning, however, is expensive (see section

on Management for Recreation Values).

The practice in the Southwestern United States for

management of pine stands with dwarf mistletoe is to

consider uneven-aged management where 25 percent

or fewer of the stems are infected. Individual tree

selection is used where fewer than 15 percent of stems

are infected; and group selection of trees in patches of

less than 1 ha where 15 to 25 percent of stems are

infected. Where more than 25 percent of trees are

infected, even-aged management is used. Because
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 91

Management Strategies for Dwarf Mistletoe: Silviculture Muir and Geils

larger trees tolerate more dwarf mistletoe infection

without deleterious effects, Conklin (2000) proposes

cutting and selection guidelines based on tree size and

infection severity (table 8-2).

One of the major challenges for management of

infested uneven-aged stands is the dispersal of dwarf

mistletoe seed from infected overstory trees to the

understory (Mathiasen 1989, Bloomberg and Smith

1982). Although the predominant opinion has been

that dwarf mistletoe intensifies rapidly after a partial

cutting or disturbance such as windthrow, there are

exceptions. Shaw and Hennon (1991) and Trummer

and others (1998) describe the relatively slow spread

and intensification of hemlock mistletoe in Alaska.

Situations such as these are good candidates for uneven-aged management. Geils and Mathiasen (1990)

provide equations for the increase in DMR for Douglas-fir in uneven-aged, multistory stands. Maffei and

others (1999) describe an exercise to develop similar

equations for other species and incorporate the results

in the Dwarf Mistletoe Model (Forest Health Technology Enterprise Team 2002). Because spatial relations

are paramount in uneven-aged, multistory stands, the

spatial-statistical model (Robinson and others 2002)

provides another means for determining expected

mistletoe spread.

In view of the uncertainties and potential adverse

effects from selection and partial cutting in infected

stands, use of the appropriate criteria for selecting and

retaining trees is especially important. Overcutting

reduces growing stock and possibly accelerates spread

of dwarf mistletoe; undercutting and leaving more

infected trees allows severe damage and unacceptable

impacts. Cutting cycles and intensity of cutting can be

adjusted to maintain healthy stands. Monitoring stand

and infestation characteristics is especially important, as is the employment of well trained and highly

skilled individuals who can recognize and evaluate

dwarf mistletoe infection and apply complex marking

guides. Although it is a challenge, management of

infested, uneven-aged stands is possible (Hawksworth

1978a, Roth and Barrett 1985).

Prescribed Burning ______________

Prescribed burning is a potential silvicultural treatment applicable to even-aged and uneven-aged stands

or forests. Historically, wildfire is an important ecological factor in many Western forest ecosystems and

a strong determinant of mistletoe distribution and

abundance (chapter 5 and Zimmerman and Leven

1984). In recent years, burning has been prescribed to

maintain or reestablish desired stand conditions. Prescribed burning for treatment in dwarf mistletoeinfested stands can be used for stand replacement or

mistletoe reduction.

Table 8-2—Dwarf mistletoe ratings for leave trees in selection

cuttings in Southwestern ponderosa pine.

Tree dbh (cm) Maximum DMR per tree

<10 0

10-15 1

16-20 2

>21+ 3

Based on Conklin (2000), acceptable rating of leave trees assumes a 20-year cutting cycle; a maximum of rating of 3 is allowed

for trees that are intended for timber purposes.

Muraro (1978) and Zimmermann and others (1990)

describe the use of fire as an economical method for

replacing lodgepole pine stands that are overstocked

and severely infested. Lodgepole pine, however, has a

number of unusual silvicultural and fire ecology characteristics that make this species suitable for such

treatment but that are not shared by all forest types.

Prescribed burning is usually a silvicultural tool for

reduction of fuels where forest type and condition

permit. Moderately to severely infected trees may be

more vulnerable to fire because of lower crowns, witches’

brooms, and accumulation of debris and resin. A goal

of prescribed burning can be the differential killing of

infected trees with discrimination of more severely

infected trees and consequently a reduction in average

stand infestation (Conklin and Armstrong 2001). Fire

intensity and distribution can be directed at specific

trees or groups of trees using techniques such as

removing or piling duff and selecting upslope/upwind

or down slope/downwind ignition points. In some

stands, dwarf mistletoe infestation generates openings or gaps where infected trees survive fire (Wanner

and Tinnin 1989). An approach for infected lodgepole

pine or Douglas-fir stands is to replace these with

more fire-resistant species such as ponderosa pine by

a series of light fires over a period of several years. In

any case, prescribed burning requires careful design

and execution by experts (Muraro 1978). Numerous

variables such as fuel loading and condition, stand

structure, objectives for burn, weather, and other

factors must be considered. Although prescribed fire

will remain primarily a treatment for other forest

management purposes, additional research and development (for example, on fire behavior, fuel distribution, and brooms) can enhance its potential as a tool

in dwarf mistletoe infested stands.

Models to Assess Treatment

Opportunities___________________

When considering more complex or controversial silvicultural treatments such as sanitation and selection

cuttings in uneven-aged stands or thinning of imma-
92 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Geils Management Strategies for Dwarf Mistletoe: Silviculture

ture, even-aged stands, it is helpful to undertake a

detailed, site-specific analysis of potential impacts

and benefits. Such evaluations typically include a

summary of current conditions, potential growth of an

infested stand, costs and effects of treatments, and

projected outcome with treatment. A variety of factors

are important to consider, such as tree age, stand

structure, stand density, species composition, site

index, and years to next treatment. Useful mistletoe

data are incidence (percent of stems infected), severity

(DMR), area and pattern of infestation, and length of

time the stand has been infested. The most feasible

approach for summarizing information, making projections, and displaying results is with a forest growth

and yield simulation model that includes the dynamics and effects of dwarf mistletoe infestation.

Numerous computer models are available that simulate various aspects of tree or stand development for

dwarf mistletoe infected trees or infested stands. Strand

and Roth (1976) describe a population model for young

pine with Arceuthobium campylopodum. Baker and

others (1982) predict stand impacts on spruce from A.

pusillum. For hemlock forests with A. tsugense,

Bloomberg and Smith (1982) model second-growth

stands and Trummer and others (1998) model old-age

stands. Myers and others (1971) introduce a growth

and yield program of mistletoe-infested pine that,

through many iterations and updates, has become the

Dwarf Mistletoe [Impact] Model (DMIM) described by

Hawksworth and others (1995). The DMIM is an

operational tool supported by the U.S. Department of

Agriculture, Forest Service and available on the

Internet (Forest Health Technology Enterprise Team

2002). The DMIM functions with the Forest Vegetation Simulator (FVS) to model tree and stand dynamics and provide a number of presimulation and

postprocessor features for data preparation, simulation control, and display (Forest Management Service

Center 2001). Development currently under way for

the DMIM includes improvement of overstory to understory spread (Maffei and others 1999). Robinson

and others (2002) describe a process-oriented simulation model derived from the same origin as the DMIM

but with additional capabilities to represent features

of the mistletoe life cycle and crown canopy. This

spatial-statistical model has a potential for examining

such integrated mistletoe treatments as the silvicultural deployment of biological control agents.

The conversion of mistletoe control from eradication

with large clear-cuts for timber production to sustained, uneven-aged management for ecological services has greatly increased the complexity of silvicultural assessments. Dwarf mistletoe simulation models

are most useful to silviculturalists for addressing

these complex situations, in which numerous factors

interact over a long period. In chapter 5, we identify

some of these interacting factors and effects; in chapter 6, we describe some procedures for acquiring data;

in chapter 7 and earlier in this chapter, we outline

treatments available to silviculturalists for managing

infested stands. Simulation models permit planners to

evaluate a number of treatment alternatives and to

compare the long-term results before committing on

the ground to a single, “experiment” in the sense of

adaptive management (Holling 1978). Elaborate simulation models such as the DMIM–FVS incorporate a

huge volume of research and experience. These models simplify an analysis by conducting the tedious

bookkeeping and arithmetic required for such processes as computing statistics and applying growth

functions. Because these processes are coded in the

program, they are documented and can be reexecuted

numerous times. The analyst is able to focus on formulating the problem, generating possible solutions, evaluating results, and documenting the overall activity.

Models are a simplification of a reality that is more

or less “correct” and hopefully at least insightful.

Although models are especially useful for novel situations, confidence in their predictions is supported by

comparisons to the actual performance of benchmark

stands. A useful set of benchmark stands represents

the range of conditions and treatments silviculturalists

are likely to consider (Taylor and Marsden 1997).

Models are usually evaluated for sensitivity to a number of factors (Chen and others 1993). Knowledge of

which factors a system is sensitive or insensitive to is

useful to the planner, as these suggest what data are

required to achieve high levels of accuracy or precision, and what treatments may be effective. The DMIM

has numerous stochastic functions and is apparently

sensitive to mistletoe incidence (percent of infected

trees) at low levels (Chen and others 1993). This may

well reflect real situations where a small infestation of

only several trees could either spread throughout the

stand or be isolated in one packet and eventually

expire. A single simulation represents one likely outcome. Gregg and Hummel (2002) describes a

bootstrapping facility for FVS that simplifies execution of multiple simulations to obtain information on

the distribution (mean and dispersion) of outcomes. It

is not necessary to simulate every infested stand to be

managed. Most stands can be clustered into groups

with similar conditions and treatment regimes. An

analysis of these typical situations provides local guidelines that can be applied to all similar stands. Special

analyses are then conducted for unusual cases and

may contribute to the portfolio of guidelines.

Although the scope, availability, and applicability of

current models for dwarf mistletoes are limited, their

chief value is in the ability to determine quantitative

effects and impacts of dwarf mistletoes under various

stand conditions and treatment regimes. In so doing,
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 93

Management Strategies for Dwarf Mistletoe: Silviculture Muir and Geils

models provide forest managers with a rational framework for decisionmaking.

Management for Recreation, Wildlife

Habitat, and Other Ecosystem

Values _________________________

It is becoming increasingly evident that active forest

management by silvicultural treatment is necessary

to sustain or enhance desirable stand conditions where

trees or stands are infested by dwarf mistletoe. The

particular conditions desired for different objectives

vary: for recreation sites, live trees that are not a

hazard; for some wildlife species, dense tree cover for

screening; for other wildlife species, large openings

with a few big trees and mistletoe brooms for nesting

and roosting. Forests are not static, and trees, especially mistletoe-infected trees, have short lives. Forest

management, working with the opportunity and capability provided by a site or stand, can influence vegetation development, including mistletoe, to meet a variety of objectives.

Management of dwarf mistletoe in recreation, administration, and home sites has the fundamental

objective to maintain a safe and pleasant environment

(Scharpf and others 1988). Although in these areas

there is a low tolerance for mistletoe damage, trees are

sufficiently valuable to justify repeated, individual

treatment such as pruning branches. Methods are

outlined as Treatments in Developed Recreation Sites.

Where wildlife habitat is an important consideration, it may be desirable to maintain or encourage

features resulting from mistletoe infections, such as

snags and witches’ brooms. The same factors that can

be manipulated to reduce mistletoe spread, intensification, and effects can also be used to enhance these

processes and produce a continuing supply of dead and

diseased trees. Examples are outlined in the section

Treatments for Wildlife Habitat and Other Ecosystem

Values.

Treatments in Developed Recreation Sites

In developed, intensively managed sites, treatments

of dwarf mistletoe are needed to protect human life

and property, and aesthetic and recreational values.

Scharpf and others (1988) outline general principles

and strategies for managing infested recreation sites

and for maintaining individual trees or stands. They

emphasize that specific management objectives and

constraints for each site should be carefully considered and incorporated in the action plan.

The primary interests in developed, intensively

used sites are to reduce the negative effects of dwarf

mistletoe on tree vigor, longevity, and hazard, and to

prevent mistletoe spread into healthy trees (Wood and

others 1979). The first opportunity to do this is at the

time of site selection and establishment. Spread from

adjacent infested areas is slow and easy to control. Site

planning and layout can achieve eradication by sanitation of light or patchy mistletoe infestations; hardy,

immune species can be planted. The value of early

control is appreciated when long-term costs of treatment and site replacement are recognized. Recreation

sites range in size and level of intensity from campgrounds to National Parks (Hansen 1997, Lightle and

Hawksworth 1973, Maffei 1984). Various techniques

and concepts of even-aged or uneven-aged silviculture

can be adapted for special uses. For example, a site

may be laid out to remove an infested block of trees; or

a portion of the infected trees may be removed on a

periodic schedule to encourage establishment of healthy

trees (Johnson 1998, Pronos 1995). A common feature

of recreation sites is inspection and treatment of potentially hazardous trees on a relatively frequent

schedule. Although branch pruning is rarely done in

commercial forests to produce clear bole wood, pruning infected branches and brooms in high value sites is

a common practice.

Pruning mistletoe-infected and broomed branches

is used to maintain and improve tree vigor and to

reduce hazard (Hawksworth and Johnson 1993, Maffei

1992). The most suitable candidates for branch pruning are trees having: infections in the lower half of the

crown only; a DMR of 3 or less, and if smaller than 13

cm in diameter, with no bole infections or branch

infections closer than 10 cm from the bole. Mark and

Hawksworth (1974) have concluded that infections on

tree boles larger than 13 cm have little effect on growth

and produced few seeds, and they are therefore not a

management concern. Aerial shoots on a branch but

within 10 cm of the bole probably emerge from an

endophytic system that has already reached the bole.

Because most trees can tolerate removal of up to half

the live crown, general practice is to prune all live

branches to two whorls above the highest visibly

infected branch. Mistletoe infestations in a tree usually include a number of latent (invisible, incubating

infections) and other easily overlooked infections. Most

of the missed infections appear in 3 to 5 years;

reinspection and repeated pruning are appropriate.

Such treated trees often show dramatic recovery in

crown vigor. Trees with severe infections, however,

such as those with infections throughout the lower

crown or in the upper crown, are not likely to respond

but likely to soon die. The proper consideration for

these trees is whether the value of retaining them for

a few more years is greater than the risk they pose for

infecting other trees.
94 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir and Geils Management Strategies for Dwarf Mistletoe: Silviculture

Broom pruning can also prolong the life and crown

vigor of individual pine trees (Lightle and Hawksworth

1973, Scharpf and others 1988). In this method, the

emphasis is on removing branches with witches’ brooms

rather than removing all visibly infected branches.

Hadfield (1999) describes the hazard in high traffic

areas from breakage of brooms in species with large or

brittle witches’ brooms. Pruning these may also be

justified.

Treatments for Wildlife Habitat and Other

Ecosystem Values

From certain perspectives and in some situations,

dwarf mistletoe infestations have beneficial impacts

for associated species and communities (Mathiasen

1996, Monning and Byler 1992, Tinnin and others

1982). In old-growth forests, dwarf mistletoes may

exert a different set of effects on infected trees and

display different dynamics (Hawksworth and others

1992a, Trummer and others 1998). Special management strategies and silvicultural treatments for infested stands are required where the objectives are to

maintain and enhance wildlife habitat, old-growth

character, and other ecosystem values.

As described in chapter 5, dwarf mistletoe infection

produces mistletoe shoots, fruits, diseased branches,

brooms, distorted crowns and boles, detritus, diseased

and insect-infested trees, snags, and eventually logs.

Infestations alter succession, disturbance regimes,

and vegetation pattern of the landscape. Within limits, these features favor some species (or groups),

inhibit other species, and are essentially neutral to

most (Watson 2001). By influencing the spread and

intensification of mistletoe and the environment around

infected trees, managers are able to affect mistletoe

infestations and ecological effects. The specific goals of

a treatment depend on specific management objectives such as identification of featured species. For

example, Reynolds and others (1992) describe guidelines for the northern goshawk that include consideration of mistletoe and other forest disturbance agents

(also see Steeger and Hitchcock 1998).

Most of the recent interest in research and development of management recommendations has focused

on snags, brooms, birds, and mammals. Bennetts and

others (1996) describe a study of passerine bird diversity in a Colorado Front Range ponderosa pine forest.

They suggest greater bird diversity is associated with

increased mistletoe infestation (24 of 28 species positively associated); the key limiting resource for the

birds in this situation may be snags. Parker (2001)

reports a similar study in a northern Arizona ponderosa pine forest. He finds, however, a more complex

situation with four species positively associated with

mistletoe (cavity-nesting birds), five species with a

negative association (avoiding infested areas), and

seven with no relation (indifferent). Fairweather (1995)

and Parks and others (1999b) describe mistletoe control treatments in which infected trees were killed but

left standing for woodpeckers and other cavity-nesting

animals. Although these snags are used, they remained standing for only a few years. Studies of broom

use by wildlife include work by Parks and others

(1999a), Hedwall (2000), and Garnett (2002). These

studies identify which birds and mammals use witches’

brooms, how they use it (for nesting and roosting), and

what kinds of brooms are preferred. This information

is useful to determine if retaining certain brooms is a

potential benefit for a favored species. Information

still lacking is knowledge of how the number and

distribution of snags and brooms relates to levels of

mistletoe infestation and to wildlife populations and

the dynamics (rates of generation and loss) of these

features.

Marshall (1996) discusses management lessons, implications, and research needs from a project to manage infested stands for northern spotted owl in southwestern Oregon. Maffei (2002) presents results of an

analysis for a similar situation also in Oregon, and for

maintaining owl habitat. Although owls use mistletoe

brooms for nesting, vegetation changes and disturbance stimulated by the mistletoe (such as fire) lead to

loss of critical owl habitat. The analyses demonstrate

use of an infection index that represents desired condition (relative to owls and mistletoe) and application

of the FVS-DMIM in a landscape planning exercise.

These projects illustrate how mistletoe information

can be integrated with wildlife criteria to design treatment regimes that benefit long-term survival of a

featured species. Complex situations involving numerous ecological relationships are not amenable to

simple guidelines defining which trees to cut and

which to retain; rather, they require an adaptive

management process of analysis, simulation, experimenting, monitoring, and revision (Holling 1978).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 95

Appendices

Appendix A: References __________

Acosta D.H.; Rodriguez, T.D.A. 1989. Infestacion de

Arceuthobium vaginatumsubsp. vaginatum(Willd.) sobre

Pinus hartwegii (Lindl.) en el Cerro San Miguel, Desierto

de Los Leones, Mexico, DF. En: V Simposio sobre

Parasitologia Forestal; 1989 Octobre 4–6; Cd. Juarez,

Chih: Sociedad Mexicana de Entomologia. 44.

Acosta, P.R.; Delgado, M.J.; Cervantes, S.P. 1992. La vegetation del Estado de Tlaxcala, Mexico. Publ. 6. Tizatlan,

Tlaxcala: Hardin Botanico Tizatlan. 31 p.

Adams, D.H.; Frankel, S.J.; Lichter, J.M. 1993. Considerations when using ethephon for suppressing dwarf mistletoe and leafy mistletoe infestations in ornamental landscapes. Journal of Arboriculture. 19(6):351–357.

Aho, P.E. 1982. Indicators of cull in western Oregon conifers.

Gen. Tech. Rep. PNW-144. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Forest and Range Experiment Station. 17 p.

Alexander, M.E.; Hawksworth, F.G. 1975. Wildland fires

and dwarf mistletoes: a literature review of ecology and

prescribed burning. Gen. Tech. Rep. RM-14. Fort Collins,

CO. U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station. 12 p.

Alexander, R.R. 1986. Silvicultural systems and cutting

methods for old-growth lodgepole pine forests in the Central Rocky Mountains. Gen. Tech. Rep. RM-127. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 31 p.

Alfaro, R. 1985. Survey and appraisal of dwarf mistletoe in

second-growth western hemlock stands. In: Muir, J.A., ed.

Proceedings of a workshop on management of hemlock

dwarf mistletoe; 1983 August 15–16; Burnaby, BC. Forest

Pest Management Rep. 4. Victoria, BC: British Columbia

Ministry of Forests, Forest Protection Branch: 10–21.

Allen, T.F.H.; Hoekstra, T.W. 1992. Toward a unified ecology. New York: Columbia University Press. 384 p.

Allred, W.S.; Gaud, W.S. 1994. Characteristics of ponderosa

pines and Abert squirrel herbivory. Southwest Naturalist.

39(1):89–100.

Alosi, M.C.; Calvin, C.L. 1985. The ultrastructure of dwarf

mistletoe (Arceuthobium spp.) sinker cells in the region of

the host secondary vasculature. Canadian Journal of

Botany. 63:889–898.

Amaranthus, M.P.; Weigand, J.F.; Abbott, R. 1998. Managing high-elevation forests to produce American matsutake

(Ticholoma magnivelare), high-quality timber, and nontimber forest products. Western Journal of Applied Forestry. 13:120–128.

Anderson, R.L.; Kaufert, F.H. 1959. Brooming response of

black spruce to dwarf mistletoe infection. Forest Science.

5:356–364.

Andrade Escobar, V.; Cibrián Tovar, D. 1980. Evaluación de

poblaciones de muérdago enano (Arceuthobium globosum

Hawks y Wiens y A. vaginatumWilld) en bosques de Pinus

hartwegiiLindl en Zoquiapan, Edo. de México. En: Primer

simposio nacional sobre parasitologia forestal; 1980

Febrero 18–19. Uruapan, Michoacán: Sociedad Mexicana

de Entomologia: 238–253.

Andrews, S.R.; Daniels, J.P. 1960. A survey of dwarf mistletoes in Arizona and New Mexico. Station Pap. 49. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 17 p.

Anonymous. 1982. Dwarf mistletoe biocontrol. The IPM

Practitioner. 4(8):5.

Ashworth, V.E.T.M. 2000. Phylogenic relationships in

Phoradendron (Viscaceae) inferred from three regions of

the nuclear ribosomal cistron. II. The North American

species of Phoradendron. Aliso. 19(1):41–53.

Atsatt, P.R. 1983a. Host-parasite interactions in higher

plants. In: Lange, O.L.; Nobel, P.S.; Osmond, C.B.; Ziegler,

H., eds. Physiological plant ecology. III. Encyclopedia of

plant physiology, Volume 12C. New York: Springer-Verlag:

519–535.

Atsatt, P.R. 1983b. Mistletoe leaf shape: a host morphogen

hypothesis. In: Calder, M.; Berhhardt, P., eds. The biology

of mistletoes. Sydney: Academic Press: 259–275.

Baker, F.A.; Durham, S. 1997. Dwarf mistletoe impact on

jack pine infection, volume loss and mortality curves.

Unpublished report submitted to: Weyehauser, Canada;

1997 March 11. Logan: Utah State University, Department of Forest Resources. 10 p.

Baker, F.A.; French, D.W. 1991. Radial enlargement of

mortality centers caused by Arceuthobium pusillum Peck

in black spruce stands. Forest Science. 37:364–367.

Baker, F.A.; French, D.W.; Rose, D.W. 1982. DMLOSS: a

simulator of losses in dwarf mistletoe infested black spruce

stands. Forest Science. 28:590–598.

Baker, F.A.; Knowles, K.; Meyer, T.R.; French, D.W. 1989.

Aerial application of ethylene-releasing chemicals fail to

promote abscission of dwarf mistletoe aerial shoots on jack

pine. Forestry Chronicle. 65:194–195.

Baker, F.A.; Knowles, K.; Slivitzky. 1992. Impact of dwarf

mistletoe on jack pine forests in Manitoba. Plant Disease.

76:1256–1259.

Baloch, G.M.; Ghani, M.A. 1980. Parasitic weed plants and

their natural enemies with special reference to Pakistan:

a review. Pakistan Journal of Forestry. 30:2–13.

Baranyay, J.A. 1966. Fungi from dwarf mistletoe infection

in western hemlock. Canadian Journal of Botany. 44:597–

604.

