United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

General Technical Report

PSW-GTR-225

October 2009

Review of Literature on 

Climate Change and 

Forest Diseases of 

Western North America

John T. Kliejunas, Brian W. Geils, Jessie Micales 

Glaeser, Ellen Michaels Goheen, Paul Hennon, 

Mee-Sook Kim, Harry Kope, Jeff Stone, Rona Sturrock, 

and Susan J. Frankel

DEPAR TMENT OF AGRICULT URE
Authors 

John T. Kliejunas is a retired regional forest pathologist, U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Region, 1323 Club Drive, Vallejo, 

CA 94592. Brian W. Geils is a research plant pathologist, U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station, Southwest Forest 

Science Complex, 2500 South Pine Knoll Drive, Flagstaff, AZ 86001. Jessie 

Micales Glaeser is a research plant pathologist, U.S. Department of Agriculture, 

Forest Service, Northern Research Station, One Gifford Pinchot Drive, Madison, 

WI 53726. Ellen Michaels Goheen is a forest pathologist, U.S. Department of 

Agriculture, Forest Service, Forest Health Protection, Southwest Oregon Forest 

Insect and Disease Service Center, 2606 Old Stage Road, Central Point, OR 97502. 

Paul Hennon is a research forest pathologist, U.S. Department of Agriculture, 

Forest Service, Forestry Sciences Laboratory, 2770 Sherwood Lane, Suite 2A, 

Juneau, AK 99801. Mee-Sook Kim is a plant pathologist, U.S. Department of 

Agriculture, Forest Service, Rocky Mountain Research Station, 1221 South Main 

Street, Moscow, ID 83843 and Assistant Professor, Department of Forest Resources, 

Kookmin University, Seoul, Korea 136-702. Harry Kope is the provincial forest 

pathologist, Forest Practices Branch, British Columbia Ministry of Forests and 

Range, 9th Floor, 727 Fisgard St., Victoria, BC, Canada V8W 1R8. Jeff Stone is 

a research professor, Department of Botany and Plant Pathology, Cordley Hall 

2082, Oregon State University, Corvallis, OR 97331. Rona Sturrock is a research 

scientist, forest pathology, Natural Resources Canada, Canadian Forest Service, 

Pacific Forestry Centre, 506 West Burnside Road, Victoria, BC, Canada V8Z 1M5. 

Susan J. Frankel is a plant pathologist, U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station, 800 Buchanan Street, West Annex 

Building, Albany, CA 94710-0011; sfrankel@fs.fed.us.

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Abstract

Kliejunas, John T.; Geils, Brian W.; Glaeser, Jessie Micales; Goheen, Ellen 

Michaels; Hennon, Paul; Kim, Mee-Sook; Kope, Harry; Stone, Jeff; 

Sturrock, Rona; Frankel, Susan J. 2009. Review of literature on climate 

change and forest diseases of western North America. Gen. Tech. Rep. 

PSW-GTR-225. Albany, CA: U.S. Department of Agriculture, Forest 

Service, Pacific Southwest Research Station. 54 p.

A summary of the literature on relationships between climate and various types 

of tree diseases, and the potential effects of climate change on pathogens in 

western North American forests is provided. Climate change generally will lead 

to reductions in tree health and will improve conditions for some highly damaging 

pathogens. Sections on abiotic diseases, declines, canker diseases, root diseases, 

Phytophthoras, foliar diseases, stem rusts of pine, mistletoes, and wood decays 

present some examples of potential disease effects with predicted climate change. 

The effects of climate change on hosts, pathogens, and their interaction will have 

numerous, mostly adverse, consequences to forest ecosystems.

Keywords: Climate change, forest pathogens, forest tree diseases.
Contents

 1 Introduction

 2 Background

 2 Climate Change

 3 Climate Change and Disturbance in Forest Ecosystems

 5 Effects of Climate Change on Forest Pathogens

 6 Abiotic Diseases

 8 Declines

10 Canker Diseases

12 Root Diseases

14 Phytophthora Species

16 Foliar Diseases

19 Stem Rusts of Pine

23 Mistletoes

26 Wood Decays

28 Summary of Climate Change and Forest Diseases

29 Implications

31 Conclusions

32 Acknowledgments

32 English Equivalents

33 References
1

Review of Literature on Climate Change and Forest Diseases of Western North America

Introduction

The evidence is clear that one of the features that characterizes 

the world’s climate is instability... 

We now come to the present century and find general agreement 

that during the first 40 or more years the warming trend has not 

only been continuing but possibly accelerating... 

As we explore what these changes in long-term weather mean with 

respect to forest diseases, we must realize that while mean rises or 

drops in temperature or moisture are important, the extremes which 

vary with these means may be more important.

—George Hepting, 1963

This literature review is the result of the “Climate Change and Western Forest 

Diseases” project sponsored by the U.S. Department of Agriculture Forest 

Service (USDA FS) Western Wildland Environmental Threat Assessment Center 

(WWETAC) and the Pacific Southwest Research Station (PSW). The project builds 

on a June 2007 workshop on climate change and forest insects and diseases held by 

WWETAC (Beukema and others 2007) and a workshop in October 2007 that was 

sponsored by the USDA FS, WWETAC, and PSW in cooperation with the 2007 

Western International Forest Disease Work conference.

Paul Hennon

Decline of yellow-cedar on western Chichagof Island in southeast Alaska.
2

GENERAL TECHNICAL REPORT PSW-GTR-225

The objectives of the Climate Change and Western Forest Diseases 

project are to:

1. Provide high-quality information on climate change and forest diseases 

to land managers and forest pathologists.

2. Integrate forest pathology and forest pathologists into climate change 

policy, management, and science.

3. Identify forest diseases that threaten ecosystems undergoing rapid 

climate change.

4. Determine needs for research, management, and extension to address 

future climate change and forest disease issues.

This literature review is intended to inform managers, decisionmakers, 

scientists, and others about the potential effects of occurring and predicted 

climate change on forest pathogens and forest pathogen/host interactions.

Background

Climate Change

The world’s climate is changing (Houghton and others 1996, IPCC 2007, Wigley 

1999). During the past 100 years, average global temperatures have risen by about 

0.8 °C. Although Earth’s climate has fluctuated for millions of years (Jackson and 

Overpeck 2000, Russell 1941), the current rate of warming has been unprecedented 

over the last millenium (IPCC 2007, Mann and others 1999). In the conterminous 

United States, average temperature has risen by 0.5 °C, and precipitation has 

increased by 5 to 10 percent (National Assessment Synthesis Team 2000). In 

addition, the variance in temperature and precipitation is increasing. Further, the 

frequency and severity of extreme weather effects may be increasing (Hopkin and 

others 2005).

Causes of climate change include changes in atmospheric concentrations of 

greenhouse gases and aerosols, and changes in land cover and solar radiation. The 

concentration of carbon dioxide, the most important anthropogenic greenhouse gas, 

has risen dramatically since preindustrial times. Models of the carbon cycle project 

that the concentration of carbon dioxide will increase from 379 parts per million 

(ppm) in 1995 to 540 to 970 ppm by 2100 (IPCC 2001, 2007). Even if human 

sources of greenhouse gas emissions were stabilized, warming would continue 

for centuries owing to time lags associated with climate processes and feedbacks 

(IPCC 2007).
3

Review of Literature on Climate Change and Forest Diseases of Western North America

Average global temperature is projected to increase from 1 to 6 °C from 1990 

to 2100. Increases across much of the United States may be even greater (Houghton 

and others 2001). Increases in winter temperature and more frequent droughts are 

predicted throughout much of the Western United States under most climate change 

models (USDA Forest Service 2003). Precipitation may increase slightly in winter 

months, but the duration of seasonal heat and drought is likely to increase (USDA 

Forest Service 2003). Projections for the Pacific Northwest (Mote and others 2003), 

based on output from eight global circulation models, are illustrative. Temperature 

in the Pacific Northwest is projected to increase from 0.5 to 2.5 °C by 2020 and 

from 1.5 to 3.2 °C by 2040. Precipitation across the region is projected to increase 

except during summer. In British Columbia, snowmelt may occur earlier and more 

rapidly, with a reduction in summer streamflows (Hamann and Wang 2006).

There is consensus that the direct and indirect effects of climate change on 

forest dynamics may be profound (Aber and others 2001, Ayres and Lombardero 

2000, Dale and others 2001). Climate change will affect the geographic distribution 

of vegetation types, ecosystem processes such as primary production, and the 

distribution and abundance of individual species of trees and other plants (Malcolm 

and Pitelka 2000, Malcolm and others 2006). In British Columbia, the projected 

future climate may result in the disappearance of subboreal and montane climate 

regions, resulting in the loss of suitable habitat for many ecologically and 

commercially important species of conifers (Hamann and Wang 2006).

