United States Department of Agriculture

Forest Rocky Mountain General Technical Report

Service Research Station RMRS-GTR-342 May 2016

Forest Root Diseases 

Across the United States
Lockman, I. Blakey; Kearns, Holly S.J., eds. 2016. Forest root diseases across the United 

States. Gen. Tech. Rep. RMRS-GTR-342. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 55 p. 

Abstract

The increasing importance and impacts of root diseases on the forested ecosystems 

across the United States are documented in this report. Root diseases have long-term 

impacts on the ecosystems where they reside due to their persistence onsite. As a group of 

agents, they are a primary contributor to overall risk of growth loss and mortality of trees in 

the lower 48 States. Root diseases kill trees, decay wood, slow tree growth, predispose trees 

to other mortality agents, and cause trees to fail or fall over. In this manner, these diseases 

reduce timber volume, alter forest composition and structure, impair ecosystem function, and 

decrease carbon sequestration. Root diseases cause tree failures that can result in serious 

injury, damage, or death, and are thus important components in hazard tree management. 

Without adequate knowledge of root diseases and their roles in forested ecosystems, our 

ability to manage their negative impacts and build resilient forest landscapes is severely 

compromised.

Keywords: Armillaria, Heterobasidion, Leptographium, Onnia, schweinitzii, Phellinus 

sulphurascens, Phytophthora, Ganoderma

Authors: 

I. Blakey Lockman—Plant Pathologist, Northern Region, Missoula, Montana

Holly S.J. Kearns—Plant Pathologist, Pacific Northwest Region, Sandy, Oregon

James T. Blodgett—Plant Pathologist, Rocky Mountain Region, Rapid City, South Dakota

Phil Cannon—Regional Plant Pathologist, Pacific Southwest Region, Vallejo, California

Michelle Cram—Plant Pathologist, Southern Region, Athens, Georgia

MaryLou Fairweather—Plant Pathologist, Southwestern Region, Flagstaff, Arizona

Greg Filip—Regional Plant Pathologist, Pacific Northwest Region, Portland, Oregon

John Guyon—Plant Pathologist, Intermountain Region, Ogden, Utah

Manfred Mielke—Area Plant Pathologist, Northeastern Area, St. Paul, Minnesota

Robin Mulvey—Plant Pathologist, Alaska Region, Juneau, Alaska 

Lori Winton—Plant Pathologist, Alaska Region, Anchorage, Alaska

John Withrow—Quantitative Ecologist, Cherokee Nation Technologies, Contractor for 

Forest Health Technology Enterprise Team, Fort Collins, Colorado

Photos: Pete Angwin, James Blodgett, Michelle Cram, MaryLou Fairweather, Greg Filip, 

Stephen Fraedrich, John Guyon, Holly Kearns, Blakey Lockman, and Don Owen
Forest Root Diseases Across the United States

Editors: I. Blakey Lockman and Holly S.J. Kearns 

Acknowledgments

Editorial Reviews and Additional Material:

Pete Angwin, Frank Betlejewski, Gregg DeNitto, Ellen Goheen, Sue Hagle, Zack Heath, Ned 

Klopfenstein, Bruce Moltzan, Bill Schaupp, Dave Shaw, Eric Smith, James Worrall, Mark 

Zweifler, and many unnamed forest health specialists who contributed to various parts of this 

paper

Map Reviews:

Pete Angwin, Aaron Bergdahl, Kelly S. Burns, Kristen Chadwick, Brennan Ferguson, Ellen 

Goheen, Linda Haugen, Marcus Jackson, Alan Kanaskie, Helen Maffei, Danielle Martin, Mike 

McWilliams, Dan Miller, Isabel Munck, Brent Oblinger, Amy Ramsey, Angel Saavedra, James 

Worrall, Paul Zambino, and many unnamed forest health specialists who supplied information 

for the root disease maps

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 or federal agencies, or both, before they 

can be recommended.

CAUTION:

PESTICIDES

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.

i

All Rocky Mountain Research Station publications are published by U.S. Forest Service employees and 

are in the public domain and available at no cost. Even though U.S. Forest Service publications are not 

copyrighted, they are formatted according to U.S. Department of Agriculture standards and research 

findings and formatting cannot be altered in reprints. Altering content or formatting, including the cover 

and title page, is strictly prohibited.
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Executive Summary

Summary

•  Root diseases are the most damaging group of forest diseases in the United States

(fig. 1).

•  Root diseases, as a group of agents, are one of the three primary contributors to overall

risk of tree mortality in the lower 48 States, according to the National Insect and

Disease Forest Risk Assessment (NIDFRA) (Krist et al. 2014).

•  Root diseases have long-term impacts on forest ecosystems due to their persistence

onsite.

•  All forest types and tree species are affected by one or more root diseases.

•  Root diseases kill trees, decay wood, slow tree growth, predispose trees to other mortality agents, and cause trees to fail or fall over.

•  Root diseases reduce timber volume, alter forest composition and structure, impair

ecosystem function, and decrease carbon sequestration.

•  Root diseases alter forest biodiversity and wildlife habitat by creating canopy openings, snags, and down woody material, which could have either a positive impact or a

negative impact on wildlife.

•  Root diseases cause tree failures that can result in serious injury, damage, or death, and

are thus important components in hazard tree management.

Background

•  Most root diseases are caused by fungi; the exceptions are those caused by

Phytophthora species, which are water molds.

Figure 1—Root diseases are predicted to cause extensive mortality across the contiguous United States 

over the next 15 years. Root diseases have an impact in Alaska and Hawaii, but there are too few data 

for modeling risk from root diseases in those States. (Krist et al. 2014).
iv

•  Most root diseases decay lignin and cellulose in tree roots, compromising water and

nutrient uptake and vertical stability of infected trees.

•  Damage from root diseases is chronic and can develop over long periods, frequently

without readily visible symptoms, so is not easily detected by aerial detection surveys.

•  Root diseases, as a group of agents, are predicted to cause well over 1 billion ft2 (93 

million m2) of basal area mortality over 15 years (2013 to 2027) (Krist et al. 2014).

This damage amounts to the death of 1.4 billion trees of 12 inches (30.5 cm) diameter

at breast height—more than the damage predicted for oak decline/gypsy moth, mountain pine beetle, or southern pine beetle (fig. 2).

Future Direction and Needs

•  Techniques to measure distribution and severity of root diseases are very limited

or even nonexistent in some locations. Newer technologies need to be evaluated to

improve our ability to recognize root diseases and measure their impacts over large

areas.

•  Long-term monitoring is necessary to properly measure and evaluate the impacts of

root diseases on growth and mortality of host trees, site factors that affect presence

and severity, and interactions between root diseases and other biotic and abiotic

agents. As our nation’s forests begin to undergo the stresses of climate change, longterm monitoring will become even more crucial for evaluating root diseases and other

agents.

•  Information on the basic biology of some of the root disease organisms is lacking. The

role of many of these organisms and their function within the ecosystems they occupy

are not well understood.

•  Some sites are so severely affected by root diseases that the current forest cannot be

economically managed, and restoration funding becomes necessary to shift forest

composition to root-disease-tolerant species.

Figure 2—Root diseases are among the greatest mortality risks to forests in the United States, predicted 

to cause more than 1 billion ft2 (93 million m2) of mortality over 15 years (2013–2027) (Krist et al. 2014).
v

•  Without adequate knowledge of root diseases and their roles in forested ecosystems,

our ability to manage their impacts and build resilient forest landscapes is critically

compromised.

Highlights by U.S. Forest Service Region

For each U.S. Forest Service region (see fig. 1), the extent of forest land area at greatest

risk of losses from root diseases and the effects of the most prevalent root diseases are summarized below.

Northern Region (R1)

•  Root diseases will put 7.6 million acres (3.1 million ha) at risk of losing 15 percent or

more of total basal area between 2013 and 2027, according to NIDFRA predictions

(Krist et al. 2014).

•  Of the 7.6 million acres (3.1 million ha) most threatened by root diseases, 1 out of every 4 acres or hectares (a total of 1.9 million acres, or 769,000 ha) is at risk of losing

25 percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  Armillaria root disease, Heterobasidion root disease, laminated root rot, schweinitzii

root and butt rot, and tomentosus root rot are the main root diseases in this region.

•  According to data collected by the U.S. Forest Service, Forest Inventory and Analysis

(FIA) crews, nearly 5.8 million acres (2.3 million ha) are currently infested with one

or more root diseases (fig. 3), which cause an estimated loss of more than 166 million

ft3 per year (4.7 million m3 per year).

•  Factors contributing to this high level of root diseases are fire suppression shifting

species composition toward species susceptible to root diseases and the introduction

of white pine blister rust, which has significantly reduced the amount of western white

pine, a root-disease-tolerant species.

Figure 3—Acres of forest land affected by root diseases, by root disease severity class, in the 

Northern Region (R1). Nearly 5.8 million acres are currently infested with some level of root diseases 

as determined by aboveground symptoms (Lockman et al. 2015). 
vi

Rocky Mountain Region (R2)

•  Root diseases will put more than 2.3 million acres (930,000 ha) at risk of losing

15 percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  Of the approximately 2.3 million acres (930,000 ha) most threatened by root diseases,

1 out of every 7 acres or hectares (a total of 310,000 acres, or 125,000 ha) is at risk

of losing 25 percent or more of total basal area between 2013 and 2027, according to

NIDFRA predictions (Krist et al. 2014).

•  Armillaria root disease has its greatest impact in spruce/fir and mixed-conifer forests.

•  Heterobasidion root disease can be found within the range of white fir in Colorado,

and occurs in pines in central Nebraska.

•  Black stain root disease infests

2 to 4 percent of pinyon

pine stands in southwestern

Colorado (fig. 4), causing

losses of more than 60 percent

on infested sites.

•  White mottled rot occurs in

aspen throughout the region,

causing significant hazards in

recreation sites.

•  Schweinitzii root and butt rot

and tomentosus root rot also

contribute to the damage in

this region.

Southwestern Region (R3)

•  Root diseases will put more than 830,000 acres (336,000 ha) at risk of losing 15

percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  Of the approximately 830,000 acres (336,000 ha) most threatened by root diseases,

12,000 acres (4,800 ha) are at risk of losing 25 percent or more of total basal area between 2013 and 2027, according

to NIDFRA predictions (Krist

et al. 2014).

•  Armillaria root disease contributes to dieback and mortality

in old-growth spruce/fir forests

and has the greatest impact on

white fir.

•  Heterobasidion root disease

occurs on 9 percent of mixedconifer forests and 14 percent of

spruce/fir forests (fig. 5).

Figure 4—Black stain root disease mortality center in a pinyon pine stand in southwestern Colorado (photo: H. Kearns, 

U.S. Forest Service).

Figure 5—Stump-centered mortality caused by 

Heterobasidion root disease (photo: U.S. Forest Service).
vii

•  Tomentosus root rot in spruce is a serious problem in recreation sites with infected trees.

•  White mottled rot occurs in aspen throughout the region, causing significant hazards in

recreation sites.

Intermountain Region (R4)

•  Root diseases will put more than 108,000 acres (44,000 ha) at risk of losing 15 percent

or more of total basal area between 2013 and 2027, according to NIDFRA predictions

(Krist et al. 2014).

•  Armillaria root disease is found on

cool, moist alpine sites, infecting

mainly true firs.

•  Tomentosus root rot (fig. 6) occurs

in spruce forests, causing pockets of

disease and mortality, which are especially challenging on recreation sites.

•  White mottled rot is common in older

aspen stands and causes tree failure.

•  Heterobasidion root disease commonly occurs in spruce/fir forests, causing

lower-stem decay and windthrow.

Pacific Southwest Region (R5)

•  Root diseases will put more than 1 million acres (405,000 ha) at risk of losing 15

percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  More than 8 million acres (3.2 million ha) of

commercial forest land on all ownerships are

adversely affected by some level of root disease.

•  Heterobasidion root disease is estimated to infect

more than 12 percent of California’s 16 million

acres (6.5 million ha) of commercial forest land,

with annual losses estimated at 9.25 million ft3

(262,000 m3).

•  Eighteen percent of true fir stands are negatively

affected by Heterobasidion root disease.

•  Twenty percent of pine stands in eastern

California have been deemed nonproductive due

to infection by Heterobasidion root disease.

•  Port-Orford-cedar root disease (fig. 7), caused

by a nonnative, invasive pathogen, has infested

about 9 percent of Port-Orford-cedar forest

types.

•  Basic information on the distribution and impacts of root diseases for Hawaii is severely lacking.

Figure 6—Tomentosus root rot causes significant 

decay in roots and basal stem wood of live, standing trees (photo: B. Lockman, U.S. Forest Service).

Figure 7—Dead and dying PortOrford-cedar along a California 

waterway infested with Port-Orfordcedar root disease (photo: D. Owens, 

California Department of Forestry and 

Fire Protection, used with permission).
viii

Pacific Northwest Region (R6)

•  Root diseases will put almost 2.8 million acres (1.1 million ha) at risk of losing 15

percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  Of the approximately 2.8 million acres (1.1 million ha) most threatened by root diseases, 47,000 acres (19,000 ha) are at risk of losing 25 percent or more of total basal

area between 2013 and 2027, according to NIDFRA predictions (Krist et al. 2014).

•  Armillaria root disease, Heterobasidion root disease, laminated root rot, and PortOrford-cedar root disease are the biggest contributors to damage.

•  The largest known centers of Armillaria root disease mortality occur in this region

(fig. 8).

•  In eastern Oregon, 30 percent of ponderosa pine and

nearly 100 percent of true

fir stands are infested with

Heterobasidion root disease.

•  Laminated root rot causes

mortality on 5 to 16 percent

of Douglas-fir forest types in

western Oregon.

•  Port-Orford-cedar root disease,

caused by a nonnative, invasive

pathogen, has infested about

9 percent of Port-Orford-cedar

forest types.

