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Forest Plant Diseases and Climate Change (2008)


Susan J. Frankel, Sudden Oak Death Research Program, Pacific Southwest Research Station

The current versions of this paper (2012) are available at


Forest plant diseases are stongly influenced by weather and climate. For forest pathogenic fungi, bacteria, viruses, and other microorganisms, the temperature and moisture conditions interacting with seasonal phenology, and stress on the host determine infection severity and distribution. Extreme weather, i.e., drought or typhoons, can kill large expanses of trees directly by overwhelming tree physiological capability and structural strength. Expected changes in climate coupled with the increasing stresses of invasive species and lack of fire are creating conditions conducive for many forest plant diseases. Patterns and rates of wood decay, caused by forest fungi, are also expected to change, which will influence forest carbon cycles.

Current Examples

Warming, changes in precipitation, and weather extremes are already influencing forest plant diseases in western North America. The following examples cover the entire range of forest diseases from purely abiotic factors (Alaska yellow-cedar decline and drought-related decline) a weak native pathogen (red band needle blight - Dothistroma septosporum) and an aggressive nonnative pathogen (sudden oak death - Phytophthora ramorum).

Alaska yellow-cedar decline. Earlier snow melt exposes shallow fine roots to colder temperatures and results in spring freeze injury that is killing millions of yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach) in Alaska (Hennon and D'Amore 2007, Hennon and Shaw 1997).

[img_assist|nid=3606|title=|desc=Alaska yellow-cedar decline on western Chichagof Island.|link=none|align=middle|width=630|height=396]

Red band needle blight (Dothistroma septosporum). In British Columbia, red band needle blight is causing unprecedented mortality in lodgepole pine (Pinus contorta Dougl. ex Loud.) plantations and mature stands. The mortality is adjacent to areas devastated by mountain pine beetle (Dendroctonus ponderosae). The disease outbreak is driven by increases in summer precipitation that are beyond the range of previously recorded weather patterns (Woods et. al 2005).

[img_assist|nid=3607|title=|desc=Lodgepole pine plantation in Northwest British Columbia heavily impacted by red band needle blight caused by Dothiostroma pini.|link=none|align=middle|width=450|height=338]

Sudden oak death (Phytophthora ramorum). Sudden oak death-related tree mortality is driven by extreme weather events. In California and Oregon, sudden oak death mortality rates abruptly increase and then subside. The patterm is driven by heavy rains and extended wet weather during warm periods which create optimal infection conditions. Infected trees suffer a reduced capacity to manage water, but survive until high temperatures and extended dry periods overwhelm their vascular capability, resulting in death. Two cycles of this pattern have been noted in California: 1998-2001 and 2005 -2008 (Frankel 2007). The Bay Area experienced an all-time record for rainy days in March 2006 with 25 days of rain; followed in July with the longest string of hot weather ever recorded.

[img_assist|nid=3608|title=|desc=Sudden oak death on tanoak (Lithocarpus densiflorus (Hook. & Arn.) Rehd.) in Marin County, California.|link=none|align=middle|width=516|height=373]

Drought-related decline. Forests that are already stressed by overstocking, pathogens, or climatic conditions such as drought may not survive additional climatic stress (Winnett 1998). In southern California, the worst drought in the recorded history of the region, in combination with stress induced by overstocking, dwarf mistletoe (Arceuthobium spp.) and annosus root disease (Fomes annosus (Fr.) Bref.), predisposed conifers to bark beetle attack. The tree mortality provided fuels for fall 2003 wildfires, which caused over $2.5 billion in damage (Keeley et. al 2004, Kliejunas et. al Draft. 2008). Recent drought in Arizona and New Mexico is causing decline of pinyon pine (Pinus edulis Engelm.) across 12 000 km2 of the Southwest (Breshears et. al 2005).