Climate Change and Disturbance in Forest Ecosystems

Forest ecosystems are affected both directly and indirectly by natural disturbances 

including but not limited to fire, drought, insects and pathogens, hurricanes, 

windstorms, and ice storms (McNulty and Aber 2001). The distribution, frequency, 

and severity of these disturbances is affected by changes in climate (Dale and 

others 2000, Joyce and others 2001, Walther and others 2002). When disturbances 

exceed a forest’s ability to maintain a stable state (that is, its resilience), fundamental changes in forest structure and function are inevitable (Drever and others 2006, 

Gunderson 2000). Impacts of disturbances and thus of climate change are seen 

over a broad spectrum of spatial and temporal scales (Peterson 2000). As climate 

changes, outbreaks of insects and diseases in forests may become more common 

(McNulty and Aber 2001). Disease reflects interaction among a susceptible host, a 

virulent pathogen, and environmental conditions that support the pathogen. Therefore, any climatic change that increases host susceptibility to a pathogen or results 

in an environment more suitable for the pathogen will result in increased incidence 

of disease.

When disturbances 

exceed a forest’s 

ability to maintain 

a stable state (that 

is, its resilience), 

fundamental changes 

in forest structure and 

function are inevitable.
4

GENERAL TECHNICAL REPORT PSW-GTR-225

Some species of trees can evolve rapidly in response to changes in temperature 

(Jump and others 2006). However, it is unclear how trees will be affected by 

interactions among changes in climate and disturbances such as fire, insects, and 

pathogens (Anderson and others 2004). Determinants of forest response to insects 

and pathogens include the category, host specificity, and aggressiveness of the 

pathogen and the phytosociology and functional importance and distinctiveness 

of the host (Ayres and Lombardero 2000, Harrington 2002, Logan and others 

2003, Lovett and others 2006).The forested area annually impacted by insects and 

pathogens in the United States is approximately 45 times the area affected by fire 

(20 400 000 and 450 000 ha, respectively), with an economic impact almost five 

times as great (Dale and others 2001, Logan and others 2003, insect and disease 

data from USDA 1997). The USDA FS (2004) estimated that the area of insect- and 

disease-caused tree mortality increased from about 1.2 million ha in 1997 to about 

4.9 million ha in 2003. Because dead trees serve as fuel for catastrophic wildfires, 

insects and pathogens often play key roles in the occurrence and severity of the 

forest’s second greatest disturbance agent, fire (Bergeron and Leduc 1998, Hepting 

and Jemison 1958).

Forests already stressed by high tree density (overstocking), pathogens, or 

climatic conditions such as drought may not survive additional climatic stress 

(Winnett 1998). Forests in southern California provide an example. In the early 

2000s, a severe drought, the worst in the recorded history of the region, in combination with stress induced by overstocking, dwarf mistletoe (Arceuthobium spp.), 

and annosus root disease (caused by Heterobasidion annosum), predisposed 

conifers to bark beetle attack. The resulting bark beetle epidemic and subsequent 

tree mortality provided fuel for wildfires that occurred in late October and early 

November 2003. More than 300 000 ha of brush and timber burned during 1 week. 

Twenty-six lives and 3,360 homes were lost. The fires caused more than $2.5 billion 

in damage (Keeley and others 2004) and dramatically altered ecosystem structure 

and function. Similarly, recent mortality of pinyon pine (Pinus edulis Engelm.) 

across 12 000 km2

 of the Southwestern United States was attributed to the stress 

of pathogens and drought that left trees susceptible to infestation by bark beetles 

(Breshears and others 2005).
5

Review of Literature on Climate Change and Forest Diseases of Western North America

Effects of Climate Change on Forest Pathogens

Key points:

Although we are uncertain how specific forest pathogens will 

respond to climate change, existing knowledge allows us to 

draw some general inferences.

• The distribution of pathogens and diseases, and their 

influence on the status and trend of forests, will change. 

Increases in temperature may allow some diseases to 

expand their latitudinal and elevational ranges. The influence of pathogens on the status and function of forests 

may change coincident with changes in species composition and climate. If managers facilitate migration of tree 

species to new locations, we should expect new diseases 

to affect those trees.

• Climate change will alter the epidemiology of plant diseases. Prediction of disease outbreaks will be more difficult 

in periods of rapid climate change and unstable weather.

• In a rapidly changing climate, the rate at which pathogens 

evolve and overcome host resistance may increase.

• Because abiotic factors such as temperature and moisture 

affect host susceptibility to pathogens and pathogen 

aggressiveness, changes in interactions between biotic 

diseases and abiotic stressors may represent the most 

substantial effect of climate change on plant diseases.

• Climate change may facilitate invasion by new nonnative 

pathogens. New epidemics may occur as a result. 

• Many pathogens currently are limited by winter 

temperature, and seasonal increases in temperature are 

expected to be greatest during winter. Accordingly, both 

overwintering survival of pathogens and disease severity 

are likely to increase.

• The effects of climate change on individual plant diseases 

will depend on the ecosystem and climate conditions.

Most plant pathogens are strongly 

influenced by environmental 

conditions and the vigor of the host. 

Climate change will directly affect 

the pathogen, the host, and the interaction between them, resulting in 

changes in disease impacts (Brasier 

2005, Burdon and others 2006). 

Global climate change ultimately is 

expressed at the microclimatic scale, 

the scale at which plant pathogens 

reproduce, disperse, and infect 

their hosts (Waggoner 1965). Host 

condition, thus susceptibility to 

pathogens, also is affected by cycles 

and extremes of temperature and 

moisture (see Ayers and Lombardero 2000, Hopkin and others 2005).

The following sections summarize the literature on relationships 

between climate and various types 

of tree diseases, and the potential 

effects of climate change on pathogens in western North American 

forests. Rather than summarizing 

the entire body of literature on the 

effects of weather (temperature and 

moisture) on the host, pathogen, and 

disease, each section presents some 

examples of potential disease effects 

with predicted climate change.
6

GENERAL TECHNICAL REPORT PSW-GTR-225

Abiotic Diseases

Agents of abiotic forest disease include, but are not limited to, air pollutants; 

extremes of temperature, precipitation, and water availability; and nutrient 

deficiencies. Abiotic environmental factors affect plant disease in several ways.

First, abiotic diseases can result from direct effects of the environment on the 

host. Direct effects of environmental extremes (for example, drought, low or high 

temperatures) are generally negative (Desprez-Loustau and others 2006). An 

increase in average temperature may accelerate photochemical reaction rates and 

increase the concentration of water vapor in the lower atmosphere, which likely will 

increase concentrations of ozone (Sitch and others 2007). Ozone 

reduces the ability of plants to sequester carbon dioxide and can 

increase tree susceptibility to root disease (James and others 1980a, 

1980b).

Second, factors such as temperature and moisture affect host 

susceptibility and pathogen aggressiveness. For example, severity 

of the foliage disease Swiss needle cast (caused by Phaeocryptopus 

gaeumannii) in western Oregon is strongly correlated with winter temperature, 

which affects growth of the pathogen (Manter and others 2005). Water stress was 

associated positively with the severity of effects of Sphaeropsis sapinea on pines 

(Bachi and Peterson 1985; Blodgett and others 1996, 1997; Paoletti and others 

2001). Drought in northwestern 

Alberta in the 1960s and 1980s 

decreased resistance of aspen 

(Populus spp.) stands to fungal 

cankers and wood-boring insects 

(Hogg and others 2002). In the 

Southwestern United States, widespread mortality of pinyon (Pinus 

monophylla Torr. & Frem.) and 

juniper (Juniperus osteosperma

(Torr.) Little and J. occidentalis

Hook.) is associated with several 

years of drought; drought, insects, 

and disease are responsible for 

local pinyon mortality rates that 

can approach 100 percent (Shaw 

and others 2005).

Key point:

• Drought stress exacerbates 

the damage to trees from 

many pathogens, particularly 

facultative parasites. 

Salt damage on Jeffrey pine, Pinus jeffreyi.

Kim Camilli, California Department of Forestry and Fire Protection 

Kim Camilli, California Dept. of 

Forestry and Fire Protection 
7

Review of Literature on Climate Change and Forest Diseases of Western North America

Third, abiotic factors also function 

as predisposing, inciting, or contributing 

agents in tree declines (Manion 1991). 

Stress depletes a tree’s carbohydrate 

reserves, increasing its vulnerability to 

insects and pathogens (Wargo and Haack 

1991).

The incidence of abiotic diseases 

associated with environmental extremes is 

expected to increase as climate changes, as 

are diseases caused by interactions between 

biotic and abiotic agents (Boland and others 

2004). Mortality of seedlings and saplings 

is a likely consequence of severe drought. 

Mature trees are less vulnerable to water 

limitation because they have deep roots and 

substantial reserves of carbohydrates and 

nutrients. However, severe or prolonged 

drought may render even mature trees less 

resistant to insects or disease (Hanson and 

Weltzin 2000, Joyce and others 2001).

Tree mortality in response to drought will become more common as temperature increases. Of particular concern is regional-scale mortality of overstory trees, 

which rapidly would alter land cover and ecosystem function. These changes would 

last for decades (Breshears and others 2005, Folke and others 2004).