Southern Region (R8)

•  Root diseases will put more than 1.6 million acres (647,000 ha) at risk of losing 15

percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  Of the approximately 1.6 million acres (647,000 ha) most threatened by root diseases,

1 out of every 9 acres or hectares (a total of 310,000 acres, or 125,000 ha) is at risk of

losing 25 percent or more of total basal

area between 2013 and 2027, according

to NIDFRA predictions (Krist et al. 2014).

•  Heterobasidion root disease is predicted

to cause losses of more than 15 percent

of the pine basal area on 2.9 million acres

(1.2 million ha) between 2013 and 2027.

•  High losses from Heterobasidion root

disease (fig. 9) are related to the shift

in tree species from longleaf pine to

loblolly pine.

Figure 8—Mortality pocket caused by Armillaria root 

disease, U.S. Forest Service, Pacific Northwest Region 

(photo: G. Filip, U.S. Forest Service).

Figure 9—Canopy openings due to mortality 

from Heterobasidion root disease-caused mortality in planted pine in Texas (photo: G. Mason, 

U.S. Forest Service).
ix

•  Littleleaf disease (fig. 10) is found on 35 percent

of the commercial range of shortleaf pine, infesting 1.4 million acres (567,000 ha) and causing

estimated losses of more than $15 million per

year.

•  Red root and butt rot affects mature and oldgrowth pine trees, slash pine infected with

fusiform rust, and eastern white pine plantations

older than 30 years.

Northeastern Area (R9)

•  Root diseases will put nearly 430,000 acres

(174,000 ha) at risk of losing 15 percent or

more of total basal area between 2013 and 2027,

according to NIDFRA predictions (Krist et al.

2014).

•  Of the almost 430,000 acres (174,000 ha) most threatened by root diseases, 1 out of

every 3 acres or hectares (a total of 135,000 acres, or 55,000 ha) is at risk of losing

25 percent or more of total basal area between 2013 and 2027, according to NIDFRA

predictions (Krist et al. 2014).

•  Heterobasidion root disease is well distributed in New York and causes damage in

thinned pine stands in Michigan and Wisconsin.

•  Armillaria root disease readily infects stressed trees and is often associated with declines caused by interacting agents.

Alaska Region (R10)

•  Basic information on the distribution and impacts of root diseases is severely lacking.

•  Heterobasidion root disease is known to occur in southeast Alaska in old-growth western hemlock and Sitka spruce forests.

•  Tomentosus root rot is thought to be widespread throughout spruce stands in southcentral and interior Alaska.

•  Armillaria is known to occur in many forest types, hastening the death of stressed trees.

Figure 10—Impacts of littleleaf disease on shortleaf pine, U.S. Forest 

Service, Southern Region (photo: 

U.S. Forest Service, Bugwood.org).
x
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Contents

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Summary . .

Background . .

Future Direction and Needs . 

Highlights by U.S. Forest Service Region  

Introduction 

Armillaria Root Disease Armillaria Species 

Biology and Ecology 

Occurrence 

Impacts 

Management  

Heterobasidion Root Disease Heterobasidion Species .

Biology and Ecology .

Occurrence .

Impacts .

Management  .

Laminated Root Rot Phellinus sulphurascens Pilát 

(formerly P. weirii (Murrill) Gilb. Douglas-fir Form) . . . . . . . . . . . . . . .15

Biology and Ecology .

Occurrence .

Impacts .

Management  .

Black Stain and Related Root Diseases Leptographium Species .

Biology and Ecology .

Occurrence .

Impacts .

Management  .

Schweinitzii Root and Butt Rot Phaeolus schweinitzii (Fr.) Pat. .

Biology and Ecology .

Occurrence .

Impacts .

Management  .
xii

Tomentosus Root Rot and Red Root and Butt Rot Onnia Species .

Biology and Ecology .

Occurrence .

Impacts .

Management  .

Phytophthora Root Diseases Phytophthora Species  .

Biology and Ecology .

Port-Orford-Cedar Root Disease Phytophthora lateralis

Tucker and Milbrath .

Occurrence .

Impacts .

Management  .

Littleleaf Disease Phytophthora cinnamomi Rands .

Occurrence .

Impacts .

Management  .

Sand Pine Root Disease Phytophthora cinnamomi Rands .

Occurrence .

Impacts .

Management  .

White Mottled Rot Ganoderma applanatum (Pers.) Pat. .

Biology and Ecology .

Occurrence .

Impacts .

Management  .

Conclusions .

References  .

Appendix. Common and Scientific Names for Host Tree 

Species Used in This Paper  .
USDA Forest Service RMRS-GTR-342. 2016. 1

Introduction

Root diseases are the most damaging group of diseases that cause volume loss of forest

trees in the United States (Krist et al. 2014). All forest types and tree species are affected by

one or more root diseases. These pathogens kill trees, decay wood, slow tree growth, predispose trees to other damaging agents, and cause trees to break or fall over. As a result, root

diseases affect timber volume (Smith 1984), forest composition and structure, ecosystem

function (Hagle et al. 2000), personal safety (Filip and Goheen 1982a; International Tree

Failure Database 2010), and carbon sequestration (Raymond et al. in review; Washington

State Academy of Sciences 2013). All but one of the root diseases discussed in this paper

are caused by native pathogens, and as such they have been long-term associates of the forest tree species they affect and enduring components of the forest ecosystems they occupy.

Management practices, fire suppression, climate change, and the introduction of exotic tree

pathogens probably have altered the historic roles of root diseases and, in many cases, exacerbated their effects.

Most root diseases are caused by fungi that typically decay lignin and cellulose in roots

of host trees, thus compromising the trees’ ability to absorb water and nutrients and often

leading to tree failure when structural roots become severely decayed (Manion 1991). Root

pathogens may also invade and block crucial water-conducting tissues and create necrotic

lesions in the phloem that eventually girdle and kill host trees. By weakening host trees, root

diseases can predispose them to windthrow and other mortality agents, such as bark beetles

(Scolytinae subfamily) or drought. Root diseases also affect the growth of host trees, and

major growth impacts often occur before a tree displays symptoms of infection aboveground

(Cruickshank et al. 2011; Thies 1983).

Root disease pathogens have long-term impacts on the forest ecosystems they occupy

due to their longevity onsite (Hagle et al. 2000), and today’s forests most likely reflect

millennia of effects from root diseases. These pathogens can remain on the site long after

infected trees die. They survive saprophytically in root systems for years to decades, thus

becoming relatively permanent components of the site. This long-term site occupancy by

root pathogens, along with such factors as aspect, soils, and moisture regimes, can affect the

productivity of a site. Site productivity is generally reduced due to growth loss and mortality

of host trees, which in turn affects carbon sequestration. By selectively removing susceptible

host trees and leaving tolerant or resistant species, root diseases often change successional

pathways by altering forest structure and composition. Conversely, these openings also allow for increased diversity in structure and species of plant life (Hansen and Goheen 2000;

Kearns and Jacobi 2005). In addition to creating canopy openings, root diseases create snags

and down woody material, all of which affect wildlife habitat.

Root diseases also threaten personal safety and property in the form of hazard trees (Filip

and Goheen 1982a; Marosy and Parmeter 1989). Hazard trees are trees that pose a major

safety risk because of their propensity to fall due to mechanical failure, often as a result of

root decay. Root diseases also have a large but unmeasured impact on recreation sites (Filip

et al. in preparation).

Most root-disease pathogens spread from infected root systems to the roots of neighboring trees via root-to-root contact or pathogen growth between adjacent root systems. Year

after year, root diseases kill millions of trees across the United States within large pockets

of mortality or as scattered small groups or individual trees, thereby causing long-term
2 USDA Forest Service RMRS-GTR-342. 2016.

openings in the forest canopy. Root disease fungi occupy roots of infected trees and most

remain onsite for years to decades after death of the host, ready to infect subsequent generations of trees. Consequently, damage from root diseases is chronic and protracted.

As a group of agents, root diseases are predicted to cause mortality of more than 1 billion

ft2 (93 million m2) of tree basal area by 2027 (Krist et al. 2014). This volume is equivalent

to the death of 1.4 billion trees of 12 inches (30.5 cm) diameter at breast height. Year-to-year

changes in damage from root diseases are not easily detected and are often overshadowed

by more fast-acting agents, such as bark beetles. However, cumulative losses from root

diseases are predicted to be greater than losses from other agents, such as oak decline/gypsy

moth (Lymantria dispar Linnaeus), pine beetles (Dendroctonus spp.), or fire (fig. 11), and

damage from root diseases is predicted to increase under current climate change scenarios

(Kliejunas 2011; Kliejunas et al. 2009; Klopfenstein et al. 2009; Washington State Academy

of Sciences 2013).

An example from research on forest land in the U.S. Forest Service’s Northern Region

illustrates the severity of damage from root diseases compared to other agents. In northern

Idaho and western Montana, forested land area affected by root diseases has been calculated

to be more than 5.7 million acres (2.3 million ha) as determined by aboveground symptoms

on U.S. Forest Service, Forest and Inventory Analysis (FIA) plots (Lockman et al. 2015).

Based on these acreage estimates, the volume lost from all root diseases in northern Idaho

and western Montana is estimated to be almost 166 million ft3 (4.7 million m3) per year.

In contrast, volume lost to mountain pine beetles (Dendroctonus ponderosae Hopkins) and

fire combined is estimated to be just over 150 million ft3 (4.2 million m3) per year (average

from 2000 to 2009), even including the current mountain pine beetle epidemic (Kegley et al.

2011). Furthermore, previous studies have shown that root disease commonly predisposes

trees to attack by mountain pine beetles (Bartos and Schmitz 1998; Fuller 1983; Goheen and

Hansen 1993; Kulhavy et al. 1984; Tkacz and Schmitz 1986).

Several models were used to project impacts from root diseases and the results were

compiled in the 2013–2027 National Insect and Disease Forest Risk Assessment (NIDFRA)

(Krist et al. 2014) (fig. 12). Risk is defined as the expectation that 25 percent or more of

the host basal area will be lost over the next 15 years, though risk is also reported in the

Figure 11—Root diseases are among the greatest mortality risks to forests in the United States, predicted to cause more than 1 billion ft2 (93 million m2) of mortality over 15 years (Krist et al. 2014).
USDA Forest Service RMRS-GTR-342. 2016. 3

following categories of loss: no loss, 1 to 4 percent loss, 5 to 14 percent loss, 15 to 25 percent loss, and 25 percent or greater loss (Krist et al. 2014). Root diseases as a group are one

of the three primary contributors to overall risk of mortality in the lower 48 states, according

to the NIDFRA models (figs. 11 and 12). Although root diseases are known to affect forested

lands in Alaska and Hawaii, basic information about the distribution and impacts of root

diseases is lacking for these States, so risk could not be calculated.

The NIDFRA models predict that during the 15-year period from 2013 to 2027 root

diseases will put almost 2.6 million acres (1 million ha) at risk of losing 25 percent or

more of total basal area across the United States. The risk of predicted basal area loss

of 25 percent or more varies by U.S. Forest Service region (Krist et al. 2014) (fig. 12).

The Northern Region has the most acreage at risk of the largest expected basal area loss

with 1.9 million acres (769,000 ha), and the Rocky Mountain Region has more than

310,000 acres (125,000 ha) at risk of the highest basal area loss. The Southern Region

has about 170,000 acres (69,000 ha) and the Northeastern Area has nearly 135,000 acres

(55,000 ha) in the highest predicted basal area loss category. The Pacific Northwest Region

and Southwestern Region have fewer acres at risk of the highest basal area loss, with

47,000 acres (19,000 ha) and 12,000 acres (4,800 ha), respectively. The Pacific Southwest

and Intermountain Regions have very few acres with predicted basal area losses of 25 percent and greater. However, basal area losses of 15 to 24 percent are predicted on more than

1 million acres (405,000 ha) in the Pacific Southwest Region and more than 108,000 acres

(44,000 ha) in the Intermountain Region. As mentioned above, basic information on the

distribution and impacts of root diseases is severely lacking in the Alaska Region, so risk of

root disease-related losses was not modeled for this region.

Figure 12—Root diseases are predicted to cause extensive mortality across the contiguous United States over 

the next 15 years (Krist et al. 2014). Root diseases have an impact in Alaska and Hawaii, but there are too few 

data for modeling risk from root diseases in those States.
4 USDA Forest Service RMRS-GTR-342. 2016.

The agents modeled in NIDFRA to calculate risk from root diseases were Armillaria

root diseases, Heterobasidion root diseases, laminated root rot, and Port-Orford-cedar root

disease. This paper discusses these four root diseases, as well as others that have significant

impacts across the United States (table 1 and see the earlier section on “Highlights by U.S.

Forest Service Region” for regional summaries). Armillaria species are widespread across

the country and cause the most damage in the Pacific Northwest and Northern Regions.

Heterobasidion species are also widely distributed; they cause significant losses in the

Southern and Pacific Southwest Regions and are having an increasing impact in the Pacific

Northwest and Northern Regions. Heterobasidion irregulare causes damage in the Lakes

States and localized areas elsewhere in the Northeastern Area. Leptographium species are

broadly distributed but have the most impact in the western regions. Phellinus sulphurascens

is restricted to the Pacific Northwest and Northern Regions, where it is considered the most

important individual natural disturbance agent in affected areas (Hansen and Lewis 1997).

Diseases caused by Onnia species and Phaeolus schweinitzii are widely distributed throughout the United States but represent more of a problem as decay agents of structural roots in

the West and the Southern Region and spruce plantations in the Lake States. Ganoderma 

applanatum affects aspen stands, and causes notable damage in the Rocky Mountain Region.

Phytophthora species affect pine forests in the Southern Region. The exotic Phytophthora 

lateralis causes a significant root disease in Port-Orford-cedar in the coastal areas of Oregon

and California. Recent research suggests species of Phytophthora are also involved in widespread declines of oak in the East.

In 1984, Smith estimated the volume of timber lost annually to root diseases in the

western United States to be 237.4 million ft3 (6.7 million m3). This volume accounted for

about 18 percent of the total mortality in the western United States at that time. Impacts of

root diseases nationwide have not been assessed since that report, although regional and

local estimates have been made (Byler et al. 1990; Hansen and Goheen 2000; James et al.