Baranyay, J.A. 1970. Lodgepole pine dwarf mistletoe in

Alberta. Publ. 1286. Ottawa, ON: Canada Department of

Fisheries and Forestry. 22 p.
96 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Baranyay, J.A.; Hawksworth, F.G.; Smith R.B. 1971. Glossary of dwarf mistletoe terms. BC P-2-71. Victoria, BC:

Department of the Environment, Canadian Forestry Service, Pacific Forest Research Centre. 42 p.

Baranyay, J.A.; Safranyik, L. 1970. Effect of dwarf mistletoe

on growth and mortality of lodgepole pine in Alberta. Publ.

1285. Canadian Forestry Service, Department of Fisheries and Forestry. 19 p.

Barlow, B.A. 1964. Classification of the Loranthaceae and

Viscaceae. Proceedings Linnaean Society of New South

Wales II. 89:268–272.

Barlow, B.A. 1983. Biogeography of Loranthaceae and

Viscaceae. In: Calder, M.; Bernhardt, P., eds. The biology

of mistletoes. Sydney: Academic Press: 19–46.

Barney, C.W.; Hawksworth, F.G.; Geils, B.W. 1998. Hosts of

Viscum album. European Journal of Forest Pathology.

28:187–208.

Barrett, J.W.; Roth, L.F. 1985. Response of dwarf mistletoeinfested ponderosa pine to thinning: 1. Sapling growth. Res.

Pap. PNW-330. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range

Experiment Station. 15 p.

Bates, C.G. 1927. Better seeds, better trees. Journal of

Forestry. 25:130–144.

Beatty, J.S.; Filip, G.M.; Mathiasen, R.L. 1997. Larch dwarf

mistletoe. Forest Insect and Disease Leaflet 169. [revised

1988]. Washington, DC: U.S. Department of Agriculture,

Forest Service. 8 p. [Online]. Available: http://

www.fs.fed.us/r6/nr/fid/fidls/fidl169.htm. [2002, June 29].

Becker, H.; Schmoll, H. 1986. Mistel: Arzneipfanze,

Brauchtum, Kunstmotiv. Stuttgart: Wissenschaftliche

Verlagsgesellschaft. 132 p.

Bello Gonzales, M.A. 1984. Estudio de muérdagos

(Loranthaceae) en la region Tarasca, Michoacán. Bol.

Téch. 102. México, DF: Secretaría de Agricultura y Recursos

Hidráulicos, Instituto Nacional de Investigaciones

Forestales. 62 p.

Bello Gonzales, M.A.; Gutierrez Garduño, M. 1985. Clave

para la identificación de la familia Loranthaceae en la

porción del Eje Neovolcánico localizado dentro del Estado

de Michoacán. Ciencia Forestal. 10(54):3–33.

Bennetts, R.E.; White, G.C.; Hawksworth, F.G.; Severs, S.E.

1996. The influence of dwarf mistletoe on bird communities in Colorado ponderosa pine forests. Ecological Applications. 6:899–909.

Besse, W.J.; Dunsworth, G.; Perry, J. 2001. The forest

project: three-year review and update. Ecoforestry.

16(4):10–17.

Bloomberg, W.J.; Smith, R.B. 1982. Measurement and simulation of dwarf mistletoe infection of second-growth western hemlock on southern Vancouver Island. Canadian

Journal of Forest Research. 12:280–291.

Bolsinger, C.L. 1978. The extent of dwarf mistletoe in six

principal softwoods in California, Oregon and Washington, as determined from forest survey records. In: Scharpf,

R.F.; Parmeter, J.R., Jr., tech. coords. Proceedings of the

symposium on dwarf mistletoe control through forest

management; 1978 April 11–13; Berkeley, CA. Gen. Tech.

Rep. PSW-31. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range

Experiment Station: 45–54.

Bonga, J.M. 1964. An unusual witches’ broom on black

spruce caused by eastern dwarf mistletoe Arceuthobium

pusillum on Picea mariana. Forest Science. 10:77–78.

Booth, C.; Samuels, G.J. 1981. Nectria neomacrospora nom.

nov.: a new name for Nectria macrospora (Wollenw.)

Oullette. Transaction British Mycological Society. 77(3):

645.

Bradley, A.F.; Noste, N.V.; Fisher, W.C. 1992. Fire ecology of

forests and woodlands in Utah. Gen. Tech. Rep. INT-287.

Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 128 p.

Brandt, J.P.; Brett, R.D.; Knowles, K.R.; Sproule, A. 1998.

Distribution of severe dwarf mistletoe damage in westcentral Canada. Special Rep. 13. Edmonton, AB: Natural

Resources Canada, Canadian Forest Service, Northern

Forestry Centre. 27 p.

Briede, J-W.; Mexal, J.G.; Lynch, A.M.; Hawksworth, F.G.

1991. Shoot growth and development of Douglas-fir and

white fir in New Mexico. In: Baumgartner, D.M; Lotan,

J.E., eds. Interior Douglas-fir: the species and its management; 1990 February 27–March 1; Spokane, WA. Pullman, WA: Washington State University Cooperative Extension: 155–159.

British Columbia Ministry of Forests. 1995. Dwarf mistletoe

management guidebook. Forest practices code of British

Columbia. Victoria, BC: British Columbia Ministry of

Forests. 20 p. [Online]. Available: http://www.for.gov.bc.ca/

tasb/legsregs/fpc/fpcguide/dwarf/dwarftoc.htm. [2002,

June 30].

British Columbia Ministry of Forests. 2000. Growth and

yield stand modelling PrognosisBC. [Homepage of

PrognosisBC, British Columbia Ministry of Forests].

[Online]. Available: http://www.fro.gov.bc.ca/research/

gymodels/progbc/index.htm. [2002, July 7].

Broshot, N.; Larson, L.; Tinnin, R. 1986. Effects of

Arceuthobium americanum on twig growth of Pinus

contorta. Res. Note. PNW-RN-453. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Research Station. 6 p.

Broshot, N.E.; Tinnin, R.O. 1986. The effect of dwarf mistletoe on starch concentrations in the twigs and needles of

lodgepole pine. Canadian Journal of Forest Research.

16:658–660.

Browner, C.H. 1985. Plants used for reproductive health in

Oaxaca, Mexico. Economic Botany. 39:482–504.

Buckland, D.C.; Marples, E.G. 1953. Management of western hemlock infested with dwarf mistletoe. British Columbia Lumberman. 36(5):50–51, 136, 140.

Bunderson, E.D.; Weber, D.J.; Nelson, D.L. 1986. Diseases

associated with Juniperus osteosperma and a model for

predicting their occurrence with environment site factors.

Great Basin Naturalist. 46:427–440.

Burke, H.R. 1975. A new species of Smilcraulax from Mexico,

with key to species of the genus (Coleoptera: Curculionidae).

Entomological News. 86:167–171.

Byler, J.W. 1978. The pest damage inventory in California.

In: Scharpf, R.F.; Parmeter, J.R., Jr., tech. coords. Proceedings of the symposium on dwarf mistletoe control

through forest management; 1978 April 11–13; Berkeley, CA. Gen. Tech. Rep. PSW-31. Berkeley, CA: U.S.

Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station: 162–171.

Byler, J.W.; Cobb, F.W., Jr. 1972. The occurrence and pathogenicity of Nectria fuckeliana on dwarf mistletoe in California. Canadian Journal of Botany. 50:1162.

Caballero Deloya, M. 1968. Los principales enemigos del

bosque in los estados de Baja California, Chihuahua,

Durango, Nayarit y Sonora, Bosques de clima templado y

frío. Publ. 5. México, DF: Direccion General del Inventario

Nacional Forestal. 20 p.

Caballero Deloya, M. 1970. La frecuencia de los daños al

bosque (areas de coníferas en Zacatecas, Sinaloa y Jalisco).

Publ. 14. México, DF: Direccion General del Inventario

Nacional Forestal. 28 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 97

Calder, D.M. 1983. Mistletoes in focus: an introduction. In:

Calder, M.; Bernhardt, P., eds. The biology of mistletoes.

Sydney: Academic Press: 1–17.

Calder, M.; Bernhardt P., eds. 1983. The biology of mistletoes. Sydney: Academic Press. 348 p.

Calvin, C.L.; Wilson, C.A. 1996. Endophytic system. In:

Hawksworth, F.G.; Wiens, D. Dwarf mistletoes: biology,

pathology, and systematics. Agric. Handb. 709. Washington, DC: U.S. Department of Agriculture, Forest Service:

113–122.

Calvin, C.L.; Wilson, C.A.; Varughese, G. 1991. Growth of

longitudinal stands of Phoradendron juniperinum

(Viscaceae) in shoots of Juniperus occidentalis. Annals of

Botany. 67:153–161.

Cerezke, H.F.; Emond, F.J. 1989. Forest insect and disease

conditions in Alberta, Saskatchewan, Manitoba, and the

Northwest Territories in 1987. Inf. Rep. NOR-X-300.

Edmonton, AB: Forestry Canada, Northern Forestry Centre. 25 p.

Cházaro B., M. 1989a. Loranthaceae (“muérdagos”) del

centro y sur del Estado de Veracruz. En: IV simposio

nacional sobre parasitologia forestal; 1987 October 28–30;

Durango, Drg. Publicación Especial 60. Instituto Nacional

de Investigaciones Forestales y Agropecularis: 604–612.

Cházaro B., M. 1989b. Los muérdago (Loranthacease) del

Estado de Jalisco. En: IV Simposio sobre parasitologia

forestal; 1989 Octobre 4–6; Cd. Juarez, Chih: 45.

Cházaro B., M. 1990. Los muérdago del Estado de Jalisco.

Biosphera. 1:3–7.

Cházaro B., M.; Huerta Martínez, F.M.; Patiño Beltrán,

R.M.; Sánchez Fornué, R.; Lomelí Mijes, E.; Flores Macías,

A. 1992. Los muérdagos (Loranthaceae) de Jalisco,

parásitas poco conocidas. Ciencia y Desarrollo. 17(102):70–

85.

Cházaro B., M.; Oliva R., H. 1987a. Loranthaceas del centro

de Veracruz y zona limítrofe de Puebla. I. Cactaceas y

Succulentas Mexicanas. 32:55–60.

Cházaro B., M.; Oliva R., H. 1987b. Loranthaceas del centro

de Veracruz y zona limítrofe de Puebla. II. Cactaceas y

Succulentas Mexicanas. 32:78–86.

Cházaro B., M.; Oliva R., H. 1988a. Loranthaceas del centro

de Veracruz y zona limítrofe de Puebla. IV. Cactaceas y

Succulentas Mexicanas. 33:42–48.

Cházaro B., M.; Oliva R., H. 1988b. Loranthaceas del centro

de Veracruz y zona limítrofe de Puebla. V. Cactaceas y

Succulentas Mexicanas. 33:71–75.

Chen, C.; Marsden, M.; Taylor, J.E. 1993. Sensitivity analysis of the Interim Dwarf Mistletoe Impact Modeling System for the user. Rep. MAG-93-10. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Forest Pest

Management Methods Application Group. 13 p.

Childs, T.W. 1960. Drought effects on conifers in the Pacific

Northwest, 1958–59. Res. Note 182. Portland, OR: U.S.

Department of Agriculture, Pacific Northwest Forest and

Range Experiment Station. 5 p.

Cibrián Tovar, D.; Campos Bolanos, R.; Pineda Torres, C.;

Guerrero Alarcon, E.; Olvera Olmos, V. 1980. Aspectos

biologicos del genero Arceuthobium. En: Primer simposio

nacional sobre parasitologia forestal; 1980 Febrero 18–19.

Uruapan, Michoacán: Sociedad Mexicana de Entomologia:

229–237.

Conkle, M.T.; Critchfield, W.B. 1988. Genetic variation and

hybridization of ponderosa pine. In: Baumgartner, D.M.;

Lotan, J.E., eds. Ponderosa pine, the species and its

management, proceedings of the symposium; 1987 September 29–October 1; Spokane, WA. Pullman, WA: Washington State University, Cooperative Extension: 27–43.

Conklin, D.A. 2000. Dwarf mistletoe management and forest health in the Southwest. Albuquerque, NM: U.S.

Department of Agriculture, Forest Service, Southwestern

Region. 30 p.

Conklin, D.A. 2002. Emergence of latent dwarf mistletoe

infection in young ponderosa pine regeneration: 10-year

monitoring of Whitetail A&B Project at Mescalero. R3-02-

02. Albuquerque, NM: U.S. Department of Agriculture,

Forest Service, Southwestern Region, Forestry and Forest

Health. 8 p.

Conklin, D.A.; Armstrong, W.A. 2001. Effects of three prescribed fires on dwarf mistletoe infection in southwestern

ponderosa pine. Rep. R3-01-02. Albuquerque, NM: U.S.

Department of Agriculture, Forest Service, Southwestern

Region, Forestry and Forest Health. 17 p.

Current Vegetation Survey. 2002. [Homepage of Current

Vegetation Survey, Region 6, Forest Service, U.S. Department of Agriculture]. [Online]. Available: http://

www.fs.fed.us/r6/survey/. [2002, July 8].

Daniel, J.T.; Templeton, G.E.; Smith, R.J.; Fox, W.T. 1973.

Biological control of northern jointvetch in rice with endemic fungal disease. Weed Science. 21:303–307.

Daugherty, C.M; Mathiasen, R.L. 1999. Adult sex ratio of

Phoradendron juniperinum in ten severely infected

Juniperus monosperma in northern Arizona. Madroño.

46(4):169–176.

Davidson, A.G.; Prentice, R.M. 1967. Important forest insects and diseases of mutual concern to Canada, the

United States and Mexico. Ottawa, ON: Ministry of Forestry and Rural Development. 248 p.

Dawson, T.E.; Ehleringer, J.R. 1991. Ecological correlates of

seed mass variation in Phoradendron juniperinum, a

xylem-tapping mistletoe. Oecology. 85:332–342.

Dawson, T.E.; Ehleringer, J.R.; Marshall, J.D. 1990a. Sexratio and reproductive variation in the mistletoe

Phoradendron juniperinum (Viscaceae). American Journal of Botany. 77:584–589.

Dawson, T.E.; King, E.J.; Ehleringer, J.R. 1990b. Age structure of Phoradendron juniperinum (Viscaceae), a xylemtapping mistletoe: Inferences from a non-destructive morphological index of age. American Journal of Botany.

77:573–583.

de Jong, M.D.; Scheepens, P.C.; Zadoks, J.C. 1990. Risk

analysis for biological control: a Dutch case study in

biocontrol of Prunus serotina by the fungus

Chondrostereum purpureum. Plant Disease. 74:189–194.

De la Puente, J.M. 1966. Forest disease and insect resistance

in Latin America. In: Gerhold, H.D.; Schreiner, E.J.;

McDermott, R.E.; Winieski, J.A., eds. Breeding pest-resistant trees. Oxford: Pergamon Press: 31–34.

DeBach, P., ed. 1964. Biological control of insect pests and

weeds. New York: Reinhold Pulishing. 844 p.

Deeks, S.J.; Shamoun, S.F.; Punja, Z.K. 2001. In vitro

germination and development of western hemlock dwarf

mistletoe. Plant Cell, Tissue, and Organ Culture. 66:97–

105.

Deeks, S.J.; Shamoun, S.F.; Punja, Z.K. [In press]. Histopathology of callus and germinating seeds of Arceuthobium

tsugense subsp. tsugense infected by Cylindrocarpon

cylindroides and Colletotrichum gloeosporioides. International Journal of Plant Sciences.

DeMars, D.J. 1980. Estimates of the occurrence of dwarf

mistletoe on the Deschutes National Forest. Res. Note

PNW–364. Portland, OR: U.S. Department of Agriculture,

Forest Service, Pacific Northwest Forest and Range Experiment Station. 8 p.
98 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

DeNitto, G., comp. 2002. Montana forest insect and disease

conditions and program highlights—2001. Rep. 02-1.

Missoula, MT: U.S. Department of Agriculture, Forest

Service, Northern Region, State and Private Forestry. 42 p.

Dixon, G.E.; Hawksworth, F.G.1979. A spread and intensification model for southwestern dwarf mistletoe in ponderosa pine. Forest Science. 25(1):43–52.

Dobie, J.; Britneff, A.A. 1975. Lumber grades and volumes

from lodgepole pine infected with dwarf mistletoe. Wood

and Fiber. 7:104–109.

Dooling, O.J. 1978. Survey methods to determine the distribution and intensity of dwarf mistletoes. In: Scharpf, R.F.;

Parmeter, J.R., Jr., tech. coords. Proceedings of the symposium on dwarf mistletoe control through forest management; 1978 April 11–13; Berkeley, CA. Gen. Tech. Rep.

PSW-31. Berkeley, CA: U.S. Department of Agriculture,

Forest Service, Pacific Southwest Forest and Range Experiment Station: 36–44.

Dowding, E.S. 1931. Wallrothiella arceuthobii, a parasite

of jack-pine mistletoe. Canadian Journal of Research.

5:219–230.

Drummond, D.B. 1978. Approaches to determining volume

losses due to dwarf mistletoe on a Westwide basis. In:

Scharpf, R.F.; Parmeter, J.R., Jr., tech. coords. Proceedings of the symposium on dwarf mistletoe control through

forest management; 1978 April 11–13; Berkeley, CA. Gen.

Tech. Rep. PSW-31. Berkeley, CA: U.S. Department of

Agriculture, Forest Service, Pacific Southwest Forest and

Range Experiment Station: 55–61.

Drummond, D.B. 1982. Timber loss estimates for the coniferous forest of the United States due to dwarf mistletoes.

Rep. 83-2. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Forest Pest Management, Methods

Application Group. 24 p.

Eav, B. B.; Marsden, M. A. 1988. Pest models for the Inland

Mountain West. In: Schmidt, W. C., comp. Future forests

of the Mountain West: a stand culture symposium; 1986

September 29–October 3; Missoula, MT. Gen. Tech. Rep.

INT-243. Ogden, UT: U.S. Department of Agriculture,

Forest Service, Intermountain Research Station. 402 p.

Edwards, D. 2001. Hemlock dwarf mistletoe in the Long

Beach Model Forest. Long Beach Model Forest. [Online].

Available: http://www.lbmf.bc.ca. [2002, April 20].

Edwards, D.N. 2002. Incidence of hemlock dwarf mistletoe

in variable retention cutblocks in the Long Beach Model

Forest. Canadian Plant Disease Survey 82: 128–132.

[Online]. Available: http://res2.agr.gc.ca/london/pmrc/

english/report/disease2002.html. [2002, August 2].

Ehleringer, J.R.; Cook, C.S.; Tieszen, L.L. 1986. Comparative water use and nitrogen relationships in a mistletoe

and its host. Oecology. 68:279–284.

Ellis, D.E. 1946. Anthracnose of dwarf mistletoe caused by

a new species of Septogloeum. Journal Elisha Mitchell

Scientific Society. 62:25–50.

Englerth, G.H. 1942. Decay of western hemlock in western

Oregon and Washington. Bull. 50. New Haven, CT: Yale

University, School of Forestry. 53 p.

Equiluz Peidra, T. 1991. Biosystematics of the Mexican and

Central American white pines. In: Proceedings, symposium on white pine provenances and breeding; August 5–

11; Montreal, Quebec. Gen. Tech. Rep. NE-155. Radnor,

PA: U.S. Department of Agriculture, Forest Service, Northeast Forest Experiment Station: 4–10.

Escudero Moreno, M.; Cibrián Tovar, D. 1985. Determinacion

del periodo de dispersion de Arceuthobium globosum

grandicaule en la region Central de Mexico. En: Memoria

de los simposia nacional de parasitologia forestal. II y III.

Publicacion Especial 46. Mexico, DF: Secretaria de

Agricultura y Recursos Hidraulicolis: 342–351.

Etheridge, D.E. 1973. Wound parasites causing tree decay in

British Columbia. Forest Pest Leaflet 62. Victoria, BC:

Canadian Department of the Environment, Pacific Forest

Research Centre. 15 p.

Everett, R.; Schelihaas, D.; Spurbeck, D.; Ohison, P.; Keenum,

D.; Anderson, D. 1997. Structure of northern spotted owl

nest stands and their historical conditions on the eastern

slope of the Pacific Northwest Cascades, USA. Forest

Ecology and Management. 94:1–14.

Fairweather, M.L. 1995. Post-treatment biological evaluation of the 1981 Tusayan dwarf mistletoe (DM) control

project. Rep. R3-95-1. Albuquerque, NM: U.S. Department

of Agriculture, Forest Service, Southwest Region, Forest

Pest Management. 6 p.

Felix, L.S. 1970a. The biology of Phoradendron bolleanum

subspecies pauciflorum on Abies concolor. Berkeley: University of California. 194 p. Dissertation.

Felix, L.S. 1970b. Hyperparasitism and autoparasitism in

true mistletoes. Plant Disease Reporter. 54:15.

Ferrell, G.T. 1974. Moisture stress and fir engraver (Coleoptera: Scolytidae) attack in white fir infected by true

mistletoe. Canadian Entomologist. 106:315–318.

Filip, G.M. 1984. Dwarf mistletoe and Cytospora canker

decrease grand fir growth in central Oregon. Forest Science. 30:1071–1079.

Filip, G.M.; Beatty, J.S.; Mathiasen, R.L. 2000. Fir dwarf

mistletoe. Forest Insect and Disease Leaflet 89. [revised]. Washington, DC: U.S. Department of Agriculture, Forest Service. 8 p. [Online]. Available: http://

www.fs.fed.us/r6/nr/fid/firdm.pdf. [2002, June 29].

Filip, G.M.; Colbert, J.J.; Parks, C.A.; Seidel, K.W. 1989.

Effects of thinning on volume growth of western larch

infected with dwarf mistletoe in northeastern Oregon.

Western Journal of Applied Forestry. 4:143–145.

Filip, G.M.; Colbert, J.J.; Shaw, C.G., III; Hessburg, P.F.;

Hosman, K.E. 1993. Influence of dwarf mistletoe and

western spruce budworm on growth and mortality of

Douglas-fir in unmanaged stands. Forest Science. 39:465–

477.

Fisher, J.T. 1983. Water relations of mistletoes and their

hosts. In: Calder, D.M.; Bernhardt, P., eds. The biology of

mistletoes. Sydney: Academic Press: 161–184.

Flora of North America Editorial Committee. 1993. Volume

2, Pteridophytes and gymnosperms. New York: Oxford

University Press. 475 p.

Flora Mesoamericana. 2002. [Homepage of Flora

Mesoamericana, Missouri Botanical Garden, Universidad

Nacional Autonoma de Mexico and The Natural History

Museum of London]. [Online]. Available: http://

www.mobot.org/MOBOT/fm/welcome.html. [2002, July

20].

Forest Health Monitoring Program. 1994. Forest health

monitoring: a national strategic plan. Research Triangle

Park, NC: U.S Department of Agriculture, Forest Service,

Forest Health Monitoring Program. 13 p. [Online]. Available: http://www.na.fs.fed.us/spfo/fhm/index.html. [2002,

June 30].

Forest Health Protection. 2002. Forest insect and disease

conditions in the United States 2000. U.S. Department of

Agriculture, Forest Service. 100 p. [Online]. Available:

http://www.fs.fed.us/foresthealth/annual_i_d_conditions/

index.html. [2002, June 29].
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 99

Forest Health Technology Enterprise Team. 2002. Dwarf

Mistletoe Model. [Homepage of Forest Health Technology

Enterprise Team, Forest Service, U.S. Department of

Agriculture]. [Online]. Available: http://www.fs.fed.us/

foresthealth/technology/dm.html. [2002, June 28].

Forest Insect and Disease Survey. 1994. Forest depletions

caused by insects and diseases in Canada 1982–1987. Inf.

Rep. ST-X-8. Ottawa, Canada: Canadian Forestry Survey,

Forest Insect and Disease Survey. 9 p.

Forest Inventory and Analysis. 2002. [Homepage of Forest

Inventory and Analysis, Forest Service, U.S. Department

of Agriculture]. [Online]. Available: http://wwwfia.fs.fed.us.