A variety of pests, pathogens, and parasites can infect trees stressed by climatic 

extremes (Jurskis 2005). An increased incidence of summer drought would make 

trees more vulnerable to attack by fungi whose activity is dependent on host stress, 

particularly root pathogens, wound colonizers, and latent colonizers of sapwood 

(Broadmeadow 2002, Desprez-Loustau and others 2006, Lonsdale and Gibbs 2002, 

Redfern and Hendry 2002). Pathogens that are more likely to infect stressed hosts 

will benefit from the effects of heat and drought on forest and urban trees (Boland 

and others 2004, Schoeneweiss 1975).

Changes in climate including increased temperature, increased evapotranspiration, and extreme weather events may create unusually high levels of stress in forest 

stands (Columbia Mountains Institute of Applied Ecology 2005, Sturrock 2007). 

Forests already stressed by high tree density, pathogens, or atmospheric conditions 

may not survive the additional climatic stress (Winnett 1998).

Ozone injury on blue elderberry, causing interveinal necrosis.

Don Duriscoe
8

GENERAL TECHNICAL REPORT PSW-GTR-225

Declines

Tree decline is characterized by a slow and progressive deterioration in condition 

or vigor, decreased growth accompanied by branch dieback on mature trees, and a 

complex etiology often involving abiotic and biotic factors (Manion 1991, Manion 

and Lachance 1992). Long-term predisposing factors (such as unfavorable site, 

age, climate, and genetics) decrease the resistance of trees to short-term inciting 

factors (such as frost and defoliation). Inciting factors weaken the trees and, in 

conjunction with contributing factors (such as opportunistic pathogens), ultimately 

kill the trees.

Decline and mortality of western white pine (Pinus monticola 

Dougl. ex D. Don), referred to as pole blight, was observed in the 

Inland Empire region of the Pacific Northwest in the 1930s and 

1940s. The decline appeared to be associated with the predisposing 

factor of shallow soils that held little moisture and the contributing 

factors of canker pathogens and root deterioration from Armillaria 

root rot (Leaphart 1958, Wellington 1954). Climate records indicated 

that between 1916 and 1940, regional precipitation was lower and 

temperature higher than during the past 280 years (Leaphart and Stage 1971). Thus, 

drought was an inciting factor in the pole blight, with shallow soils the predisposing 

factor and secondary fungi contributing factors.

Key point:

• The incidence and magnitude of forest declines are 

predicted to increase as local 

populations of trees are 

stressed by climate change.

Aspen stand nearly completely dead owing to sudden aspen decline, Mancos-Dolores Ranger 

District, San Juan National Forest, Colorado. 

Phil Kemp

James Worrall
9

Review of Literature on Climate Change and Forest Diseases of Western North America

Mortality of yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach), 

known as yellow-cedar decline, occurs on about 200 000 ha in southeast Alaska 

and British Columbia (Hennon and others 2005, Wittwer 2004). Despite much 

study, no biotic causes could be identified for the decline, which began in the 

late 19th century (Hamm and others 1988, Hansen and others 1988, Hennon 

1990, Hennon and others 1986, Hennon and McWilliams 1999, Shaw and others 

1985). The distribution of yellow-cedar decline is associated with low snowpack 

in winter and spring, which is consistent with the assumption that snow protects 

yellow-cedar. Seasonal air and soil temperatures consistently were correlated 

with yellow-cedar decline, but saturated soils, acidic soils, aluminum toxicity, and 

calcium deficiency were not. The areas with greatest yellow-cedar mortality had 

temperatures that were higher in the spring and summer, lower in winter, and had 

greater daily ranges. Snow delays soil warming. Without the snow’s protection, 

yellow-cedar roots were damaged during warm spring days interspersed with 

overnight frost (Schaberg and others 2007). Thus, the inciting factor of climate 

change (climate warming) is hypothesized to be the primary cause of yellow-cedar 

decline (Beier and others 2008; Hennon and others 2006, 2008).

Sudden aspen decline, the premature death of aspen (Populus spp.) stands, 

recently has begun to occur in southern Colorado, Utah, and Nevada. The decline 

is characterized by severe, rapid branch dieback and crown thinning. Widespread 

and severe drought from 2000 to 2005 may have caused stress in aspens, particularly in mature stands at lower elevations, reducing the trees’ resistance to secondary infections and infestations. The decline is associated with predisposing factors 

(low elevations, south to west aspects, open stands, and mature trees), inciting 

factors (hot, dry conditions), and contributing factors (secondary insects and pathogens including Cytospora canker [caused by Valsa sordida Nitschke], aspen bark 

beetle [Trypophloeus populi Hopkins, and Procryphalus mucronatus LeConte], 

poplar borer [Saperda calcarata Say], and bronze poplar borer [Agrilus liragus

Barter and Brown]) (Forest Health Management 2008, Worrall and others 2008).
10

GENERAL TECHNICAL REPORT PSW-GTR-225

Canker Diseases

Cankers are diseases that result in death of sections on branches or main trunks 

of trees. Cankers are usually caused by the infection of bark tissues by plant 

pathogens. Although numerous canker pathogens are capable of attacking vigorous trees, many canker-causing fungi are more likely to cause damage to trees 

with heat and drought stress (Schoeneweiss 1975, 1981). Outbreaks or epidemics 

of these stress-related canker diseases often follow water stress and freezing 

stress (Schoeneweiss 1981). Most published studies refer to a positive association 

between drought and disease, or to synergistic effects of drought and disease. 

Most canker diseases are caused by facultative parasites such as Botryosphaeria, 

Sphaeropsis, Cytospora, and Biscognauxia (Hypoxylon) (Desprez-Loustau and 

others 2006).

Botryosphaeria dothidea attacks a wide range of host 

trees, but causes serious damage only to those that are 

weakened or under environment stress (Ma and others 2001). 

Drought stress and winter injury have been associated with 

increased infection and canker expansion of B. dothidea 

(Brown and Hendrix 1981).

Severity of Sphaeropsis shoot blight, caused by the fungus 

Sphaeropsis sapinea (Fr.) Dyko & B. Sutton, consistently has 

been associated with water stress (Bachi and Peterson 1985; 

Blodgett and others 1996, 1997; Paoletti and others 2001). 

Cankers caused by Septoria musiva Peck on inoculated water-stressed trees were 

significantly larger than those on nonstressed trees (Maxwell and others 1997).

The distribution of pitch canker (caused by Fusarium circinatum Nirenberg 

& O’Donnell) is limited by low temperatures. Inoculation trials during winter 

yielded low infection rates (Inman and others 2008). Increases in minimum 

winter temperature may increase the range of the pathogen in the Sierra Nevada 

and elsewhere (Battles and others 2006).

The pathogen Biscogniauxia mediterranea (De Not.) Kuntze (Hypoxylon 

mediterranea) historically was widespread in oak woodlands in the 

Mediterranean, with a northern range limit of southern Tuscany. However, in 

recent years, the pathogen has been detected on damaged oaks in Slovenia (Jurc 

and Ogris 2006), and it is likely to continue moving north as climate warms.

Key point:

• Many stem canker and dieback 

pathogens are more successful 

when attacking stressed trees. 

Water and drought stress, and 

therefore the incidence of canker 

and dieback pathogens, will 

increase as climate changes.

Tom Gordon, University of California-Davis
11

Review of Literature on Climate Change and Forest Diseases of Western North America

Climate change may increase the incidence of many canker diseases. Most 

canker-causing fungi benefit from heat and drought stress to forest and urban 

trees (Boland and others 2004, Broadmeadow 2002, Lonsdale and Gibbs 2002, 

Schoeneweiss 1975). For example, incidence of Sphaeropsis sapinea on pines 

is likely to increase as drought becomes more common. The incidence of some 

other canker pathogens, such as Thyronectria austro-americana (Speg.) Seeler 

on honeylocust (Gleditsia triacanthos L.) and Cryphonectria cubensis Bruner on 

Eucalyptus spp., may decrease as climate changes. Incidence of these pathogens 

is associated with high rainfall, and their hosts display some resistance to drought 

stress (Despres-Loustau and others 2006).

Closeup views of a resin-soaked pitch canker lesion caused by Fusarium circinatum

on Monterey pine, Pinus radiata, from a tree in coastal California near Monterey. 

Tom Gordon, University of California-Davis
12

GENERAL TECHNICAL REPORT PSW-GTR-225

Root Diseases

Tree stress is one of the major factors affecting the incidence and spread of root 

diseases. Several root pathogens, in particular Heterobasidion annosum (Fr.) Bref. 

and Armillaria spp., both of which are major pathogens of western forest conifers, 

are more aggressive when hosts are stressed. Climate change could increase the 

incidence and spread of root pathogens like these if host trees become stressed. In 

addition, climate change could alter the relative fitness of various mycorrhizal fungi 

and other microbes that currently suppress root disease. The protective effects of 

mycorrhizae may change in response to changes in soil temperature or moisture 

(Broadmeadow 2002).

Some reports suggest that hot and dry conditions typical of prolonged drought will increase incidence and spread of root diseases in 

forests. However, it is difficult to predict how diverse root diseases will 

be affected by different climate scenarios. Although empirical data 

are limited, a rise in atmospheric carbon dioxide and increased growth 

of tree roots may result in an increase in severity or frequency of root 

disease. More extensive root systems would increase the probability of 

invasion (O’Neill 1994). This increase could be offset by increased plant 

vigor and disease resistance (Runion and others 1994).