1984). The purpose of this paper is twofold: to update the information in Smith (1984) and

to document the importance of root diseases and the impacts they are having on U.S. forest

ecosystems.

Armillaria Root Disease 

Armillaria Species

Biology and Ecology

Armillaria species are among the most damaging and broadly distributed forest tree

pathogens in the world and the contiguous United States (fig. 13) (Shaw and Kile 1991;

Tainter and Baker 1996). Several species in the genus Armillaria are pathogenic to varying degrees to North American conifers and hardwoods of all ages. Armillaria ostoyae 

(Romagn.) Herink, Herink, J.,1 A. mellea (Vahl: Fr.) Kummer, A. calvescens Bérubé &

1 An earlier classified species, A. solidipes Peck, has been proposed to replace A. ostoyae

(Burdsall and Volk 2008), but a vote is pending to conserve A. ostoyae taxon (Hunt et al. 2011;

Redhead et al. 2011).
USDA Forest Service RMRS-GTR-342. 2016.

5

Table 1—Root disease pathogens and the U.S. Forest Service regions in which they have significant impact.

USFS region 

name (numeric 

designation)

Armillaria 

root disease

(Armillaria

spp.)

Heterobasidion 

root disease

(Heterobasidion

spp.)

Laminated root 

rot 

(Phellinus

sulphurascens)

Black stain and 

related root 

diseases

(Leptographium

spp.)

Schweinitzii 

root and butt rot 

(Phaeolus

schweinitzii)

Tomentosus 

root rot and 

red root and 

butt rot 

(Onnia spp.)

Phytophthora 

root diseases

(Phytophthora

spp.)

White 

mottled rot 

(Ganoderma

applanatum)

Northern Region (R1) X X X X X

Rocky Mountain Region (R2) X X X X X XSouthwestern Region (R3) X X X XIntermountain Region (R4) X X X XPacific Southwest Region (R5)

X X X X X

Pacific Northwest Region (R6) X X X X X X

Southern Region (R8) X X X XNortheastern Area (R9) X X X X X XAlaska Region (R10) X X X
6 USDA Forest Service RMRS-GTR-342. 2016.

Dessur., A. sinapina Bérubé & Dessur., A. gallica Marxmüller & Romagn., A. gemina

Bérubé & Dessur., A. tabescens (Scop.) Emel, and A. altimontana Brazee, B. Ortiz, Banik,

& D.L. Lindner (formerly NABS X) cause root disease throughout the United States.

These Armillaria species affect most of the important tree species in the Northeastern Area

(Blodgett and Worrall 1992; Brazee and Wick 2009). The main cause of Armillaria root

disease in western conifers is A. ostoyae, whereas the other Armillaria species tend to be less

pathogenic or only saprophytic (Filip et al., in preparation; Shaw and Kile 1991). However,

A. mellea is a significant pathogen on hardwoods in California (Baumgartner and Rizzo

2001), and A. sinapina and A. gallica are thought to be the main cause of root disease in

hardwoods in Wyoming and South Dakota (Blodgett 2015; Blodgett and Lundquist 2011).

Armillaria species can persist on a site for decades in the coarse roots of infected trees

(Redfern and Filip 1991). One A. ostoyae clone in Oregon is causing disease on nearly 2,400

acres (970 ha) and has been estimated to be at least 1,900 years old (Ferguson et al. 2003).

Once established on a root of a living host, the fungus invades and kills the cambium of the

root and then decays the root tissues, which compromises the absorption of water and nutrients, causing direct mortality or greater susceptibility to stem failure and attack by insects

and other pathogens (Baumgartner et al. 2011; Shaw and Kile 1991). Spread occurs when

the fungus moves from an infected root system to neighboring trees by root-to-root contact,

or by limited growth of rhizomorphs through the soil. Rhizomorphs are root-like structures

Figure 13—Distribution of hosts and damage caused by Armillaria root disease (Armillaria spp.) at the county 

level across the contiguous United States. In green counties, the hosts are present, but the specific root disease 

has not yet been detected although not all areas have been surveyed. In orange counties, both the tree hosts and 

the root disease are present. In red counties, the root disease is considered damaging in multiple forest stands to 

such an extent that management is affected.
USDA Forest Service RMRS-GTR-342. 2016. 7

Figure 14—Rhizomorphs of an Armillaria species 

(photo: J. O’Brien, U.S. Forest Service, Bugwood.org).

(fig. 14) that are able to grow through the upper soil layers for several feet and directly infect

tree roots. The reproductive structures of Armillaria species are mushrooms (fig. 15), the

spores of which appear to play a limited role in spreading the disease (Baumgartner et al.

2011).

Occurrence

Pathogenic species of Armillaria are distributed throughout most of the forested regions

of the continental United States, Alaska, and Hawaii; however, the impacts they have

on forest ecosystem services vary greatly by species of Armillaria, host, and site. In the

Northeastern Area, the primary hosts are species of oak, maple, beech, birch, aspen, balsam

fir, spruce, and pine (see the appendix for tree species nomenclature). Armillaria root disease

occurs throughout the Rocky Mountain Region and primary hosts include Douglas-fir,

true firs, spruce, and lodgepole and ponderosa pines, and aspen, narrowleaf and plains cottonwoods, balsam poplar, bur oak, paper birch, green ash, and other hardwoods. Interior

Douglas-fir and true firs are the most susceptible tree species in the Pacific Northwest and

Northern Regions, but pine and other species may be highly susceptible on some sites, especially if the trees are maladapted to the site. In the Southwestern Region, interior Douglas-fir

and Engelmann spruce sustain the most damage, but in some areas ponderosa pine is also

a prominent host (Omdal et al. 2004). In Alaska and the Southern Region, Armillaria is

not usually the primary cause of tree mortality, but is an opportunist on many tree species

affected by other factors. In Hawaii, Armillaria species have been found associated with

declining stands, but the overall incidence on the Hawaiian islands is unknown (Hanna et al.

2007; Kim et al. 2010).

Impacts

Armillaria root disease affects many forest products and services including recreation,

timber, and aesthetics. Along with causing indirect tree mortality through snapping or

uprooting, Armillaria causes direct tree mortality, growth reduction, and wood loss due to

decay, and predisposes trees to other lethal agents. Mortality due to infection by A. ostoyae 

in the Northern Region is greatest on warm, moist habitats though large disease pockets

also develop on dry sites and cold, high-elevation sites (Byler et al. 1990). Armillaria root

disease is broadly distributed throughout much of the Intermountain Region but causes little

direct mortality except in seedlings and saplings regenerating near infected stumps of the

Figure 15—Mushrooms of Armillaria ostoyae

(photo: B. Lockman, U.S. Forest Service).
8 USDA Forest Service RMRS-GTR-342. 2016.

previous generation; however, rot develops in the basal stem wood (butt) of mature trees in

these areas (Hagle 2010). In the Pacific Northwest Region, disease is severe on grand fir,

white fir, and interior Douglas-fir, and sometimes on pines and other species. Armillaria root

disease is generally worse east of the Cascade Mountains and in southwestern Oregon but

does cause high levels of damage on some harsh sites in western Oregon and Washington,

especially in areas with compacted soils (Goheen and Willhite 2006). In the Southwestern

and Intermountain Regions, Armillaria root disease is contributing to dieback and mortality

in old-growth spruce and true fir forests, but causing only minor losses in younger forests

except in areas with substantial soil disturbance. In the Rocky Mountain Region, a recent,

systematic survey across forest types in Wyoming found Armillaria species causing disease in 22 percent of surveyed locations (J.T. Blodgett, U.S. Forest Service, Forest Health

Protection, Rapid City, SD, pers. comm., March 5, 2015). In southeast Alaska, Armillaria

root disease is a leading cause of heart rot of western hemlock and Sitka spruce.

Mortality from Armillaria root disease may occur in patches up to tens of acres in size

or as scattered individual tree mortality (fig. 16). In the West, changes in forest tree species composition have resulted in an increase in species more susceptible to Armillaria

root disease, such as interior Douglas-fir, subalpine fir, white fir, and grand fir (Filip et al.

2007; Filip et al., in preparation; Hagle et al. 1995, 2000). Tree species composition has

changed primarily because of fire exclusion but also because of other factors including the

introduction of Cronartium ribicola Fisch. (the cause of white pine blister rust). In northern Idaho and western Montana, long-term plots (Hagle 1985) and analyses indicate that

Armillaria root disease infests more than 12 million acres (4.9 million ha), and almost 2.5

million of those acres (1 million ha) will have greater than 25-percent mortality of mature

stems by 2027 (Krist et al. 2007). Damage is most severe on the best growing sites, and

severely affected stands often convert to brush fields in the absence of tree species tolerant

to Armillaria root disease. In Colorado, annual volume losses by Armillaria in subalpine fir

have been estimated to be 370,800 ft3 (10,500 m3) (Allen et al. 2011; Fuller and Johnson

1985). In severely infested Pacific Northwest forests, tree mortality caused by A. ostoyae

ranges from 25 to 50 ft3 per acre per year (1.7 to 3.5 m3 per ha per year) (Filip et al. 2010).

In addition to tree mortality, Armillaria significantly affects tree growth and site productivity. Recent research has shown that many infected trees have greatly reduced growth with no

aboveground symptoms (Bloomberg and Morrison 1989; Cruickshank 2011; Cruickshank

et al. 2011).

Figure 16—Distinct pockets of mortality from Armillaria root disease as seen from the air in central 

Montana (left) (GoogleTM Earth 2014) and on the ground in Oregon (right) (photo: G. Filip, U.S. Forest 

Service).
USDA Forest Service RMRS-GTR-342. 2016. 9

In the Northeastern Area, past mortality from Armillaria root disease most commonly

occurred as scattered individual trees. More recently, impact from Armillaria appears to

be increasing due to synergistic effects of exotic insects and diseases and extreme weather

events including ice storms and severe drought. Although rarely quantified, in forest plantations Armillaria species can cause 3 to 50 percent mortality over a rotation (Wargo and

Harrington 1991), and some of the most widely distributed species also frequently cause butt

rot (Brazee and Wick 2009). Detrimental impacts of many Armillaria species are often associated with environmental stresses, such as drought and late spring frost (Wargo 1996). In

the Northeastern Area where turkey (Meleagris gallopavo), deer (Odocoileus spp.), and bear

(Ursus spp.) populations are highly dependent on oaks and beech, Armillaria root disease

often results in mast reduction and loss of the oak type.

Armillaria root disease predisposes trees to insects (Bartos and Schmitz 1998; Fuller

1983; Goheen and Hansen 1993; Hertert et al. 1975; Kulhavy et al. 1984; Tkacz and Schmitz

1986) and other diseases (Filip et al., in preparation). During periods between bark beetle

epidemics, trees affected by root disease foster endemic beetle populations. In the Front

Range of Colorado and the Black Hills of South Dakota, 62 percent and 75 percent of ponderosa pine attacked by mountain pine beetle, respectively, were also infected by Armillaria

root disease (Fuller 1983). In the Northeastern Area, Armillaria species are associated with

several native and exotic insect defoliators. For example, losses of oak trees attacked by

Armillaria species after defoliation by the exotic gypsy moth have been reported throughout

the Northeast. Armillaria species have also been associated with native defoliating insects of

oaks and maples (Wargo 1996) and partial cuttings and disturbance (Wargo and Harrington

1991). Considering this association with environmental stress, insect defoliators, and disturbance, one would expect Armillaria-related mortality to increase due to severe storms, insect

defoliator outbreaks, and fragmented landscapes. Under a changing climate, the incidence of

and impact from Armillaria root disease are expected to increase. If growing conditions also

become drier, then the impact from Armillaria root disease will be even greater due to the

increased stress on the host trees (Kliejunas 2011; Kliejunas et al. 2009; Klopfenstein et al.

2009). Armillaria root disease is also a common contributing factor in many forest declines

such as oak decline (Kile et al. 1991; Wargo and Harrington 1991).

Armillaria also has a large but unmeasured impact on recreation sites (Filip et al., in

preparation). Live trees often fall due to mechanical failure resulting from root decay, which

is a major safety concern in these sites. Armillaria root disease is a concern in all hazard

tree mitigation efforts in developed sites. It is also an important consideration in vegetation

restoration projects where susceptible hosts are a significant component.

Management

The primary focus for management of Armillaria root disease is observing which species

on the site appear to be tolerant or resistant and favoring those species during thinning and

regeneration (Filip et al. 2010; Filip et al., in preparation; Hadfield et al. 1986; Hagle and

Shaw 1991; McDonald et al. 2005; Rippy et al. 2005; Shaw et al. 2009). Prevention of soil

compaction and tree wounding during management activities is encouraged. Where groups

of infected trees, or disease centers, are recognized, these centers can be managed differently

from uninfected areas around the centers. The removal of infected stumps during regeneration may significantly reduce root disease in the future stand (Hagle and Shaw 1991;

Morrison et al. 2014), but this approach has not been widely implemented in the United
10 USDA Forest Service RMRS-GTR-342. 2016.

States. Initial trials of biological control agents for Armillaria root disease are promising and

warrant further investigation (Chapman et al. 2004).