[2002, July 8].

Forest Management Service Center. 2000. Forest Vegetation Simulator (FVS). [October 12-last update]. [Homepage

of Forest Health Management Service Center, Forest

Service, U.S. Department of Agriculture]. [Online]. Available: http://www.fs.fed.us/fmsc/fvs/. [2002, June 28].

Forest Service. 2002. [Homepage of Tools for Forest Vegetation Management, Forest Service, U.S. Department of

Agriculture]. [Online]. Available: http://www.fs.fed.us/

vegtools/. [2002, July 7].

Frankel, S.; Adams, D. 1989. Reduction of dwarf mistletoe

with the plant growth regulator ethephon. Forest Pest

Management Rep. 89-1. San Francisco: U.S. Department

of Agriculture, Forest Service, Pacific Southwest Region.

5 p.

Frankel, S.; Dale, J.; Hart, D. 1989. A biological evaluation

of pest conditions at five southern California ski areas,

San Bernardino National Forest. Rep. 89-5. Albany, CA:

U.S. Department of Agriculture, Forest Service, Pacific

Southwest Region. 21 p.

Frazer, J.G. 1930. The golden bough. London: MacMillian

and Company. 12 volumes.

Freeman, C.E.; Worthington, R.D.; Corral, R.D. 1985. Some

floral nectar—sugar compositions from Durango and

Sinaloa, Mexico. Biotropica. 17:309–313.

French, D.W.; Meyer, M.P.; Anderson, R.L. 1968. Control of

dwarf mistletoe on black spruce. Journal of Forestry.

66:359–360.

Funk, A.; Baranyay, J.A. 1973. Additional fungi and gall

disease of dwarf mistletoe swellings in western hemlock.

Canadian Plant Disease Survey. 53:182.

Funk, A.; Smith, R.B. 1981. Potebniamyces gallicola n. sp.,

from dwarf mistletoe infections in western hemlock. Canadian Journal of Botany. 59:1610–1612.

Funk, A.; Smith, R.B.; Baranyay, J.A. 1973. Canker of dwarf

mistletoe swellings on western hemlock caused by Nectrica

funkeliana var. macrospora. Canadian Journal of Forest

Research. 3:71–74.

Gallina, S. 1988. Importancia del injerto (Phoradendron sp.)

para el venado. Southwestern Naturalist. 33:21–25.

García, A.M. 1977. Patología vegetal práctica. Ed. Limusa.

México. 156 p.

Garnett, G.N. 2002. Wildlife use of witches’ brooms induced

by dwarf mistletoe in ponderosa pine forests of northern

Arizona. Flagstaff: Northern Arizona University. Thesis.

Gehring, C.A.; Whitham, T.G. 1992. Reduced mycorrhhizae

on Juniperus monosperma with mistletoe: the influence of

environmental stress and tree gender on a plant parasite

and a plant-fungal mutualism. Oecologia. 89:298–303.

Geils, B.W. 2001a. Dwarf mistletoe. In: Maloy, O.C.; Murray,

T.D., eds. Encyclopedia of plant pathology. New York:

John Wiley & Sons, Inc.: 388–389.

Geils, B.W. 2001b. Leafy mistletoe. In: Maloy, O.C.; Murray,

T.D., eds. Encyclopedia of plant pathology. New York:

John Wiley & Sons, Inc.: 613–616.

Geils, B.W.; Hawksworth, F.G.; Janssen, J.E. 1991. Longevity of ponderosa pine parasitized by southwestern dwarf

mistletoe. Supplement to Bulletin of the Ecological Society of America. 72(2):122.

Geils, B.W.; Mathiasen, R.L. 1990. Intensification of dwarf

mistletoe on southwestern Douglas-fir. Forest Science.

36:955–969.

Gentry, H.S. 1946. Notes on the vegetation of Sierra Surotato

in northern Sinaloa. Bulletin of Torrey Botanical Club.

73:451–462.

Gibson, I.A.S. 1978. Informe sobre una visita a Mexico.

Ciencia Forestal. 3(12):40–53.

Gilbert, J.A. 1984. The biology of dwarf mistletoes

(Arceuthobiumspp.) in Manitoba. Winnipeg: University of

Manitoba. 138 p. Thesis.

Gilbert, J.A. 1988. Some aspects of the reproductive biology

of Arceuthobium americanum in Manitoba. Winnipeg:

University of Manitoba. Dissertation.

Gill, F.S. 1990. Ornithology. New York: W.H. Freeman and

Company. 660 p.

Gill, L.S. 1935. Arceuthobium in the United States. Trans.

Conn. Acadamy Arts and Science: 32:111–245.

Gill, L.S. 1956. Summary of chemical tested for controlling

western species of dwarf mistletoes (Arceuthobium spp.).

In: Proceedings 3rd annual western international forest

disease work conference; 1955 December; Spokane, WA:

18–25.

Gill, L.S.; Hawksworth, F.G 1954. Dwarf mistletoe control in

southwestern ponderosa pine forests under management.

Journal of Forestry. 1954:347–353.

Gill, L.S.; Hawksworth, F.G. 1961. The mistletoes, a literature review. Tech. Bull. 1242. Washington, DC: U.S.

Department of Agriculture. 87 p.

Gleick, J. 1988. Chaos: making a new science. Penguin. 352

p.

Gregg, J.W. 1991. The differential occurrence of the mistletoe, Phoradendron juniperinum, on its host Juniperus

osteosperma in the Western United States. Salt Lake City:

University of Utah, Department of Biology. 78 p. Thesis.

Gregg, T.F.; Hummel, S.S. 2002. Assessing sampling uncertainty in FVS projections using a bootstrap resampling

method. In: Crookston, Nicholas L.; Havis, Richard N.,

comps. Second Forest Vegetation Simulator (FVS) conference; 2002 February 12–14; Fort Collins, CO. Proc.

RMRS-P-25. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Gregg, T.F.; Michaels Goheen, E. 1997. Current vegetation

survey in the Pacific Northwest Region: a direct link to FVS

and the insect and disease extensions. In: Teck, R.; Moeur,

M.; Adams. J., comps. Proceedings: Forest Vegetation Simulator conference; 1997 February 3–7; Fort Collins, CO. Gen.

Tech. Rep. INT-GTR-373. Ogden, UT: U.S. Department of

Agriculture, Forest Service, Intermountain Research Station: 182–190.

Grimble, D.G.; Beckworth, R.C. 1993. Temporal changes in

presence of late instar Mitoura spinetorum (Lycaenidae)

in eastern Oregon. Journal of the Lepidopterists’ Society.

47(4):329–330.

Gutierrez Vilchis, L.H.; Salinas Quinard, R. 1989. Algunos

aspectos fenologicos del muérdago enano Arceuthobium

vaginatum vaginatum (Viscaceae) en el Desierto del Los

Leones, D.F. En: V simposio sobre parasitologia forestal;
100 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

1989 Octobre 4–6; Cd. Juarez, Chih: Sociedad Mexicana

de Entomologia. 43.

Guyon, J.; Munson, S. 1991. Biological evaluation of insect

and disease conditions of the Carson and Bridgeport

Ranger Districts, Toiyabe National Forest. Rep. R4-91-02.

Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Region, Forest Pest Management. 45

p.

Hadfield, J.S. 1999. Douglas-fir dwarf mistletoe infection

contributes to branch breakage. Western Journal of Applied Forestry. 14(1):5–6.

Hadfield, J.S.; Mathiasen, R.L.; Hawksworth, F.G. 2000.

Douglas-fir dwarf mistletoe. Forest Insect and Disease

Leaflet 54. [revised 2000]. Washington, DC: U.S. Department of Agriculture, Forest Service. 9 p.

Hadfield, J.S.; Russell, K. 1978. Dwarf mistletoe management in the Pacific Northwest. In: Scharpf, R.F.; Parmeter,

J.R., Jr., tech. coords. Proceedings of the symposium on

swarf mistletoe control through forest management; 1978

April 11–13; Berkeley, CA. Gen. Tech. Rep. PSW-31.

Berkeley, CA: U.S. Department of Agriculture, Forest

Service, Pacific Southwest Forest and Range Experiment

Station: 73–81.

Hanover, J.W. 1966. Tree improvement for disease resistance in Western United States and Canada. In: Gerhold,

H.D.; Schreiner, E.J.; McDermott, R.E.; Winieski, J.A.,

eds. Breeding pest-resistant trees. Oxford: Pergamon Press:

53–56.

Hansen, D. 1997. Insect and disease conditions in Bryce

Canyon National Park detected during August 1995. Rep.

R4-97-06. Ogden, UT: U.S. Department of Agriculture,

Forest Service, Intermountain Region, Forest Health Protection. 44 p.

Hawksworth, F.G. 1952. Unusual hosts for two southwestern mistletoes. Plant Disease Reporter. 36(6):246.

Hawksworth, F.G. 1961. Dwarf mistletoe of ponderosa pine

in the Southwest. Tech. Bull. 1246. Washington, DC: U.S.

Department of Agriculture. 112 p.

Hawksworth, F.G. 1965. Life tables for two species of dwarf

mistletoe. I. Seed dispersal, interception, and movement.

Forest Science. 11:142–151.

Hawksworth, F.G. 1972. Biological control of the mistletoes.

In: Nordin, V.J., comp. Biological control of forest diseases. Fifteenth Congress of the International Union of

Forestry Research Organizations; 1971 March; Gainesville,

FL. Ottawa, ON: Canadian Forestry Service: 83–92.

Hawksworth, F.G. 1977. The 6 class dwarf mistletoe rating

system. Gen. Tech. Rep. RM–48. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 7 p.

Hawksworth, F.G. 1978a. Biological factors of dwarf mistletoe in relation to control. In: Scharpf, R.F.; Parmeter, J.R.,

Jr., tech. coords. Proceedings of the symposium on dwarf

mistletoe control through forest management; 1978 April

11–13; Berkeley, CA. Gen. Tech. Rep. PSW-31. Berkeley,

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station: 5–

15.

Hawksworth, F.G. 1978b. Intermediate cuttings in mistletoe-infested lodgepole pine and southwestern ponderosa

pine stands. In: Scharpf, R.F.; Parmeter, J.R., Jr., tech.

coords. Proceedings of the symposium on dwarf mistletoe

control through forest management; 1978 April 11–13;

Berkeley, CA. Gen. Tech. Rep. PSW-31. Berkeley, CA:

U.S. Department of Agriculture, Forest Service, Pacific

Southwest Forest and Range Experiment Station: 86–92.

Hawksworth, F.G. 1979. Phoradendron in the Western

United States. In: Drummond, D.B., comp. Proceedings

26th western international forest disease work conference; 1978 October 24–28; Tucson, AZ. Davis, CA: U.S.

Department of Agriculture, Forest Service, Forest Insect

and Disease Management, Methods Application Group:

21–31.

Hawksworth, F.G. 1980. Los muérdagos enanos

(Arceuthobium) y su importancia en la silvicultura de

México. En: Primer simposio nacional sobre parasitologia

forestal; 1980 Febrero 18–19. Uruapan, Michoacán:

Sociedad Mexicana de Entomologia: 207-228.

Hawksworth, F.G. 1983. Mistletoes as forest parasites. In:

Calder, M.; Bernhardt, P., eds. The biology of mistletoes.

Sydney: Academic Press: 317–333.

Hawksworth, F.G. 1987. Taxonomia y distribucion de

Arceuthobium en Mexico y Centro America. En: Simposio

nacional sobre parasitiologia forestal y IV Reunion sobre

Plagas y enfermedades forestales; Octobre 28–30; Durango,

Dgo. Instituto Nacional de Investigaciones Forestales y

Agropecuarias: 559–591.

Hawksworth, F.G. 1991. Coevolution of Mexican white pines

and their dwarf mistletoe parasites. In: Proceedings, symposium on white pine provenances and breeding; August

5–11; Montreal, Quebec. Gen. Tech. Rep. NE-155. Radnor,

PA: U.S. Department of Agriculture, Forest Service, Northeast Forest Experiment Station: 23–28.

Hawksworth, F.G.; Cibrián Tovar, D. 1985. Observaciones

sobre las enfermedades de árboles forestales en el norte de

México y sur de los Estados Unidos. En: Memoria de los

simposia nacionales de parasitologia forestal II y III.

Publicación Especial 46. México DF: Secretaría de

Agricultura y Recursos Hidráulicos, Institito Nacional de

Investigaciones Forestales: 57–66.

Hawksworth, F.G.; Dixon, C.S.; Krebill, R.G. 1983.

Peridermium bethelii: a rust fungus associated with lodgepole pine dwarf mistletoe. Plant Disease. 67:729–733.

Hawksworth, F.G.; Dooling, O.J. 1984. Lodgepole pine dwarf

mistletoe. Forest Insect and Disease Leaflet 18. [revised

2001]. Washington, DC: U.S. Department of Agriculture,

Forest Service. 11 p. [Online]. Available: http://

www.fs.fed.us/r6/rn/fid/fidls/fidl18.htm. [2002, June 29].

Hawksworth, F.G.; Edminster, C.B. 1981. Carlos Bates’

dwarf mistletoe resistant ponderosa pines: a postscript

after half a century. Res. Note RM-412. Fort Collins, CO:

U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station. 4 p.

Hawksworth, F.G.; Geils, B.W. 1985. Vertical spread of

dwarf mistletoe in thinned ponderosa pine in Arizona.

Res. Note. RM-460. Fort Collins, CO: U.S. Department of

Agriculture, Forest Service, Rocky Mountain Forest and

Range Experiment Station. 3 p.

Hawksworth, F.G.; Geils, B.W. 1990. How long do mistletoeinfected ponderosa pines live? Western Journal of Applied

Forestry. 5:47–48.

Hawksworth, F.G.; Geils, B.W. 1996. Biotic associates. In:

Hawksworth, F.G.; Wiens, D. 1996. Dwarf mistletoes:

biology, pathology, and systematics. Agric. Handb. 709.

Washington, DC: U.S. Department of Agriculture, Forest

Service: 73–89.

Hawksworth, F.G.; Graham, D.P. 1963a. Dwarf mistletoe on

spruce in the Western United States. Northwest Science.

37:31–38.

Hawksworth, F.G.; Graham, D.P. 1963b. Spread and intensification of dwarf mistletoe in lodgepole pine reproduction. Journal of Forestry. 61:587–591.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 101

Hawksworth, F.G.; Hinds, T.E. 1964. Effects of dwarf mistletoe on immature lodgepole pine stands in Colorado. Journal of Forestry. 62:27–32.

Hawksworth, F.G.; Hinds, T.E.; Johnson, D.W.; Landis, T.D.

1977a. Silvicultural control of dwarf mistletoe in young

lodgepole pine stands. Tech Rep. R2-10. Lakewood, CO:

U.S. Department of Agriculture, Forest Service, Rocky

Mountain Region, State and Private Forestry. 13 p.

Hawksworth, F.G.; Johnson, D.W. 1989a. Biology and management of dwarf mistletoe in lodgepole pine in the Rocky

Mountains. Gen. Tech. Rep. RM-169. Fort Collins, CO:

U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station. 38 p.

Hawksworth, F.G.; Johnson, D.W. 1989b. Mistletoe control

with ethephon. In: van der Kamp, B.J., comp. Proceedings

36th annual western international forest disease work

conference; 1988 September 19–23; Park City, UT.

Vancouver, BC: University of British Columbia, Department of Forest Sciences: 29–45.

Hawksworth, F.G.; Johnson, D.W. 1993. Protect your trees

from dwarf mistletoe. Gen. Tech. Rep. RM-225. Fort Collins,

CO: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station. 8 p.

Hawksworth, F.G.; Lusher, A.A. 1956. Dwarf mistletoe

survey and control on the Mescalero Apache Reservation,

New Mexico. Journal of Forestry. 54:384–390.

Hawksworth, F.G.; Marsden, M.A. 1990. Permanent plots

for quantifying damage caused by western dwarf mistletoes and their spread and intensification. Res. Note RM498. Fort Collins, CO: U.S. Department of Agriculture,

Forest Service, Rocky Mountain Forest and Range Experiment Station. 6 p.

Hawksworth, F.G.; Moir, W.H.; Janssen, J.E. 1992a. Effects

of dwarf mistletoe in old-growth lodgepole pine stands at

Fraser Experimental Forest, Colorado. In: Kaufmann,

M.R.; Moir, W.H.; Bassett, R.L., tech. coords. Old-growth

forest in the Southwest and Rocky Mountain Regions,

proceedings of a workshop; 1992 March 9–13; Portal, AZ.

Gen. Tech. Rep. RM-213. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain

Forest and Range Experiment Station: 60–65.

Hawksworth, F.G.; Scharpf, R.F. 1978. Literature on the

dwarf mistletoes: damage and control. In: Scharpf, R.F.;

Parmeter, J.R., Jr., tech. coords. Proceedings of the symposium on dwarf mistletoe control through forest management; 1978 April 11–13; Berkeley, CA. Gen. Tech. Rep.

PSW-31. Berkeley, CA: U.S. Department of Agriculture,

Forest Service, Pacific Southwest Forest and Range Experiment Station: 178–179.

Hawksworth, F.G.; Scharpf, R.F., tech coords.1984. Biology

of dwarf mistletoes. Gen. Tech. Rep. RM-111. Fort Collins,

CO: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station: 53–61.

Hawksworth, F.G.; Scharpf, R.G. 1981. Phoradendron on

conifers. Forest Insect and Disease Leaflet 164. Washington, DC: U.S. Department of Agriculture, Forest Service.

7 p.

Hawksworth, F.G.; Scharpf, R.S.; Marosy, M. 1991. European mistletoe continues to spread in Sonoma County.

California Agriculture. 45(6):39–40.

Hawksworth, F.G.; Shaw, C.G., III. 1984. Damage and loss

caused by dwarf mistletoes in coniferous forests of Western North America. In: Wood, R.K.S.; Jellis, G.J., eds.

Plant diseases: infection, damage and loss. Oxford:

Blackwell Scientific Publications. 327 p.

Hawksworth, F.G.; Wicker, E.F.; Scharpf, R.F. 1977b. Fungal parasites of dwarf mistletoes. Gen. Tech. Rep. RM-36.

Fort Collins, CO: U.S. Department of Agriculture, Forest

Service, Rocky Mountain Forest and Range Experiment

Station. 14 p.

Hawksworth, F.G.; Wiens, D. 1965. Arceuthobiumin Mexico.

Brittonia. 17:213–238.

Hawksworth, F.G.; Wiens, D. 1966. Observations on witches’

broom formation, autoparasitism, and new hosts in

Phoradendron. Madroño. 18(7):218–224.

Hawksworth, F.G.; Wiens, D. 1972. Biology and classification of dwarf mistletoes (Arceuthobium). Agric. Handb.

401. Washington, DC: U.S. Department of Agriculture,

Forest Service. 234 p.

Hawksworth, F.G.; Wiens, D. 1977. Arceuthobiumin Mexico:

additions and range extensions. Brittonia. 29:411–418.

Hawksworth, F.G.; Wiens, D. 1989. Two new species, nomenclatural changes, and range extensions in Mexican

Arceuthobium (Viscaceae). Phytologia. 66:3–11.

Hawksworth, F.G.; Wiens, D. 1993a. Viscaceae—mistletoe

family. Journal of Arizona-Nevada Academy of Science.

27(2):241–245.

Hawksworth, F.G.; Wiens, D. 1993b. Viscaceae, mistletoe

family. In: Hickman, J.C., ed. Jepson manual, higher

plants of California. Berkeley: University of California

Press: 1092–1097.

Hawksworth, F.G.; Wiens, D. 1996. Dwarf mistletoes: biology, pathology, and systematics. Agric. Handb. 709. Washington, DC: U.S. Department of Agriculture, Forest Service. 410 p. [Online]. Available: http://www.rms.nau.edu/

publications/ah_709/. [2002, May 2].

Hawksworth, F.G.; Wiens, D.; Nickrent, D.L. 1992b. New

Western North American taxa of Arceuthobium

(Viscaceae). Novon. 2:204–211.

Hawksworth, F.G.; Williams-Cipriani, J.C; Eav, B.B.; Geils,

B.G.; Johnson, R.R.; Marsden, M.A.; Beatty, J.S.; Schubert,

G.D; Robinson, D.C.E. 1995. Dwarf mistletoe impact modeling system: user’s guide and reference manual. Rep.

MAG-95-2. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Forest Pest Management, Methods

Application Group. 120 p. [Online]. Available: http://

www.fs.fed.us/foresthealth/technology/d_mod/dm_ug.pdf.

[2002, June 28].

Hedwall, S.J. 2000. Bird and mammal use of dwarf mistletoe-induced witches’ brooms in Douglas-fir in the Southwest. Flagstaff: Northern Arizona University. 66 p. Thesis.

Hennon, P.E.; Beatty, J.S.; Hildebrand, D. 2001. Hemlock

dwarf mistletoe. Forest Insect and Disease Leaflet 135.

Washington, DC: U.S. Department of Agriculture, Forest

Service. 8 p. [Online]. Available: http://www.fs.fed.us/r6/

nr/fid/fidls/hemlock-dm.pdf. [2002, June 29].

Hernandez Cuevas, L.V. 1991. Los muéradagos

(Loranthaceae) de la region central del Estado de Tlaxcala.

Publ. 4. Tizatlan, Tlaxcala: Jardin Botanico de Tizatlan.

38 p.

Hernandez Cuevas, L.V.; Acosta P., R.; Galinado F., G.L.

1992. Los muérdago enanos (Arceuthobium spp.) en los

bosques de pino del Volcan la Malintzi, Estados del Tlaxcala

y Puebla. Bol. 15. Tizatlan, Tlaxcala, Mexico: Jardin

Botanico Tizatlan. 58 p.

Hessburg, P.F.; Mitchell, R.G.; Filip, G.M. 1994. Historical

and current roles of insects and pathogens in eastern

Oregon and Washington forested landscapes. Gen. Tech.

Rep. PNW-GTR-327. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research

Station. 72 p.
102 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Hildebrand, D.M. 1995. Arceuthobium tsugense—hemlock

dwarf mistletoe: a species of special concern? Portland,

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Region. 54 p.

Hildebrand, D.M; Mathiasen, R.L.; Beatty, J.S. 1997. Mountain hemlock dwarf mistletoe (Arceuthobium tsugense

subsp. mertensianae), 1995 general regional survey in

Washington. Rep. R6-97-01. Portland, OR. U.S Department of Agriculture, Forest Service, Pacific Northwest

Region, Natural Resources, Forest Insects and Diseases.

5 p.

Hinds, T.E.; Hawksworth, F.G. 1965. Seed dispersal velocity

in four dwarf mistletoes. Science. 148:517–519.

Hitchcock, C.; Cronquist, A.; Ownbey, M.; Thompson, J.W.

1964. Vascular plants of the Pacific Northwest. Part 2.

Seattle: University of Washington Press.

Hodge, J.; Humphreys, N.; Van Sickle, G.A. 1994. Surveys of

forest health in managed stands in British Columbia,

1993. Victoria, BC: Department of Natural Resources

Canada, Canadian Forestry Service, Pacific Forestry Centre.

Holling, C.S. 1978. Adaptive environmental assessment and

management. London: John Wiley & Sons.

Holling, C.S. 1992. Cross-scale morphology, geometry, and

dynamics of ecosystems. Ecological Monographs.

62(4):477–502.

Holling, C.S.; Meffe, G.K. 1996. Command and control and

the pathology of natural resource management. Conservation Biology. 10(2):328–337.

Horst, R.K. 2002. Westcott’s plant disease handbook. 6th ed.

Hingham, MA: Kluwer Academic Publishers. +1000 p.

Hreha, A.M.; Weber, D.J. 1979. Distribution of Arceuthobium

divaricatum and Phoradendron juniperinum on the South

Rim of Grand Canyon, Arizona. Southwestern Naturalist.

24(4):625–636.

