Indicator: Forest diseases

Tree diseases can be detrimental to stand productivity, largely by affecting the rate of growth or mortality in the forest.

  • The environment plays a critical role in the disease process, as environmental conditions may directly influence disease, affect the ability of a pathogen to spread and infect hosts, or influence host susceptibility to pathogens.
  • In addition to native forest pathogens, exotic pathogens have become established in Canada. One such pathogen, Cronartium ribicola, the causal agent of white pine blister rust, has had a major impact on five-needle pines in Canada since its introduction at the beginning of the 20th century.

Diseases include a wide range of infections caused by bacteria, viruses and other microorganisms (called pathogens, collectively), as well as abnormalities and disturbances to the normal structure and growth of a tree.

Tree disease symptoms usually develop as a result of complex relationships between susceptible trees, environmental conditions and pathogens. They can affect all parts of a tree including its roots, stems and foliage.

Trees stressed by adverse environmental conditions can become more susceptible to pathogen impacts as a result of their reduced ability to mount a defence response.

Why is this indicator important?

  • Diseases can result in a range of impacts, from volume loss to decreased wood quality and ultimately tree mortality.
  • In a commercial forestry context, when the primary objective is the production of high quality forest products, impacts of disease are considered negative.
  • In a forest managed for biodiversity and conservation, forest disease can be viewed as an agent of nutrient cycling and habitat creation.

What is the outlook?

  • Climate change will affect both hosts and pathogens. However, predicting exactly how host and pathogen interactions will change is difficult. Managers will need to monitor forests carefully in order to respond rapidly to changes in forest pathogens.
  • The root disease pathogen Armillaria ostoyae is expected to cause increased mortality in trees already affected by drought conditions induced by climate change.
  • Regulations developed in response to past pathogen introductions are an important element in the strategy to reduce future damage from exotic pathogens.
Photo: A lodgepole pine killed by Armillaria root disease
A lodgepole pine killed by Armillaria root disease.