Heterobasidion Root Disease 

Heterobasidion Species

Biology and Ecology

Heterobasidion root disease (fig. 17) in North America is caused by the fungi

Heterobasidion irregulare Garbelotto & Otrosina and H. occidentale Otrosina &

Garbelotto. Most coniferous species (and a few hardwoods) are susceptible to infection by

Heterobasidion species, although the two species have distinct host specificities. In general, 

H. irregulare infects pine species, whereas H. occidentale infects other species (Otrosina

and Garbelotto 2010). Both species produce perennial fruiting structures (sporophores or

conks) that release wind-disseminated spores (Asiegbu et al. 2005; Hodges 1969; Hsiang et

al. 1989) (fig. 18). Infections caused by H. irregulare spores are usually limited to the tops

of freshly cut stumps in pine species (Otrosina and Cobb 1989), whereas H. occidentale

spores readily infect freshly cut stumps and fresh stem wounds in other species (Otrosina

and Garbelotto 2010). Heterobasidion species in the newly infected stump or tree may then

infect a neighboring tree via root-to-root contact, leading to the development of disease

Figure 17—Distribution of hosts and damage caused by Heterobasidion root disease, (a) Heterobasidion 

occidentale and (b) H. irregulare, at the county level across the contiguous United States. In green counties, the hosts are present, but the specific root disease has not yet been detected although not all areas 

have been surveyed. In orange counties, both the tree hosts and the root disease are present. In red 

counties, the root disease is considered damaging in multiple forest stands to such an extent that management is affected.

a
USDA Forest Service RMRS-GTR-342. 2016. 11

b

Figure 17—(Continued).

centers (fig. 19). Although both species have been

isolated from “nonhost” stumps without causing

disease in neighboring “host” trees (Kliejunas

1986; Lockman 1993; Otrosina et al. 1992),

interior Douglas-fir and true firs are occasionally

infected when growing near infected pine stumps.

Occurrence

Heterobasidion species exist across the continental United States and Alaska, but their impacts

and the host species affected differ by region. In

the Southern Region, Heterobasidion root disease

affects all pines but has the greatest impact on

the commercial species including loblolly, slash,

shortleaf, and longleaf pines. In the Northeastern

Area, Heterobasidion root disease has been found

causing damage in one county in Minnesota, and

is widespread in Wisconsin and Michigan, where it

is most damaging to red and jack pines after thinning (Stanosz et al. 1995; Wisconsin Department

of Natural Resources 2003), especially on sandy

sites. True firs, interior Douglas-fir, and ponderosa

pine are the most susceptible and affected species

in the Northern Region; true firs, hemlocks, and

Figure 19—Pockets of mortality due to 

Heterobasidion root disease in planted 

pine near Nacogdoches, TX. (photo: G. 

Mason, U.S. Forest Service, Bugwood.

org).

Figure 18—Heterobasidion fruiting structures in a fir stump (photo: H. Kearns, U.S. 

Forest Service).
12 USDA Forest Service RMRS-GTR-342. 2016.

ponderosa pine are the most affected species in the Pacific Northwest Region. Across the

Pacific Southwest Region, Heterobasidion species are widespread and can be found on

nearly every coniferous species (Bega and Smith 1966; Schmitt et al. 2000; Smith 1989),

although true firs, ponderosa pine, and Jeffrey pine are the most severely affected trees. In

the Intermountain Region, Heterobasidion species can be found on every coniferous species

except limber pine, whitebark pine, and western larch (Tegethoff 1973), and H. occidentale

is common in subalpine fir stands in high-elevation areas. In the Rocky Mountain Region,

H. occidentale is common in forests with white fir (Worrall et al. 2010). In coastal Alaska,

low-level impacts from Heterobasidion root disease are reported in young stands of hemlock

and Sitka spruce, and in old-growth timber in southeast Alaska.

Creation of stumps from harvesting provides entry courts for the spores of

Heterobasidion species, favoring development of the disease (fig. 20). In one survey on

the Fremont National Forest in southern Oregon,

true fir stands with multiple harvest entries had

a 100-percent incidence of Heterobasidion root

disease, compared to 12 percent of stands with no

history of harvesting (Goheen and Goheen 1989).

In the Southern Region, soil texture is an important

factor in the severity of the disease (Alexander et

al. 1975); soils at high risk for Heterobasidion root

disease are characterized as sand, sandy loam, or

loamy sand deeper than 10 inches (25 cm), with

a low water table (Morris and Frazier 1966). It is

thought that well-drained, sandy soils increase potential drought stress of the host, which may favor

subsequent growth of the pathogen in the roots

(Towers and Stambaugh 1968). Sandy soils also allow for increased root connections (Kuhlman 1974)

and foster the ability of spores to percolate through

the soil and infect roots directly (Kuhlman 1969).

In the Northeastern Area, Heterobasidion root disease is widespread in New York south of

the Adirondack Mountains but is severe only in pine plantations on sandy sites in the eastern

and southeastern part of the state. Plantations on former agricultural sites may sustain more

damage than those on continuously forested sites.

Impacts

Root and butt rot caused by Heterobasidion species progresses from resin-soaked and

stained wood, to a white pocket rot or white stringy rot. Trees affected by Heterobasidion

root disease often appear healthy until more than half the roots, or the majority of the root

collar, are infected. Trees with Heterobasidion root disease often become windthrown or are

attacked by bark beetles prior to crown symptoms of decline (Schmitt et al. 2000; Sinclair

and Lyon 2005).

Heterobasidion root disease can reduce tree growth and subsequently increase host

susceptibility to bark beetle attack (Alexander 1989; Ferrell and Smith 1976; Wisconsin

Department of Natural Resources 2003). Radial growth losses of southern pines have

been documented to range from 6 to 36 percent; height growth was reduced by 40 percent

Figure 20—Mortality around stump infected with Heterobasidion root disease 

(photo: G. Filip, U.S. Forest Service).
USDA Forest Service RMRS-GTR-342. 2016. 13

in trees with greater than 50-percent colonization (Alexander 1989). Trees stressed by

Heterobasidion root disease are often attacked by bark beetles such as Ips species, Scolytus 

species, and Dendroctonus terebrans Olivier (Barnard 1999; Ferrell and Smith 1976;

Livingston et al. 1983; Morris 1970). This root disease can also play a major role in southern

pine beetle attack (Dendroctonus frontalis Zimmermann) (Alexander et al. 1980, 1981).

Heterobasidion root disease causes serious economic loss by way of mortality, growth

loss, and the necessity of early harvest of a lower value product. In the Southern Region,

the NIDFRA model (Krist et al. 2014) indicates that 38 million acres (15.4 million ha) of

loblolly-slash-shortleaf pine forest are at risk of some level of loss from Heterobasidion root

disease, with more than 3.3 million acres (1.3 million ha) expected to lose at least 15 percent

of pine basal area in the next 15 years. More than 380,000 acres (153,800 ha) of eastern

white pine are at some level of risk, with 167,000 of those acres (67,600 ha) expected to

lose at least 25 percent of the host basal area over 15 years. The value of timber products

harvested in the Southern Region in 2007 was estimated at $12 billion (Hanson et al. 2010)

with softwood accounting for about 69 percent of these products (Wear et al. 2007). The

monetary losses from Heterobasidion root disease in the Southeast are in the millions of dollars per year.

In several forested areas of eastern Oregon, 30 percent of pine stands are estimated to

be infested by Heterobasidion root disease; infested stands support only 77 percent of the

stocking and 44 percent of the basal area when compared to uninfested stands (Goheen and

Goheen 1989). Heterobasidion root disease in true fir stands east of the Cascade Mountains

has increased significantly due to the creation of large stumps from selective harvesting. In the Northern Region, an estimated 4 million acres (1.6 million ha) are affected by

H. occidentale. Greater than 25-percent mortality over 15 years is expected on more than

770,000 of those acres (311,600 ha) (S.K. Hagle, unpublished data).2 Another 114,000 acres

(46,100 ha) of ponderosa pine are affected by H. irregulare in western Montana

(B. Lockman, unpublished data).2

In the Rocky Mountain and Southwestern Regions, few data are available for

Heterobasidion species, but frequency is thought to be low for both species (Worrall et al.

2010). In the Southwestern Region, annual mortality rates were estimated at 1 percent in

the pine type, 9 percent in mixed-conifer forests, and 14 percent in spruce-fir forests (Wood

1983); however, rates are increasing in the wake of selective harvesting, aging stands, and

drought. In contrast, it is thought that lack of large-tree harvesting coupled with extensive

stand-replacing fires has led to a significant reduction of H. irregulare in southern Idaho

(J.T. Hoffman, personal communication, May 31, 2011). In the Pacific Southwest Region,

Heterobasidion root disease is estimated to infect approximately 12.5 percent of the State’s

16 million acres (6.5 million ha) of commercial forest land, resulting in annual losses of

about 222 million board feet (MMBF) (18.5 million ft3, 524,000 m3) (California Forest

Pest Council 1988). This equates to an annual loss approaching $50 million (July 2014–

December 2014 average stumpage values for ponderosa pine; California State Board of

Equalization 2014). In southeast Alaska, a Heterobasidion species is one of three fungi

contributing to a one-third volume loss in old-growth timber (Filip and Morrison 1998).

2 Data on file with B. Lockman, U.S. Forest Service, Forest Health Protection, Missoula, MT.
14 USDA Forest Service RMRS-GTR-342. 2016.

Heterobasidion root disease is also a threat to visitor safety and property in the form of

hazard trees on public lands (Filip and Goheen 1982a; Marosy and Parmeter 1989). National

forests across the nation have an estimated annual visitation rate of more than 173 million

people (U.S. Forest Service 2010). Tree failures from root disease are a great concern; for all

entries into the International Tree Failure Database through July 2010, 35.2 percent of tree

failures were due to root failures (International Tree Failure Database 2010). Although actual

injury and property loss from hazard trees are infrequent, managers of recreation sites are

likely to be held liable and sued for compensation (Anderson 1988).

Heterobasidion root disease also affects carbon sequestration. The southeastern forests

are estimated to sequester 100 million metric tons of carbon dioxide per year, or a third of

U.S. forests’ capacity to absorb carbon dioxide (Jose 2007). Losses to Heterobasidion root

disease are estimated to be greater than 15 percent of the 2012 host basal area on 3.6 million acres (1.5 million ha) by 2027 (Krist et al. 2014). And, as the climate changes, losses

from Heterobasidion root disease are expected to increase (Kliejunas et al. 2009; Nilson et

al. 1999). In the Southern Region, infected stands will probably be harvested prematurely,

altering carbon sequestration for years. These areas could also be converted to a land use

other than forestry that is seen as more profitable, which may greatly reduce future carbon

sequestration.

Management

Preventing the introduction of the disease onto a site is the best method available for

reducing the impact from Heterobasidion species in forests. This approach is especially applicable in pine stands, where stem wounds are less likely to become a site for infection than

in species with thinner bark. Establishment of Heterobasidion from spores can be prevented

by using stump treatments, which is the primary method to prevent Heterobasidion root

disease (Asiegbu et al. 2005). Treatments to prevent stump infection include two borate

salts: sodium tetraborate decahydrate (SporaxTM) and disodium octaborate tetrahydrate

(Cellu-TreatTM).3 Registrations for these products are necessary for use, but such registrations are dependent on the financial feasibility to the producers. As of this writing, the

registration for Sporax, the most commonly used stump treatment in the United States,

is scheduled to be discontinued due to the high cost of maintaining its registration. Local

strains of the fungus Phlebiopsis gigantea (Fr.) Jülich are currently being tested for the control of Heterobasidion root disease in the Southern Region. Additional work is necessary to

get this product labeled for use in the United States.

Silvicultural techniques are also available for minimizing the impacts from this disease,

such as minimizing wounds to residual trees, converting a site to tree species more tolerant to Heterobasidion root disease, and managing density to decrease root contacts and

improve drought tolerance of residual trees (Alexander 1989; Asiegbu et al. 2005; Rippy

et al. 2005). In the Southern Region, new pine stands are planted on a wide spacing (200 to

3 The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S.

Department of Agriculture or the Forest Service of any product or service to the exclusion of others

that may be suitable.
USDA Forest Service RMRS-GTR-342. 2016. 15

400 trees per acre, 494 to 988 trees per ha) to delay thinning and thus reduce future infections (Alexander 1989). Wide spacing also decreases the potential for root-to-root contact,

thus decreasing the spread potential if the fungus does enter the stand. Minimizing wounding of

residual trees is highly recommended in the Pacific Northwest (Hadfield et al. 1986; Petersen

1989; Rippy et al. 2005), especially in thin-barked species, such as hemlock and true fir (Russell

1989). Wounds are considered the main point of entry in these thin-barked species.

Although expensive and not widely used, removal of stumps is a method that has been

applied to high-value sites such as seed orchards (Alexander 1989). In recreation areas that

have large numbers of affected trees, it may be necessary to close a site permanently or at

least until the susceptible overstory is removed (West 1989).

Laminated Root Rot 

Phellinus sulphurascens Pilát (formerly P. weirii

(Murrill) Gilb. Douglas-fir Form)

Biology and Ecology

Phellinus sulphurascens Pilát (formerly P. weirii (Murrill) Gilb. Douglas-fir form) causes

laminated root rot (fig. 21), which affects Douglas-fir and other commercially important

conifer species in the northwestern United States. Phellinus sulphurascens is a fungus that

produces a mostly annual, crust-like fruiting structure, or sporophore. Spores are dispersed

by wind or water, occasionally landing on a substrate that is suitable for their survival

Figure 21—Distribution of hosts and damage caused by laminated root rot (Phellinus sulphurascens) at 

the county level across the contiguous United States. In green counties, the hosts are present, but the 

specific root disease has not yet been detected although not all areas have been surveyed. In orange 

counties, both the tree hosts and the root disease are present. In red counties, the root disease is considered damaging in multiple forest stands to such an extent that management is affected.
16 USDA Forest Service RMRS-GTR-342. 2016.

(Nelson 1976). The mycelium that grows from a germinated spore must establish on suitable

wood and then outcompete other fungi in order to survive. Once established, it spreads from

woody substrates to the roots of live trees through direct root contact with infested wood.

The fungus invades and kills the cambium of the root and then decays the dead root tissues.

It may eventually travel up the root to colonize the root collar, and girdle the tree.

Many conifers in the northwestern United States are occasionally infected, but Douglasfir, mountain hemlock, grand fir, and white fir are the most susceptible species in the Pacific

Northwest Region (Hadfield and Goheen 1986; Hansen and Goheen 2000). In the Northern

Region, Douglas-fir and grand fir are the most common hosts, with subalpine fir commonly

infected when growing in proximity to grand fir (Thies and Sturrock 1995).

