Armillaria root disease (Armillaria spp.)

Last Review/Updated: January 6, 2003

Armillaria root disease refers to diseases caused by Armillaria species. It is one of the most important diseases of forest trees and, in particular, one of the most serious diseases of young conifers in Alberta.

Primary Hosts and Distribution

  • All tree species in the province

Armillaria root disease is found throughout Alberta.

Disease Cycle

In general, Armillaria has two stages: a saprophytic phase (obtaining nutrients from dead organic matter) and a parasitic stage (obtaining nutrients from living tree hosts).

Saprophytic

When in the saprophytic phase, it infects old roots or stumps of dead trees.

Parasitic

In the parasitic phase, Armillaria extends shoestring-like rhizomorphs into the soil to infect new living hosts. The disease can also spread from root to root or by root grafts.

How it spreads:

Armillaria slowly grows up into the root, girdling the root collar and stem. When the root is infected, a creamy-white mycelial fan (fungal sheet) develops and colonizes the wood under the bark. Spores from mushrooms, which develop in clusters over infected roots, could potentially spread the disease.

Detection and Damage

Look for the following signs and symptoms:

  • White mycelial fans found between the bark and wood around the root collar and affected roots of the tree.
  • Abnormal amount of resin produced around the root collar of the tree.
  • Dark brown rhizomorphs on roots or in the soil, and often brown or honey-coloured mushrooms at the base of the tree in the fall.
  • Dull green, yellow, or reddish-brown foliage (needles) over the whole tree (true for young trees only).

Trees under stress (i.e. those predisposed to attack by other factors) are more susceptible to Armillaria. The fungus can quickly kill small trees. Larger trees are affected by having reduced growth but can survive for a longer period of time. Indirectly, these weakened trees are more prone to blowdown and to pests.

Management

A well-managed and healthy stand is more resistant to Armillaria infection than one that is not. For information on Armillaria resource material, click here. Some important management techniques include:

  • Surveying and periodically monitoring stands;
  • Conducting preharvest surveys to identify disease centres.
  • Removing or treating healthy stumps following logging operations to reduce the spread of the disease. This technique has not been proven in Alberta but has been effective in BC;
  • Removing infected stumps and roots of dead and diseased trees;
  • Avoiding stand thinning in subsequent regeneration if more than 10 small disease centres are found per hectare. Avoid these centres when replanting or remove stumps and roots within 20 m.;
  • Conducting push-over logging where feasible;
  • Planting more resistant species nearer infected stumps and less resistant species further away. The only known long-term resistant species in Alberta is white birch;
  • Reducing the amount of soil compaction when harvesting;
  • Not damaging stems and roots during harvesting operations.