Several species of Armillaria cause root disease in forest trees, 

resulting in mortality or reduced growth (Shaw and Kile 1991). Armillaria root disease caused by A. ostoyae (Romag.) Herink is responsible 

for volume losses of 2 to 3 million m3

/year of timber in the forests 

of Canada’s Pacific Northwest (Morrison and Mallett 1996). Some 

Armillaria species are primary pathogens and attack healthy trees in western 

forests, whereas other species act as secondary agents, infecting after hosts have 

experienced a predisposing stress agent such as drought, other diseases, insects, or 

reductions in local habitat quality. The incidence of Armillaria root disease is likely 

to increase under warmer or drier conditions (Shaw and Kile 1991, U.S. Office of 

Technology Assessment 1993). Such increases not only will lead to declines in tree 

growth but may increase the incidence or severity of insect attacks on stressed trees 

(Battles and others 2006). In eastern forests, Piercey-Normore and Bérubé (2000) 

found that stress from insect defoliation predisposed host trees to root infection by 

A. ostoyae, and significant damage from Armillaria has been observed in forests 

that have been stressed by drought (Wargo and Harrington 1991). In the Pacific 

Northwest, where the mean annual temperature currently is below the optimum 

(25 °C) for Armillaria growth, a warmer climate is likely to result in increased 

prevalence and rate of spread of root disease (IPCC 2001).

Key point:

• Simultaneous increases 

in temperature and 

decreases in precipitation are likely to increase 

incidence and spread of 

root diseases in forests. 

However, the response 

of root diseases to 

climate change differs 

among species and 

climate scenarios.

Phytosphere Research
13

Review of Literature on Climate Change and Forest Diseases of Western North America

Adverse impacts from annosus root disease also are expected to increase 

as climate changes. The pathogen H. annosum generally is more common and 

damaging on drier sites (see Puddu and others 2003, for example). Nilson and 

others (1999) suggested that warming will provide more favorable environmental 

conditions for H. annosum to spread, increasing damage to trees and economic loss.

Laminated root rot, caused by Phellinus weirii (Murr.) Gilb., commonly kills 

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and several other conifer species (Thies and Sturrock 1995). The distribution and occurrence of this pathogen 

is closely associated with the host tree species, especially Douglas-fir. If climate 

change will affect the host distribution and conditions, Phellinus spp. can be less or 

more threatening to the host species depending on other environmental conditions.1

Forest soils can buffer roots from changes in air temperature, so a few degrees 

increase in air temperature may not affect the fungus substantially.

Because occurrence and distribution of root rot pathogens and their host species 

are closely linked, it is essential to predict distribution of host species in response 

to climate change. A modeling approach is available to explain current and predict 

future distributions of forest trees using climate variables (Rehfeldt and others 

2006). However, the coarse grain of such models precludes prediction of changes 

in microclimate that may affect both hosts and root pathogens dramatically.

1 Thies, W.G. 2008. Personal communication. Research forest pathologist (retired), 

Pacific Northwest Research Station, Forest Sciences Laboratory, 3200 SW Jefferson 

Way, Corvallis, OR 97331.

John W. Hanna

John W. Hanna

Armillaria root disease mortality center in northern Idaho. Armillaria fruiting body.
14

GENERAL TECHNICAL REPORT PSW-GTR-225

Phytophthora Species

Numerous species in the genus Phytophthora parasitize a wide variety of host 

plants, including forest trees. Although some Phytophthora species are host 

specific, others are generalists. The success of Phytophthora partly reflects its 

ability to produce sporangia and zoospores rapidly when environmental conditions 

are favorable, increasing inoculum levels from extremely low to high within a 

few days or weeks. The effects of most Phytophthora species are greatest when 

temperatures are relatively mild to warm. Moisture—from sources 

including rainfall, dew deposition, and irrigation—is the main 

environmental control on population size of the pathogen. The 

multicyclic nature of Phytophthora-caused plant diseases has resulted 

in severe epidemics in forest and agricultural systems worldwide.

Phytophthora cinnamomi Rands, a widespread pathogen in 

tropical and subtropical regions, causes diseases in the root and 

stem-base of a wide range of broadleaved and coniferous species, and 

has been well studied in diverse forest ecosystems. The fungus-like 

organism is transmitted in the soil and requires warm, wet soils to 

infect roots. The pathogen is currently present but has limited effects 

in conifer forests of the Pacific Northwest. Although Douglas-fir is susceptible 

to the pathogen, the winter months are currently too cold for effective zoospore 

production and infection, and the summer months are currently too dry (Roth 

and Kuhlman 1966). If, as projected, winters in the Pacific Northwest become 

Key point:

• Increases in winter 

temperatures of 0.5 to 

1.5 °C will allow for 

increases in survival of 

Phytophthora cinnamomi

propagules, resulting in 

increased negative impacts 

from P. cinnamomi.

Sudden oak death mortality caused by Phytophthora ramorum near Big Sur, California.

Kerri Frangioso, University of California-Davis
15

Review of Literature on Climate Change and Forest Diseases of Western North America

warmer, P. cinnamomi may infect Douglas-fir. Epidemic losses of trees owing to P. 

cinnamomi in New Zealand tend to be restricted to years when rainfall is unusually 

high in autumn and late spring, when soil temperatures are sufficient to support 

sporulation. Sporulation of P. cinnamomi did not occur at typical soil temperatures 

during the wet winter months in New Zealand (Chee and Newhook 1965).

Not only increases in mean temperature but increases in the duration of 

relatively warm temperature may increase production of inoculum and northward 

expansion of P. cinnamomi and similar organisms (Brasier and Scott 1994, Hepting 

1963). Phytophthora cinnamomi is likely to become more prevalent across Europe. 

Models indicate that activity of the pathogen across the United Kingdom and 

continental Europe is likely to increase substantially as a result of higher year-round 

temperatures and northward expansion of the pathogen (Brasier 1996, Brasier and 

Scott 1994). In Europe, winter temperatures limit survival of the pathogen and its 

effects on oaks, especially Quercus robur L. and Q. rubra L.

Bergot and others (2004) compared the geographic ranges of P. cinnamomi 

in France from 1968 to 1998 and projections for 2070 through 2099. Predictions 

of winter survival of the pathogen were based on temperature in the phloem of 

infected trees, which in turn was estimated from a regional climate scenario 

derived from a global circulation model. Projected increases in winter temperatures 

between the two periods were 0.5 to 5.1 °C. Higher annual rates of P. cinnamomi 

survival were predicted, resulting in a potential range expansion of 100 to several 

hundred kilometers eastward from its current range along the Atlantic coast. Using 

the CLIMEX model, Desprez-Loustau and others (2007) estimated that predicted 

warming would be favorable to most of the pathogens they studied, especially those 

like P. cinnamomi for which minimum winter temperatures limit survival. 

Warming may increase habitat quality for P. cinnamomi in northern California. 

Garbelotto and others (2006) reported infections of P. cinnamomi among coast live 

oaks (Q. agrifolia Nee.) in southern California (San Diego County). Inoculations 

in February (average temperature of 19 °C) resulted in small lesions (26 ± 15 mm), 

whereas September (average temperature of 24 °C) inoculations resulted in larger 

lesions (135 ± 68 mm), consistent with previous research that suggested colder 

temperatures are unfavorable to the pathogen.

Increased mortality from other Phytophthora species, including P. ramorum

Werres, De Cock & Man in`t Veld, the cause of sudden oak death, is expected if 

temperature increases in the summer and precipitation increases in the spring. 

These changes will lead to increases in inoculum production by the pathogen 

(Frankel 2007, Rizzo and others 2005). Phytophthora ramorum is addressed in 

more detail in the following section.
16

GENERAL TECHNICAL REPORT PSW-GTR-225

Foliar Diseases

Because they tend to be strongly influenced by weather, pathogens that cause 

foliar diseases are among the pathogen groups most likely to be affected by changing climate. Sporulation and infection often occur within a narrow temperature 

range. Spore release usually coincides with periods of precipitation. Spores of 

foliar pathogens are typically dispersed by wind during wet conditions, and spore 

germination and infection typically require free moisture on the foliar surface. The 

severity of these “threshold diseases” (Hepting 1963) usually varies in response to 

annual weather patterns, and is likely to increase in regions that become warmer 

and wetter. Conversely, increased incidence of drought likely will 

decrease the severity of some foliar diseases. Timing of sporulation of many foliar pathogens is synchronous with emergence and 

development of new tissue on host plants. Changes in climate that 

favor development of susceptible host foliage during favorable 

conditions for the pathogen also will increase outbreaks of foliage 

disease. Elevated concentrations of carbon dioxide increase host 

growth and pathogen fecundity (Chakraborty and Datta 2003, 

Chakraborty and others 2000a, Coakley and others 1999); when 

humidity also increases, the incidence and severity of foliar diseases are likely to 

increase (Coakley and others 1999, Manning and von Tiedemann 1995).