Laminated root rot is a “disease of the site”; established mycelia of this fungus are essentially permanent on a site (Thies

and Sturrock 1995). The fungus

spreads underground through tree root

systems and survives for many decades

in dead roots and stumps. Phellinus 

sulphurascens kills trees in expanding

patches that can be from a few trees

to many acres in size (fig. 22). These

patches may be dispersed throughout

a stand. Trees of all ages are killed as

their roots are destroyed by the fungus.

Many also die from windthrow following decay of support roots.

Occurrence

Laminated root rot is best known and probably most damaging in coastal Douglas-fir

forests of the Pacific Northwest Region. Surveys indicate that nearly 9 percent of the

Douglas-fir forest area in western Oregon is occupied by laminated root rot centers in which

half of the Douglas-firs have been killed (Hansen and Goheen 2000). In interior Pacific

Northwest forests, laminated root rot can cause severe damage where Douglas-fir and true

fir exist. It is also a major disease in forests of the Northern Region, where it causes problems on a narrow range of habitats; cool, moist habitats typified by western hemlock and

warm, moist grand fir habitat types make up most of the infested sites. Byler et al. (1990)

found sites on the Lolo National Forest in western Montana on western hemlock habitat type

to have far higher probability of laminated root rot than other habitat types.

Impacts

Significant amounts of decay can occur in root systems before trees infected with laminated root rot begin showing obvious crown symptoms (fig. 23). Slowed growth is usual

the first observable symptom with cumulative growth loss dependent on the age of the tree.

Diameter growth is more affected by laminated root rot than is height growth in coastal

Douglas-fir (Harrington and Thies 2007; Thies 1983). Bloomberg and Wallis (1979) detected

reduced radial increment in infected trees lasting 30 years or more before death in mature

Douglas-fir; smaller trees had shorter periods of growth decline before death. The largest declines in growth occur in trees with the highest rates of decay (Harrington and Thies 2007).

Figure 22—Large, expanding mortality pocket from laminated root rot (Phellinus sulphurascens) in northwestern 

Montana (photo: B. Lockman, U.S. Forest Service).
USDA Forest Service RMRS-GTR-342. 2016. 17

Laminated root rot is especially damaging

if susceptible hosts are regenerated in areas where the disease was already present

in previous stands.

In coastal forests, grand fir is the most

susceptible species, followed by Douglasfir. Nelson and Sturrock (1993) measured

grand fir mortality rates averaging about 31

percent in 17 to 20 years for trees planted

around infected stumps in coastal Oregon

and British Columbia. Rates for coastal

Douglas-fir averaged about 26 percent. Filip

and Schmitt (1979) found mortality rates of 20 percent in Douglas-fir (mostly coastal variety)

and 23 percent in grand fir. Similar rates of mortality were measured in the Northern Region on

randomly placed plots, although interior Douglas-fir had a higher mortality rate than grand fir.

A net loss in basal area of 35 percent for Douglas-fir and 20 percent for grand fir was measured.

Monitored trees were at least 5 inches (12.7 cm) diameter at breast height at the start of the study

and were followed over a 20-year period (S.K. Hagle, unpublished data).

In the Northern Region, it is estimated that nearly 740,000 acres (299,500 ha) are infested

with P. sulphurascens. Of these, more than 150,000 acres (60,700 ha) have mortality rates of

at least 25 percent of the basal area every 15 years. Large canopy gaps with over 75-percent

canopy loss caused by laminated root rot are estimated to occupy more than 15,000 acres

(6,100 ha) of the most productive forest land types in the Northern Region (S.K. Hagle,

unpublished data). Douglas-fir and grand fir forests on warm, moist site types are damaged

the most (Byler et al. 1990). Estimates of area occupied by laminated root rot in the Pacific

Northwest Region vary from 5 to 11 percent of Douglas-fir forests, and the annual volume

lost is estimated to range from 32 million to 83 million ft3 (906,000 to 2.4 million m3)

(Washington State Academy of Sciences 2013).

Laminated root rot significantly affects developed sites throughout its range, and is

considered the most serious root disease affecting developed-recreation sites in the Pacific

Northwest Region (Filip and Goheen 1982a). Campgrounds have been closed due to

the hazards represented by P. sulphurascens (K.L. Chadwick, personal communication,

February 27, 2013).

In contrast with the negative impacts that P. sulphurascens has on forest stands, its presence can lead to an increase in plant community diversity. Laminated root rot is a major

factor in shaping forest structure, composition, and processes, such as successional trajectories, wherever it occurs (Hansen and Goheen 2000).

Impacts from root diseases potentially could increase under current climate change scenarios

due to the expected increased stress of their host trees (Woods et al. 2010). The incidence and

spread of P. sulphurascens is not tied to host stress, but is more dependent on the distribution of

the hosts, which may be affected by climate change (Hansen and Goheen 2000).

Management

Methods for managing laminated root rot fall into two general categories: silvicultural control

of the disease and removal or reduction of the inoculum (infected substrate wood material)

(Hadfield et al. 1986; Rippy et al. 2005; Thies and Sturrock 1995). Silvicultural control includes

Figure 23—Extensive decay from laminated root rot 

(Phellinus sulphurascens) in roots of live Douglas-fir 

(photo: G. Filip, U.S. Forest Service).
18 USDA Forest Service RMRS-GTR-342. 2016.

planting and managing for tree species tolerant to laminated root rot in an approach called species

conversion. The easiest time to accomplish this is at the final harvest, when an infected stand can

be regenerated to more tolerant species. Favoring tolerant species during any intermediate entry

into an infected stand can also be used to decrease losses to laminated root rot as well as reduce

the inoculum on the site. Wide spacing of young stands is another method of controlling disease

within an infested stand. Wide spacing (more than 13 feet, 4 meters, between trees) reduces root

contacts between trees, which in turn reduces the spread of the fungus through the root systems.

This method should be used with great caution, as periodic entries are required to remove natural

regeneration, but the method is appropriate for Douglas-fir age 60 years or less growing on deep

soils (Thies and Sturrock 1995).

Removal or reduction of inoculum can be accomplished by stump removal and push-over

logging (Hadfield et al. 1986; Thies and Sturrock 1995). Both techniques remove the majority of infected root systems from the site, thus removing the inoculum and greatly reducing

carryover of the disease into the next generation of trees. Inoculum removal or reduction

allows managers to grow Douglas-fir on high-value sites infested with laminated root rot.

Both techniques can cause extensive disturbance on a site with mixed effects on growth of

the subsequent stand (Morrison et al. 2014; Zabowski et al. 2008).

Inoculum reduction by the use of biological control agents antagonistic to P. sulphurascens is a possibility, but a suitable biological control agent has yet to be found (Thies and

Sturrock 1995). Use of chloropicrin for chemical reduction of inoculum in stumps has been

tested with no detectable decrease in replacement-stand mortality resulting from laminated

root rot (Thies and Westlind 2006).

Black Stain and Related Root Diseases 

Leptographium Species

Biology and Ecology

Several species of Leptographium are associated with root diseases of conifers in the United

States. In the western United States, black stain root disease (BSRD) (fig. 24), a wilt-like disease

of conifers caused by the native, insect-vectored, fungal pathogen Leptographium wageneri 

(Kendr.) Wingf. (Harrington and Cobb 1983, 1988), is considered the most important. In terms

of pathogenesis, BSRD diverges from typical root diseases by neither causing wood decay

nor invading the cambium prior to tree death (Cobb 1988). Leptographium wageneri is commonly found in ponderosa, lodgepole, and Jeffrey pines (var. ponderosum Harrington & Cobb);

Douglas-fir (var. pseudotsugae Harrington & Cobb); and pinyon and singleleaf pinyon pines

(var. wageneri (Kendr.) Wingf.). It is considered one of the five most damaging root diseases in

western forests (Hadfield et al. 1986) and is widespread across much of the range of its hosts, but

incidence and severity, and thus the importance to forest management, vary greatly (Ferguson

2009). Because L. wageneri is an insect-vectored pathogen and the vectors are generally attracted

to weakened trees, this fungus often occurs in complexes with other root disease fungi (Filip and

Goheen 1982b; Goheen and Filip 1980; Morrison and Hunt 1988). It has been found colonizing

the same roots of Douglas-fir as Armillaria ostoyae (Byler et al. 1983; Goheen and Hansen 1978;

Morrison and Hunt 1988), Phellinus sulphurascens (Filip and Goheen 1982b; Goheen and Filip

1980; Morrison and Hunt 1988; Witcosky 1989), and a Heterobasidion species (Kelsey et al.

1998).
USDA Forest Service RMRS-GTR-342. 2016. 19

Figure 24—Distribution of hosts and damage caused by black stain root disease (a, b, and c: three varieties of Leptographium wageneri) at the county level across the contiguous United States. In green counties, 

the hosts are present, but the specific root disease has not yet been detected although not all areas have 

been surveyed. In orange counties, both the tree hosts and the root disease are present. In red counties, 

the root disease is considered damaging in multiple forest stands to such an extent that management is 

affected.

b

a
20 USDA Forest Service RMRS-GTR-342. 2016.

In the eastern United States, two other species of Leptographium cause damage,

L. procerum (Kendr.) Wingf. and L. terebrantis Barras & Perry. Both species are associated with red pine decline in the Lakes States. Red pine decline is caused by a complex of

interacting organisms in which both L. procerum and L. terebrantis are considered weakly

pathogenic and consistently work in concert with insects. Among these insects are root

collar weevil (Hylobius radicis Buchanan), pales weevil (H. pales Herbst), red turpentine

beetle (Dendroctonus valens LeConte), pitch-eating weevil (Pachylobius picivorus Germar),

and Hylastes porculus Erichson, which feed on freshly cut stumps and the lower stems

and roots of red pine and carry the fungi into the lower stem and root system (Erbilgin and

Raffa 2002; Klepzig et al. 1991). Bark beetles (Ips pini Say and I. grandicollis Eichhoff) are

ultimately responsible for the tree’s death (Erbilgin and Raffa 2002; Klepzig et al. 1991).

Leptographium terebrantis is also present in the Rocky Mountain Region and has caused

significant damage in some stands (Blodgett 2013; Holah 1993).

Leptographium procerum is the causal agent of procerum root disease, which results in

root lesions, basal cankers, and occasionally death of pine species, especially young (less

than 20-year-old) eastern white pine. Procerum root disease is often associated with pines

stressed by poor sites, weevil infestations of the major roots and root collar, and other root

pathogens (Alexander et al. 1988; Sinclair and Lyon 2005). Once established in a pine

plantation with interconnected root systems, Leptographium species can spread to healthy

trees through root connections. The wood of colonized roots is resin soaked, and the infected

trees become stressed because they have reduced capacity to absorb and move water and

synthesize defensive compounds (fig. 25). These stressed trees continue to attract beetles

that feed on the lower stem, particularly the red turpentine beetle (Erbilgin and Raffa 2002;

Klepzig et al. 1991).

c

Figure 24—(Continued).
USDA Forest Service RMRS-GTR-342. 2016. 21

Occurrence

Although Leptographium wageneri can be found throughout northern Idaho and

Montana, BSRD is generally observed only in conjunction with other root pathogens considered to be the primary agent of mortality (Byler et al. 1983), or in offsite ponderosa pine

plantations (Ferguson 2009). This disease is present throughout the range of pinyon pine in

western Colorado, where it is generally found on the more productive pinyon sites (Landis

and Helburg 1976). A 1998

ground survey in southwestern

Colorado found an estimated

2 to 4 percent of pinyon pine

were infected (W.R. Jacobi,

unpublished data)4 (fig. 26).

In Nevada, Utah, and southern

Idaho, BSRD is found on just

over 3,000 acres (1,200 ha)

of pinyon pine stands used

for Christmas tree production

(J. Guyon, personal communication, April 24, 2011),

where it is often most apparent

at the beginning of a period of

drought. In the central Sierra

Nevada Mountains, a high proportion of the largest and most active infection sites in ponderosa pine occur in moist, cool areas, and spread of the pathogen appears to be more rapid

in gullies and small creek drainages (Wilks et al. 1985).

4 Data on file with W.R. Jacobi, Professor Emeritus, Colorado State University, Department of

Bioagricultural Sciences and Pest Management, Fort Collins, CO.

Figure 25—Characteristic 

staining of the water-conducting 

tissue in the sapwood of a pine 

with black stain root disease 

(Leptographium wageneri var. 

wageneri) (photo: W. Jacobi, 

Colorado State University, 

Bugwood.org).

Figure 26—Black stain root disease center (Leptographium

wageneri var. wageneri) in pinyon pine, southwestern Colorado 

(photo: H. Kearns, U.S. Forest Service).
22 USDA Forest Service RMRS-GTR-342. 2016.

In Washington, Oregon, and northern California the disease is most commonly found

in 10- to 30-year-old Douglas-fir plantations and dense natural regeneration west of the

Cascade Crest and in ponderosa pine of all ages east of the Cascade Mountains. In southwestern Oregon, BSRD was detected

in 19 percent of five hundred 10- to

30-year-old Douglas-fir plantations,

compared to 1 percent with Armillaria

root disease and 7 percent with laminated root rot (Hessburg et al. 2001).

Approximately 80 percent of mortality

centers in Douglas-fir are associated

with disturbed areas: road edges,

clear-cut margins, thinnings, areas

with drainage problems, and places

where previous soil disturbance such

as compaction and displacement have

occurred (Goheen and Hansen 1978;

Hansen 1978). To date there has been

no systematic effort to map L. wageneri in the western United States, but

historical records of L. wageneri have

been cataloged in the California Insect

and Disease Atlas (CAIDA) database

(CAIDA 2014) (fig. 27).

In the eastern United States, procerum root disease causes mortality and growth losses in

eastern white pine and red pine primarily in young pine and Christmas tree plantations. In

the Northeastern Area, root diseases caused by Leptographium species occur mainly in the

Lake States, often in red pine planted on lower quality pin oak sites, where the root diseases

are associated with red pine decline. In the Southeast, the species play a role in the decline

of pines damaged by various causes.

