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Indicator: Forest diseases

Forest diseases are caused by pathogens, which are biotic agents that disrupt normal tree functions. Abiotic factors such as changing weather regimes, too much or too little precipitation, or extreme temperatures may affect their growth and development and can increase the susceptibility of trees to forest diseases.

  • In natural settings, native diseases caused by native pathogens assist nutrient cycling, create habitat and enhance biodiversity.
  • In commercial forests, diseases may be managed to prevent losses in tree volume and wood quality.
  • Tree breeding and silviculture have been used to manage disease and optimize growth.
  • Environmental conditions are important for moderating the interaction between the tree and the pathogen.

All tree parts are affected by disease:

  • Foliar pathogens cause reduced photosynthetic capability.
  • Stem pathogens cause structural problems that disrupt water and nutrient flow and lead to breakage or decreased wood quality.
  • Root pathogens cause reduced water and nutrient uptake.

Disease is not always lethal, but disease effects often reduce growth and decrease productivity.

Rust coloured spruce needles infected by Chrysomyxa ledicola
White spruce needles infected by Chrysomyxa ledicola (causal agent of spruce needle rust)

Spruce needle rust can be common in years in which environmental conditions are favourable for the pathogen to be active. The orange aeciospores released from infected needles can create an orange film on the surface of lakes and rivers near heavily infected trees.

Why is this indicator important?

  • In the worst case, diseased trees can die; at the very least, volume and wood quality are reduced.
  • Diseases can impact a tree’s growth rate, volume and wood quality and may ultimately kill the tree. Forest managers manage disease to reduce losses and enhance forest resilience.

What is the outlook?

  • Climate change will alter future interactions between trees and pathogens, resulting in uncertainty over future losses to disease.
  • Future climatic conditions are likely to increase tree death from drought and floods in the Prairie provinces and may induce changes in the natural distribution of tree species.
  • Introduced exotic pathogens, such as Cronartium ribicola (the causal agent of white pine blister rust), have resulted in the significant loss of native trees.
  • The oak wilt pathogen, Bretziella fagacearum, is present in the northern United States and Diplodia corticola, the causal agent of bot canker, is present in Maine. Both pathogens are geographically close to Canada, but the diseases have not been detected here.

What reporting frameworks does this indicator support?

Sources and information
Photo credit
  • Photo of infected white spruce needles by Tod Ramsfield

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

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