Information on interactions between specific foliar diseases and weather is 

limited, but general inferences can be based on the known dependence of these 

pathogens on weather. The incidence and severity of most foliar diseases will likely 

increase if late winter and spring become wetter and warmer, but may decrease if 

summer becomes drier (Broadmeadow and Ray 2005). The effect of increasing 

night temperature and, in turn, changes in humidity, will differ among pathogens 

(Coakley and others 1999, Harvell and others 2002).

Swiss needle cast of Douglas-fir, caused by the native pathogen Phaeocryptopus gaeumannii (Rohde) Petrak, occurs throughout the range of the host (Boyce 

1940). Although previously considered a minor disease caused by a weak pathogen, 

a severe epidemic has occurred since the early 1990s in the coastal fog belt of 

Oregon (Hansen and others 2000, Kanaskie and others 2004). Although the causes 

of the current outbreak are not known definitively, habitat quality for the pathogen 

may have been increased by development of dense Douglas-fir plantations in moist, 

coastal locations where Douglas-fir was not historically dominant. Manter and 

others (2005) reported that abundance of fruiting bodies and premature needle 

loss were positively correlated with mean daily temperature in winter and cumulative leaf wetness in spring, and developed a temperature-based model to predict 

Key point:

• The majority of foliar pathogens 

are likely to increase in 

occurrence and severity as 

temperature and precipitation 

increase in late winter and 

early spring.

Alan Kanaskie, 

Oregon Department of Forestry
17

Review of Literature on Climate Change and Forest Diseases of Western North America

geographic variation in disease severity. Although the authors could not project 

changes in disease severity based on winter mean daily temperature, they detected 

a relationship between recent regional climate patterns and the observed increase 

in the disease. Average temperatures for the period from January to March have 

increased by approximately 0.2 to 0.4 °C per decade since 1966 in the coastal area 

of Oregon and Washington. In New Zealand, where Douglas-fir is not native, 

winter mean temperature was the climate variable most closely correlated with 

severity of disease caused by P. gaeumannii, accounting for about 80 percent of 

the variation (Stone and others 2007).

Climate change provided ideal temperature and humidity for the recent 

increased incidence and severity of Dothistroma needle blight (caused by 

Dothistroma septosporum (Dorog) Morelet; Mycosphaerella pini Rostr.) on 

lodgepole pine (Pinus contorta var. latifolia Dougl. ex Loud.) in Canada (Woods 

and others 2005). The impact of the native needle pathogen traditionally has 

been low, rarely causing damage. However, the needle blight is currently causing 

extensive defoliation and mortality in plantations of lodgepole pine in northwestern 

British Columbia. These plantations, heavily planted since 1990 and covering 

close to 40 000 ha in northwest British Columbia, are most severely affected; 

Alan Kanaskie, Oregon Department of Forestry

Black fruiting bodies of the Swiss needle cast fungus, Phaeocryptopus gaeumannii, on the underside 

of Douglas-fir foliage.
18

GENERAL TECHNICAL REPORT PSW-GTR-225

entire plantations of lodgepole pine are failing. Even mature pine trees are dying 

(Woods 2003). Previously, Dothistroma needle blight has caused defoliation and 

reduced host growth—but not mortality—only in exotic plantations (mainly Pinus 

radiata D. Don) in the Southern Hemisphere (Gibson 1972). In British Columbia, 

the pathogen is now attacking a native host, and causing mortality even in mature 

stands, rather than merely retarding growth via defoliation (Bradshaw 2004).

Noting that the current epidemic of Dothistroma needle blight coincided with 

an increase in frequency of warm rain events during the mid to late 1990s, Woods 

and others (2005) analyzed climate data for the region. They found a strong positive 

relationship between increased precipitation and increased disease severity. Their 

studies demonstrated that a relatively small change in climate can have serious 

implications for a tree species, particularly if that change passes an environmental 

threshold that restricted the development of a pathogen.

Drought produces favorable conditions for some foliar pathogens. A reduction 

in the number of summer rain days may reduce the incidence of Marssonina leaf 

spot (caused by Marssonina spp.) of poplar (Populus sp.) (Lonsdale and Gibbs 

2002). Wagener (1959) reported increased damage to ponderosa pine (Pinus ponderosa Dougl. ex Laws.) from the needle cast pathogen Davisomycella medusa 

(Dearn.) Darker during periods of drought. An unusually wet summer in 1999 

may have precipitated an increase in the abundance of the needle cast pathogen 

D. ponderosae (Staley) Dubin. Subsequent drought years allowed that pathogen to 

develop and kill infected needles of ponderosa pine on the San Juan National Forest 

in Colorado (Worrall and Sullivan 2002).

The effects of introduced or emerging pathogens may increase as a result 

of climate change and forest management. Phytophthora ramorum recently 

emerged and is strongly influenced by climate. In addition to causing lethal stem 

infections in oaks (Quercus spp.) and tanoak (Lithocarpus densiflorus [Hook. & 

Arn.] Rehder), P. ramorum causes nonlethal foliar infections in a range of other 

species. Inoculum of P. ramorum increases rapidly with heavy rains associated 

with El Niño events (Davidson and others 2005). Dry conditions are unfavorable. 

Chlamydospores and zoospores of the pathogen can survive more that 30 days 

under moist conditions in the laboratory, but survive less than 30 minutes when 

the relative humidity is less than 30 percent (Davidson and others 2002). Venette 

and Cohen (2006) used the CLIMEX model to identify areas of the United States 

suitable for development of P. ramorum. Increased precipitation during spring 

would favor development of the pathogen in areas where it already occurs and 

expansion of the pathogen to new locations.
19

Review of Literature on Climate Change and Forest Diseases of Western North America

Stem Rusts of Pine

Stem rusts of pine (Cronartium) are aerially-dispersed, biotrophic, fungal pathogens 

that have coevolved with their hosts. The complexity of their life cycles differ 

(Ziller 1974). In western North America, the introduced and invasive C. ribicola

(J.C. Fischer in Rabenh.) is the best known and most serious blister rust of white 

pines (Pinus subg. Strobus) (Mielke 1943). Comandra blister rust (C. comandrae

Peck) and western gall rust (Peridermium harknessii J. P. Moore) are important 

native pathogens of other pines (subg. Pinus). Except for their spores, which are 

exposed to the atmosphere, these rusts require living hosts. Microcyclic rusts such 

as P. harknessii spread directly from pine to pine; Cronartium rusts, however, must 

alternate between the pine host (pycnial and 

aecial stages) and an angiosperm host (uredial, 

telial, and basidial stages). The abundance 

of these rusts is therefore influenced both by 

weather and by host populations—each of which 

is affected by climate change. Severe epidemics 

of stem rust profoundly affect the genetics and 

demography of the infested pine (Kinloch 2003). 

Phylogeo-graphy, biogeography, and ecology of 

the stem rust pathosystem provide the basis for 

assessing rust-climate interactions, threats to 

forest eco-systems, and potential mitigations.

Spore development, dispersal, and germination of rust fungi are directly affected by abiotic 

factors (for example, see Chang and Blenis 1989, 

Powell 1972). The environmental requirements for rust infection are complex and 

have been especially well-documented for C. ribicola (Bega 1960, Mielke 1943, 

Spaulding 1922, Van Arsdel 1954). Epidemiology and ecophysiology are well studied for P. harknessii, C. comandrae, and C. ribicola (Van Arsdel and others 2006). 

These pathosystems commonly display a wave year phenomenon when especially 

favorable weather facilitates substantial increases in rust spread and intensification 

(Jacobi and others 2002, Peterson 1971). Because stem rusts respond to weather 

events, monthly average climate variables at the regional or even site level may not 

be sufficient for predicting rust hazard. Increasing heat and aridity may decrease 

infection either by inhibiting the rust or defoliating the telial host (Kimmey 1944, 

Kimmey and Wagener 1961). In some dry locations unfavorable for the repeating 

stage on the telial host, C. coleosporioides J.C. Arthur drops the uredial stage of 

its life cycle (van der Kamp 1993). Because infection is usually through stomates, 

Key points:

• Although rusts can adapt to a wide range of 

environmental conditions, their tolerances are 

unknown. Under a changing climate, the 

incidence of rusts will be determined chiefly by 

host distribution. Rusts are likely to remain the 

cause of damaging and lethal diseases.

• Typically, rusts increase in intensity and 

distribution in “wave years” during which the 

weather is especially favorable for sporulation, 

dispersal, and infection. As climate changes, 

the frequency of such wave years is expected 

to change.

David Shaw
20

GENERAL TECHNICAL REPORT PSW-GTR-225

whatever affects stomatal opening (for example, carbon dioxide concentration, 

drought) affects infection.