Impacts

In the Northeastern Area, Leptographium infections in red pine and eastern white pine

result in mortality and growth losses, with mortality between 20 and 50 percent in some

plantations (Sinclair and Lyon 2005). Live pinyon pine density in discrete, recently formed

BSRD mortality centers in southwestern Colorado was reduced by an average of 63 percent,

and 76 percent of pinyon in these centers were infected by L. wageneri (Kearns and Jacobi

2005). In Oregon, Washington, and northern California, L. wageneri is considered to cause

moderate impact to forest resources. Disease centers range in size from 1 tree to 10 acres

(4 ha, several hundred trees) and contain groups of dead trees interspersed with and surrounded by symptomatic and healthy live trees. In a study of young (5- to 28-year-old)

Douglas-fir plantations in Washington and Oregon, Hansen and Goheen (1988) reported that

31 percent of the trees were killed. In the most heavily affected Douglas-fir plantations in

southwestern Oregon and northwestern California, stocking levels were found to be reduced

by as much as 50 percent (DeNitto 1982; Goheen et al. 1983, 1984, 1985). In northern Idaho

and Montana, it is rare for BSRD to be found acting as a primary pathogen of Douglas-fir

Figure 27—Historical records of black stain root disease (Leptographium wageneri) in California Insect 

and Disease Atlas (CAIDA May 2014). 
USDA Forest Service RMRS-GTR-342. 2016. 23

or pines, and the disease is not considered a major concern in forest management. Although

annual mortality is relatively small, BSRD is a concern to those who harvest pinyon pine for

Christmas trees or utilize pine nuts for food in the Intermountain Region. It has been speculated that the occurrence of BSRD, at least L. wageneri var. pseudotsugae, could increase in

areas where climate change physiologically stresses the host trees (Ayres and Lombardero

2000).

Black stain root disease can result in increased plant and animal diversity from tree mortality and gap creation. In pinyon pine stands affected by BSRD in southwestern Colorado,

shrub layer composition, diversity, and cover did not differ between disease centers and

unaffected surrounding woodlands, but cover and composition of the herbaceous layer

were significantly different (Kearns and Jacobi 2005). In the northeastern United States,

root disease centers caused by Leptographium species are associated with increased species

diversity related to openings and hardwood encroachment.

Management

In southwestern Colorado, tree removal, soil fumigation, and placement of a mechanical

polyethylene barrier between healthy and diseased trees were evaluated for their ability

to limit lateral spread of BSRD centers in pinyon pine. None of the treatments was effective at preventing expansion of mortality centers (Sharon and Johnson 1987). At this time,

management of BSRD in pinyon pine is limited to identification of high-hazard sites,

cultural techniques to maintain tree vigor and spacing to avoid tree-to-tree transmission, and

replacing susceptible pinyon pine with nonsusceptible junipers (Cobb 1988). In Douglas-fir

plantations in the Pacific Northwest, current management recommendations include planting

nonsusceptible species in existing BSRD centers and avoiding site disturbances and tree

injuries to prevent establishment of new infection centers. In high-hazard areas, which are

those within 1 mile (1.6 km) of existing BSRD centers, recommendations include the avoidance of tractor logging to minimize soil disturbance, planting of mixed species, and timing

of precommercial thinnings between June and early August (Hadfield et al. 1986; Hansen et

al. 1988; Harrington and Cobb 1988; Rippy et al. 2005). The recommendations for management in ponderosa pine are similar: identify high-hazard areas; avoid monocultures; plant

mixed species with a ponderosa pine component of less than 25 percent; and avoid tree

wounding, soil compaction, and other site disturbances (Cobb 1988; Hadfield et al. 1986).

Schweinitzii Root and Butt Rot 

Phaeolus schweinitzii (Fr.) Pat.

Biology and Ecology

Schweinitzii root and butt rot (fig. 28) is caused by the fungus Phaeolus schweinitzii (Fr.)

Pat., a root pathogen that causes extensive brown-cubical decay of host trees’ large support

roots and basal stem wood (butts). In addition, root tips are killed, causing them to terminate

in gall-like swellings that greatly reduce structural support (fig. 29). Schweinitzii root and

butt rot affects a broad list of conifer hosts and causes significant damage in Douglas-fir,

Sitka spruce, western larch, western hemlock, and western white pine in western North

America (Hagle and Filip 2010). In the Lakes States, eastern larch and white and black

spruces are most commonly damaged, whereas in the Southeast, damage occurs primarily

in mature and old-growth pine trees, young slash pines growing on old naval stores sites
24 USDA Forest Service RMRS-GTR-342. 2016.

(Blakeslee and Oak 1980), and mature

or suppressed eastern white pine stands

(Hepting and Downs 1944). Phaeolus 

schweinitzii is not considered an aggressive tree killer; rather, it is relatively slow

acting, which results in a very gradual

expression of symptoms. Seedlings can

be infected and occasionally may be

killed; however, it is generally in mature

trees that root deterioration causes a

measurable decline in tree vigor and

increased likelihood of windthrow.

The primary mode of infection is via

spores that percolate through the soil,

where the fungal pathogen invades fine

roots. Phaeolus schweinitzii frequently

infects young trees and can remain

inactive for many years until heartwood

is formed within the stem and larger

lateral roots. Subsequently, the fungus

Figure 29—Gall-like swelling of Douglas-fir roots 

caused by schweinitzii root and butt rot (Phaeolus

schweinitzii) (top) and P. schweinitzii fruiting bodies 

(bottom left and right) (photos: H. Kearns, U.S. Forest 

Service).

Figure 28—Distribution of hosts and damage caused by schweinitzii root and butt rot (Phaeolus 

schweinitzii) at the county level across the contiguous United States. In green counties, the hosts are 

present, but the specific root disease has not yet been detected although not all areas have been surveyed. In orange counties, both the tree hosts and the root disease are present. In red counties, the root 

disease is considered damaging in multiple forest stands to such an extent that management is affected.
USDA Forest Service RMRS-GTR-342. 2016. 25

becomes active in the heartwood, causing brown-cubical decay (Hagle and Filip 2010).

Affected trees become stressed due to deterioration of their root systems and often are attacked by other root pathogens, especially Armillaria species, and insects like Douglas-fir

beetle (Dendroctonus pseudotsugae Hopkins) (Hadfield et al. 1986; Hagle and Filip 2010).

Because other co-occurring damaging agents are easier to identify than P. schweinitzii,

the impact and distribution of schweinitzii root and butt rot are probably underestimated.

Although trunk wounds caused by fire or mechanical injury are most likely not infected

directly by spores, wounds may exacerbate decay in previously infected roots and butts

(Hagle and Filip 2010). Stumps infected with P. schweinitzii are not considered important

sources of inoculum, and there is little spread of the fungus from tree to tree through root

contacts (Hagle and Filip 2010).

Occurrence

Schweinitzii root and butt rot is widely distributed and one of the most common root

and butt rots across North America. It is typically found scattered throughout stands rather

than in discrete root disease centers. It is very common in old-growth stands in the western

United States (Hagle and Filip 2010). An older survey of eastern white pine near Asheville,

North Carolina, found 4.5 percent of white pines were affected by P. schweinitzii (Hepting

and Downs 1944). Currently, an estimated 93,000 acres (37,600 ha) of eastern white pine

are considered at high risk for schweinitzii root and butt rot due to age and host frequency

(M. Cram, unpublished data).5

Impacts

Schweinitzii root and butt rot affects many forest resources including recreation, timber,

wildlife habitat, and other forest values and can interact with other agents of forest change.

Scattered mortality, especially of dominant trees, and individual windthrown and broken

trees are typical. This disease is a major concern in recreation areas with a significant

component of older Douglas-fir, western larch, or Sitka spruce because of the high risk of

property damage or personal injury from windthrow and breakage. In addition, schweinitzii

root and butt rot can extend up into the lower stems of trees, leading to significant volume

loss (Hagle and Filip 2010).

Schweinitzii root and butt rot can provide suitable habitat for cavity-nesting animals. In

addition, decayed butts and roots infected with P. schweinitzii disintegrate, leaving behind

brown-rot residues that are very stable and provide important properties, such as increased

water-holding capacity, to the organic component of forest floors. These organic components

may constitute up to 30 percent of the soil volume in the upper layers (Hagle and Filip

2010). Climate change that increases stress to host trees will likely increase the impacts from

agents that decay the roots and butts of trees, including P. schweinitzii.

5 Data on file with M. Cram, U.S. Forest Service, Forest Health Protection, Athens, GA.
26 USDA Forest Service RMRS-GTR-342. 2016.

Management

When managing sites with schweinitzii root and butt rot, maintaining good growth and

vigor through density management is vital (Hagle and Filip 2010). However, partial harvests

that retain predominantly Douglas-fir can result in increased mortality through windthrow,

Douglas-fir beetle attack, and breakage of leave trees. It is also imperative to rule out the

presence of more aggressive root pathogens on the site, such as A. ostoyae, when considering density reduction for managing schweinitzii root and butt rot. Where possible, other

species such as ponderosa pine and lodgepole pine, which are considered more resistant,

should be retained on the site (Hagle and Filip 2010). There is little risk of infection and

mortality of regeneration, especially if the trees will be harvested before age 150 (Hadfield

et al. 1986; Hagle and Filip 2010). Most pines are more resistant to the disease than are

Douglas-fir or Sitka spruce, so they should be favored in revegetation plantings, especially

in a timber or recreation setting.

Tomentosus Root Rot and Red Root and Butt Rot 

Onnia Species

Biology and Ecology

Two species of Onnia cause significant root disease in the continental United States: Onnia 

tomentosa (Fr.) P. Karst., which causes tomentosus root rot, and Onnia circinata (Fr.) P. Karst.,

which causes red root and butt rot (figs. 30 and 31). Tomentosus root rot is primarily a disease

of Engelmann, white, and blue spruces, but it has been reported in several other hosts including

lodgepole pine, western white pine, Douglas-fir, western hemlock, western larch, ponderosa pine,

and grand and subalpine firs. Onnia circinata is a primary pathogen affecting sand pine over the

Figure 30—Distribution of hosts and damage caused by tomentosus root rot (Onnia tomentosa) at the 

county level across the contiguous United States. In green counties, the hosts are present, but the specific root disease has not yet been detected although not all areas have been surveyed. In orange counties, 

both the tree hosts and the root disease are present. In red counties, the root disease is considered damaging in multiple forest stands to such an extent that management is affected.
USDA Forest Service RMRS-GTR-342. 2016. 27

age of 20 and one of the likely causes of sand pine root disease (Barnard et al. 1985). Other pines

affected include slash pine with basal fusiform rust cankers (Boyce 1967) and eastern white pine

in plantations older than 30 years (Hepting and Downs 1944). Red root and butt rot also affects

mature and old-growth pine trees in the Southeast and is thought to contribute to root and butt rot

in lodgepole pine in the Northern Region.

Onnia species most likely enter host trees through direct contact with infected roots or by

way of spores that land on wounded roots or root collars (Lewis and Hansen 1991; Sinclair and

Lyon 2005). The fungus colonizes the roots and basal stem wood (butts) of the host and over

time causes a characteristic white pocket rot of the roots and butts, often culminating in death

(fig. 32). Spread occurs primarily by root contacts or fungal mycelia growing short distances

through the soil, but spread via spores can also occur. Trees affected by tomentosus root rot are

Figure 32—Decay caused by tomentosus root rot (Onnia tomentosa) in Engelmann spruce roots (left), 

and O. tomentosa fruiting body (right) (photos: B. Lockman, U.S. Forest Service).

Figure 31—Distribution of hosts and damage caused by red root and butt rot (Onnia circinata) at the 

county level across the contiguous United States. In green counties, the hosts are present, but the 

specific root disease has not yet been detected although not all areas have been surveyed. In orange 

counties, both the tree hosts and the root disease are present. In red counties, the root disease is considered damaging in multiple forest stands to such an extent that management is affected.
28 USDA Forest Service RMRS-GTR-342. 2016.

often reservoirs for endemic populations of spruce beetles (Dendroctonus rufipennis Kirby) and

spruce engraver beetles (Ips spp.) (Lewis and Lingren 2002). When windthrow events trigger an

outbreak, these beetles can move from the root disease centers, or groups of root-disease-infected

trees, into the surrounding forest, causing extensive mortality.

Occurrence

The Onnia species are broadly distributed throughout North America (figs. 30 and 31).

Tomentosus root rot occurs primarily in temperate spruce forests. It is not known to be widespread in most forests, but when present it can occur as disease pockets (stand openings) that

are very difficult to manage. Soil moisture and nutrition, as well as position on the slope,

influence where these stand-opening root disease pockets occur.

Red root and butt rot is most often found on pines. Gilbertson and Ryvarden (1986) describe O. circinata as being distributed throughout North America, and illustrate its presence

in all the western States but Nevada. During the preparation of this current report, the distribution of O. circinata was documented in only a few of the States reported in Gilbertson and

Ryvarden (1986). A possible explanation for this discrepancy is that the fruiting structures

(sporophores) of O. circinata and of O. tomentosa are extremely difficult to differentiate in

the field, and most documentation of presence and damage have probably been credited to

O. tomentosa. It is important to resolve this confusion in the future, as proper identification

is necessary for management recommendations.

Impacts

Onnia-caused root diseases can have significant impacts on forest resources including

recreation, wildlife habitat, timber, and other forest values. The death of trees, windthrow,

reduced growth, loss of volume after decay moves up into the stem, and the spread of the

disease into nearby trees reduce timber

volume and value and site productivity.

Where spruce occurs on developed sites,

the presence of O. tomentosa may result in

damage to property or may endanger life

due to falling trees (fig. 33), resulting in

the permanent closing of developed sites

(Lockman and Steed 2014). Stand openings

caused by tomentosus root rot influence the

structure of the forest canopy, which could

have either a positive impact or a negative

impact on wildlife habitat, depending on

whether wildlife species favor openings or a

continuous canopy.