Hunt, K. 1971. A comparison of kraft pulping of sound and

dwarf mistletoe-infected western hemlock wood. Inf. Rep.

VP-X-78. Vancouver, BC: Canada Department of Fisheries and Forestry, Forest Products Laboratory. 7 p.

Hunt, R.S. 1993. Abies. In: Flora of North America Editorial

Committee. Flora of North America north of Mexico,

Volume 2. New York: Oxford University Press: 354–362.

Hunt, R.S.; Owens, J.N.; Smith, R.B. 1996. Penetration of

western hemlock, Tsuga heterophylla, by the dwarf mistletoe Arceuthobium tsugense, and development of the parasite cortical system. Canadian Journal of Plant Pathology.

18:342–346.

Hunt, R.S.; Smith, R.B. 1978. Natural infection of two new

hosts by hemlock dwarf mistletoe in British Columbia.

Canadian Plant Disease Survey. 58:31–32.

Jensen, K. 2000. Colletotrichum gloeosporioides (Penz.) Penz.

& Sacc. f. sp. Malvae. Fungal disease of mallow. Agriculture and Agri-Food Canada Web. [Online]. Available:

http://res2.agr.ca/lethbridge/weedbio/agents/

acolglom.htm. [2002, July 8].

Johnson, D.W. 1992. Effects of application rate and timing

of ethephon treatments on abscission of ponderosa pine

dwarf mistletoe 4 years following treatment. Tech. Rep.

R2-54. Lakewood, CO: U.S. Department of Agriculture,

Rocky Mountain Region, Forest Pest Management. 9 p.

Johnson, D.W. 1994. Dwarf mistletoe suppression program

activity review. In: Proceedings joint western forest insect

work conference and western international forest disease

work conference. Albuquerque, NM: 102–107.

Johnson, D.W. 1998. Evaluation of Hidden Lakes dwarf

mistletoe pruning study, Routt National Forest, Colorado.

Tech. Rep. R2-61. Lakewood, Co: U.S. Department of

Agriculture, Forest Service, Rocky Mountain Region, Renewable Resources. 8 p.

Johnson, D.W.; Hawksworth, F.G.; Drummond. D.B. 1981.

Yield loss of lodgepole pine stands to dwarf mistletoe in

Colorado and Wyoming National Forests. Plant Disease.

65:437–438.

Johnson, D.W.; Hildebrand, D.M.; Hawksworth, F.G. 1989.

Effects of application rate and timing of ethephon treatments on abscission of ponderosa pine dwarf mistletoe.

Tech. Rep. R2-44. Lakewood, CO: U.S. Department of

Agriculture, Forest Service, Rocky Mountain Region, Forest Insect and Disease Management. 7 p.

Johnson, D.W.; Yarger, L.C.; Minnemeyer, C.D.; Pace, V.E.

1976. Dwarf mistletoe as a predisposing factor for mountain pine beetle attack of ponderosa pine in the Colorado

Front Range. Tech. Rep. R2-4. Lakewood, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain

Region, Forest Insect and Disease Management. 7 p.

Johnson, W.F. 1977. Manager’s handbook for black spruce in

the North-Central States. Gen. Tech. Rep. NC-34. St.

Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station. 18 p.

Julien, M.H. 1982. Biological control of weeds: a world

catalogue of agents and their target weeds. Farnham

Royal Slough, UK: Commonwealth Institute of Biological

Control. 108 p.

Kimmey, J.W.; Mielke, J.L. 1959. Western dwarf mistletoe

on ponderosa pine. Forest Pest Leaflet 40. Washington,

DC: U.S. Department of Agriculture, Forest Service. 7 p.

Kipfmueller, K.F.; Baker, W.L. 1998. Fires and dwarf mistletoe in a Rocky Mountain lodgepole pine ecosystem. Forest

Ecology and Management. 108:77–84.

Knutson, D.M. 1970. Effects of dwarf mistletoe infection on

ponderosa pine xylem. Northwest Science. 44:64.

Knutson, D.M. 1978. Biological and chemical control of

dwarf mistletoe. In: Scharpf, R.F.; Parmeter, J.R., Jr.,

tech. coords. Proceedings of the symposium on dwarf

mistletoe control through forest management; 1978 April

11–13; Berkeley, CA. Gen. Tech. Rep. PSW-31. Berkeley,

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station:

151–155.

Knutson, D.M. 1983. Physiology of mistletoe parasitism and

disease responses in the host. In: Calder, M.; Bernhardt,

P., eds. The biology of mistletoes. Sydney: Academic Press:

295–316.

Knutson, D.M.; Hutchins, A.S. 1979. Wallrothiella

arceuthobiiinfecting Arceuthobium douglasii: culture and

field inoculation. Mycologia. 71:821–828.

Knutson, D.M.; Tinnin, R. 1980. Dwarf mistletoe and host

tree interactions in managed forests of the Pacific Northwest. Gen. Tech. Rep. PNW-111. Portland, OR: U.S Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 19 p.

Knutson, D.M.; Tinnin, R. 1986. Effects of dwarf mistletoe

on the response of young Douglas-fir to thinning. Canadian Journal of Forest Research. 16:30–35.

Kolb, T. E. 2002. Ecofisiologia del parasitismo en el reino

vegetal. En: Saez-Lopez, J.A.; Catalan, P.; Saez, L., eds.

Plantas parasitias del la Peninsula Iberica e Islas Baleares.

Madrid: Mundi-Prensa: 57–85.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 103

Kope, H.H.; Shamoun, S.F. 2000. Mycoflora associates of

western hemlock dwarf mistletoe plants and host swellings collected from southern Vancouver Island, British

Columbia. Canadian Plant Disease Survey. 80:144–147.

Kope, H.H.; Shamoun, S.F.; Oleskevich, C. 1997. First report of Colletotrichum gloeosporioides on Arceuthobium

tsugensesubsp. tsugensein Canada. Plant Disease. 81:1095.

Korstian, C.F.; Long, W.H. 1922. The western yellow pine

mistletoe: effect on growth and suggestions for control.

Bull. 1112. Washington, DC: U.S. Department of Agriculture. 35 p.

Kuijt, J. 1955. Dwarf mistletoes. Botanical Review.

21(10):569–626.

Kuijt, J. 1960a. The distribution of dwarf mistletoes,

Arceuthobium, in California. Madroño. 15:129–139.

Kuijt, J. 1960b. Morphological aspects of parasitism in the

dwarf mistletoes (Arceuthobium). Publ. Botany 30. Berkeley, CA: University of California: 337–436.

Kuijt, J. 1963. Distribution of dwarf mistletoes and their

fungus hyperparasites in Western Canada. National

Museum of Canada Bulletin. 186:134–148.

Kuijt, J. 1964. Critical observations on the parasitism of

New World mistletoes. Canadian Journal of Botany.

42:1243–1278.

Kuijt, J. 1967. On the structure and origin of the seedling of

Psittacanthus schiedeanus(Loranthaceae). Canadian Journal of Botany. 45:1497–1506.

Kuijt, J. 1968. Mutual affinities of Santalalean families.

Brittonia. 20(2):136–147.

Kuijt, J. 1969a. The biology of parasitic flowering plants.

Berkeley: University of California Press. 246 p.

Kuijt, J. 1969b. Observations on Wallrothiella arceuthobii,

a fungus parasite of dwarf mistletoes. Canadian Journal

of Botany. 47:1359–1365.

Kuijt, J. 1970. Seedling establishment in Psittacanthus

(Loranthaceae). Canadian Journal of Botany. 48(4):705–

711.

Kuijt, J. 1975a. The genus Cladocolea (Loranthaceae). Journal Arnold Arboretum. 56(3):265–335.

Kuijt, J. 1975b. The identity of Struthanthus haenkei

(Spirostylis haenkei) (Loranthaceae). Canadian Journal

of Botany. 53:249–255.

Kuijt, J. 1983. Status of the genera Aetanthus and

Psathyranthus (Loranthaceae). Candollea. 38:661–672.

Kuijt, J. 1987. Novelties in Mesoamerican mistletoes

(Loranthacease and Viscacease). Annuals Missouri Botanical Garden. 74:511-532.

Kuijt, J. 1988. Monograph of the Eremolepidaceae. Systematic Botanical Monographs. 18.

Kuijt, J. 1996. Cataphylls and taxonomy in Phoradendron

and Dendrophtthora (Viscaceae). Acta Bot. Neerl. 45:263–

277.

Lamont, B. 1983. Germination of mistletoes. In: Calder, M.;

Bernhardt, P., eds. The biology of mistletoes. Sydney:

Academic Press: 129–143.

Lei, S.A. 1997. Host-parasite relationship between Utah

juniper and juniper mistletoe in the Spring Mountains of

southern Nevada. In: Monsen, S.B.; Stevens, R., comps.

Proceedings, ecology and management of pinyon-juniper

communities within the Interior West; 1997 September

15–18; Provo, UT. RMRS-P-9. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Research Station: 99–104.

Lichter, J.M.; Reid, M.S.; Berry, A.M. 1991. New methods for

control of leafy mistletoe (Phoradendron spp.) on landscape trees. Journal of Arboriculture. 17(5):127–130.

Lightle, P.C.; Hawksworth, F.G. 1973. Control of dwarf

mistletoe in a heavily used ponderosa pine recreation

forest: Grand Canyon, Arizona. Res. Pap. RM-106. Fort

Collins, CO: U.S. Department of Agriculture, Forest

Service, Rocky Mountain Forest and Range Experiment

Station. 22 p.

Lightle, P.C.; Weiss, M.J. 1974. Dwarf mistletoe of ponderosa pine in the Southwest. Forest Insect and Disease

Leaflet 19. [revised 1998]. Washington, DC: U.S. Department of Agriculture, Forest Service. 7 p. [Online]. Available: http://www.fs.fed.us/r6/nr/fid/fidl/fidl19.htm. [2002,

June 29].

Linhart, Y.B.; Synder, M.A.; Gibson, P.J. 1994. Differential

host utilization by two parasites in a population of ponderosa pine. Oecologia. 98:117–120.

Linnane, J.P. 1987. Fort Bayard National Cemetery. File

Rep. 3420, December 7, 1987. Albuquerque, NM: U.S.

Department of Agriculture, Forest Service, Southwestern

Region. 2 p.

Livingston, W.H.; Blanchette, R.A.; Brenner, M.L.; Zuzek,

K.J. 1985. Effective use of ethylene-releasing agents to

prevent spread of eastern dwarf mistletoe on black spruce.

Canadian Journal of Forest Research. 15:872–876.

Livingston, W.H.; Brenner, M.L. 1983. Ethephon stimulates

abscission of eastern dwarf mistletoe aerial shoots on

black spruce. Plant Disease. 67:909–910.

Livingston, W.H.; Brenner, M.L., Blanchette, R.A. 1984.

Altered concentrations of abscisic acid, indole-3-acetic

acid, and zeatin riboside associated with eastern dwarf

mistletoe infections on black spruce. In: Hawksworth,

F.G.; Scharpf, R.F., tech coords. Biology of dwarf mistletoes. Proceedings of the symposium; 1984 August 8; Fort

Collins, CO. Gen. Tech. Rep. RM-111. Fort Collins, CO:

U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station: 53–61.

Lopez De Buen, L.; Ornelas, J.F. 1999. Frugivorous birds,

host selection and the mistletoe Psittacanthus schiedeanus,

in central Veracruz, Mexico. Journal Tropical Ecology.

15:329–340.

Madgiral Sanchez, X. 1967. Contribucion al conocimiento

del la ecologia del los bosques de oyamel (Abies religiosa)

en el Valle de Mexico. Bol. Tech. 18. Instituto Nacional de

Investigaciones Forestales. 94 p.

Maffei, H. 1984. Control of dwarf mistletoe at the Grand

Canyon: results after a third of a century. In: Hunt, R.S.,

comp. Proceedings 32nd annual western international

forest disease work conference; 1984 September 25–28;

Taos, NM: 59–60.

Maffei, H. 1992. Pruning guidelines for dwarf mistletoe

infected ponderosa pine. Bend, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Region,

Deschutes National Forest. 13 p.

Maffei, H. 2002. A methodology for modeling the spatial and

temporal effects of vegetation management alternatives

on late successional habitat in the Pacific Northwest. In:

Crookston, Nicholas L.; Havis, Richard N., comps. Second

Forest Vegetation Simulator (FVS) conference; 2002 February 12–14; Fort Collins, CO. Proc. RMRS-P-25. Ogden,

UT: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Research Station.

Maffei, H.; Arena, J.V. 1993. Forest insect and disease risk

and occurrence maps in GIS: application to integrated

resource analysis. In: Liebhold, A.M.; Barrett, H.R., eds.

Proceedings; spatial analysis and forest pest management; 1992 April 27–30; Mountain Lakes, VA. Gen. Tech.

Rep. NE-175. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment

Station: 8–14.
104 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Maffei, H.; Beatty, J. 1988. Changes in the incidence of dwarf

mistletoes over 30 years in the Southwest. In: van der

Kamp, B.J., comp. Proceedings 36th annual western international forest disease work conference; 1988 September 19–23; Park City, UT. Vancouver, BC: University of

British Columbia, Department of Forest Science: 88–90.

Maffei, H.; Stone, J.; David, L.; Gregg, T.; Adams, J.; Geils,

B. 1999. Development of the understory enhancement for

the dwarf mistletoe spread and intensification model. In:

Trummer, L., comp. Proceedings 46th western international forest disease work conference; 1998 September

28–Ocober 2; Reno, NV. Anchorage, AK: U.S. Department

of Agriculture, Forest Service, State and Private Forestry:

37–46.

Magasi, L.P. 1984. Forest conditions in the Maritimes in

1983. Inf. Rep. M-X-19. Canadian Forest Service,

Maritimes Forest Research Centre. 49 p.

Makowski, R.M.D.; Mortensen, K.1992. The first

mycoherbicide in Canada: Colletotrichum gloeosporioides

f. sp. malvae for round-leaved mallow. In: Delfosse, E.S.;

Scottt, R.R., eds. Proceedings of the first international

weed control congress; Melbourne, Australia: 298–300.

Mantiri, F.; Samuels, G.J.; Rahe, J.E.; Honda, B. 2001.

Phytogenetic relationships in Neonectria species having

Cylindrocarpon anamorphs inferred from mitochondrial

ribosomal DNA sequences. Canadian Journal of Botany.

79:334–340.

Mark, W.R.; Hawksworth, F.G. 1974. How important are

bole infections in spread of ponderosa pine dwarf mistletoe? Journal of Forestry. 72:146–147.

Mark, W.R.; Hawksworth, F.G.; Oshima, N. 1976. Resin

disease: a new disease of lodgepole pine dwarf mistletoe.

Canadian Journal of Forest Research. 6:415–424.

Marler, M.; Pedeersen, D.; Michell-Olds, T.; Callaway, R.M.

1999. A polymerase chain reaction method for detecting

dwarf mistletoe infection in Douglas-fir and western larch.

Canadian Journal of Forest Research. 29:1317–1321.

Marsden, M.A.; Filip, G.M.; Hessburg, P.F. 1990. Using the

forest timber inventory for sampling the occurrence of

pests on interior Douglas-fir. In: Baumgartner, D.M.;

Lotan, J.E., comp. Interior Douglas-fir: the species and its

management; 1990 February 27–March 1; Spokane, WA.

Pullman, WA: Washington State University, Cooperative

Extension: 109–113.

Marsden, M.A.; Shaw, C.G., III; Morrison, M. 1993. Simulation of management options for stands of southwestern

ponderosa pine attacked by Armillaria root disease and

dwarf mistletoe. Res. Pap. RM-308. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 5 p.

Marshall, J.D.; Dawson, T.E.; Ehleringer, J.R. 1993. Gender-related differences in gas exchange are not related to

host quality in the xylem-tapping mistletoe Phoradendron

juniperinum (Viscaceae). American Journal of Botany.

80(6):641–645.

Marshall, J.D.; Ehleringer, J.R. 1990. Are xylem-tapping

mistletoes partially heterotrophic? Oecology. 84:244–248.

Marshall, J.G.; Dawson, T.E.; Ehleringer, J.R. 1994. Integrated nitrogen, carbon, and water relations of a xylemtapping mistletoe following nitrogen fertilization of the

host. Oecologia. 100:430–438.

Marshall, K. 1996. Management implications of maintaining Douglas-fir dwarf mistletoe brooms for wildlife use. In:

Beatty, J.S., comp. Proceedings 44th western international forest disease work conference; 1996 September

16–20; Hood River, OR. Sandy, OR: U.S. Department of

Agriculture, Forest Service, Westside Forest Insects and

Diseases Technical Center: 72–74.

Marshall, K.; Filip, G.M. 1999. The relationship of Douglasfir dwarf mistletoe (Arceuthobium douglasii) to stand

conditions and plant associations in the Southern Cascade

Mountains, Oregon. Northwest Science. 73(4):301–311.

Marshall, K.; Mamone, M.S.; Barclay, R.B. 2000. A survey of

northern spotted owl nests in Douglas-fir dwarf mistletoe

brooms in the Siskiyou Zone, Rouge River National Forest

and Ashland Resource Area, Medford District, Bureau of

Land Management. Rep. SWOFIDSC-00-3. U.S. Department of Agriculture, Forest Service, Southwest Oregon

Forest Insect and Disease Service Center. 12 p.

Martínez, M. 1948. Los Pinos Mexicanos. 2d ed. México. 361

p.

Martínez Gonzáles, F. 1983. Las plagas forestales en Mexico.

En: Primeras Jornadas Forestales Hispano-Mexicanas.

Publicación Especial 41. México DF: Secretaría de

Agricultura y Recursos Hidráulicos, Instituto Nacional de

Investigaciones Forestales: 123–128.

Mathiasen, R.L. 1978. Seed germination and infection techniques for three southwestern dwarf mistletoes. Plant

Disease Reporter. 62:64–67.

Mathiasen, R.L. 1982. Taxonomic studies of dwarf mistletoes (Arceuthobium spp.) parasitizing Pinus strobiformis.

Great Basin Naturalist. 42:120–127.

Mathiasen, R.L. 1984. Improving assessment of dwarf mistletoe infection in southwestern ponderosa pine stands. Arizona Forestry Notes 21. Flagstaff: Northern Arizona University, School of Forestry. 7 p.

Mathiasen, R.L. 1989. Management of dwarf mistletoe using uneven-aged cutting methods. In: Proceedings 9th

intermountain regional silviculture workshop; 1989 September 18–21; Jackson Hole, WY. Ogden UT: U.S. Department of Agriculture, Forest Service, Intermountain Region: 186–194.

Mathiasen, R.L. 1994. Natural infection of new hosts by

hemlock dwarf mistletoe. Res. Note RM-RN-530. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 6 p.

Mathiasen, R.L. 1996. Dwarf mistletoes in forest canopies.

Northwest Science. 70:61–71

Mathiasen, R.L. 1998a. Comparative susceptibility of conifers to larch dwarf mistletoe in the Pacific Northwest.

Forest Science. 44(4):559–568.

Mathiasen, R.L. 1998b. Infection of young western larch by

larch dwarf mistletoe in northern Idaho and western

Montana. Western Journal of Applied Forestry. 13(2):41–

46.

Mathiasen, R.L. 1999. Comparative susceptibility of subalpine firs to Douglas-fir dwarf mistletoe. Canadian Journal

of Plant Pathology. 21:45–51.

Mathiasen, R.L.; Beatty, J.S.; Hildebrand, D.M. 1995a. First

report of larch dwarf mistletoe on Pacific silver fir and on

mountain hemlock in the Cascade Mountains, Washington. Plant Disease. 79:1249.

Mathiasen, R.L.; Beatty, J.S.; Hildebrand, D.M. 1996a. First

report of lodgepole pine dwarf mistletoe on subalpine fir.

Plant Disease. 80:342.

Mathiasen, R.L.; Beatty, J.S.; Melgar, J. 2000a. First report

of Arceuthobium hondurense on Pinus tecunumanii. Plant

Disease. 84(3):372–373.

Mathiasen, R.L.; Beatty, J.S.; Pronos, J. 2002a. Pinyon pine

dwarf mistletoe. Forest Insect and Disease Leaflet 174.

Washington, DC: U.S. Department of Agriculture, Forest

Service. 7 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 105

Mathiasen, R.L.; Daugherty, C.M. 2001. Susceptibility of

foxtail pine and western white pine to limber pine dwarf

mistletoe in northern California. Western Journal of Applied Forestry. 16(2):58–60.

Mathiasen, R.L.; Geils, B.W.; Carlson, C.E.; Hawksworth,

F.G. 1995b. Larch dwarf mistletoe not found on alpine

larch. Res. Note. RM-RN-533. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 4 p.

Mathiasen, R.L.; Hawksworth, F.G. 1988. Dwarf mistletoes

on western white pine and whitebark pine in northern

California and southern Oregon. Forest Science. 34:429–

440.

Mathiasen, R.L.; Hawksworth, F.G.; Edminster, C.B. 1990.

Effects of dwarf mistletoe on growth and mortality of

Douglas-fir in the Southwest. Great Basin Naturalist.

50:173–179.

Mathiasen, R.L.; Hoffman, J.T.; Guyon, J.C.; Wadleigh, L.L.

1996b. Comparison of two roadside survey procedures for

dwarf mistletoes on the Sawtooth National Forest, Idaho.

Great Basin Naturalist. 56:129–134.

Mathiasen, R.L.; Howell, B. 2002. First report of

Arceuthobium hawksworthii in Honduras. Plant Disease.

2002(7):815.

Mathiasen, R.L.; Marshall, K. 1999. Dwarf mistletoe diversity in the Siskiyou–Klamath Mountain Region. Natural

Areas Journal. 19:379–385.

Mathiasen, R.L.; Melgar, J.; Beatty, J.; Parks, C. 2000b.

First report of Psittacanthus angustifoliuson Pinus oocarpa

and Pinus maximinoi. Plant Disease. 84(2):203.

Mathiasen, R.L.; Nickrent, D.; Daugherty, C. 2002b. First

report of Arceuthobium hondurense in Oaxaca, Mexico.

Plant Disease. 86:72.

Mathiasen, R.L.; Nickrent, D.; Parks, C.; Beatty, J.; Sesnie,

S. 2001. First report of Arceuthobium hondurense in

Mexico. Plant Disease. 85:444.

Mathiasen, R.L.; Parks, C.; Beatty, J.; Sesnie, S. 2000c. First

report of Psittacanthus angustifolius on pines in Mexico

and Guatemala. Plant Disease. 84:808.

Mathiasen, R.L.; Parks, C.G.; Geils, B.W.; Beatty, J.S. 1998.

Notes on the distribution, host range, plant size, phenology, and sex ratio of two rare dwarf mistletoes from

Central America: Arceuthobium hawksworthii and A.

hondurense. Phytologia. 84(2):154–164.

Mathiasen, R.L.; Shaw, D.C. 1998. Adult sex ratio of

Arceuthobium tsugense in six severely infected Tsuga

heterophylla. Madroño. 45(3):210–214.

McCambridge, W.F.; Hawksworth, F.G.; Edminster, C.B.;

Laut, J.G. 1982. Ponderosa pine mortality resulting from

a mountain pine beetle outbreak. Res. Pap. RM-235. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 7 p.

McGregor, M.D. 1978. Management of mountain pine beetle

in lodgepole pine stands in the Rocky Mountain area. In:

theory and practice of mountain pine beetle management

in lodgepole pine forests, proceedings of the symposium;

1978 April 25–27; Pullman, WA. Moscow, ID: University

of Idaho Press: 129–139.

McHenry, D.E. 1934. Does mistletoe kill trees at Grand

Canyon. Grand Canyon Nature Notes. 9:305–309.

McMinn, H.E. 1939. Loranthanthaceae, mistletoe family, 1.

Phoradendron. In: An illustrated manual of California

shrubs. J.W. Stacey, Inc.: 89–92.