Climate currently does not appear to limit white pine blister rust in British 

Columbia (Campbell and Antos 2000). However, climate may restrict the rust elsewhere. After rapid expansion of blister rust from its introduction near Vancouver 

(Mielke 1943), spread stalled to the east (eastern Montana), southeast (Yellowstone), 

and south (California). Because hosts occur beyond the range of blister rust, its 

limits were ascribed to unfavorable climates (Kinloch 2003). However, the rust has 

since expanded across Montana, into New Mexico, and down the Sierra Nevada 

(Geils and others 2003, Kliejunas 1985). Colonization of these generally warmer 

and drier regions could indicate either that the rust is more adaptable, or suitable 

microsites are more common, than initially thought. For example, the Yellowstone 

region was described as too cold and dry to support severe infestation of blister 

rust (Hendrickson 1970). This may be the case on the central Yellowstone plateau, 

but the effective environmental limit may be separation from critical Ribes species 

rather than a direct microclimatic effect. Severe blister rust outbreaks have occurred 

on the periphery of the Yellowstone region (for example, Teton Pass; see Kearns and 

Jacobi 2007), but a credible analysis has not been conducted to examine correlations between climate and rust severity throughout the Yellowstone region. The size 

of rust populations may be decreased by a series of extreme winter cold fronts such 

as those that purged ponderosa pine (Pinus ponderosa Dougl. ex Laws.) of comandra blister rust at Mink Creek, Idaho (Wagener 1950). Before the cold snap, there 

were infected trees; after, there were no expanding or sporulating cankers.

Ecological interactions with other species, especially insects, also affect the 

severity of rust impact. These interactions usually have direct or indirect climate 

effects and biotic responses (Hiratsuka and others 1987). For example, spermatization of the rust is mediated by pycnia-associated insects (Hunt 1985); factors 

influencing insect abundance or behavior could affect reproduction of the rust. 

Likewise, various other insects and fungi are associated with aecial sporulation 

and host necrosis, but their influence on disease progress is unclear (for example, 

Furniss and others 1972, Jacobi 1993, van der Kamp and Blenis 1996). Relationships between stem rusts and bark beetles also are also complex and vary in space 

and time (Campbell and Antos 2000, Six and Adams 2007). Lodgepole pines 

severely cankered by C. comandrae have lower sugar levels, which increases 

their susceptibility to mountain pine beetle (Dendroctonus ponderosae Hopkins) 

(Nebeker and others 1995). Rust-infected trees may provide high-quality habitat for 

bark beetles between outbreaks. Host regeneration following a bark beetle outbreak 

could provide many susceptible new hosts.
21

Review of Literature on Climate Change and Forest Diseases of Western North America David Shaw, Oregon State Universtiy

Aecia appearing on whitebark pine in the Wallowa Mountains, Oregon, in late July 2009.

Hazard models and epidemiological simulation models illustrate how climate, 

host, and some stem rusts may interact. Elevation, vegetation type, or topographic 

position are correlated with distribution or incidence of some stem-rust species 

(for example, Beard and others 1983, Howell and others 2006). Because these 

factors are assumed to be surrogates for climate, inferences about the distribution 

of the stem rusts under different climate scenarios are possible. Projections of the 

location of Cronartium comandrae outbreaks and resulting loss of tree volume have 

been based on site and weather variables (Jacobi and others 1993, Woods 2006). 

Meteorological records indicated that weather conditions considered suitable for 

infection of lodgepole pine by C. comandrae (more than 6 continuous hours during 

the months of July, August, and September in which temperature was 10 to 20 

°C and the air was nearly saturated) occurred at least every few years within the 

central Rocky Mountains. The frequency, but not the duration, of potential infection 

episodes was related to these weather patterns (Jacobi and others 2002). Patterns 

of upper level airflow conducive to long-distance transport of rust spores from a 

source to a target, coupled with data on surface conditions favorable for infection 

at a designated target, were used to identify potential periods of transport and 

infection by C. ribicola spores (Frank and others 2008). Regional warming likely 

would alter the frequency of potential infection episodes.
22

GENERAL TECHNICAL REPORT PSW-GTR-225

Numerous models for predicting outbreaks of white pine blister rust are available (Van Arsdel and others 1961). An epidemiological simulation model has been 

used to examine interactions among climate and the ecology and genetics of white 

pine blister rust (McDonald 1996, McDonald and others 1981). Northern Idaho 

and the southern Sierra Nevada in California have contrasting climates. Northern 

Idaho had severe rust epidemics as early as the 1920s; the southern Sierra became 

infested many decades later. Simulations projected outbreaks on the basis of 

typical Idaho weather and temperature response functions of rusts in that state. 

If stand development is simulated with meteorological data typical for California 

and the rust’s epidemiological parameters for Idaho, no outbreak occurs. But if the 

exercise is repeated with the parameters adjusted to represent shift in the rust’s 

response function, the model generates an outbreak (McDonald 1996). This exercise suggests that although the rust, which was adapted to cool, moist conditions 

in Idaho, initially did not survive under warmer and drier conditions in California, 

a local strain of the rust evolved that could survive in California. Ultimately, 

extremes of temperature and humidity restrict the rust, but the rust may be able 

to adapt to changing climate. The response of stem rusts to climate change can 

be predicted and managers can allow for surprise by managing for resilience 

(Drever and others 2006).
23

Review of Literature on Climate Change and Forest Diseases of Western North America

Mistletoes

The dwarf mistletoe genus Arceuthobium and the true mistletoe genus 

Phoradendron occur on various conifer and hardwood species in western 

North America (Geils and others 2002). Arceuthobium spp., parasitic seed 

plants on various Pinaceae and Cupressaceae, cause reductions in tree growth, 

seed and cone production, and wood quality, and result in increased tree mortality 

and susceptibility of the host to attacks from pathogens and insects (Hawksworth 

and Wiens 1970). In addition, Arceuthobium often form brooms that create fuel 

ladders into tree crowns (Hawksworth and Wiens 1970). Phoradendron spp., 

common on hardwoods and some conifers, are less damaging 

to their hosts than Arceuthobium because they have leaves 

and produce chlorophyll, but heavy infections can result in 

significant tree stress (Geils and others 2002).

Mistletoes may play a significant role in tree mortality 

as trees become stressed by drought and other changes in 

climate. For example, the interactive effects of Arceuthobium

and bark beetles are responsible for 40 to 60 percent of the 

pine mortality in southern California during years of average 

precipitation. Mortality is more frequent in the presence 

of other stress factors, such as drought, aerial oxidants, 

or extremely high density of trees (Schultz and Allison 

1982, Schultz and Kliejunas 1982). Wood and others (1979) 

attributed 90 percent of the mortality of Jeffrey pines (Pinus 

jeffreyi Grev. & Balf.) at Laguna Mountain in southern California to a combination 

of California flatheaded borer (Melanophila californica Van Dyke), western dwarf 

mistletoe (A. campylopodum Engelm.), and annosus root disease. In the central 

Sierra Nevada, red fir (Abies magnifica A. Murr.) are predisposed to fir engraver 

(Scolytus ventralis LeConte) attack by several stress factors, including severe 

drought, high tree density, annosus root disease, and dwarf mistletoe (Frankel and 

others 1988). Childs (1960) reported that death of conifer branches in the Pacific 

Northwest following an unusually hot summer in 1958 and an unusually dry 

summer in 1959 was particularly widespread in trees stressed by dwarf mistletoe 

and other agents. He also suggested that the death of branches infected by dwarf 

mistletoe actually may reduce inoculum and thus reduce spread of the disease.

Key points:

• Both genera of mistletoes common 

to the Western States can infect 

healthy host trees. Severe infection 

can induce stress and predispose 

hosts to serious damage in 

combination with other biotic agents 

and drought.

• Mistletoe reproduction is often 

limited by cold temperatures, so 

warming could allow for geographic 

range expansion. Paul Dunham
24

GENERAL TECHNICAL REPORT PSW-GTR-225

The combined stress from dwarf mistletoe, drought (especially via reduction in 

soil moisture), and other agents often results in tree mortality. Surveys during the 

1976–77 drought in California estimated that 12.3 million trees (equivalent to about 

30 000 m3 of wood) had died. All sites and forest types were affected. Forest insects 

and pathogens contributed to 98 percent of the lost volume. Of this loss, 65 percent 

was the result of a combination of dwarf mistletoe and root diseases predisposing 

the hosts to drought stress (Byler 1978, Craig 1979). More recent surveys in 

California also indicated that trees infected with dwarf mistletoe were the first 

to die during drought (Byler 1978). Dwarf mistletoe predisposed many stands to 

insect attack and has induced 60 to 80 percent of all Jeffrey pine mortality in years 

of severe drought (Jenkinson 1990). In ponderosa pine in southwest Colorado, the 

occurrence and severity of effects of dwarf mistletoe were positively correlated 

with dry sites (Merrill and others 1987). Wilson and Tkacz (1992) reported that 

severe drought, dense stand conditions, and presence of pinyon dwarf mistletoe (A. 

divaricatum Engelm.) predisposed pinyon pine (Pinus edulis Engelm.) in northern 

Arizona to attack by pinyon ips, Ips confusus (LeConte). Gehring and Whitham 

(1995) found that levels of the true mistletoe Phoradendron juniperinum Engelm. 

ex Gray in junipers (Juniperus monosperma (Engelm.) Sarg.) growing in droughty 

cinder soils were three times those in sandy-loam soils.

