Root Diseases — Biology & History

Distribution of major root diseases

The maps in this PDF depict the range of Armillaria root disease, laminated root rot, Tomentosus root rot, blackstain root disease (Douglas-fir variety), blackstain root disease (pine variety) and Annosus across British Columbia.

Host susceptibility and symptomology

Trees of all ages can be affected by root disease. For detailed information on field identification of all major root diseases in B.C., see the Field Guide to Forest Damage in British Columbia (Joint Report No. 17). 

Disease detection occurs mainly at the stand and tree levels. Table 2 describes the signs and symptoms for infected stands and trees. Primary detection of root disease may also be assisted by using air photos and satellite imagery.

Root disease and stand dynamics

Fungal root pathogens are widespread throughout most forested ecosystems in B.C. They play a significant but often subtle role as biological agents of change that, in part, help regulate the structure and composition of forests. With most forest resource management objectives, root diseases increase management complexity and risk for a number of reasons. This section provides a broader insight into the behaviour of root diseases and how they affect stand dynamics in natural and managed forests.

Root diseases often act as opportunists, rapidly colonizing new food sources. Creating new food sources, such as roots and stumps by harvesting or stand management practices can lead to a dramatic increase in the amount of root disease on a site. Foresters creating prescriptions should be aware of several key biological principles when contemplating management activities.

  1. Root disease inoculum (mainly stumps) remains infectious for up to 35 years, depending on fungal species and inoculum size.
  2. Root disease incidence increases with increasing host susceptibility and increasing proportion of susceptible species.
  3. Root disease incidence and damage may accumulate over time without appropriate treatment.
  4. Root diseases infect trees and subsequently spread from tree to tree via root contacts or grafts, spores, and in the case of Armillaria, by rhizomorphs.
  5. Armillaria root disease may rapidly spread throughout a stump when quiescent root lesions are triggered by brushing, thinning, or partial cut harvesting.
  6. Root diseases may infect healthy hosts after contact with inoculum on or in stumps and root material remaining in the soil after fire, windthrow, harvesting, or thinning.

Unexposed, residual inoculum has an infective lifespan of up to 35 years, meaning that regeneration or residual trees are at risk of becoming infected from this inoculum for a substantial period of time.

Root diseases are very persistent in natural ecosystems and are linked directly to major ecosystem processes. The effects of root disease can result in increased fuel loading either directly or indirectly through association with insects, especially bark beetles. In fire dependent ecosystems the major agents of disturbance are fire alone, fire–root disease, or fire–root disease–insect combinations. In ecosystems which are not fire dependent, root diseases in association with wind, snow, ice, or insects are common agents of disturbance.