Meinecke, E.P. 1912. Parasitism of Phoradendron

juniperinum libocedri Engelm. Reprinted from: Proceedings of the Society of American Foresters. Vol. VII, No.

1.1912 March. Washington, DC: Judd & Dettweiler, Inc.:

35–41.

Melgar, J.; Berrios, M.; Mathiasen, R.; Howell, B. 2001. First

report of Psittacanthus angustifolius on Psidium guineese

and Pinus tecunumanii in Honduras. Plant Disease.

85:1120.

Melgar, J.; Mathiasen, R.; Howell, B. 2001. First report of

Arceuthobium globosum subsp. grandicaule in Honduras.

Plant Disease. 85:563.

Merrill, L.M.; Byler, J.W.; Corse, T.; Reedy, T.; Hall. L.D.

1988. An evaluation of the effectiveness of dwarf mistletoe

suppression in uneven-aged stands on the Flathead Indian Reservation. Forest Pest Management Rep. 89-6.

Missoula, MT: U.S. Department of Agriculture, Forest

Service, Northern Region. 9 p.

Merrill, L.M.; Hawksworth, F.G.; Johnson, D.W. 1985. Evaluation of a roadside survey procedure for dwarf mistletoe on

ponderosa pine in Colorado. Plant Disease. 69:572–573.

Mielke, J.L. 1959. Infection experiments with Septogloeum

gillii, a fungus parasitic on dwarf mistletoe. Journal of

Forestry. 57:925–926.

Missouri Botanical Garden. 2002. VAST nomenclatural database (revision 1.5). [Homepage of Missouri Botanical

Garden]. [Online]. Available: http://mobot.mobot.org/W3T/

Search/vast.html [2002, May 2].

Mistletoe Center. 2002. [Homepage of The Mistletoe Center,

Rocky Mountain Research Station, Forest Service, U.S.

Department of Agriculture]. [Online]. Available: http://

www.rms.nau.edu/mistletoe. [2002, July 2].

Monning, E.; Byler, J. 1992. Forest health and ecological

integrity in the Northern Rockies. Rep. 92-7. Missoula,

MT: U.S. Department of Agriculture, Forest Service, Northern Region, Forest Pest Management. 19 p.

Moody, B.H., comp. 1992. Forest insect and disease conditions in Canada, 1989. Ottawa, ON: Forestry Canada,

Forest Insect and Disease Survey. 117 p.

Moody, B.H.; Amirault, P.A. 1992. Impacts of major pests on

forest growth and yield in the Prairie Provinces and

Northwest Territories: a literature review. Inf. Rep. NORX-324. Edmonton, AB: Forestry Canada, Northern Forestry Centre. 35 p.

Moore, M. 1979. Mistletoe. In: Medicinal plants of the

Mountain West. Santa Fe: Museum of New Mexico Press.

107–109.

Morris, M.J.; Wood, A.R.; Den Breeyen, A.D. 1998. Development and registration of a fungal inoculant to prevent regrowth of cut wattle tree stumps in South Africa, and brief

overview of other bioherbicide projects currently in

progress. In: Main, D.S.; Burge, M.N., eds. IV international bioherbicides workshop, programme and abstracts;

1998 August 6–7; University of Strathclyde, Glasgow,

Scotland: 15.

Mortenson, K. 1998. Biological control of weeds using microorganisms. In: Boland, G.J.; Kuykendall, L.D., eds. Plantmicrobe interactions and biological control. New York:

Marcel Dekker Inc.: 223–248.

Muir, J.A. 1967. Occurrence of Colletotrichum gloeosporioides

on dwarf mistletoe (Arceuthobium americanum) in Western Canada. Plant Disease Reporter. 51:789–799.

Muir, J.A. 1970. Dwarf mistletoe spread in young lodgepole

pine stands in relation to density of infection sources. BiMon. Res. Notes. 26:49. Canada Department of Fisheries

and Forests.
106 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Muir, J.A. 1973. Cylindrocarpon gillii, a new combination

for Septogloeum gillii on dwarf mistletoe. Canadian Journal of Botany. 51:1997–1998.

Muir, J.A. 1977. Effects of the fungal hyperparasite

Colletotrichum gloeosporioides of dwarf mistletoe

(Arceuthobium americanum) on young lodgepole pine.

Canadian Journal of Forest Research. 7:579–583.

Muir, J.A. 1989. European leafy mistletoe. In: Pest Management Progress 8. Victoria BC: British Columbia Ministry of Forests and Lands.

Muir, J.A. 2002. Survey of dwarf mistletoe (Arceuthobium

americanum Nutt. ex Engelm.). Canadian Plant Disease

Survey 82: 133–135. [Online]. Available: http://

res2.agr.gc.ca/london/pmrc/english/report/

disease2002.html. [2002, August 2].

Muir, J.A., ed. 1993. Proceedings of a workshop on management of hemlock dwarf mistletoe. BC Pest Management

Rep. 4. Victoria, BC: British Columbia Ministry of Forests.

58 p.

Muir, J.A.; Robins, J.K. 1973. Detection of dwarf mistletoe of

jack pine on aerial photographs. Plant Disease Reporter.

57:951–954.

Munns, E.N. 1919. Effect of fertilization on the seed of

Jeffrey pine. Plant World. 22:138–144.

Muraro, S. J. 1978. Prescribed fire—a tool for the control of

dwarf mistletoe in lodgepole pine. In: Scharpf, R.F.;

Parmeter, J.R., Jr., tech. coords. Proceedings of the symposium on dwarf mistletoe control through forest management; 1978 April 11–13; Berkeley, CA. Gen. Tech. Rep.

PSW-31. Berkeley, CA: U.S. Department of Agriculture,

Forest Service, Pacific Southwest Forest and Range Experiment Station: 124–127.

Mushtaque, M.; Baloch, G.M. 1979. Possibilities of biological

control of mistletoes, Loranthus spp., using oligophagous

insects from Pakistan. Entomophaga. 24:73–81.

Myers, C.A.; Hawksworth, F.G.; Stewart, J.L. 1971. Simulating yields of managed, dwarf-mistletoe-infected lodgepole pine stands. Res. Pap. RM-72. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 15 p.

Myren, D.T.; Gross, H.L. 1977. Distribution of organisms

causing important forest tree diseases in Ontario based on

collections recorded by the Forest Insect and Disease

Survey Unit. Rep. O-X-262. Sault Ste. Marie, ON: Department of Fisheries and the Environment, Canadian Forestry Service, Great Lakes Forestry Research Centre. 136

p.

North American Forestry Commission (NAFC). 2002.

[Homepage of the North American Forestry Commission].

[Online]. Available: http://www.fs.fed.us/global/nafc/

welcome.html. [2002, July 2].

Najera Martinez, F.; Gonzales Gandarilla, J.F.; Cibrian

Tovar, J. 1987. Los muérdago enanos en el Estado de

Durango en Mexico. En: IV simposio nacional sobre

parasitologia forestal y IV reunion sobre enfermedades

forestales; 1987 Octobre 28–30; Durango, Dgo. Instituto

Nacional de Investigaciones Forestales y Agropecuarias:

592–603.

Nebeker, T.E.; Schmitz, R.F.; Tisdale, R.A.; Hobson, K.R.

1995. Chemical and nutritional status of dwarf mistletoe,

Armillaria root rot, and comandra blister rust infected

trees which may influence tree susceptibility to bark

beetle attack. Canadian Journal Botany. 73:360–369.

Nevill, R.J.; Wood, C. 1995. Hemlock dwarf mistletoe and

decay organisms associated with western hemlock and

amabilis fir at the Montane Alternative Silvicultural

Systems (MASS) research site in British Columbia. In:

Arnott, J.T.; Besse, W.J.; Mitchell, A.K.; Peterson, J., eds.

Montane Alternative Silvicultural Systems (MASS), proceedings of a workshop; 1995 June 7–8; Courtenay, BC.

FRDA Rep. 238. BC: Canada-BC Partnership Agreement

of Forest Research Development. 122 p.

Nicholls, T.H.; Egeland, L.; Hawksworth, F.G.; Johnson,

D.W.; 1987a. Control of dwarf mistletoe with ethephon. In:

Cooley, S.J., comp. Proceedings 34th annual western

international forest disease work conference; 1986 September 8–12; Juneau, AK. Portland, OR: U.S. Department

of Agriculture, Forest Service, Forest Pest Management:

78–85.

Nicholls, T.H.; Egeland, L.; Hawksworth, F.G.; Johnson,

D.W.; Robbins, M.K. 1987b. Control of dwarf mistletoe

with a plant growth regulator. In: Proceedings of a

technical conference on management of subalpine forests: building on 50 years of research; 1987 July 6–9;

Silver Creek, CO. Gen. Tech. Rep. RM-149. Fort Collins,

CO: U.S. Department of Agriculture, Forest Service,

Rocky Mountain Forest and Range Experiment Station:

154–156.

Nicholls, T.H.; Hawksworth, F.G.; Merrill, L.M. 1984. Animal vectors of dwarf mistletoe, with special reference to

Arceuthobium americanum on lodgepole pine. In:

Hawksworth, F.G.; Scharpf, R.F., tech. coords. Biology of

dwarf mistletoes: proceedings of the symposium; 1984

August 8; Fort Collins, CO. Gen. Tech. Rep. RM-111. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 102–110.

Nickrent, D.L. 2002. [June 8, last update]. Parasitic plant

connection. [Homepage of D. L. Nickrent, Department of

Plant Biology, Southern Illinois University at

Carbondale]. [Online]. Available: http://

www.science.siu.edu/parastic-plant/index.html. [2002,

July 5].

Ostry, M.E.; Nicholls, T.H. 1979. Eastern dwarf mistletoe on

black spruce. Forest Insect and Disease Leaflet 158. Washington, DC: U.S. Department of Agriculture, Forest Service. 7 p.

Page, J.M. 1981. Drought accelerated parasitism of conifers

in the mountain ranges of northern California, USA.

Environmental Conservation. 8:217–226.

Parker, A.J.; Parker, K.C. 1994. Structural variability of mature lodgepole pine stands on gently sloping terrain in Taylor

Park Basin, Colorado. Canadian Journal of Forest Research.

24:2020–2029.

Parker, A.K. 1970. Growth of Wallrothiella arceuthobii on

artificial media. Canadian Journal of Botany. 48:837–

838.

Parker, T.J. 2001. Bird communities in dwarf mistletoe

infested ponderosa pine forests. Flagstaff, AZ: Northern

Arizona University. 90 p. Thesis.

Parks, C.; Bull, E.; Tinnin, R.O.; Shepherd, J.F.; Blumton, A.K.

1999a. Wildlife use of dwarf mistletoe brooms in Douglas-fir

in northeast Oregon. Western Journal of Applied Forestry.

14:100–105.

Parks, C.; Conklin, D.A.; Bednar, L.; Maffei, H. 1999b.

Woodpecker use and fall rates of snags created by killing

ponderosa pine infected with dwarf mistletoe. Res. Pap.

PNW-RP-515. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station.

11 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 107

Parks, C.; Hoffman, J.T. 1991. Control of western dwarf

mistletoe with the plant-growth regulator ethephon. Res.

Note PNW-RN-506. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Forest and

Range Experiment Station. 4 p.

Parks, C.G.; Bull, E.L. 1997. American martin use of rust

and dwarf mistletoe brooms in northeastern Oregon.

Western Journal of Applied Forestry. 12(4):131–133.

Parmeter, J.R., Jr. 1978. Forest stand dynamics and ecological factors in relation to dwarf mistletoe spread, impact,

and control. In: Scharpf, R.F.; Parmeter, J.R., Jr., tech.

coords. Proceedings of the symposium on dwarf mistletoe

control through forest management; 1978 April 11–13;

Berkeley, CA. Gen. Tech. Rep. PSW-31. Berkeley, CA:

U.S. Department of Agriculture, Forest Service, Pacific

Southwest Forest and Range Experiment Station: 16–30.

Parmeter, J.R., Jr.; Hood, J.R.; Scharpf, R.F. 1959.

Colletotrichum blight of dwarf mistletoe. Phytopathology.

49:812–815.

Parmeter, J.R., Jr.; Scharpf, R.F. 1963. Dwarf mistletoe or

red fir and white fir in California. Journal of Forestry.

61:371–374.

Parmeter, J.R., Jr.; Scharpf, R.F. 1982. Stem infection by

dwarf mistletoe in California true firs. Res. Pap. PSW165. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. 7 p.

Pearson, G.A. 1912. The influence of age and condition of the

tree upon seed production in western yellow pine. Circ.

196. Washington, DC: U.S. Department of Agriculture,

Forest Service. 11 p.

Perry, J.P., Jr. 1991. The Pines of Mexico and Central

America. Portland, OR: Timber Press. 231 p.

Piirto, D.D.; Crews, D.L.; Troxell, H.E. 1974. The effects of

dwarf mistletoe on the wood properties of lodgepole pine.

Wood and Fiber. 6:26–35.

Plant Names Project. 1999. [Homepage of International

Plant Names Index]. [Online]. Available: http://

www.ipni.org. [2002, May 1].

Pousette, J. 1991. Impact of dwarf mistletoe on growth in

lodgepole pine forests. Internal Rep. PM-PB-60. Victoria,

BC: British Columbia Ministry of Forests. 12 p.

Power, J.M.; D’Eon, S.P. 1991. Quantifying pest-caused

forest depletion using geographic information systems

and database technologies. Inf. Rep PI-X-105. Petawawa,

ON: Forestry Canada, Petawawa National Forestry Institute. 17 p.

Pronos, J. 1995. Can dwarf mistletoe be eliminated from

recreation areas? In: Proceedings 43rd western international forest disease work conference; Whitefish, MT: 88–

92.

Quick, C.R. 1963. Chemical control. Unit IX. Leafy mistletoes (Phoradendron spp.). In: Proceedings 10th western

international forest disease work conference; 1962 October 15–19; Victoria, Alberta (sic): 97–98.

Quick, C.R. 1964. Experimental herbicide control of dwarf

mistletoe on some California conifers. Res. Note PSW-47.

Berkeley, CA: U.S. Department of Agriculture, Forest

Service, Pacific Southwest Forest and Range Experiment

Station. 9 p.

Ramsfield, T.D. 2002. Investigations into a biological control

strategy for lodgepole pine dwarf mistletoe. Vancouver,

BC: University of British Columbia, Graduate Studies,

Department of Forest Sciences. 185 p. Ph.D. Thesis.

Ramsfield, T.D.; Shamoun, S.F.; van der Kamp, B.J. 1999.

Fungal parasites of lodgepole pine dwarf mistletoe in

British Columbia. Canadian Journal of Plant Pathology.

21:204.

Reich, R.M.; Lundquist, J.; Bravo, V.A. 2000. Spatial relationship of resident and migratory birds and canopy openings in disease ponderosa pine forests. Environmental

Modeling and Software. 15:189–197.

Reich, R.M.; Mielke, P.W.; Hawksworth, F.G. 1991. Spatial

analysis of ponderosa pine trees infected with dwarf

mistletoe. Canadian Journal of Forest Research. 21:1808–

1815.

Reid, N.; Florentino Caldera, H.; Marmolejo M., J.G. 1987.

Intensidad de infeccion de cuatro especies de muérdago

enano Arceuthobium en El Cerro El Potosi, Nuevo Leon.

En: IV simposio nacional sobre parasitologia forestal y IV

reunion sobre y enfermedades forestales; 1987 Octobre

28–30; Durango, Dgo. Instituto Nacional de

Investigacciones Forestales y Agropecuarias: 613–624.

Reid, N.; Standford Smith, M.; Yan, Z. 1995. Ecology and

population biology of mistletoes. In: Lowman, M.D.;

Nadcarni, N.M., eds. Forest canopies. San Diego, CA:

Academic Press: 285–310.

Reid, N.; Yan, Z. 2000. Mistletoes and other phanerogams

parasitic on eucalypts. In: Keane, P.J.; Kile, G.A.; Podger,

F.D.; Brown, B.N., eds. Diseases and oathogens of

eucalypts: 353–383.

Reséndiz Reséndiz, J.; Martínez Hernández, J.F.; Carbajal

Vera, I.J. 1989. Contribucion al conocimientodel la

distribucion geografica, altitudinal y hospederos del

muérdago verdadero Psittacanthus sp. En: V simposio

sobre parasitologia forestal; 1989 Octobre 4–6; Juarez,

Chihuahua. México, DF: Sociedad Mexicana de Entomogia,

AC: 46.

Rey, L.; Sadik, A.; Fer, A.; Renaudin, S. 1991. Trophic

relations of the dwarf mistletoe Arceuthobium oxycedri

with its host Juniperus oxycedrus. Journal of Plant Physiology. 138:411–416.

Rey, L.; Sadik, A.; Fer, A.; Renaudin, S. 1992. Etude de

quelques aspects du metolisme carbone et azote chez

l’Arceuthobium oxycedri, gui nain du genevrir. Canadian

Journal of Botany. 70:1709–1216.

Reynolds, R.T.; [and others]. 1992. Management recommendations for the northern goshawk in the Southwestern

United States. Gen. Tech. Rep. RM-217. Fort Collins, CO:

U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station. 90 p.

Richardson, K.S.; van der Kamp, B.J. 1972. The rate of

upward advance and intensification of dwarf mistletoe on

immature western hemlock. Canadian Journal of Forest

Research. 2:313–316.

Ridings, W.H. 1986. Biological control of stranglervine in

citrus, a researcher’s view. Weed Science. Supplement 1:

31-32.

Ringnes, D.B.; Stover, P.; Scharpf, R.F. 1996. Dwarf mistletoe resistance in ponderosa pine: selection and testing

protocols. In: Moreno, M.T.; Cubero, J.I.; Berner, D.;

Musselman, L.J.; Parker, C., eds. Advances in parasitic

plant research. Proceedings 6th parasitic weed symposium. Cordoba, Spain: Junta de Andalucia, Consejeria de

Agricultura y Pesca: 690–696.

Robbins, K.; Johnson, D.W.; Hawksworth, F.G.; Nicholls,

T.H. 1989. Aerial application of ethephon is ineffective in

controlling lodgepole pine dwarf mistletoe. Western Journal of Applied Forestry. 4:27–28.

Robins, J.K. 1972. Dwarf mistletoe aerial surveying method:

results of preliminary trials in the Athabasca Forest.

Misc. Rep. NOR-Y-10. Edmonton, AB: Department of the

Environment, Canadian Forest Service, Northern Forest

Research Center. 6 p.
108 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Robinson, D.C.E.; Geils, B.W.; Muir, J. 2002. A spatial

statistical model for the spread of dwarf mistletoe within

and between stands. In: Crookston, Nicholas L.; Havis,

Richard N., comps. Second Forest Vegetation Simulator

(FVS) conference; 2002 February 12–14; Fort Collins, CO.

Proc. RMRS-P-25. Ogden, UT: U.S. Department of

Agriculture, Forest Service, Rocky Mountain Research

Station. [Presentation online]. Available: http://

www.essa.com/downloads/FVS_2002_DMSummary.pdf.

[2002, September 5].

Rodriguez Angeles, A. 1983. Muérdago enano sobre Abies,

Pinus y Pseudotsugae de Mexico. Ciencia Forestal.

8(45):7–45.

Roeser, J. 1926. The importance of seed source and the

possibilities of forest tree breeding. Journal of Forestry.

24:38–51.

Rolston, H., III. 1994. Conserving natural value. New York:

Columbia University Press. 259 p.

Roth, L.F. 1953. Pine dwarf mistletoe on the Pringle Falls

Experimental Forest. Res. Note. 91. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 3 p.

Roth, L.F. 1971. Dwarf mistletoe damage to small ponderosa

pines. Forest Science. 17:373–380.

Roth, L.F. 1974a. Resistance of ponderosa pine to dwarf

mistletoe. Silvae Genetica. 23:116–120.

Roth, L.F. 1974b. Juvenile susceptibility of ponderosa pine

to dwarf mistletoe. Phytopathology. 64:689–692.

Roth, L.F. 1978. Genetic control of dwarf mistletoe. In:

Scharpf, R.F.; Parmeter, J.R., Jr., tech. coords. Symposium proceedings: dwarf mistletoe control through forest

management;. 1978 April 11–13; Berkeley, CA. Gen. Tech.

Rep. PSW-31. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest Range and

Experiment Station: 69–72.

Roth, L.F. 2001. Dwarf mistletoe-induced mortality in northwest ponderosa pine growing stock. Western Journal of

Applied Forestry. 16(3):136–141.

Roth, L.F.; Barrett. J.W. 1985. Response of dwarf mistletoeinfested ponderosa pine to thinning: 2. Dwarf mistletoe

propagation. Res. Pap. PNW-331. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 20 p.

Ruhland, B.M.; Calvin, C.L. 2001. Morphological aspects of

seedling establishment in four temperate region

Phoradendron (Viscaceae) species. Madroño. 48(2):79-89.

Ryan, M.G. 1990. Growth and maintenance respiration in

stems of Pinus contorta and Picea engelmannii. Canadian

Journal of Forest Research. 20:48–57.

Safranyik, L; Nevill, R.; Morrison, D. 1998. Effects of stand

density management on forest insects and diseases. Tech.

Trans. Note 12. Victoria, BC: Natural Resources Canada,

Canadian Forest Service, Pacific Forestry Centre. 4 p.

Sala, A.; Carey, E.; Callaway, R. 2001. Dwarf mistletoe

affects whole-tree water relations of Douglas-fir and western larch primarily through changes in leaf to sapwood

ratios. Oecologia. 126:42–52.

Salas, P.M.A. 1988. Aves dispersoras de muérdago. Informe

Técnico. Secretaría de Agricultura y Recursos Hidráulicos,

Instituto Nacional de Investigaciones Forestales y

Agropecularis.

Schaffer, B.; Hawksworth, F.G.; Beemsterboer, P. 1983a.

Effects of dwarf mistletoe and vigor classes on electrical

resistance in lodgepole pine. Forest Science. 29:124–126.

Schaffer, B.; Hawksworth, F.G.; Jacobi, W.R. 1983b. Effects of comandra blister rust and dwarf mistletoe on

cone and seed production of lodgepole pine. Plant Disease. 67:215-217.

Schaffer, B.; Hawksworth, F.G.; Wullscheger, S.D.; Reid,

C.P.P. 1983c. Cytokinin-like activity related to host reactions to dwarf mistletoes. Forest Science. 29:66–70.

Scharpf, R.F. 1964. Cultural variation and pathogenicity of

the Colletotrichum blight fungus of dwarf mistletoe. Phytopathology. 54:905–906.

Scharpf, R.F. 1969a. Cytospora abietisassociated with dwarf

mistletoe on true firs in California. Phytopathology.

59:1657–1658.

Scharpf, R.F. 1969b. Dwarf mistletoe on red fir: infection

and control in understory stands. Res. Pap. PSW-50.

Berkeley, CA: U.S. Department of Agriculture, Forest

Service, Pacific Southwest Forest and Range Experiment

Station. 8 p.

Scharpf, R.F. 1970. Seed viability, germination and radicle

growth of dwarf mistletoe in California. Res. Pap. PSW59. Berkeley, CA: U.S. Department of Agriculture, Forest

Service, Pacific Southwest Forest and Range Experiment

Station. 18 p.

Scharpf, R.F. 1972. Summation of tests for chemical control

of dwarf mistletoe. In: Proceedings 19th annual western

international forest disease work conference; 1971 September 13–17; Medford, OR: 80–83.

Scharpf, R.F. 1984. Host resistance to dwarf mistletoes. In:

Hawksworth, F.G.; Scharpf, R.F., tech. coords. Biology of

dwarf mistletoes, symposium proceedings;. 1984 August

8; Fort Collins, CO. Gen. Tech. Rep. RM-111. Fort Collins,

CO: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station: 70–76.