Harry Kope

Lodgepole pine dwarf mistletoe.
25

Review of Literature on Climate Change and Forest Diseases of Western North America

Because climate limits the ranges of many dwarf mistletoes, climate change 

likely will result in extensions in range. Southwestern dwarf mistletoe (A. 

vaginatum (Willd.) J. Presl ssp. cryptopodum) does not occur throughout the 

entire range of ponderosa pine; distinct northern, upper elevational, and lower 

elevational limits exist (Mark and Hawksworth 1976). Surveys of occurrence, slope 

association, and severity of several dwarf mistletoe species in Colorado revealed 

elevational zones specific to each host-parasite interaction (Williams 1971). For 

example, A. vaginatum was present to the upper elevational range of its host (2800 

m), but absent below 1860 m. Below 2130 m, ponderosa pine is vigorous; relatively 

high temperature may increase its resistance to mistletoe. Lodgepole pine dwarf 

mistletoe (Arceuthobium americanum Nutt ex. Engelm.) on lodgepole pine occurs 

at the lower but not at the upper elevational limits of its host (Hawksworth 1956). 

The elevational limit for the mistletoe may reflect that the growing season at 

higher elevations is not long enough for the fruit to mature before severe autumn 

frosts occur. Although ponderosa pine in the Black Hills is susceptible to dwarf 

mistletoe infection, the parasite is absent from the Black Hills and Bearlodge 

Mountains, apparently because early onset of the cold season kills young embryos 

of an essentially tropical mistletoe (Alexander 1987). Bloomberg (1987) suggested 

that differences in incidence and severity of hemlock dwarf mistletoe (A. tsugense

[Rosendahl] G.N. Jones) among geographic areas (low in Alaska, moderate to high 

in British Columbia, and moderate in Washington and Oregon) reflect the effects 

of climate on seed production and spread, stand composition and growth rate, and 

dwarf mistletoe biotypes. Lodgepole pine dwarf mistletoe is a damaging pathogen 

of jack pine (Pinus banksiana Lamb.) in western Canada, but is absent from jack 

pine forests in the colder, more northern areas, again suggesting limitation by low 

temperature (Brandt and others 2004).

In California, the range of several Phoradendron species is limited by low 

temperatures. Wagener (1957) reported that occasional low temperatures restrict P. 

densum Torr. on western juniper (Juniperus occidentalis Hook.) and P. pauciflorum 

Torr. on white fir (Abies concolor (Gord. & Glend.) Lindl.) to their present limits 

and account for their absence at the northern extent of their hosts’ ranges. A warming climate likely would extend the range of these Phoradendron species.
26

GENERAL TECHNICAL REPORT PSW-GTR-225

Wood Decays

Wood decay is caused by fungi that either break down holocellulose and lignin 

(white rots) or holocellulose but not lignin (brown rots). The decay is often 

described according to its position in the tree (for example, root rot, butt rot, 

stem rot) (Tainter and Baker 1996). Decay of wood in living trees and of woody 

debris is heavily dependent on moisture and temperature (Boddy 1983, Griffin 

1977, Loman 1965). Some decay fungi are quite tolerant of high temperatures and 

dry conditions and are often found in the outer layers of slash piles, whereas other 

fungi require more water and lower temperatures and are restricted to the center 

or bottom of slash piles (Loman 1962, 1965; Spaulding 1929, 1944).

Heart-rot fungi are pathogens that colonize the central 

portion of living trees. Although many heart-rot fungi are host 

generalists and have a broad geographic range, others are more 

restricted and colonize specific tree species (Tainter and Baker 

1996). Actively growing trees can often limit fungal colonization via active chemical and physical barriers that restrict decay 

to the central column (Shigo 1984). However, trees under stress, 

particularly drought stress, may have fewer reserves available 

for host defense. Low humidity and dry conditions make treewound surfaces less favorable for spore germination, possibly 

resulting in fewer established infections. Certain heart-rot fungi 

can persist in dry wood for many years, so colonization can resume under more 

favorable conditions (Wright 1934).

The breakdown of woody debris is primarily caused by numerous species of 

sap-rot fungi that saprotrophically degrade downed wood, recycling nutrients back 

into the soil. Many of these fungi are not host specific (Gilbertson and Ryvarden 

1986); therefore, there is a wide array of decay fungi available to colonize a 

specific substrate under a given set of conditions. Wood moisture content is the 

most important influence on decay and is dependent on humidity, precipitation, 

and uptake of water from ground contact. Decay fungi cannot degrade wood 

with moisture content below 30 percent (fiber saturation point; moisture content 

expressed on a dry-weight basis), and upper moisture limits generally do not exceed 

90 percent, owing to lack of oxygen (Griffin 1977). Wood temperature is the second 

most important influence on fungal activity. Fungi can generally grow when wood 

temperature exceeds 0 °C, the temperature at which free water in the wood cell 

wall is available to the fungus. The optimum temperature for growth varies among 

species; most decay fungi have temperature optima between 20 to 35 °C. For 

Key points:

• Decay of wood is dependent on 

moisture and temperature. 

• With climate warming, white rot 

fungi, which break down carbon 

compounds as well as lignin, may 

move northward. This would alter 

carbon sequestration rates by 

accelerating carbon breakdown.

Jessie Micales Glaeser
27

Review of Literature on Climate Change and Forest Diseases of Western North America

Fruiting body of the oyster mushroom (Pleurotus), a common 

wood decay organism. 

Jessie Micales Glaeser

example, Gloeophyllum sepiarium (Fr.) Karst and 

Phlebia subserialis (Bourdot & Galzin) Donk., 

with temperature optima of 38 °C and 31 °C, 

respectively (Loman 1962), can colonize casehardened logs and the upper portions of slash piles 

under conditions of high temperatures and relatively 

low moisture (Loman 1962, 1965; Spaulding 

1929, 1944). Coniophora puteana and Stereum 

sanguinulentum, isolated from lower portions of 

the same slash piles, have temperature optima of 

24 °C and 20 to 24 °C, respectively (Loman 1962).

Brown rot fungi are commonly associated with 

conifers, and white rots with hardwoods (Gilbertson 

1980, 1981; Nobles 1958). Hardwoods that are currently limited by freezing temperatures are likely 

to spread northward and replace conifers as the 

climate warms (Shafer and others 2001). Expansion 

of hardwood taxa may increase the overall occurrence of white rot fungi. Because white rots break 

down carbon compounds as well as lignin, carbon 

sequestration rates will be altered by expansion of 

this category of pathogens.

Dead wood can represent a substantial fraction 

of stored carbon in forest ecosystems, and decay 

rates of coarse woody debris may be affected by 

climate change. Kueppers and others (2004) found 

differences in biomass and rates of decay along an elevational gradient, suggesting 

that warming will lead to a loss of dead wood carbon from subalpine forest. Models 

of decomposition of woody debris suggest that an increase in regional warming and 

drying in the Western United States will increase rates of decay, leaving less debris 

on the forest floor and increasing release of carbon dioxide, except when limited by 

extremely xeric conditions (see Yin 1999, for example). Therefore, it appears that 

a changing climate will substantially influence the development of wood decay, 

especially in the decomposition of downed wood and slash. The potential effects 

of climate on individual heart-rot diseases are unknown.
28

GENERAL TECHNICAL REPORT PSW-GTR-225

Summary of Climate Change and Forest Diseases

Because there is considerable uncertainty about the type and magnitude of 

climate changes that will occur, the responses of forest ecosystems and forest 

tree pathogens to climate change are also uncertain (Millar and others 2007). 

Information on interactions among the environment, forest pathogens, and their 

hosts is limited. Effects of climate changes on host physiology, adaptation or 

maladaptation, and population genetics that affect host-pathogen interactions 

will be altered by climate change in ways essentially unknown.

Despite these uncertainties, the potential effects of climate change on forest tree pathogens can be projected by analyzing current relationships between 

the behavior of individual pathogens and a suite of abiotic and biotic variables 

(Lonsdale and Gibbs 1994). The environmental conditions that many individual 

diseases can exploit have been quantified. Based on existing knowledge, it 

is reasonable to infer that climate change generally will lead to reductions in 

tree health and will improve conditions for some highly damaging pathogens. 

Drought stress will exacerbate the impacts of many pathogens. Increases in 

winter temperatures will increase the occurrence and severity of pathogens such 

as Phytophthora. Increases in spring precipitation will increase the occurrence 

and severity of foliar pathogens, whereas increases in autumn precipitation will 

promote expansion of stem rusts. Predictions are relatively feasible and credible 

for pathogens whose range or activities primarily are limited by temperature. It 

is more difficult to predict how climate change may affect pathogens dependent 

on interactions with other organisms such as insect vectors (Lonsdale and Gibbs 

1994). Few data are available to validate predicted interactions among hosts, 

pathogens, and climate (Runion 2003).

Generalizations about the effects of climate change on forest ecosystems and 

forest pathogens in western North America are difficult because different hosts 

and pathosystems in different locations will have distinct responses to climate 

change (Runion 2003, Sturrock 2007). For example, decreases in precipitation 

may drive changes in host-pathogen relationships in Arizona, whereas increases 

in temperature may have the greatest influence on these relationships in Alaska. 