[img_assist|nid=3609|title=|desc=Widespread decline of pinyon pine (Pinus edulis) on the Carsen National Forest, viewed from Highway 285, six miles (9.7 km) from Ojo Caliente, New Mexico. |link=none|align=middle|width=516|height=314]

Predicted Trends

  • Although uncertainty exists in estimating how forest diseases will respond to climate change, some general statements may be helpful in predicting logical responses of forest diseases to various environmental conditions, new rates of change, and exposure to new climate conditions.
  • Changes will occur in the type, amount and relative importance of forest diseases. With warming, some diseases may be able to occur farther north or at higher elevations than under current climate conditions.
  • Pathogens may play new roles in the health and functioning of newly forming forests because these conditions will be distinctive from any previous forest. If managers facilitate migration of tree species in new environments, then new disease situations might be expected.
  • The epidemiology of plant diseases will be altered. Prediction of disease outbreaks will be more difficult in periods of rapidly changing climate and unstable weather.
  • In a rapidly changing environment, host resistance to pathogens may be overcome more rapidly owing to accelerated pathogen evolution, a result of the shorter regeneration time of fungi and bacteria relative to trees.
  • Warmer winters will likely lead to greater overwintering success of pathogens, increasing disease severity.
  • Interactions between biotic (mostly fungal) and abiotic diseases (such as drought) may represent the most important effects of climate change on forest diseases.
  • Changes in wood decay rate and pattern could be heavily affected by climate change because decay in both living and dead trees is intimately dependent on moisture and temperature regimes. An alteration in decay rate would similarly alter forest carbon cycling. This is a good example of the type of disease that will be influenced by coarse weather patterns (warmer mean temperature during the growing season, and an extended growing season).

English Equivalents:
1 kilometer (km) = 0.621 miles
1 square kilometer (km2) = 0.386 square miles

Frankel, Susan J. 2008. Forest Plant Diseases and Climate Change. (May 20, 2008). U.S. Department of Agriculture, Forest Service, Climate Change Resource Center.

Hepting, G. 1963. Climate and forest diseases. Annual review of phytopathology. 1: 31-50.

Kliejunas, J.T.; Geils, B.; Glaeser, J.M.; Goheen, E.M.; Hennon, P.; Mee-Sook K.; Kope, H.; Stone, J.; Sturrock, R.; Frankel, S.J. [Draft. February 21, 2008]. Climate and forest diseases of western North America: a literature review. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 36 p.

Sturrock, R.N. 2007. Climate change effects on forest diseases: an overview. In: Jackson, M.B., comp. Proceedings 54th annual western international forest disease work conference: 51-55.

Breshears, D.D.; Cobb, N.S.; Rich, P.M.; Price, K.P.; Allen, C.D.; Balice, R.G.; Romme, W.H.; Kastens, J.H.; Floyd, M.L.; Belnap, J.; Anderson, J.J.; Myers, O.B.; Meyer, C.W. 2005. Regional vegetation die-off in response to global-change type drought. Proceedings of the National Academy of Sciences of the United States of America. 102: 15144-15148.

Frankel, S.J. 2007. Climate change's influence on sudden oak death, PACLIM 2007. May 13-15. 2007, Monterey, CA. (May 13, 2008)

Hennon, P.; D'Amore, D. 2007. The mysterious demise of an ice-age relic: exposing the cause of yellow-cedar decline. Science Findings. Issue 93. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 6 p.

Hennon, P.; Shaw, C.G., III. 1997. The enigma of yellow-cedar decline: What is killing these long lived, defensive trees? Journal of Forestry. 95(12): 4-10.

Keeley, J.E.; Fotheringham, C.J.; Moritz, M.A. 2004. Lessons from the October 2003 wildfires in southern California. Journal of Forestry. 102: 26-31. (May 13, 2008)

Kliejunas, J.T.; Geils, B.; Glaeser, J.M.; Goheen , E.M.; Hennon, P.; Mee-Sook K.; Kope, H.; Stone, J.; Sturrock, R. and Frankel, S.J. [Draft. February 21, 2008]. Climate and forest diseases of Western North America: a literature review. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 36 p. (May 13, 2008)

Winnett, S.M. 1998. Potential effects of climate change on U.S. forests: a review. Climate Research. 11: 39-49. (May 13, 2008)

Woods, A.J.; Coates, K.D.; Hamann, A. 2005. Is an unprecedented Dothistroma needle blight epidemic related to climate change? Bioscience. 55:761-769. (May 13, 2008)