Scharpf, R.F. 1987. Resistance of Jeffrey pine to dwarf

mistletoe. In: Weber, H.C.; Forstreuter, W., eds. Proceedings of the 4th international symposium on parasitic

flowering plants; 1987 August 2–7; Philipps Universitat,

Marburg, Germany: 745–753.

Scharpf, R.F.; Bynum, H.H. 1975. Cytospora canker of true

firs. Forest Pest Leaflet 146. Washington, DC: U.S. Department of Agriculture, Forest Service. 5 p.

Scharpf, R.F.; Geils, B.; Wiens, D.; Parker, C.; Forstreuter,

W. 1997. Parasitic plants. In: Hansen, E.M.; Lewis, K.J.,

eds. Compendium of conifer diseases. St. Paul, MN: American Phytopathological Society: 36–40.

Scharpf, R.F.; Hawksworth, F.G. 1966. Hosts and distribution of Uredo phoradendri. Mycologia. 58:811–812.

Scharpf, R.F.; Hawksworth, F.G. 1968. Dwarf mistletoe on

sugar pine. Forest Pest Leaflet 113. Washington, DC: U.S.

Department of Agriculture, Forest Service, 4 p.

Scharpf, R.F.; Hawksworth, F.G. 1993. Mistletoes. In:

Scharpf, R.S., tech. coord. Diseases of Pacific Coast conifers. Agric. Handb. 521. Albany, CA: U.S. Department of

Agriculture, Forest Service, Pacific Southwest Research

Station: 112–135.

Scharpf, R.F.; Kinlock, B.B.; Jenkinson, J.L. 1992. One seed

source of Jeffrey pine shows resistance to dwarf mistletoe.

Res. Pap. PSW-RP-207. Albany, CA: U.S. Department of

Agriculture, Forest Service, Pacific Southwest Forest

Range and Experiment Station. 8 p.

Scharpf, R.F.; Parameter, J.R., Jr. 1967. The biology and

pathology of dwarf mistletoe, Arceuthobium

campylopodum f. abietinum, parasitizing true firs (Abies

spp.) in California. Tech. Bull. 1362. Washington, DC:

U.S. Department of Agriculture. 42 p.

Scharpf, R.F.; Parmeter, J.R., Jr., tech coords. 1978. Proceedings of the symposium on dwarf mistletoe control

through forest management. Gen. Tech. Rep. PSW-31.

Berkeley, CA: U.S. Department of Agriculture, Forest

Service, Pacific Southwest Forest and Range Experiment

Station. 190 p.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 109

Scharpf, R.F.; Parmeter, J.R., Jr. 1982. Population dynamics of dwarf mistletoe on young true firs in the Central

Sierra Nevada, California. Res. Pap. PSW-161. Albany,

CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. 9

p.

Scharpf, R.F.; Roth, L.F. 1992. Resistance of ponderosa pine

to western dwarf mistletoe in central Oregon. Res. Pap.

PSW-RP-208. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range

Experiment Station. 9 p.

Scharpf, R.F.; Smith, R.S.; Vogler, D. 1988. Management of

western dwarf mistletoe in ponderosa and Jeffrey pines in

forest recreation areas. Gen. Tech. Rep. PSW-103. Berkeley, CA: U.S. Department of Agriculture, Forest Service,

Pacific Southwest Research Station. 11 p.

Scharpf, R.F.; Vogler, D. 1986. Western dwarf mistletoe

infects understory Jeffrey pine seedlings on Cleveland

National Forest, California. Res. Note PSW-386. Berkeley, CA: U.S. Department of Agriculture, Forest Service,

Pacific Southwest Forest and Range Experiment Station.

2 p.

Schmitt, C.L. 1996. BMPMZ management guide, management of ponderosa pine infected with western dwarf mistletoe in northeastern Oregon. BMZ-96-07. Portland, OR:

U.S. Department of Agriculture, Forest Service, Pacific

Northwest Region, Blue Mountains Pest Management

Zone.

Schmitt, C.L. 1997. BMPMZ management guide, management of Douglas-fir infected with dwarf mistletoe in the

Blue Mountains of northeastern Oregon and southwestern Washington. BMZ-97-02. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Region, Blue Mountains Pest Management Zone.

Schwandt, J.; Page, D. 1978. Surveying Idaho forests for

dwarf mistletoe by helicopter. Rep. 78-6. Coeur d’Alene,

ID: Idaho Department of Lands, Forest Insect and Disease

Control. 7 p.

Shamoun, S.F. 1998. Development of biological control strategy for management of dwarf mistletoes. In: Sturrock, R.,

comp. Proceedings 45th western international forest disease work conference; 1997 September 15–19; Prince

George, BC. Victoria, BC: Canadian Forest Service, Pacific Forestry Centre. 36–42.

Shamoun, S.F. 2000. Application of biological control to

vegetation management in forestry. In: Spencer, N.R., ed.

Proceedings of the X international symposium on biological control of weeds; 1999 July 4–14; Montana State

University, Bozeman, MT: 87–96.

Shamoun, S.F.; Hintz, W.E. 1998. Development and registration of Chondrostereum purpureum as a mycoherbicide

for hardwood weeds in conifer reforestation sites and

utility rights-of-way. In: Main, D.S.; Burge, M.N., eds. IV

international bioherbicides workshop, programme and

abstracts; 1998 August 6–7; University of Strathclyde,

Glasgow, Scotland: 14.

Shamoun, S.F.; Ramsfield, T.D.; Shrimpton, G.; Hintz, W.E.

1996. Development of Chondrostereum purpureum as a

mycoherbicide for red alder (Alnus rubra) in utility rightsof-way. In: Comeau, P.; Harper, G., eds. Proceedings of the

1996 expert committee on weeds. Victoria, BC: British

Columbia Ministry of Forests, Research Branch: 9.

Shaw, C.G., III; Hennon, P.H. 1991. Spread, intensification,

and upward advance of dwarf mistletoe in thinned young

stands of western hemlock in southeast Alaska. Plant

Disease. 75:363–367.

Shaw, D.C.; Freeman, E.A.; Mathiasen, R.L. 2000. Evaluating the accuracy of ground-based hemlock dwarf mistletoe

rating: a case study using the Wind River canopy crane.

Western Journal Applied Forestry. 15(1):8–14.

Shaw, D.C.; Weiss, S.B. 2000. Canopy light and the distribution of hemlock dwarf mistletoe (Arceuthobium tsugense

[Rosendahl] G.N. Jones subsp. tsugense) aerial shoots in an

old-growth Douglas-fir/western hemlock forest. Northwest

Science. 74(4):306–315.

Sinclare, W.A.; Lyon, H.H.; Johnson, W.T. 1987. American

mistletoes and dwarf mistletoes In: Diseases of trees and

shrubs. Ithaca, NY: Comstock Publishing Association:

420–435.

Singh, P. 1981. Eastern dwarf mistletoe on black spruce in

Newfoundland. Phytopathology. 71:255.

Singh, P.; Carew, G.C. 1989. Impact of eastern dwarf mistletoe in black spruce forests in Newfoundland. European

Journal of Forest Pathology. 19:305–322.

Smith, B.S. 1992. Lucy Bishop Millington, nineteenth-century botanist: her life and letters to Charles Horton Peck,

State Botanist of New York. Huntia. 8:111–153.

Smith, E.L.; McMahan, A.J. 2002. Eco-physiology approach

to projecting tree stress and vigor in FVS. In: Crookston,

Nicholas L.; Havis, Richard N., comps. Second Forest

Vegetation Simulator (FVS) conference; 2002 February

12–14; Fort Collins, CO. Proc. RMRS-P-25. Ogden, UT:

U.S. Department of Agriculture, Forest Service, Rocky

Mountain Research Station.

Smith, R.B. 1969. Assessing dwarf mistletoe on western

hemlock. Forest Science. 15:277–285.

Smith, R.B. 1985. Hemlock dwarf mistletoe biology and

spread. In: Muir, J., ed. Proceedings, workshop on management of hemlock dwarf mistletoe; 1983 August 15–16;

Burnaby, BC. Pest Management Rep. 4. Victoria, BC:

British Columbia Ministry of Forests: 4–10.

Smith, R.B.; Funk, A. 1980. Assessing Nectra macrospora as

a biological control agent for hemlock dwarf mistletoe. Bimonthly Progress Report. Ottawa: Canada Department of

Forestry. 36(3):11–12.

Smith, R.B.; Wass, E.F. 1976. Field evaluation of ecological

differention of dwarf mistletoe on shore pine and western

hemlock. Canadian Journal of Forest Research. 6:225–

228.

Smith, R.B.; Wass, E.F. 1979. Infection trials with three

dwarf mistletoe species within and beyond their known

ranges in British Columbia. Canadian Journal of Plant

Pathology. 1:47–58.

Smith, R.S. 1977. Overstory spread and intensification of

hemlock dwarf mistletoe. Canadian Journal of Forest

Research. 7:632–640.

Smith, R.S. 1983. Diseases of eastside pine types. In: Robson,

T.F.; Standford, R.B., eds. Management of the eastside

pine type in northeastern California; proceedings of a

symposium; 1982 June 15–17; Susanville, CA. Publ. SAF

83-06. Northern California Society of American Foresters:

79–81.

Smith, R.S.; Wass, E.F.; Meagher, M.D. 1993. Evidence of

resistance to hemlock dwarf mistletoes (Arceuthobium

tsugense) in western hemlock (Tsuga heterophylla) clones.

European Journal of Forest Pathology. 23:163–170.

Snyder, M.A.; Fineschi, B.; Linhart, Y.B.; Smith, R.H. 1996.

Multivariate discrimination of host use by dwarf mistletoe

Arceuthobium vaginatum subsp. cryptopodum: inter- and

intraspecific comparisons. Journal of Chemical Ecology.

22(2):295–305.

Sproule, A. 1996a. Branch age in jack pine at the time of

dwarf mistletoe infection. Forestry Chronicle. 72(3):307.
110 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Sproule, A. 1996b. Impact of dwarf mistletoe on some aspects

of the reproductive biology of jack pine. Forestry Chronicle.

72(3):303–306.

Srivastava, L.M.; Esau, K. 1961. Relation of dwarf mistletoe

(Arceuthobium) to the xylem tissues of conifers. II. Effect

of the parasite on the xylem anatomy of the host. American

Journal of Botany. 48:209–215.

Stage, A.R.; Crookston, N.L.; Wiitala, M.R. 1986. Procedures for including pest management activities in forest

planning using present or simplified planning methods.

In: Bailey, R.G., tech. coord. Lessons from using FORPLAN:

proceeding of a planning and implementation workshop;

1986 April 29–May 1; Denver, CO. Washington, DC: U.S.

Department of Agriculture, Forest Service, Land Management Planning Systems: 202–215.

Standley, P.C. 1920. Loranthaceae. In: Trees and shrubs of

Mexico. Contributions from the United States National

Herbarium, Volume 23. Washington DC: Smithsonian

Institution: 222–235.

Steeger, C.; Hitchcock, C.L. 1998. Influence of forest structure and diseases on nest-site selection by red-breasted

nuthatches. Journal of Wildlife Management. 62(4):1349–

1358.

Stevens, R.E.; Hawksworth, F.G. 1970. Insects and mites

associated with dwarf mistletoes. Res. Pap. RM-59. Fort

Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 12 p.

Stevens, R.E.; Hawksworth, F.G. 1984. Insect-dwarf mistletoe associations: an update. In: Hawksworth, F.G.; Scharpf,

R.F., tech. coords. Biology of dwarf mistletoes: proceedings of the symposium; 1984 August 8; Fort Collins, CO.

Gen Tech. Rep. RM-111. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain

Forest and Range Experiment Station: 94–101.

Stewart, J.L. 1978. Overview of the dwarf mistletoe problem. In: Scharpf, R.F.; Parmeter, J.R., Jr., tech. coords.

Symposium proceedings: dwarf mistletoe control through

forest management;. 1978 April 11–13; Berkeley, CA.

Gen. Tech. Rep. PSW-31. Portland, OR: U.S. Department

of Agriculture, Forest Service, Pacific Southwest Forest

Range and Experiment Station: 2–4.

Stipe, L.E.; Kohler, S.; Hagle, S.; Hastie, L. 1992. Montana

forest pest conditions and program highlights, 1991. Rep.

92-2. Missoula, MT: U.S. Department of Agriculture,

Forest Service, Northern Region, Forest Pest Management. 27 p.

Strand, M.A.; Roth, L.F. 1976. Simulation model for spread

and intensification of western dwarf mistletoe in thinned

stands of ponderosa pine saplings. Phytopathology. 66:888–

895.

Sutton, G.M. 1951. Dispersal of mistletoes by birds. Wilson

Bulletin. 63(4):235–237.

Tainter, F.H.; French, D.W. 1971. The role of wound periderm in the resistance of eastern larch and jack pine to

dwarf mistletoe. Canadian Journal of Botany. 49:501–

504.

Taylor, J.E. 1995. Western larch dwarf mistletoe and ecosystem management. In: Schmidt, W.C.; McDonald, K.J.,

comp. Ecology and management of Larix forests: a look

ahead, proceedings of an international symposium; 1992

October 5–9; Whitefish, MT. Gen. Tech. Rep. INT-GTR319. Ogden, UT: U.S. Department of Agriculture, Forest

Service Intermountain Research Station: 310–313.

Taylor, J.E.; Marsden, M.A. 1997. Permanent plots for

studying the spread and intensification of larch dwarf

mistletoe and the effects of the parasite on growth of

infected western larch on the Flathead Indian Reservation, Montana. Rep. 97-5. U.S. Department of Agriculture,

Forest Service, Northern Region, Forest Health Protection. 5 p.

Taylor, R.J.; Patterson, T.F.; Harrod, R.J. 1994. Systematics

of Mexican spruce—revisited. Systematic Botany. 19(1):47–

59.

TeBeest, D.O.; Templeton, G.E. 1985. Mycoherbicides:

progress in the biological control of weeds. Plant Disease.

69:6–10.

Thomson, A.J.; Muir, J.A.; Lewis, K.J. 1997. Variability in

sub-regional impacts of dwarf mistletoe on mature lodgepole pine. Forestry Chronicle. 73:371–375.

Thoreau, H.D. 1858. The journal of Henry David Thoreau,

February 12, 1858. Boston: Houghton Mifflin. [1962 Dover

reprint, Vol. 2:278–279.]

Tinnin, R.O. 1998. An alternative to the 6-class dwarf

mistletoe rating system. Western Journal of Applied Forestry. 13:64–65.

Tinnin, R.O. 2001. Effect of dwarf mistletoe on bole taper

and volume in young Douglas-fir. Western Journal of

Applied Forestry. 16(1):5–8.

Tinnin, R.O.; Forbes, R.B. 1999. Red squirrel nests in witches’

brooms in Douglas-fir trees. Northwest Naturalist. 80:17–

21.

Tinnin, R.O.; Hawksworth, F.G.; Knutson, D.M. 1982.

Witches’ broom formation in conifers infected by

Arceuthobium spp.: an example of parasitic impact upon

community dynamics. American Midland Naturalist.

107:351–359.

Tinnin, R.O.; Knutson, D.M. 1980. Growth characteristics of

the brooms on Douglas-fir caused by Arceuthobium

douglasii. Forest Science. 26:149–158.

Tinnin, R.O.; Knutson, D.M. 1985. How to identify brooms in

Douglas-fir caused by dwarf mistletoe. Res. Note PNW426. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 8 p.

Tinnin, R.O.; Parks, C.G.; Knutson, D.M. 1999. Effects of

Douglas-fir dwarf mistletoe on trees in thinned stands in

the Pacific Northwest. Forest Science. 45(3):359–365.

Tkacz, B.M. 1989. Incorporating pest management into land

management decisions. In: Tecle, A.; Covington, W.W.;

Hamre, R.H., tech. coords. Multiresource management of

ponderosa pine forests; 1989 November 14–16; Flagstaff,

AZ. Gen. Tech. Rep. RM-185. Fort Collins, CO: U.S.

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 60–64.

Tong Jun; Ren Wei. 1980. Preliminary studies on the disease

cycle of Arceuthobium chinense. Journal Yunnan Forestry

College. 1:19–25.

Trelease, W. 1916. The genus Phoradendron, a monographic

revision. Urbana: University of Illinois Press. 224 p.

Tropical Agriculture Research and Training Center. 1992.

Forest pests in Central America. Tech. Man. 3. Turrialba,

Costa Rica: Tropical Agriculture Research and Training

Center. 49 p.

Trummer, L.M.: Hennon, P.E.; Hansen, E.M.; Muir, P.S.

1998. Modeling the incidence and severity of hemlock

dwarf mistletoe in 110-year-old wind-disturbed forests in

southeast Alaska. Canadian Journal of Forest Research.

28:1501–1508.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 111

Turner, N.J.; Szczawinski, A.F. 1991. Common poisonous

plants and mushrooms of North America. Portland, OR:

Timber Press: 250.

Unger, L. 1992. Dwarf mistletoes. Forest Pest Leaflet 44.

Victoria, BC: Canadian Forest Service. [Online]. Available:

http://www.pfc.cfs. nrcan.gc.ca/diseases/hforest/Pests/

dmistle_e.html. [2002, June 29].

Urness, P.J. 1969. Nutritional analysis and in vitro digestibility of mistletoes browsed by deer in Arizona. Journal of

Wildlife Management. 33(3):499–505.

USDA, NRCS. 2001. The PLANTS database, Version 3.1.

[Homepage of Natural Resources Conservation Service,

U.S. Department of Agriculture]. [Online]. Available: http:/

/plants.usda.gov/. [2002, July 2].

Valdivia Sanchez, J.J. 1964. Pathogia forestal. I. El muérdago

enano (Arceuthobium spp.) en los bosques de la zona

noreste de Michoacán. Bol. 15. Comision Forestal

Michoacan: 1–67.

van der Kamp, B.J.; Hawksworth, F.G. 1985. Damage and

control of the major diseases of lodgepole pine. In:

Baumgartner, D.M.; Krebill, R.G.; Arnott, J.T.; Weetman,

G.F. Lodgepole pine the species and its management,

symposium proceedings; 1984 May 8–10 and May 14–16;

Spokane, WA, and Vancouver, BC. Pullman WA: Office of

Conferences and Institutes, Cooperative Extension: 125–

131.

Vandygriff, J.C.; Rasmussen, L.A.; Rineholt, J.F. 2000. A

novel approach to managing fuel wood harvest using bark

beetle pheromones. Western Journal Applied Forestry.

15(4):183–188.

van Ommeren, R.J.; Whitham, T.G. 2002. Changes in

interactions between juniper and mistletoe mediated by

shared avian frugivores: parasitism to potential mutualism. Oecologia. 130:281–288.

Van Sickle, G.A.; Wegwitz, E. 1978. Silvicultural control of

dwarf mistletoes in young lodgepole pine stands in Alberta

and British Columbia. Rep. BC-X-180. Victoria, BC: Environment Canada, Canadian Forest Service, Pacific Forestry Centre. 11 p.

Vasek, F.C. 1966. The distribution and taxonomy of three

western junipers. Brittonia. 18:350–372.

Vázquez Collazo, I. 1986. Sistema de evaluación de 4 clases

para muérdago verdadero (Psittacanthus spp.). Archivo

Téch. México DF: Secretaría de Agricultura y Recursos

Hidráulicos, Instituto Nacional de Investigaciones

Forestales y Agropecularis.

Vázquez Collazo, I. 1989. Ciclo biologico y fenologia del

muérdago verdadero (Psittacanthus calyculatus (DC.) Don)

en el C.E.F. Barranca de Cupatitzio. En: V simposio sobre

parasitologia forestal; 1989 Octobre 4–6; Juarez, Chih.

Mexico, DF: Sociedad Mexicana de Entomogia. 42 p.

Vázquez Collazo, I. 1991. Notas sobre polinazcion y dispersion de Arceuthobium vaginatum(Willd.) Presl (Viscaceae).

La Ciencia y el Hombre [Universidad Veracruzana]. 8:101–

109.

Vázquez Collazo, I. 1993a. Contribución al conocimiento del

muérdago verdadero (Psittacanthus spp.) en México. En:

II simposio sobre detección, evaluación y Manejo de

Muérdagos en coníferas. México: Secretaría de Agricultura

y Recursos Hidráulicos, SFF.

Vázquez Collazo, I. 1993b. Determinacion de “tipo de sitio” para

evaluar rodales infectados por muérdago enano

(Arceuthobium globosum Hawksworth et Wiens). Ciencia

Forestal. 18(73):77–103.

Vázquez Collazo, I. 1994a. Intensidad del muestreo para

evauar rodales infestados por muérdago enano

(Arceuthobium globosum Hawksworth & Wiens). Foll.

Tech. 17. Uraupan, Michoacán: Secretaria de Agricultura

y Recursos Hidraulicos, Instituto Nacional de

Investigaciones Forestales y Agropecuaries, Centro

Investigaciones del Pacifico Centro. 23 p.

Vázquez Collazo, I. 1994b. Determinacion de la epoca y dosis

del Esteron 47M para el control del muérdago verdadedo

(Psittacanthus calyculatus) en mezquite (Prosopis

laevigata). Ciencia Forestal. 19(76):27–50.

Vázquez Collazo, I.; Pérez Chávez, R. 1989. Efecto del

parasitismo del muérdago verdadero (Psittacanthus spp.)

en la producción de semilla de tres especies de pino. En: V

simposio sobre parasitologia foresta; 1989 Octobre. 4–6;

Juarez, Chihuahua. México, DF: Sociedad Mexicana de

Entomogia, AC: 64.

Vázquez Collazo, I.; Pérez Chávez, R.; Pérez Chávez, R.

1982. Efecto del parasitismo del muérdago (Psittacanthus

schiedeanus Cham. and Schlecht Blume) en el desarrollo

de tres especies del género Pinus. Ciencia Forestal.

40(7):48–64.

Vázquez Collazo, I.; Pérez Chávez, R.; Pérez Chávez, R.

1985. Efecto del parasitismo del muérdago, Psittacanthus

schiedeanus (Cham. and Schleicht) Blume, en el desarrollo

de tres especies del genero Pinus. En: Memoria de los

simposia nacionales de parasitologia forestal II y III.

Publicacion Especial 46. México, DF: Secretaría de

Agricultura y Recursos Hidráulicos: 47–56.

Vázquez Collazo, I.; Perez Chavez, R.; Perez Chavez, R.

1986. Control quimico del muérdago verdadero

(Psittacanthus sp.) en la Sierra Purepecha (Meseta

Tarasca). Ciencia Forestal. 59(1):106–126.

Vega Rojas, E.J. 1976. El muérdago, enfermedad en bosques

de conferas y hojosas. Bosques y Fauna. 13(1):3–19.

Venkata Rao, C. 1963. On the morphology of the calyculus.

Journal of Indian Botanical Society. 42:618–628.

Vera Gaxiola, F. 1984. Observiones dasmetricas en un rodal

de Pinus hartwegii Lind. atacado por muérdago. En:

Memoria de los simposia nacionales de parasitologia forestal II y III. Publicacion Especial 46. Sociedad Mexicana

de Entomologia: 39–46.

Wagener, W.W. 1957. The limitation of two leafy mistletoes

of the genus Phoradendron by low temperatures. Ecology.

38:142–145.

Wagener, W.W. 1965. Dwarf mistletoe removal and

reinvasion in Jeffrey and ponderosa pine, northeastern

California. Res. Note PNW-73. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Research Station. 8 p.

Wagner, M.R.; Mathiasen, R.L. 1985. Dwarf mistletoepandora moth interaction and its contribution to ponderosa pine mortality in Arizona. Great Basin Naturalist.

45:423–426.

Wall, R.E. 1994. Biological control of red alder using stem

treatments with the fungus Chondrostereum purpureum.

Canadian Journal of Forest Research. 24:1527–1530.