Tree species differ significantly in their ability to adapt to increases in temperature and atmospheric concentrations of carbon dioxide and their tolerance of 

disturbances (Lemmen and Warren 2004).

Based on existing 

knowledge, it is 

reasonable to infer 

that climate change 

generally will lead 

to reductions in tree 

health and will improve 

conditions for some 

highly damaging 

pathogens. 
29

Review of Literature on Climate Change and Forest Diseases of Western North America

Implications

Various models have been used to predict how climate change will affect forest 

ecosystems. Some of these models have been used to predict the response of 

individual pests to climate change (Baker and others 2000, Logan and others 2003). 

For example, CLIMEX has been used to explore the response to climate change of 

P. cinnamomi (Desprez-Loustau and others 2007) and P. ramorum (Venette and 

Cohen 2006). Unfortunately, as sophisticated as some climate models have become, 

many do not account for or consider all the important factors involved in the effects 

of climate change. Current climate model projections do not include pathogen 

and insect impacts on vegetation, and most models do not incorporate the effects 

of pathogens as agents of regime change (Folke and others 2004). It is clear that 

climate change will alter current host-pathogen interactions.

Although the impacts of individual disturbances such as forest pathogens 

on forest structure and function have been studied, there is little research on the 

interactions of climate and disturbance (Dale and others 2000). Thus, the extent to 

which climate change will affect the frequency, severity, or magnitude of disturbances is difficult to predict (Loehle and LeBlanc 1996). Research on impacts of 

climate change on plant pathogens has been limited, with most work concentrating 

on the effects of a single disturbance on the tree host, or the interaction of a single 

disturbance and climate. Disturbances may interact in a cumulative or cascading 

manner, with increases in one type of disturbance increasing the potential for other 

types of disturbances. For example, drought in southern California forests increased 

mortality of trees from insects and root pathogens. The cumulative mortality 

resulted in extremely high fuel loads that facilitated severe wildfire.

The role of abiotic stressors, pathogens, and their synergistic interactions with 

the host often are not included in climate change scenarios (Loehle and LeBlanc 

1996, Scherm 2004). There is consensus that climate change likely will stress 

trees and increase their vulnerability to insects, pathogens, and emerging diseases 

(Brasier 2001, McNulty and Aber 2001).

Maladaptation of hosts and their pathogens resulting from climate change 

could alter sporulation and rates of development of the pathogen, modify host 

resistance and phenology, and result in changes in the physiology of host-pathogen 

interactions. For example, increased carbon dioxide concentrations will affect 

plant diseases by changing host physiology and anatomy, such as lowered nutrient 
30

GENERAL TECHNICAL REPORT PSW-GTR-225

concentration, greater accumulation of carbohydrates in leaves, more waxes, extra 

layers of epidermal cells and increased fiber content, and greater number of mesophyll cells (Chakraborty and others 1998). Two important effects of elevated carbon 

dioxide on host-pathogen interactions will be a delay in initial establishment of a 

pathogen because of modifications in pathogen aggressiveness and host susceptibility, and increased fecundity of pathogens (Coakley and others 1999). An increase 

in carbon dioxide may increase tree canopy size and density, resulting in a higher 

microclimate relative humidity, and a subsequent increase in foliar and rust diseases 

Manning and Tiedemann 1995).

Additional consequences of climate change are shifts in the geographical 

distribution of host and pathogen and altered impacts of the pathogen on the host 

(Coakley and others 1999). For example, a warmer climate may result in extensions 

of northern range limits for some pathogens currently limited by winter extremes 

(Loehle and LeBlanc 1996). The extension of the range of the subtropical pathogen 

P. cinnamomi to more northern latitudes is often given as an example. Warmer 

temperatures at northern latitudes and higher elevations may favor other pathogens 

and insect vectors as well, resulting in increased damage to hosts and to forest 

ecosystems. Relatively rapid changes in climate may result in host resistance to 

pathogens being overcome more rapidly, owing to accelerated pathogen evolution, 

a result of the shorter regeneration time of fungal pathogens relative to trees. This 

decrease in host resistance may also result in greater than expected ecosystem damage. Some studies suggest that with regional warming and drying, subalpine forests 

will be a net source of carbon (Kueppers and Harte 2005). Increases in temperature 

at northern latitudes may result in greater activity by decay fungi and potentially 

reduce carbon sequestration in northern forests. The literature on decline diseases, 

such as pole blight and yellow-cedar decline, provides examples of how a tree 

species can be affected by a change in climate that affects host physiology and site 

conditions.

The interactions and synchrony developed over time are crucial components of 

the sustainable ecosystems existing under current climatic conditions. As the climate changes, these components will change, resulting in changes in host–pathogen 

relationships. The failure to consider and include detrimental changes such as an 

increase of pathogen species favored by warmer temperatures or increased number 

of insect species capable of vectoring pathogens in climate change models where 

data are available means that those models underestimate, or in some instances 

overestimate, the effects of climate change on forest ecosystems.
31

Review of Literature on Climate Change and Forest Diseases of Western North America

Conclusions

The effects of climate change on hosts, pathogens, and their interaction will have 

numerous, mostly adverse, consequences to forest ecosystems. Most of these 

consequences are not currently accounted for in climate change models. Some 

key points that emerged from this literature review are:

•	 Climate change will alter the frequency, intensity, duration, and timing of

disturbances in forest ecosystems. These disturbances include fire, drought, 

introduced species, outbreaks of insects and pathogens, hurricanes, windstorms, ice storms, and landslides (Dale and others 2001). The relative role 

of pathogens and other stressors will differ among ecosystems (Ayres and 

Lombardero 2000).

•	 Climate change will affect the geographical distribution of plant	diseases,	

the losses they cause, and the efficacy of disease management strategies.

•	 Climate change may precipitate irreversible changes in ecosystem state

(Folke and others 2004, Pascual and Guichard 2005). Ecosystems may be 

resilient to individual stresses, but multiple simultaneous stresses could 

seriously impact status and sustainability of forests (Aber and others 2001), 

especially forests on marginal sites.

•	 Risk	assessments developed previously for invasive and emergent	diseases	

can provide a framework for organizing and presenting information on 

interactions between forest disease and climate change (for example, Yang 

2006). The information needs of decisionmakers differ, but all assessments 

should communicate assumptions, potential bias, uncertainty, and appropriate applications. Assessments on forest disease-climate change should 

include an adequately supported and responsive infrastructure that includes 

research, stakeholders, and decisionmakers at various agencies and levels 

(Hain 2006).

•	 Efforts to model host-pathogen interactions in forest ecosystems would

benefit from active networking among multiple disciplines across a gradient of scales (Garrett and others 2006, Lundquist and Hamelin 2005, Proulx 

and others 2005). Models must consider the synergistic impacts of climate 

change and other environmental stressors.
32

GENERAL TECHNICAL REPORT PSW-GTR-225

•	 As	climate changes, managers and policymakers will need input and guidance from pathologists in the form of viable management alternatives. 

Pathologists are uniquely trained to understand host-pathogen-environment 

interactions as integrated systems (Bruck and Shafer 1991). Achieving 

management objectives for forest ecosystems as climate changes will 

require multiple approaches, including models, formal assessments, distributed learning networks, and adaptation policies that are responsive to 

a wide variety of environmental circumstances (Norgaard and Baer 2005, 

Spittlehouse and Stewart 2003).

Acknowledgments

Contributions for individual diseases were provided by Brian W. Geils (rusts and 

mistletoes), Jessie Micales Glaeser (heart rots and wood decays), Ellen Michaels 

Goheen (Phytophthora spp.), Paul Hennon (tree declines), Mee-Sook Kim (root 

diseases), John T. Kliejunas (abiotic diseases and cankers), Harry Kope (foliar 

diseases), and Jeffrey Stone (foliar diseases).

Thanks to Pauline Spaine (U.S. Department of Agriculture, Forest Service, 

Southern Research Station) and Glen Stanosz (University of Wisconsin, Madison) 

for providing publications and citations for canker diseases, and to Charles G. 

“Terry” Shaw, U.S. Department of Agriculture, Forest Service, Western Wildland 

Environmental Threat Assessment Center; Erica Fleishman, University of California Santa Barbara, National Center for Ecological Analysis and Synthesis; Kerry 

Britton, U.S. Department of Agriculture, Forest Service, Research and Development; and James Worrall, U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Region, Forest Health Management for guidance and reviews.

Project funding was provided by U.S. Department of Agriculture, Forest 

Service, Western Wildland Environmental Threat Assessment Center and the 

Pacific Southwest Research Station.

English Equivalents

When you know: Multiply by: To f ind:

Millimeters (mm) 0.0394 Inches

Meters (m) 3.28 Feet

Cubic meters (m3

) 424 Board feet

Square kilometers (km2) .386 Square miles

Celsius (°C) 1.8 and add 32 Fahrenheit
33

Review of Literature on Climate Change and Forest Diseases of Western North America

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