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Indicator: Forest diseases

Diseases are caused by pathogens, primarily fungi in trees. All parts of a tree can become infected, leading to decreases in wood quality, productivity and occasionally death.

  • Native pathogens co-exist with their hosts, achieving a balance over millennia, and are important for nutrient cycling, biodiversity and for other ecosystem processes.
  • Disease effects from native pathogens typically occur over several growing seasons, causing gradual losses.
  • Exotic introduced pathogens have caused extensive mortality to some tree species.
  • Disease effects from exotic pathogens can occur quickly, resulting in rapid tree death.

Climate change and tree disease:

  • Weather regulates host / pathogen interactions.
  • Suitable environmental conditions when the host is susceptible can facilitate pathogen infection.
  • Under climate change, we expect differences in pathogen behaviour, as weather patterns and environmental conditions change.


It is difficult to predict exactly how forest disease will be impacted by climate change. Monitoring and being prepared to respond are important for reducing losses.

Why is this indicator important?

  • Economic losses can accrue when wood quality and volume are reduced within managed forests.
  • Tree diseases can be managed to reduce economic losses by planting less susceptible tree species, using silvicultural treatments such as brushing or thinning and selecting genetically improved stock for planting.

What is the outlook?

  • The effects of globalization, including increased trade, travel and tourism, can increase the risks of introducing exotic pathogens and their establishment, leading to increased damage to trees.
  • We must remain vigilant to avoid the introduction of exotic pathogens, such as the oak wilt pathogen, Bretziella fagacearum, which is present in the northern United States.
  • Technological improvements in DNA-based identification of forest pathogens are important tools in preventing the establishment of exotic pathogens.


What reporting frameworks does this indicator support?

  • Montréal Process: 3.a
A jack pine tree branch with an orange gall on it from an infection of Cronartium harknessii
Cronartium harknessii is a native pathogen that is the causal agent of western gall rust, a common disease of hard pines, including lodgepole pine, jack pine and ponderosa pine in Canada.
Larger image [168 kb]
Sources and information
Photo credit
  • Cronartium harknessii photo by Tod Ramsfield.

Table of contents — The State of Canada's Forests Report

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