A survey of eastern white pine near Asheville, North Carolina, found 5.3 percent of

white pine was affected by O. circinata (Hepting and Downs 1944). Currently, an estimated

93,000 acres (37,600 ha) of eastern white pine are considered at high risk for red root a

butt rot due to age and host frequency (M. Cram, unpublished data).6

6 Data on file with M. Cram, U.S. Forest Service, Forest Health Protection, Athens, GA.

Figure 33—Tree failure of live Engelmann 

spruce at a developed site in Utah due to extensive decay caused by tomentosus root rot (Onnia 

tomentosa) (photo: J. Guyon, U.S. Forest Service).
USDA Forest Service RMRS-GTR-342. 2016. 29

Climate change that increases stress to host trees will probably increase the impacts from

tomentosus root rot (Woods et al. 2010) and from other agents that decay the roots and butts

of trees, including O. circinata.

Management

Tomentosus root rot is considered a disease of the site; once the pathogen is established,

it is very difficult to manage susceptible conifer hosts (Rippy et al. 2005). Often the best

strategy is to foster nonsusceptible hosts in the stand openings (Hadfield et al. 1986) until

infected roots completely decay, a process that can take more than 30 years (Hunt and Unger

1994). In stands with low levels of disease, replanting host trees at least 10 feet (3 m) from

infected stumps can reduce the spread of the disease. Accurate and thorough root rot surveys

are needed for this tactic (Lewis et al. 2000).

There has been little research in the management of stands infested with O. circinata, and

no direct controls are currently available. As with many root diseases, the key to management is prevention of disease by management practices such as avoiding planting species

offsite, reducing rotation ages, and preventing wounding of trees during stand entry. For example, to minimize O. circinata in sand pine, planting in sites with very deep, well-drained,

sandy soils is recommended. Stands should not be allowed to become overly dense, and

trees should be harvested prior to overmaturity (Hughes and Smith 2010).

Phytophthora Root Diseases 

Phytophthora Species

Biology and Ecology

Phytophthora species belong to a group of organisms called oomycetes, commonly referred to as “water molds” because they grow, reproduce, and infect plant roots in water and

saturated soils. Excess water allows roots to become flooded for extended periods of time.

Phytophthora species produce structures called sporangia that release swimming zoospores

(fig. 34). These zoospores are attracted

by amino acids and other chemicals

exuded from nearby elongating or

wounded roots, where they encyst,

germinate, and infect the root (Roth

et al. 1987; Sinclair and Lyon 2005).

Near-saturated soil conditions are

required for germination and dispersal

of zoospores (Pscheidt 2007).

Although soil saturation is necessary for infection, once Phytophthora

is inside plant tissues, it continues to

colonize the roots regardless of soil

conditions. Root infection is progressive and ultimately leads to aboveground symptoms. Initially, only a few fine roots are

damaged. As the pathogen invades the root system, plant cells lose the ability to absorb and

transport water and nutrients. Phytophthora continues to infect more of the root system until

it reaches the root crown area, where it kills the cambium and eventually girdles the plant.

Figure 34—Spores of Phytophthora lateralis, the cause 

of Port-Orford-cedar root disease (photo: provided by 

U.S. Forest Service).
30 USDA Forest Service RMRS-GTR-342. 2016.

Some Phytophthora species produce resting spores (chlamydospores or oospores, or

both) in the infected roots that enable the fungus to survive for extended periods of time

in the soil. While in or on the soil, the root decomposes, leaving the resting spores behind.

Infested soils can then be transferred to new areas via vehicles, animals, hikers, or potted

nursery stock (Betlejewski et al. 2011). If susceptible plants are growing or planted in the

same area, soil saturation will start the disease cycle again (Parke et al. 2008; Pscheidt 2007;

Sinclair and Lyon 2005).

Plant diseases caused by Phytophthora species have been some of the most destructive historically and continue to be among the most destructive in the world today. The Irish potato famine

of the 1840s was caused by P. infestans. The current widespread mortality of trees in Australia

has been attributed to P. cinnamomi. Sudden oak death and related diseases caused by P. ramorum have killed thousands of oaks and other species in the United States (California and Oregon)

and Europe. Some forms of a destructive European root disease on alder caused by P. alni have

been found in Alaska but have not yet been reported in any States within the continental United

States. Climate changes resulting in increased winter temperatures, seasonal precipitation shift

from summer into winter, and an increased incidence of heavy rains will favor the development

and spread of diseases caused by some Phytophthora species (Kliejunas et al. 2009; Sturrock et

al. 2011). The following is a brief discussion of forest root diseases caused by two Phytophthora 

species in the United States: P. lateralis on Port-Orford-cedar in the Pacific Northwest and P. cinnamomi on pines in the Southeast.

Port-Orford-Cedar Root Disease 

Phytophthora lateralis Tucker and Milbrath

Occurrence

Port-Orford-cedar has a narrow native range in southwestern Oregon and northwestern

California but is a valuable tree species ecologically and economically (Roth et al. 1987;

USFS 2006; Zobel et al. 1985). The species has been widely planted as an ornamental

(Lawson’s cypress) throughout the United States, Canada, and Europe. Port-Orford-cedar

root disease (fig. 35) is caused by the exotic root pathogen Phytophthora lateralis Tucker

and Milbrath. About 9 percent of the mapped Federal lands with Port-Orford-cedar in

Oregon and California are infested with P. lateralis and contain dead and dying trees

(Betlejewski et al. 2003). Pacific yew is the only other native host affected but is much less

susceptible than Port-Orford-cedar and becomes infected only if growing close to PortOrford-cedar (DeNitto and Kliejunas 1991; Murray and Hansen 1997). The pathogen is

believed to have been introduced from Asia into the Pacific Northwest in the 1920s on ornamental cedars. It was first identified in native Port-Orford-cedar forests in 1952 (Betlejewski

et al. 2011; Roth et al. 1987). The disease is spread by several spore forms that are transported in water or soil, or on other infected plants. High-risk areas for infestation include

streams, drainages, low-lying areas, and areas below roads and trails. The spread of the

pathogen via root contacts and by water-borne spores results in mortality centers. Because

Port-Orford-cedar is commonly a riparian species, dead and dying Port-Orford-cedar are

often observed along infested waterways (fig. 36).
USDA Forest Service RMRS-GTR-342. 2016. 31

Figure 35—Distribution of hosts and damage caused by 

Port-Orford-cedar root disease (Phytophthora lateralis) at the 

county level across the contiguous United States. In green 

counties, the hosts are present, but the specific root disease 

has not yet been detected although not all areas have been 

surveyed. In orange counties, both the tree hosts and the 

root disease are present. In red counties, the root disease 

is considered damaging in multiple forest stands to such an 

extent that management is affected.

Figure 36—Dead Port-Orford-cedar 

killed by Port-Orford-cedar root disease (Phytophthora lateralis) along an 

infested stream in northern California 

(photo: D. Owen, California Department 

of Forestry and Fire Protection, used 

with permission).
32 USDA Forest Service RMRS-GTR-342. 2016.

Root infection leads rapidly to tree mortality with

little or no growth loss before death. Tree mortality

can occur within 2 to 3 weeks for seedlings and saplings and may take several years for large trees. In

live Port-Orford-cedar with crown symptoms, a distinctive cinnamon-colored stain adjacent to healthy

cream-colored inner bark is present at the root collar

(fig. 37). Cedar bark beetles (Phloesinus spp.) attack

some infected trees with reduced vigor.

Impacts

Mortality of Port-Orford-cedar has economic,

social, and ecological impacts. From an economic

standpoint, tree mortality can affect the volume of

lumber available in both the foreign and domestic

markets. Export values of Port-Orford-cedar today

are at least three times higher than the domestic market values (Betlejewski et al. 2003). In the past, the

Japanese placed a high value on Port-Orford-cedar;

stumpage values often exceeded $10,000 per thousand board feet (83.3 ft3, 2.4 cubic m3) for

higher lumber grades. Domestic products include paneling, decking, fence posts and rails,

arrow shafts, and boughs for the floral industry. Besides the economic value of Port-Orfordcedar, Native Americans value the tree for a variety of reasons: family homes and sweat

houses, medical purposes, regalia for religious services, and storage containers (Betlejewski

et al. 2003). Ecologically, Port-Orford-cedar is found in 43 plant associations in Oregon and

California, some of which are unique to certain drainages (Atzet et al. 1996; Jimerson and

Daniel 1994). On serpentine soils, Port-Orford-cedar may be the only or most abundant tree

species on the site.

Snags of all diameter classes provide abundant habitat for a variety of cavity-nesting

birds and wildlife (Betlejewski et al. 2003). As snags decay and fall, Port-Orford-cedar provides habitat as down woody material that remains for decades due to the decay resistance

of the wood. Port-Orford-cedar is an important species in riparian ecosystems that provides

shade and thus lower stream temperatures. Port-Orford-cedar plant associations contain

unique species and regional endemic, rare, or sensitive plants.

Management

Several tools and recommendations have been developed to manage the impacts and

spread of P. lateralis (Betlejewski et al. 2003; BLM and USFS 2004; Hadfield et al. 1986;

USFS 2006; Zobel et al. 1985). Because Port-Orford-cedar is the main host for the disease,

favoring other tree species (except Pacific yew) greatly reduces infection and mortality

on the site. Disease-resistant Port-Orford-cedar have been developed and are important in

revegetating areas that are severely affected by root disease or wildfire (Bower et al. 2000).

Resistant Port-Orford-cedar, however, are not immune to infection, and resistant stock

Figure 37—Characteristic cinnamoncolored staining of inner bark of 

Port-Orford-cedar infected with PortOrford-cedar root disease (Phytophthora 

lateralis) in California (photo: P. Angwin, 

U.S. Forest Service).
USDA Forest Service RMRS-GTR-342. 2016. 33

should not be planted in areas where eradication of the pathogen is an important management objective or where resistant but infected trees would serve as inoculum sources to

adjacent nonresistant Port-Orford-cedar.

Roadside sanitation, or removal of living Port-Orford-cedar, is an effective tool in reducing spread (Goheen et al. 2012; Hadfield et al. 1986) because the pathogen infects only

living hosts. Another common technique is separating forest operations in diseased stands

from those in disease-free locations, both in space and time. Projects should be scheduled to

be completed in the warm and dry months and discontinued when wet conditions favorable

to pathogen spread develop. Washing stations to ensure that vehicles and equipment are

thoroughly cleaned before moving them to uninfested areas are recommended (Goheen et al.

2012). When water is needed for activities such as forest road construction and maintenance

or firefighting, uninfested water sources should be used when possible, or if water is potentially infested, it should be treated with disinfectants such as chlorine bleach.

Littleleaf Disease 

Phytophthora cinnamomi Rands

Occurrence

In the southern United States, the pathogen Phytophthora cinnamomi Rands is part of a

complex of factors that causes littleleaf disease of shortleaf pine and, to a lesser extent, of

loblolly pine (fig. 38) (Campbell and Copeland 1954; Mistretta 1984). Other pine species

are rarely affected. Littleleaf disease occurs primarily in pine stands established on eroded

agricultural sites with clay soils in the southern Piedmont (Oak and Tainter 1988).

Figure 38—Distribution of hosts and damage caused by littleleaf disease (Phytophthora cinnamomi) at 

the county level across the contiguous United States. In green counties, the hosts are present, but the 

specific root disease has not yet been detected although not all areas have been surveyed. In orange 

counties, both the tree hosts and the root disease are present. In red counties, the root disease is considered damaging in multiple forest stands to such an extent that management is affected.
34 USDA Forest Service RMRS-GTR-342. 2016.

Impacts

A survey in the early 1980s showed littleleaf disease affected 35 percent of the commercial range of shortleaf pine, equaling about 1.4 million acres (567,000 ha), and caused

an estimated annual loss of $25 million (Mistretta

1984). Currently, the scale of the disease and losses

have been reduced as managers over the past 50

years have converted severely damaged shortleaf

pine stands to loblolly pine or other more tolerant

species (Sinclair and Lyon 2005). However, loblolly

pine stands on sites conducive to littleleaf disease

still undergo significant reduced growth and premature losses. The growth decline of affected loblolly

pine begins around age 20, with growth culminating

at 30 years for trees with severe crown symptoms

and 40 years for trees with light crown symptoms

(Oak and Tainter 1988). The incidence of loblolly

pine with littleleaf symptoms is 15 percent on highrisk, 14 percent on intermediate-risk, and only 1

percent on low-risk sites (Oak and Tainter 1988).

Mortality of affected trees occurs on average 6 years

after the onset of crown symptoms (Mistretta 1984)

(fig. 39).

Management

Foresters use several management methods to prevent or control littleleaf symptoms.

Littleleaf disease is prevented largely by converting affected stands to resistant species of

hardwoods or pines. Other disease control methods include improving the site by ripping

to fragment hardpans prior to planting or fertilizing pine stands on infertile soils (Mistretta

1984).

Sand Pine Root Disease 

Phytophthora cinnamomi Rands

Occurrence

Phytophthora cinnamomi Rands is the primary pathogen involved in sand pine root

disease (figs. 40 and 41), but other agents involved include Onnia circinata and, to a lesser

extent, Phaeolus schweinitzii, Leptographium procerum, Heterobasidion irregulare, and an

Armillaria species (Barnard et al. 1985). Sand pine occurs naturally on xeric upland sands in

the Florida sandhills (Brendemuehl 1990), where it occupies about 2 million acres (809,000

ha) and is a commercial species planted on infertile, droughty sands. Sand pine root disease

is found only in sand pine plantations established with nursery seedlings. Affected stands

associated with P. cinnamomi are typically located on shallow soils with heavy clay layers

or poorly drained soils, which are soils that are not characteristic of natural sand pine sites.

The occurrence of P. cinnamomi is predominantly in young (less than 15-year-old) sand pine

stands, whereas O. circinata (cause of red root and butt rot) affects mature to old-growth

stands of 20 years and older on a variety of sites (Barnard et al. 1985).