Wall, R.E.; Prasad, R.; Shamoun, S.F. 1992. The development and potential role of mycoherbicides for forestry.

Forestry Chronicle. 68:736–741.

Walters, J.W. 1978. A guide to forest diseases of southwestern conifers. Rep. R3 78-9. Albuquerque, NM: U.S. Department of Agriculture, Forest Service, Southwestern

Region, Forest Insect and Disease Management. 36 p.
112 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Wanner, J.; Tinnin, R.O. 1986. Respiration in lodgepole pine

parasitized by American dwarf mistletoe. Canadian Journal of Forest Research. 16:1375–1378.

Wanner, J.L. 1986. Effects of infection by dwarf mistletoe

(Arceuthobium americanum) on the population dynamics

of lodgepole pine (Pinus contorta). Portland, OR: Portland

State University. 103 p. Dissertation.

Wanner, J.L.; Tinnin, R.O. 1989. Some effects of infection

by Arceuthobium americanum on the population dynamics of Pinus contorta in Oregon. Canadian Journal

of Forest Research. 19:736–742.

Watson, D.M. 2001. Mistletoe—a keystone resource in forests and woodlands worldwide. Annual Reviews of Ecology and Systematics. 32:219–250.

Weir, J.R. 1915. Wallrothiella arceuthobii. Journal of Agricultural Research. 4:369–378.

Weir, J.R. 1916a. Larch mistletoe: some economic considerations of its injurious effects. Bull. 317. Washington, DC:

U.S. Department of Agriculture. 25 p.

Weir, J.R. 1916b. Mistletoe injury to conifers in the Northwest. Bull. 360. Washington, DC: U.S. Department of

Agriculture. 39 p.

Wellwood, R.W. 1956. Some effects of dwarf mistletoe on

hemlock. Forestry Chronicle. 32:282–296.

Whiting, A.F. 1950. Ethnobotany of the Hopi. Bull. 15

(revised). Flagstaff, AZ: Museum of Northern Arizona.

120 p.

Wicker, E.F. 1967a. Seed density as a klendusic factor of

infection and its impact upon propagation of Arceuthobium

spp. Phytopathology. 57:1164–1168.

Wicker, E.F. 1967b. Appraisal of biological control of

Arceuthobium campylopodum f. campylopodum by

Colletotrichum gloeosporioides. Plant Disease Reporter.

51:311–313.

Wicker, E.F. 1974. Ecology of dwarf mistletoe seed. Res. Pap.

INT-154. Ogden, UT: U.S. Department of Agriculture,

Forest Service, Intermountain Forest and Range Experiment Station. 28 p.

Wicker, E.F.; Hawksworth, F.G. 1988. Relationships of

dwarf mistletoes and intermediate stand cultural practices in the Northern Rockies. In: Schmidt, W.C., comp.

Proceedings, future forests of the Mountain West: a stand

culture symposium; 1986 September 29–October 3;

Missoula, MT. Gen. Tech. Rep. INT-243. Ogden, UT: U.S.

Department of Agriculture, Forest Service, Intermountain Research Station. 402 p.

Wicker, E.F.; Hawksworth, F.G. 1991. Upward advance,

intensification and spread of dwarf mistletoe in a thinned

stand of western larch. Res. Note RM-504. Fort Collins,

CO: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station. 4 p.

Wicker, E.F.; Shaw, C.G. 1968. Fungal parasites of dwarf

mistletoes. Mycologia. 60:372–383.

Wiens, D. 1964. Revision of the acataphyllous species of

Phoradendron. Brittonia. 16:1–54.

Wiens, D. 1970. Phoradendron. In: Correll, D.S.; Johnston,

M.C. Manual of the vascular plants of Texas. Renner:

Texas Research Foundation: 504–505.

Wiens, D.; Barlow, B.A. 1971. The cytogeography and

relationships of the viscaceous and eremolepidaceous

mistletoes. Taxon. 20:313–332.

Wiens, D.; Calvin, C.L. 1987. Epiparasitism in mistletoes.

Golden Bough (Royal Botanic Gardens, Kew). 9:3–5.

Wiens, D.; DeDecker, M. 1972. Rare natural hybridization in

Phoradendron (Viscaceae). Madroño. 21:395–402.

Wiens, D.; Hawksworth, F.G. 1990. Phoradendron sect.

Pauciflorae (Viscaceae): new species and combinations.

Unpublished manuscript, October 18. 19 p.

Wiens, D.; Nickrent, D.L.; Shaw, C.G., III.; Hawksworth,

F.G.; Hennon, P.E.; King, E.J. 1996. Embryonic and hostassociated skewed adult ratios in dwarf mistletoe. Heredity. 77:55–63.

Wiens, D.; Shaw, C.G., III. 1994. Arceuthobium hawksworthii

(Viscaceae), a new species of dwarf mistletoe from Belize.

Journal of Idaho Academy of Science. 30(1):25–32.

Wiggins, I.L. 1980. Flora of Baja California. Stanford, CA:

Stanford University Press: 139–141.

Wilcox, W.W.; Pong, W.Y.; Parmeter, J.R. 1973. Effects of

mistletoe and other defects on lumber quality in white fir.

Wood and Fiber. 4:272–277.

Wilson, C.A.; Calvin, C.L. 1996. Anatomy of the dwarf

mistletoe shoot system. In: Hawksworth, F.G.; Wiens, D.

Dwarf mistletoes: biology, pathology, and systematics.

Agric. Handb. 709. Washington, DC: U.S. Department of

Agriculture, Forest Service: 95–111.

Wilson, C.L. 1969. Use of plant pathogens in weed control.

Annual Review of Phytopathology. 7:411–434.

Wilson, J.L.; Tkacz, B.M. 1992. Pinyon ips outbreak in

pinyon juniper woodlands in northern Arizona: a case

study. In: Ffolliott, Peter F.; Gottfried, Gerald J.; Bennett,

Duane A.; Hernandez C., Victor Manuel; Ortega-Rubio,

Alfredo; Hamre, R. E., tech. cords. Ecology and management of oak and associated woodlands: perspectives in the

Southwestern United States and Northern Mexico. Gen.

Tech. Rep. RM-218. Fort Collins, CO: U.S. Department of

Agriculture, Forest Service, Rocky Mountain Forest and

Range Experiment Station: 187–190.

Wittwer, D., comp. 2002. Forest insect and disease conditions in Alaska—2001 R10-TP-102. Anchorage, AK: U.S.

Department of Agriculture, Forest Service, Alaska Region. 65 p.

Wood, C. 1986. Distribution maps of common tree diseases

in British Columbia. Inf. Rep. BC-X-281. Victoria, BC:

Canadian Forestry Service, Pacific Forestry Centre. 68 p.

Wood, R.E.; Shuft, M.J.; Schultz, D.E. 1979. An evaluation

of tree mortality in Laguna Mountain Recreation Area.

Insect and Disease Management Rep. 79-1. San Francisco: U.S. Department of Agriculture, Forest Service,

Pacific Southwest Region, Cleveland National Forest. 22

p.

Wright, E. 1942. Cytospora abietis, the cause of a canker of

true firs in California and Nevada. Journal Agricultural

Research. 65:143–153.

Zavala, I.; Zavala, M.A. 1993. Global positioning system as

a tool for ecosystem studies at the landscape level: an

application in the Spanish Mediterranean. Landscape

and Urban Planning. 24:95–104.

Zimmerman, G.T.; Laven, R.D. 1984. Ecological interrelationships of dwarf mistletoe and fire in lodgepole pine

forest. In: Hawksworth, F.G.; Scharpf, R.F., tech. coords.

Biology of dwarf mistletoes: proceedings of the symposium; 1984 August 8; Fort Collins, CO. Gen. Tech. Rep.

RM-111. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range

Experiment Station: 123–131.

Zimmermann, G.T.; Laven, R.D.; Omi, P.N.; Hawksworth,

F.G. 1990. Use of prescribed fire for dwarf mistletoe

control in lodgepole pine management. In: Alexander,

M.E.; Bisgrove, G.F., tech. coord. The art and science of

fire management; 1988 October 24–27; Kananaskis, AB.

Info. Rep. NOR-X-309. Edmonton, AB: Forestry Canada,

Northern Forestry Centre: 163–174.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 113

Appendix B: Scientific and Common Names of Trees ________________

Abies amabilis Douglas ex J. Forbesa ................................................................................................. Pacific silver fir

Abies balsamea (Linnaeus) Millera ..................................................................................................... balsam fir

Abies bifolia A. Murraya ...................................................................................................................... Rocky Mountain subalpine fir

= Abies lasiocarpa var. arizonica (Merriam) Lemmon ..................................................................... corkbark fir

Abies concolor (Gordon & Glendinning) Hildebranda ....................................................................... white fir

Abies durangensis Martinezc ............................................................................................................... Durango fir

Abies grandis (Douglas ex D. Don in Lambert ) Lindleya ................................................................. grand fir

Abies lasiocarpa (Hooker) Nuttalla ..................................................................................................... subalpine fir

Abies lowiana (Gordon) A. Murraya ................................................................................................... Sierra white fir

= Abies concolor var. lowiana (Gordon) Lemmon

Abies magnifica A. Murraya ................................................................................................................ California red fir

Abies procera Rehdera ......................................................................................................................... noble fir

Abies religiosa Lindleyc....................................................................................................................... sacred fir

Abies religiosa var. emarginata Loock & Martinezc

Abies vejarii Martinezc [as vejari] ....................................................................................................... Vehar fir

Abies vejarii subsp. mexicana (Martinez) A. Farjonc ......................................................................... Mexican fir

=Abies mexicana Martinez

Calocedrus decurrens (Torrey) Florina ............................................................................................... incense-cedar

Cupressus arizonica Greenea............................................................................................................... Arizona cypress

Cupressus arizonica var. montana (Wiggins) Littlec ......................................................................... cypress

Cupressus bakeri Jepsona .................................................................................................................... Baker cypress

Cupressus benthami Endlicherc ........................................................................................................... cypress

Cupressus goveniana Gordona ............................................................................................................ Gowen cypress

Cupressus lusitanica Millerc

................................................................................................................ cypress

Cupressus macnabiana A. Murraya..................................................................................................... MacNab cypress

Cupressus macrocarpa Hartwega. ....................................................................................................... Monterey cypress

Cupressus sargentii Jepsona.. .............................................................................................................. Sargent cypress

Juniperus ashei J. Bucholtza................................................................................................................ Ashe juniper

Juniperus ashei var. saltillensis (H.M. Hall) Silbac

Juniperus californica Carrièrea ........................................................................................................... California juniper

Juniperus deppeana Steudela............................................................................................................... alligator juniper

Juniperus flaccida Schlechtendala....................................................................................................... drooping juniper

Juniperus monosperma (Engelmann) Sargenta ................................................................................... one-seed juniper

Juniperus occidentalis Hookera........................................................................................................... western juniper

Juniperus osteosperma (Torrey) Littlea .............................................................................................. Utah juniper

Juniperus pinchotii Sudwortha ............................................................................................................ Pinchot juniper

= Juniperus erythrocarpa Cory

Juniperus scopulorum Sargenta ........................................................................................................... Rocky Mountain juniper

Larix decidua Millerc ........................................................................................................................... European larch

Larix leptolepis (Sieb. & Zuccarini) Gordonc..................................................................................... Japanese larch

 =Larix kaempfer (Lambert) Sargentc

Larix laricina (Du Roi) K. Kocha........................................................................................................ tamarack

Larix occidentalis Nuttalla ................................................................................................................... western larch

Picea abies (Linneaus) H. Karstena..................................................................................................... Norway spruce

Picea breweriana S. Watsona .............................................................................................................. Brewer spruce

Picea engelmannii Parry ex Engelmanna ............................................................................................ Engelmann spruce

Picea glauca (Moench) Vossa ............................................................................................................. white spruce

Picea mariana (Miller) Britton, Sterns, & Poggenburga .................................................................... black spruce

Picea mexicana Martinezc ................................................................................................................... Mexican spruce

Picea pungens Engelmanna. ................................................................................................................ blue spruce

Picea rubens Sargenta .......................................................................................................................... red spruce

Picea sitchensis (Bongard) Carrièrea .................................................................................................. Sitka spruce

Pinus albicaulis Engelmanna ............................................................................................................... whitebark pine

Pinus aristata Engelmanna .................................................................................................................. Colorado bristlecone pine

Pinus arizonica Engelmannb ............................................................................................................... Arizona pine

= Pinus ponderosa var. arizonica (Engelmann) Shawa

Pinus arizonica var. stormiae Martinezb ............................................................................................. pino real

Pinus attenuata Lemmona. .................................................................................................................. knobcone pine

Pinus ayacahuite C. Ehrenberg ex Schlechtendal b ............................................................................ Mexican white pine

Pinus ayacahuite var. brachyptera Shawb

Pinus balfouriana Greville & Balfoura ............................................................................................... foxtail pine

Pinus balfouriana subsp. australis Mastoguiseppe & Mastoguiseppe .............................................. Sierra foxtail pine

Pinus banksiana Lamberta ................................................................................................................... jack pine

Pinus bungeana Zuccarinic.................................................................................................................. lacebark pine

Pinus californiarum D. K. Baileyc ...................................................................................................... singleleaf pinyon

Pinus californiarum subsp. fallax (Little) D. K. Baileyc .................................................................... Arizona singleleaf pinyon

Pinus caribaea Morelet ....................................................................................................................... Caribbean pine

Pinus caribaea var. hondurensis (Sénécl) Barr. & Golf.b .............................................................................................. Honduras Caribbean pine

Pinus cembroides Zuccarinib ............................................................................................................... Mexican pinyon

Pinus cembroides subsp. orabensis D. K. Baileyb .............................................................................. Orizaba pinyon

=Pinus orizabensis (D. K. Bailey) D. K. Bailey & Hawksworth

Pinus contorta Dougl. ex Loudona ...................................................................................................... lodgepole pine
114 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Pinus contorta var. contortaa............................................................................................................... shore pine

=Pinus contorta subsp. bolanderi (Parlatore) Critchfielda................................................................. Bolander pine

Pinus contorta var. murrayana (Grenville & Balfour) Engelmanna .................................................. Sierra lodgepole pine

Pinus contorta var. latifolia Engelmann in S Watsona ....................................................................... lodgepole pine

Pinus cooperi Blancob ......................................................................................................................... pine amarillo

Pinus coulteri D. Dona......................................................................................................................... Coulter pine

Pinus culminicola Andresen & Beamanb ............................................................................................ Potosi pinyon

Pinus discolor Bailey & Hawksworthb ............................................................................................... border pinyon

Pinus douglasiana Martinezb .............................................................................................................. pino

Pinus durangensis Martinezb ............................................................................................................... ocote

Pinus edulis Engelmanna ..................................................................................................................... pinyon

Pinus engelmannii Carrièrea ................................................................................................................ Apache pine

Pinus flexilis E. Jamesa ........................................................................................................................ limber pine

Pinus halepensis Millerc ...................................................................................................................... Aleppo pine

Pinus hartwegii Lindleyb ..................................................................................................................... Hartweg pine

Pinus herrerai Martinezb ..................................................................................................................... ocote

Pinus jaliscana Perez de la Rosab ....................................................................................................... Jalisco pine

Pinus jeffreyi Greville & Balfour in A. Murraya ................................................................................ Jeffrey pine

Pinus lambertiana Douglasa ................................................................................................................ sugar pine

Pinus lawsonii Roezl ex Gordon & Glendinningcb ............................................................................. pino

Pinus leiophylla Schiede & Deppea .................................................................................................... pine chino

Pinus leiophylla var. chihuahuana (Engelmann) G.R. Shawa............................................................ Chihuahua pine

Pinus longaeva D. K. Baileya .............................................................................................................. Intermountain bristlecone pine

Pinus lumholtzii Robins & Fern.b ........................................................................................................ pino triste

Pinus maximinoi H. E. Mooreb ............................................................................................................ pino

Pinus michoacana Martinezb ............................................................................................................... pino lacio

Pinus monophylla Torrey & Fremonta. ............................................................................................... singleleaf pinyon

Pinus montezumae Lambertb ............................................................................................................... Montezuma pine

Pinus monticola Douglas ex D. Dona.................................................................................................. western white pine

Pinus mugo Turrac ............................................................................................................................... dwarf mountain pine

Pinus muricata D. Dona....................................................................................................................... Bishop pine

Pinus oaxacana Mirovb ....................................................................................................................... pino chalmaite

Pinus occidentalis Swartzc .................................................................................................................. West Indian pine

Pinus oocarpa Schiedeb ....................................................................................................................... pino prieto

Pinus oocarpa var. ochoterenia Martinez b

Pinus oocarpa var. microphylla Shawb

=Pinus praetermissa Styles & McVaughc

Pinus palustris Millera ......................................................................................................................... longleaf pine

Pinus patula Schl. & Chamissob ......................................................................................................... pino triste

Pinus pinea Linneausc. ........................................................................................................................ Italian stone pine

Pinus ponderosa Douglas ex Lawson & C. Lawsona ......................................................................... ponderosa pine

Pinus ponderosa var. scopulorum Engelmann in S. Watsona ............................................................ Rocky Mountain ponderosa pine

Pinus pringlei Shawb ........................................................................................................................... pino rojo

Pinus pseudostrobus Lindleyb ............................................................................................................. pino

Pinus quadrifolia Parlatoreb ................................................................................................................ Parry pinyon

Pinus radiata D. Dona.......................................................................................................................... Monterey pine

Pinus resinosa Aitona .......................................................................................................................... red pine

Pinus rudis Endlicherb ......................................................................................................................... pino

Pinus sabiniana Douglas ex D. Don in Lamberta ............................................................................... digger pine

Pinus strobiformis Engelmanna ........................................................................................................... southwestern white pine

Pinus strobiformis var. potosiensis Silbac

Pinus strobus Linnaeusa....................................................................................................................... eastern white pine

Pinus sylvestris Linnaeusa

. .................................................................................................................. Scotch pine

Pinus tecunumanii (Schwertfeger) Equiluz & Perryb ......................................................................... pino

Pinus teocote Schl. & Chamissob ........................................................................................................ ocotl

Pinus thunbergii Parlatore ................................................................................................................... pine

Pinus torreyana Parry ex Carrièrea ..................................................................................................... Torrey pine

Pinus virginiana Millera ...................................................................................................................... Virginia pine

Pinus washoensis H. Mason & Stockwella ......................................................................................... Washoe pine

Pseudotsuga macrocarpa (Vasey) Mayra ........................................................................................... bigcone Douglas-fir

Pseudotsuga menziesii (Mirbel) Francoa............................................................................................. Douglas-fir

Pseudotsuga menziesii var. menziesii .................................................................................................. coast Douglas-fir

Pseudotsuga menziesii var. glauca (Mayr) Francoa............................................................................ Rocky Mountain Douglas-fir

Taxodium distichum (L.) Richard var. mexicanum Gordona .............................................................. Mexican bald-cypress

=Taxodium mucronatum Tenore

Tsuga heterophylla (Rafinesque) Sargenta.......................................................................................... western hemlock

Tsuga mertensiana (Bongard) Carrièrea.............................................................................................. mountain hemlock

a Flora of North America Committee (1993)

b Perry (1991)

c

 Plant Names Project (1999)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 115

Reviewers and Contributors

David Conklin

Forest Service, Southwest Region

Albuquerque, New Mexico, USA

Mary Lou Fairweather

Forest Service, Southwest Region

Flagstaff, Arizona, USA

Dave Johnson

Forest Service, Rocky Mountain Region (retired)

Lakewood, Colorado, USA

Job Kuijt

Department of Biology

University of Victoria

Victoria, British Columbia, Canada

Jose Melgar

Departamento de Proteccion Vegetal

Fundacion Hondurena de Investigacion Agricola

La Lima, Cortes, Honduras, Central America

Don Robinson

ESSA Technologies

Vancouver, British Columbia, Canada

Appendix C: Acknowledgments _____________________________________

Technical Support

Joyce Van deWater, Illustrator

Rocky Mountain Research Station

Fort Collins, Colorado, USA

Deborah Parker

Barb Satink, Editorial Assistant

Barbara Palrmo, Editorial Assistant

Rocky Mountain Research Station

Flagstaff, Arizona, USA

Nancy Chadwick, Visual Information Specialist

Loa Collins, Editorial Assistant

Lillie Thomas, Editorial Assistant

Louise Kingsbury, Group Leader, Publishing Services

Rocky Mountain Research Station

Ogden, Utah, USA
116 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Appendix D: Glossary______________________________________________

Baranyay and others (1971) provide a complete glossary of terms and special definitions that apply to mistletoes.

Although most of our readers ought to be familiar with the concepts and terms of forestry, they may be less

comfortable with a number of other terms used here. These are primarily botanical and plant pathology terms or

words with a special application in this context.

Abundance. See incidence.

Acuminate apex. Tapering to a point with the sides more or less pinched in before reaching the tip.

Adnate. The union of unlike parts, as an inferior ovary to the calyx tube.

Allozymes. Similar proteins that provide a physiochemical trait for investigating the population genetics of

groups of plants (hosts or parasites).

Anamorph. An imperfect stage of a fungus that is taxonomically described and provides a basis for identification

and referral.

Anthesis. Period when the flower is open.

Ascospore. A spore of a fungus produced within an ascus, a saclike cell of ascomycetes in which, following meiosis,

a specific number (usually eight) of ascospores is produced.

Autocorrelation, spatial. A quantification of a relation between two entities whereby the similarity of a feature

depends on the distance between the entities. Because of mistletoe spread and intensification, the severities of

mistletoe infection on two neighbor trees tend to be more or less similar, or spatially autocorrelated.

Bark strand. A structure that ramifies throughout the inner bark of the host and from which shoots and sinkers

of the mistletoe are derived.

Blight. Rapid discoloration and death of all parts of a plant.

Bootstrapping. A statistical processing method using iteration and repeat calculation to estimate variation.

Branch girdle, segment. Girdle refers to a region on a vegetative branch or main stem of a conifer between two

annual growth segments; segment refers to a single year’s growth of a vegetative branch or main stem.

Brooming. See witches’ broom.

Callus. Undifferentiated plant tissue, usually as grown in a laboratory with artificial media.

Calyculus. A floral structure of the Loranthaceae, a vestigial whorl of bracts of the suppressed flowers of a lateral

branch inflorescence that have become adnate to the inferior ovary.

Canker, mistletoe canker. The structure and malformation of a host stem or branch caused by a disruption of

the cambium and bark as a result of dwarf mistletoe infection.

Cordate. A shape of a leaf like a stylized heart.

Cortex. Ground-tissue region of a stem or root bounded externally by epidermis and internally by the vascular

system; a primary-tissue region.

Decussate. Of leaves or scale-like leaves that are arranged in pairs that alternately cross each other.

Disjunct. Pertaining to a discontinuous range having two or more potentially interbreeding populations

separated by a distance that precludes genetic exchange by pollination or dissemination.

Dyads. See monads.

Endemic. The kind of distribution for taxa that is geographically small.

Endophytic system. The root system parts of a dwarf mistletoe within host tissues. The endophytic system

consists of bark strands within the inner bark and “sinkers” that are embedded in successively formed layers

of xylem, referred to as haustorial root system or haustorium.

Endosperm. A tissue, containing stored food, that develops from the union of a sperm nucleus and the polar nuclei

of the central cell; it is digested by the growing sporophyte either before or after the maturation of the seed; found

only in angiosperms.

Epigynous. Growing, or appearing to grow, on the summit of the ovary.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 117

Flabellate branching. Fan shaped, a branching pattern produced by the continued development of superposed

axillary buds.

Flowering, direct and indirect. Indirect flowering is the result of an intervention of a rest period between

initiation of a floral bud and anthesis, whereas direct flowering is the result of uninterrupted development of

floral buds from initiation to anthesis.

Frass. Solid larval insect excrement.

Fusiform. Spindle-shaped; broadest at the middle and tapering at both ends.