Figure 39—Symptomatic shortleaf 

pine infected with littleleaf disease 

(Phytophthora cinnamomi) (photo: 

S. Fraedrich, U.S. Forest Service).
USDA Forest Service RMRS-GTR-342. 2016. 35

Figure 40—Distribution of hosts and damage caused by sand pine root disease (Phytophthora cinnamomi and other pathogens) at the county level across the contiguous United States. In green counties, 

the hosts are present, but the specific root disease has not yet been detected although not all areas have 

been surveyed. In orange counties, both the tree hosts and the root disease are present. In red counties, 

the root disease is considered damaging in multiple forest stands to such an extent that management is 

affected.

Figure 41—Symptomatic (left) and dead and down (right) sand pines infested with sand pine root disease (Phytophthora cinnamomi and other pathogens) (photo: M. Cram, U.S. Forest Service).

Impacts

In Florida, sand pine root disease, the disease complex with which Phytophthora cinnamomi is associated, led to losses of $1 million to $2 million per year in the early 1980s

(Barnard et al. 1985).

Management

Although it is not clear that P. cinnamomi is introduced via seedlings, ensuring seedlings

are uninfested or using seed to establish plantations would decrease potential future infestations. Careful evaluation of soils prior to planting to avoid soils with poor drainage or use of

methods to fragment hardpans may assist in reducing losses related to P. cinnamomi.
36 USDA Forest Service RMRS-GTR-342. 2016.

White Mottled Rot 

Ganoderma applanatum (Pers.) Pat.

Biology and Ecology

White mottled rot (fig. 42) is caused by the fungus Ganoderma applanatum (Pers.) Pat.

It produces a flat, shelf-like fruiting structure (sporophore) at the base of trees, often near

old wounds, and usually close to the soil (fig. 43). The sporophore is commonly called the

artist’s conk, a name that comes from its use as a drawing medium by artists. The fungus

causes extensive decay of the roots and basal stem wood (butts) of trees (fig. 43). Decayed

wood is mottled white to light tan, eventually becomes spongy, and may develop black lines.

Infections occur from airborne spores or from contacts with infected roots of neighboring

trees. Conks can produce billions of airborne spores that infect wounds at the base of trees

or on roots. Root-to-root spread leads to groups of infected trees called disease centers. The

fungus can persist on sites as a saprophyte for several years after host trees die.

Occurrence

Ganoderma applanatum occurs throughout North America (Sinclair and Lyon 2005). It is

found in all 50 States within the United States as a pathogen or as a common wood-decaying

fungus of logs and stumps of deciduous and some conifer tree species. Ganoderma-caused

root disease has been reported in live trees of many species including apple, aspen, basswood, beech, birch, cherry, citrus, cottonwood, elm, hemlock, hornbeam, horse chestnut,

Figure 42—Distribution of hosts and damage caused by white mottled rot (Ganoderma applanatum) at 

the county level across the contiguous United States. In green counties, the hosts are present, but the 

specific root disease has not yet been detected although not all areas have been surveyed. In orange 

counties, both the tree hosts and the root disease are present. In red counties, the root disease is considered damaging in multiple forest stands to such an extent that management is affected.
USDA Forest Service RMRS-GTR-342. 2016. 37

black locust, honeylocust, maple, mulberry, myrtle, oak, spruce, sycamore, tuliptree, sweetgum, and willow. It is a significant problem in cottonwood across North America and in

northern hardwoods in the East.

White mottled rot is a common problem of aspen throughout its range and is the most

important root disease of aspen in the central Rocky Mountains (Hinds 1985; Ross 1976).

Asymptomatic diseased trees, those with healthy-looking crowns, frequently fall while alive.

The disease was associated with 86 percent of windthrown, live aspen trees in Colorado

(Hinds 1985). Based on two independent systematic surveys of aspen, white mottled rot

was causing disease in 47 percent of stands in Wyoming (Blodgett and Lundquist 2008) and

in 10, 22, and 28 percent of stands in three national forests in Wyoming and South Dakota

(Blodgett et al. 2014). Small trees or trees on poor sites with dry, shallow soils may be killed

before decay leads to windthrow.

Impacts

White mottled rot affects many forest products and services including recreation, timber,

and aesthetics. Where aspen grows on developed sites, this disease may result in damage to

property or endanger

life. Live, healthy-looking aspen frequently

fall when infected with

this disease (fig. 44), so

particular attention and

consideration are needed for evaluating aspen

as potential hazard

trees in developed sites.

Along with causing

indirect tree mortality

through uprooting or

Figure 43—Sporophore of Ganoderma applanatum (left), the cause of white mottled rot (right) (photos: 

M. Fairweather, U.S. Forest Service).

Figure 44—Live aspen with roots extensively decayed by Ganoderma applanatum (the cause of white mottled rot) at the time of tree failure. Note 

the sporophore of the fungus at the base of the tree (photo: J. Blodgett, 

U.S. Forest Service).
38 USDA Forest Service RMRS-GTR-342. 2016.

snapping at the base, white mottled rot causes direct tree mortality, growth reduction, and

wood loss due to decay. It also predisposes trees to other opportunistic lethal agents such

as canker pathogens and insects. Because this disease kills and decays roots, it may reduce

root-suckering in infested stands, thus reducing regeneration success. White mottled rot

combined with other diseases and insects is among the driving forces affecting vegetation

succession in the aspen cover type. Climate change that increases stress to host trees will

most likely increase the impacts from G. applanatum and other agents that decay the roots

and butts of trees (Kliejunas et al. 2009; Woods et al. 2010). Impacts from these and other

damage agents can result in extensive changes in species composition over time, often with

displacement of aspen (Worrall et al. 2013). White mottled rot causes mortality, growth loss,

and decay, all of which lead to reduced carbon sequestration.

Management

Clear options for managing white mottled rot are needed. In developed sites, hazard tree

inspectors should be trained to recognize the conks. Conks are the only clear indicator of

infection and extensive decay in live, standing trees. Fortunately, conks often develop before

trees fall or die. Species conversion away from susceptible hardwoods is encouraged in

developed sites, especially if this disease is present. Methods for preventing or reducing the

incidence of white mottled root rot once trees are infected are not available. However, decay

is often associated with wounds, so wounds should be avoided to reduce future infections.

Because this disease is a major problem of aspen and a few other deciduous tree species,

recognizing the problem and discriminating against susceptible species are also potential

management options in nondeveloped forested sites. Species conversion could include complete removal of susceptible host tree species from an infected stand, or selective removal of

aspen and other host species during thinning to favor nonhost species.

Conclusions

Root diseases are the most damaging diseases affecting forest trees in the United States

(Krist et al. 2014). Once established, root pathogens have long-term impacts on forest ecosystems and resiliency. They can affect site productivity as much as do other factors, such

as latitude, elevation, soils, moisture regimes, and exposure. Root diseases can change successional pathways by altering forest composition and structure (Hagle et al. 2000), reduce

overall productivity due to mortality and growth loss of host trees (Krist et al. 2014), affect

carbon sequestration due to growth loss and mortality of trees and decay of woody material

(Jose 2007; Krist et al. 2014), and threaten personal safety due to failures of infected trees

(Filip and Goheen 1982a; International Tree Failure Database 2010).

Although root diseases affect forested areas throughout the United States, techniques to

locate and measure the incidence and severity of root diseases are very limited or nonexistent in many locations. For example, the distribution maps herein are limited to county-level

reports of stand-level damage, rather than precise locations of individual root disease agents.

Aerial detection surveys are used to map rapid changes and mortality from forest insects,

such as bark beetles and defoliators, but root diseases cause progressive and constant mortality that is difficult to detect and diagnose with these surveys. Newer technologies need to be

evaluated to improve our ability to recognize and measure root diseases and their impacts

over large landscapes.
USDA Forest Service RMRS-GTR-342. 2016. 39

More data are needed regarding the impacts of root diseases on growth and mortality of

host trees, site factors that affect the presence and severity of root diseases, and interactions

between root diseases and other biotic and abiotic agents. Long-term monitoring is necessary to properly measure and evaluate these effects and relationships. Root diseases will

have ever-increasing impacts on trees as temperature and precipitation regimes are altered

under projected climate scenarios (Kliejunas 2011; Kliejunas et al. 2009; Sturrock et al.

2011). As our nation’s forests are subjected to the cumulative stresses of climate change,

long-term monitoring will become more crucial for evaluating incidence of and impacts

from root diseases.

Many questions about root diseases remain unanswered. Information on the basic biology and

taxonomic status is lacking for some pathogens. We do not fully understand the ecological role of

many of these root-disease pathogens, or how they function in the ecosystems where they reside.

The importance of spores for spread of some pathogens is virtually unknown. Development of

management tools, such as effective biological control agents, disease-suppressive soils, and other novel approaches, is needed for these pervasive diseases. Although precise effects of climate

change on these pathogens, their hosts, and their interactions are unknown, it is likely that many

forest trees will become stressed from climate maladaptation, which will probably increase the

deleterious impacts of many root diseases. The influences of forest management practices, such

as repeated entries into stands and soil disturbance, on root diseases need to be better understood.

Resources are needed to support research for addressing these and other important issues related

to root diseases of forests.

Many current management tools focus on changing species composition on sites heavily

affected by root diseases. In the western United States, evidence indicates that a major shift

in tree species has occurred over the last 80+ years due to fire suppression and the introduction of white pine blister rust, allowing for a significant increase in species susceptible to

root disease (Filip et al. 2007; Hagle et al. 2000). In the southeastern United States, longleaf

pine was the dominant pine on dry upland soils (Outcalt 2000; Wahlenber 1946), but fire

suppression and management have caused a shift away from longleaf pine (Chapman 1932)

to loblolly pine. Loblolly pine is more susceptible than longleaf pine to Heterobasidion root

disease on high-risk sites (Hodges 1974). Changing species composition to species that are

more tolerant to root disease can be done at the time of harvest, but some sites are so heavily

infested by root diseases that the current forest cannot be economically managed (Hagle

and Shaw 1991; Hagle et al. 1994). Restoration funding will be necessary to make the shift

toward root-disease-tolerant species in these areas.

Restoring forest resiliency is a top priority for the U.S. Forest Service and is necessary

for forests to survive and remain sustainable and productive in the face of changing climates,

major disturbances, introductions of new pests, and evolving societal needs. Adequate

knowledge of root diseases and their roles in forested ecosystems is essential to our ability to

manage their negative impacts and build resilient forest landscapes.

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Appendix. Common and Scientific Names for Host Tree 

Species Used in This Paper 

Common name Scientific name

Alder Alnus spp.

Apple Malus spp.

Ash, Green Fraxinus pennsylvanica Marsh.

Basswood Tilia spp.

Beech, American Fagus grandifolia Ehrh.

Birch Betula spp.

Birch, Paper Betula papyrifera Marsh.

Cherry Prunus spp.

Citrus Citrus spp.

Douglas-firs Pseudotsuga menziesii (Mirb.) Franco

Douglas-fir, Coastal Pseudotsuga menziesii (Mirb.) Franco var. menziesii

Douglas-fir, Interior

(= Rocky Mountain

Douglas-fir)

Pseudotsuga menziesii (Mirb.) Franco var. glauca 

(Beissn.) Franco

Elm Ulmus spp.

Firs, True Abies spp.

Fir, Balsam Abies balsamea (L.) Mill.

Fir, Grand Abies grandis (Dougl. ex D. Don) Lindl.

Fir, Subalpine Abies lasiocarpa (Hook.) Nutt.

Fir, White Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.

Hemlocks Tsuga spp.

Hemlock, Mountain Tsuga mertensiana (Bong.) Carr.

Hemlock, Western Tsuga heterophylla (Raf.) Sarg.

Honeylocust Gleditsia triacanthos L.

Hornbeam, American Carpinus caroliniana Walt.

Horse chestnut (introduced species)

Aesculus hippocastanum L.

Juniper Juniperus spp.

Larch, Eastern Larix laricina (Du Roi) K. Koch

Larch, Western Larix occidentalis Nutt.

Locust, Black Robinia pseudoacacia L.

Maple Acer spp.

Mulberry, Red Morus rubra L.

Myrtle, Oregon (= California Laurel)

Umbellularia californica (Hook. & Arn.) Nutt.

Oaks Quercus spp.

Oak, Bur Quercus macrocarpa Michx.

Oak, Pin Quercus palustris Muenchh.
USDA Forest Service RMRS-GTR-342. 2016. 55

Pines Pinus spp.

Pine, Eastern white Pinus strobus L.

Pine, Jack Pinus banksiana Lamb.

Pine, Jeffrey Pinus jeffreyi Grev. & Balf.

Pine, Limber Pinus flexilis James

Pine, Loblolly Pinus taeda L.

Pine, Lodgepole Pinus contorta Dougl. ex. Loud.

Pine, Longleaf Pinus palustris Mill.

Pine, Pinyon Pinus edulis Engelm.

Pine, Ponderosa Pinus ponderosa Dougl. ex Laws.

Pine, Red Pinus resinosa Ait.

Pine, Sand Pinus clausa (Chapm. ex Engelm.) Vasey ex Sarg.

Pine, Shortleaf Pinus echinata Mill.

Pine, Singleleaf

pinyon

Pinus monophylla Torr. & Frem.

Pine, Slash Pinus elliottii Engelm.

Pine, Western white Pinus monticola Dougl. ex D. Don

Pine, Whitebark Pinus albicaulis Engelm.

Populus Spp. Populus spp.

Aspen Populus tremuloides Michx.

Cottonwood Populus spp.

Cottonwood, Narrowleaf

Populus angustifolia James

Cottonwood, Plains Populus deltoides var. occidentalis Rydb.

Poplar, Balsam Populus balsamifera

Port-Orford-Cedar

(=Lawson’s Cypress)

Chamaecyparis lawsoniana (A. Murr.) Parl.

Spruces Picea spp.

Spruce, Black Picea mariana (Mill.) Britton, Sterns & Poggenb.

Spruce, Blue Picea pungens Engelm.

Spruce, Engelmann Picea engelmannii Parry ex Engelm.

Spruce, Sitka Picea sitchensis (Bong.) Carr.

Sweetgum Liquidambar styraciflua L.

Sycamore Platanus occidentalis L.

Tuliptree Liriodendron tulipifera L.

Willow Salix spp.

Yew, Pacific Taxus brevifolia Nutt.


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