Glabrous. Smooth, no hairs present.

Glaucous. Covered with a whitish or bluish waxy covering.

Growth loss. An expression of yield reduction that includes both lost annual production on still living trees and

lost volume to tree death.

Growth, primary and secondary. The growth of shoots and roots from inception until completion of their

expansion is primary growth. This growth is the result of apical meristems and their three primary derivative

meristems (protoderm, ground meristem, and procambium). Secondary growth results from divisions of

secondary meristems (typically the vascular cambium and phellogen) and adds circumference to the plant body.

Haustorium, primary and secondary. The primary haustorium is a wedge-like projection, arising from the

circular attachment disc of the radicle, that penetrates the outer bark extending to the host xylem. Secondary

haustoria are “sinkers” produced by bark strands that grow radially to the vascular cambium.

Holdfast. A disc-like swelling at the distal end of the radicle through which infection of the host occurs.

Host susceptibility. A subjective classification system based on the percentage of trees of the host species in

question that are infected by dwarf mistletoe within 6 m of a principal host heavily infected with the same species

of dwarf mistletoe.

Hyphae. Tubular threads of the mycelium of a fungus or similar organism.

Hypocotyl. Region of an embryo that is between the radicle and the attachment point of the cotyledons.

Incidence, abundance, distribution, severity. Incidence refers to the frequency of which host trees in a given

stand are infected by a given species of mistletoe (usually measured as percent of trees infected). Abundance

refers to the relative quantity of mistletoe in a stand or on a host (not usually quantified). Distribution describes

the spatial extant and pattern of a mistletoe species or population within a given area. Severity is a qualitative

term describing the disease situation (see infection class); high incidence along with large abundance would

result in a severe disease situation.

Incubation period. That period from infection to production of first shoots. See latency.

Infection class. A measure (generally from 0 to 6) of the relative severity of dwarf mistletoe infection for

individual trees, in contrast to host susceptibility class.

Infection, secondary infection, localized and systemic infections. Infection refers to that process in which

dwarf mistletoes successfully penetrate host tissue and initiate establishment of the endophytic system;

infection also refers to the mistletoe plant and the associated diseased host tissues. Secondary infection is

reinfection by dwarf mistletoe of already infected tissue. Localized infections (anisophasic) are those in which

the endophytic system is generally restricted to within or near (such as a few centimeters) the swollen portion

of the host, whereas systemic infections (isophasic) are those in which the endophytic system occurs within the

host terminal bud, and growth keeps pace with that of the host’s shoot apices.

Infestation. A condition in which one or more trees of a stand or group are infected.

Intensification. Increase in the number of dwarf mistletoe infections within a tree (see spread).

Internode. See node.

Latency. Phenomenon in which host tissues are infected by dwarf mistletoe but either visible symptoms of

swelling or brooming are not apparent or shoots are not present. Infections are latent during the incubation

period and when environmental conditions induce a cessation in production of shoots.
118 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Lenticel. A group of loose corky cells formed beneath the epidermis of woody plants; allows gas exchange to occur

across the periderm.

Ligulate. A property of a leaf, petal, or similar structure whereby the structure possesses a small membranous

appendage.

Meiosis. The chromosome reduction stage in formation of gametes.

-Merous. A suffix indicating division into parts; a five-merous flowers would have five sepals, five petals, five

stamens and a five-carpellate pistil (providing all these structures were present).

Monads, dyads, triads. Sets of flowers arising from a common peduncle, in singles (monads), pairs (dyads), or

triples (triads).

Mycelial stroma. A mass of vegetative hyphae in or on which spores are produced.

Mycoherbicide. A plant-killing substance based on the action of live fungi that induce disease.

Mycorrhizae. Fungus-root, a symbiotic relation of a fungus and plant root.

Node, internode. A region of the stem where a leaf or leaves diverge; the region in between nodes is an internode.

Obovate. A shape of a leaf like an oval, wider at one end (stylized egg).

Parasite, parasitism. A parasite is an organism, such as a mistletoe, that obtains sustenance from another

organism, and also completes all, or at least some, of its life cycle on that host organism. Parasitism is the typical

mode of existence or behavior of a parasite.

Pathosystem. A biotic combination consisting of a host and a pathogen; the reference is to the pair of organisms

rather than the nature of their relationship.

Pedicel. The stalk of an individual flower.

Peduncle. The stalk of an inflorescence (basal to a pedicel).

Penetration wedge. A structure in dwarf mistletoes that develops from the holdfast and initiates the infection

process.

Pericarp. The wall of the ripened ovary (fruit); consists of three layers, the exocarp (outer), mesocarp (middle),

and endocarp (inner).

Periderm, necrophylatic. A bark, cortex tissue that reacts to invasion by rapid, localized necrosis (isolating the

potential pathogen).

Phloem, primary and secondary. The principal food-conducting tissue of a plant composed mainly of sieve

elements, various kinds of parenchyma cells, fibers, and sclereids. Primary and secondary phloem are formed

during primary and secondary growth, respectively.

Phyllotaxy. The morphological arrangement of leaves.

Pistillate, stigma. Referring to the female flower, which includes an ovary, pistil, style, and stigma (which

receives the pollen).

Primary growth. See growth.

Pubescence, trichomes, puberulent, papillate-hispid, stellate. Hair-like structures on a plant surface are

epidermal glands called trichomes. A surface that bears trichomes is pubescent or puberulent if the hairs are

thin and sparse or papillate-hispid if it has “hairy bump.” Stellate hairs have a stalk and three or more branches

from a common point.

Radicle. See holdfast.

Severity. See incidence.

Sessile. Of a leaf that appears attached directly to the stem, without a petiole.

Sinker. A radially oriented structure, composed of tracheary and parenchymal elements, that originates from a

dwarf mistletoe bark strand and grows centripetally to the cambium where it becomes embedded by successive

layers of xylem.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 119

Source–sink. In the context of the mistletoe–tree interaction, the tree is the source or supplier of water and

nutrients to the mistletoe, and the mistletoe plays the role of sink or depository of water and nutrients taken

up by the host tree.

Spike, determinate, indeterminate spike. The unbranched inflorescence of a mistletoe of indeterminate type

when flowering proceeds from the base while younger flowers are formed as the spike continues to elongate or

of determinate type otherwise.

Sporodochia, conidial. An asexual reproductive structure of a fungus that produces spores by a budding

process.

Spread, vertical spread. Increase in the area of mistletoe infestation by infection of additional host trees.

Vertical spread refers to the net result of dispersal of mistletoe seeds to higher portions of the host crown.

Staminate. Referring to the male flower, which produces pollen.

Stigma. See pistillate.

Symbiont. A member of a close biotic relation whereby both species benefit, such as in mycorrhizae, and in

contrast to a pathogen that benefits to the harm of the host. The terms of these forms of mutualisms — symbiosis

and parasitism — are relative and contextual.

Sympatry. The condition in which the distributions of two species overlap and hybridization between taxa would

be possible if they were not reproductively isolated by factors other than spatial separation.

Synonymy. See taxonomy.

Systematics. See taxonomy.

Systemic infection (isophasic). Infection in which growth of the endophytic system keeps pace with the growth

of the infected host branch. See infection.

Taxon (plural, Taxa). A taxonomic unit of any rank (order, family, genus, species, subspecies, and so forth).

Taxonomy, systematics, synonymy. Taxonomy refers to the valid assignment of names to organisms based on

natural relations and rules of convention; systematics refers to the natural relations based on decent from a

common ancestor. Several (taxonomic) names may be used for an individual plant or population of plants. One

name will be preferred (for reasons dealing with the inferred relations and application of conventions); other

names are synonyms.

Terete. Of a stem, approximately cylindrical but tapering at ends.

Trichomes. See pubescence.

Triads. See monads.

Verticillate branching. Whorled, a branch pattern produced by the continued development of superposed

axillary buds.

Vertical spread. See intensification.

Viscin. Sticky material contained in the viscin cells of dwarf mistletoe fruit, which acts as the initial means of seed

attachment to the host.

Witches’ broom. An abnormally profuse, dense mass of host branches. This is a common symptom induced by

dwarf mistletoe infection, as well as other parasites and abiotic agents.

Woodrose. An ornamental object composed of the host wood deformed by a mistletoe haustorium and exposed by

removing the mistletoe tissue.
120 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

A

Abies 29

Abies amabilis 45, 50, 54, 59, 60

Abies balsamea 57

Abies bifolia 45, 50

Abies concolor 4, 26, 31, 32, 45, 49, 50, 70, 92

Abies durangensis 33, 45

Abies grandis 45, 49, 50, 54, 59

Abies lasiocarpa 45, 46, 54, 59, 60

Abies lasiocarpa var. arizonica. See Abies bifolia.

Abies lowiana 45

Abies magnifica 45, 46, 66, 70, 92

Abies procera 59

Abies religiosa 19, 20, 46

Abies vejarii 46

Alnus 32

Arbutus 29

Arceuthobium 4, 36, 39, 40, 70

Arceuthobium abietinum 44, 45, 46, 66, 83, 85

Arceuthobium abietis-religiosae 46

Arceuthobium americanum 41, 46, 66, 67, 69,

70, 76, 83, 84, 85, 86

Arceuthobium apachecum 47

Arceuthobium aureum 47, 52

Arceuthobium bicarinatum 53

Arceuthobium blumeri 47, 66

Arceuthobium californicum 48, 55

Arceuthobium campylopodum 44, 47, 48, 49, 53,

54, 55, 56, 58, 66, 83, 86, 98

Arceuthobium cyanocarpum 49, 66

Arceuthobium divaricatum 26, 49, 86

Arceuthobium douglasii 41, 50, 64, 66, 67,

83, 84, 86

Arceuthobium durangense 51, 66

Arceuthobium gillii 51, 55, 66

Arceuthobium globosum 47, 52, 61, 67

Arceuthobium guatemalense 53, 66, 67

Arceuthobium hawksworthii 52

Arceuthobium hondurense 53

Arceuthobium laricis 45, 53, 66, 86

Arceuthobium littorum 54

Arceuthobium microcarpum 54, 66

Arceuthobium monticola 55

Arceuthobium nigrum 53, 55, 66

Arceuthobium oaxacanum 56

Arceuthobium occidentale 54, 56, 66, 67, 84

Arceuthobium pendens 56

Arceuthobium pusillum 57, 66, 67, 70, 76,

84, 86, 98

Arceuthobium rubrum 56, 57

Arceuthobium siskiyouense 58

Arceuthobium strictum 58, 66

Arceuthobium tsugense 45, 58, 59, 60, 70,

83, 84, 85, 98

Arceuthobium vaginatum 51, 60, 61, 67, 84, 86

Arceuthobium verticilliflorum 36, 62

Arceuthobium yecorense 62

Arctostaphylos 29

B

biological control 8, 22, 34, 68, 82, 93, 98

black spruce. See Picea mariana

C

Calocedrus 31

Canada 5, 6, 70, 76

Alberta 46

British Columbia 4, 46, 50, 54, 59, 60

Manitoba 46, 57

Maritime Provinces 57, 70

Ontario 46, 57

Quebec 57

Saskatchewan 46, 57

Central America 15, 20, 21, 53

Chamaebatiara millefolium 31

Chamaecyparis lawsoniana 31

chemical control 8, 22, 34, 85, 93

Cladocolea 2, 3

Cupressus 19, 32

Cupressus arizonica 30, 31, 32, 33

Cupressus bakeri 30

Cupressus benthami 29, 32, 33

Cupressus goveniana 30

Cupressus lusitanica 32

Cupressus macnabiana 30

Cupressus macrocarpa 30

Cupressus sargentii 30

cypress. See Cupressus

D

Dendropemon 4

Douglas-fir. See Pseudotsuga

E

effect

damage to host 5, 21, 37, 64, 65, 66, 67, 68

parasitic physiology 21, 26

endophytic system 26, 37, 64, 67, 86

F

fire 37, 38

fuels 68, 69, 71

prescribed burning 97

Appendix E: Index ________________________________________________
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 121

fungi, decay, disease, mycorrhizae 22, 27,

34, 45, 67, 68, 82, 92

G

genetic resistance, genetic management 8, 34, 37,

86, 93

guide

field identification 2, 26, 44

silviculture 93, 94, 95, 96, 99, 100

H

haustorium 16, 18, 22

Hawksworth’s mistletoe 30, 31, 33

I

impact

ecological 5, 22, 26, 33, 68, 69, 70, 71, 90

resource 5, 22, 33, 70, 71, 78, 79, 90

infection

latent 37, 38, 64, 99

insects, defoliators, bark beetles, associated 22, 26, 34, 68, 82, 92

intensification

mistletoe 38, 64

J

jack pine. See Pinus banksiana

juniper. See Juniperus

Juniperus 29

Juniperus ashei 30

Juniperus californica 30, 31

Juniperus deppeana 29, 30, 31, 32, 33

Juniperus flaccida 30, 31, 33

Juniperus monosperma 29, 30, 31, 33

Juniperus occidentalis 30, 31

Juniperus osteosperma 29, 30, 31

Juniperus pinchotii 29, 30, 31

Juniperus scopulorum 31

L

Larix decidua 54, 59

Larix laricina 57, 87

Larix leptolepis 54

Larix lyallii 54

Larix occidentalis 49, 54, 59, 66, 70, 86, 92

life cycle, mistletoe; reproduction; pollination 16,

17, 18, 26, 35, 36, 37, 91, 98

lodgepole pine. See Pinus contorta

M

management 5, 22, 64, 65, 71, 74, 82

adaptive 98, 100

assess and monitor 5, 8, 78, 91, 97

objectives and planning 23, 34, 73, 74, 90, 93

Mexico 5, 6, 15, 70

Baja California 30, 31, 33, 49, 50

Chiapas 3, 19, 20, 21, 47, 53, 55

Chihuahua 29, 31, 32, 33, 45, 48,

50, 51, 52, 61, 62

Coahuila 30, 32, 33, 47, 48, 50, 61

Distrito Federal 46, 53, 61

Durango 29, 31, 32, 33, 48, 50, 51, 52, 55, 58, 61, 62

Guanajuato 20, 33, 55

Guerrero 19, 53

Hidalgo 29, 32, 46, 53, 55, 61

Jalisco 3, 20, 29, 33, 46, 51, 52, 53, 61

Mexico 20, 32, 33, 46, 53, 55, 61

Michoacán 3, 19, 20, 21, 33, 46, 53, 55

Morelia 20

Nayarit 29, 51, 61

Nuevo Leon 33, 46, 48, 50, 61

Oaxaca 3, 20, 21, 32, 47, 53, 55, 56, 61

Puebla 3, 27, 32, 33, 46, 53, 55, 56, 61

Querétaro 29, 55, 61

San Luis Potosi 33, 56

Sinaloa 20, 29, 51, 58, 61

Sonora 3, 29, 31, 48, 51, 52, 61, 62

Tamaulipas 20, 32, 46, 61

Tlaxcala 20, 21, 32, 46, 53, 55, 61

Veracruz 3, 19, 21, 29, 32, 53, 55, 56, 61

Yucatan 20

Zacatecas 29, 33, 55, 61

mimicry 26, 32

model

simulation 8, 64, 65, 66, 75, 77,

78, 79, 82, 95, 97, 100

molecular biology (DNA, isozyme) 27, 64, 88

O

oak. See Quercus

P

pattern

spatial autocorrelation 38, 70, 78, 79, 97

Phoradendron 4, 25, 27, 28, 33

Phoradendron bolleanum 28, 29, 32, 33

Phoradendron brachystachyum 33

Phoradendron capitellatum 29, 31

Phoradendron densum 27, 29, 31, 33

Phoradendron galeottii 32

Phoradendron juniperinum 26, 27, 31, 32, 50

Phoradendron libocedri 31

Phoradendron ligatum 31

Phoradendron longifolium 32

Phoradendron minutifolium 29, 32

Phoradendron olivae 32

Phoradendron pauciflorum 29, 32

Phoradendron rhipsalinum 27, 33

Phoradendron saltillense 30, 31, 32, 33
122 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002

Phoradendron velutinum 27

Picea abies 49, 54, 59

Picea breweriana 55, 60, 70

Picea engelmannii 46, 49, 50, 54, 55, 59

Picea glauca 46, 54, 57, 59, 70

Picea mariana 46, 57, 65, 66, 70, 86, 92

Picea mexicana 55

Picea pungens 46, 50, 55, 57, 66

Picea rubens 57, 70

Picea sitchensis 59

Pinus 29, 66

Pinus albicaulis 46, 49, 54, 60, 66

Pinus aristata 46, 49, 55, 61

Pinus arizonica 51, 52, 55, 61, 62

Pinus attenuata 48, 56, 58

Pinus ayacahuite 45, 48, 53, 66

Pinus balfouriana 49

Pinus banksiana 46, 54, 56, 57, 66, 70, 86

Pinus bungeana 56

Pinus californiarum 50

Pinus caribaea 20, 21, 56

Pinus cembroides 50, 56

Pinus contorta 45, 46, 49, 54, 58, 59, 60, 61,

65, 66, 69, 70, 86, 87, 92

Pinus cooperi 51, 52, 55, 57, 61, 62

Pinus coulteri 48, 56

Pinus discolor 50, 56

Pinus douglasiana 20, 51, 52

Pinus durangensis 51, 52, 57, 61, 62

Pinus edulis 50, 86, 92

Pinus engelmannii 4, 20, 52, 57, 58, 61, 62

Pinus flexilis 46, 47, 48, 49, 61, 66

Pinus halepensis 56

Pinus hartwegii 52, 61, 87

Pinus herrerai 20, 51, 57, 61, 62

Pinus jaliscana 3

Pinus jeffreyi 46, 48, 55, 56, 58, 65, 86, 87, 92

Pinus lambertiana 45, 48, 55, 92

Pinus lawsonii 3, 20, 52, 55, 56, 61

Pinus leiophylla 3, 19, 20, 21, 22, 27,

51, 55, 58, 62, 66

Pinus longaeva 49

Pinus lumholtzii 3, 20, 51, 55, 62

Pinus maximinoi 20, 53

Pinus michoacana 20, 32, 47, 51, 53, 56

Pinus monophylla 30, 31, 50, 92

Pinus montezumae 3, 19, 20, 21, 47, 51,

53, 55, 61

Pinus monticola 45, 48, 49, 54, 55, 59, 60

Pinus mugo 46, 49

Pinus muricata 54

Pinus oaxacana 47, 55, 56

Pinus occidentalis 4

Pinus oocarpa 20, 21, 47, 51, 53

Pinus palustris 56

Pinus patula 3, 47, 53, 55, 61

Pinus pinea 56

Pinus ponderosa 46, 48, 49, 54, 56, 58, 59,

61, 64, 66, 67, 70, 86, 87, 92

Pinus pringlei 20, 53

Pinus pseudostrobus 3, 4, 20, 32, 47,

51, 53, 55, 56

Pinus quadrifolia 50

Pinus radiata 54, 56, 59

Pinus resinosa 49, 54, 57

Pinus rhynchanthus 20

Pinus rudis 20, 53, 61

Pinus sabiniana 48, 56, 66

Pinus strobiformis 47, 48, 49, 55, 61

Pinus strobus 49, 57

Pinus sylvestris 46, 49, 54, 56, 59, 61

Pinus tecunumanii 20, 53

Pinus teocote 19, 20, 21, 52, 53, 55, 57, 58, 61

Pinus thunbergii 56

Pinus torreyana 56

Pinus virginiana 56

Pinus washoensis 49

ponderosa pine. See Pinus ponderosa

Pseudotsuga 46, 50, 59, 64, 66, 70, 86,

87, 88, 92

Psidium guineese 20

Psittacanthus 4, 15, 18, 19, 21

Psittacanthus americanus 19, 22

Psittacanthus angustifolius 16, 19

Psittacanthus calyculatus 16, 17, 18, 20, 22

Psittacanthus macrantherus 20

Psittacanthus pinicola 20

Psittacanthus rhychanthus

Psittacanthus schiedeanus 21

Q

Quercus 18, 29, 32, 33

R

rare

endemic 4, 51, 53, 54, 55, 56, 58, 62

rating system 18, 21

susceptibility 38

recreation management 34, 71, 78, 99

root disease 77

S

Sequoia 5

Sequoiadendron 5

severity

mistletoe (DMR) 38, 39, 64, 65, 66, 69, 77, 78

silviculture 8, 22, 34, 38, 75, 78, 82, 89, 90, 92

harvest and regeneration 93, 94, 96

pruning 99

sanitation 93, 94, 96, 97, 99

thinning 93, 95, 96
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-98. 2002 123

spread

dispersal 16, 36, 37, 38, 97, 98, 99

dispersal by birds 16, 17, 26, 34, 36, 56, 62, 69

vertical 38

Struthanthus 2, 3

sympatry 29, 31, 32, 33, 51, 61

T

Taxodium 3, 33

taxonomy, nomenclature, systematics v, 18,

26, 27, 28, 29, 30, 44, 45, 48, 59

Thuja 5

Tsuga canadensis 59

Tsuga heterophylla 54, 59, 60, 70, 87, 88, 92

Tsuga mertensiana 49, 54, 59, 60

U

United States of America 5, 6, 70

Alaska 59

Arizona 4, 29, 30, 31, 33, 45, 47,

48, 50, 51, 55, 61

California 4, 30, 31, 33, 45, 46, 48, 49, 50,

54, 55, 56, 58, 59, 60

Colorado 31, 46, 49, 50, 61

Idaho 31, 46, 49, 50, 54

Lake and Northeastern States 57

Montana 46, 49, 50, 54

Nevada 31, 45, 46, 49, 50

New Mexico 29, 30, 31, 47, 50, 51, 55, 61

Oregon 30, 31, 45, 46, 49, 50,

54, 55, 58, 59, 60

Texas 30, 31, 50, 61

Utah 31, 45, 46, 49, 50, 61

Washington 45, 46, 49, 50, 54, 59, 60

Wyoming 46, 49

use and folklore of mistletoe

(art, food, medicine, dye, magic) 4, 21, 34, 71

V

value

(as intangible or ethical worth) 4, 70, 71

Viscum 4

W

wildlife habitat and management 21, 27, 38, 51, 68,

69, 71, 78, 100

witches’ broom;

brooming, use by associated species 8,

26, 31, 37, 38, 39, 47, 48, 49, 50, 51, 52,

53, 54, 55, 56, 57, 62, 64, 67, 68, 69, 100
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its

programs and activities on the basis of race, color, national origin, sex, religion,

age, disability, political beliefs, sexual orientation, or marital or family status. (Not

all prohibited bases apply to all programs.) Persons with disabilities who require

alternative means for communication of program information (Braille, large print,

audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice

and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights,

Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington,

DC 20250-9410 or call (202) 720-5964 (voice or TDD). USDA is an equal

opportunity provider and employer.

ROCKY MOUNTAIN RESEARCH STATION

RMRS

The Rocky Mountain Research Station develops scientific information and technology to improve management, protection, and use of

the forests and rangelands. Research is designed to meet the needs

of National Forest managers, Federal and State agencies, public and

private organizations, academic institutions, industry, and individuals.

Studies accelerate solutions to problems involving ecosystems,

range, forests, water, recreation, fire, resource inventory, land reclamation, community sustainability, forest engineering technology,

multiple use economics, wildlife and fish habitat, and forest insects

and diseases. Studies are conducted cooperatively, and applications

may be found worldwide.

Research Locations

Flagstaff, Arizona Reno, Nevada

Fort Collins, Colorado* Albuquerque, New Mexico

Boise, Idaho Rapid City, South Dakota

Moscow, Idaho Logan, Utah

Bozeman, Montana Ogden, Utah

Missoula, Montana Provo, Utah

Lincoln, Nebraska Laramie, Wyoming

*Station Headquarters, Natural Resources Research Center,

2150 Centre Avenue, Building A, Fort Collins, CO 80526