Armillaria root disease, Armillaria ostoyae,

Fig. 129 Mushrooms at base of infected tree.	Fig. 130 Resinosis. This symptom is common around the lower bole.	Fig. 131 Typical crown symptoms of an immature tree.	Fig. 132 Mycelial fans develop between bark and outer wood.

Distribution: South of approximately 53 degrees latitude.

Host Susceptibility: Essentially all trees and many woody plant species are susceptible.

Signs & Symptoms: In stands of all ages, armillaria root disease can kill individual trees or cause large centres of mortality. Groups of dead, shade-intolerant conifers may be replaced with shade-tolerant conifers, broadleaved species, or other woody shrubs. Symptoms include chlorotic thinning foliage, reduced shoot growth, and distress cones. Basal resinosis can occur near ground level on resinous hosts. Roots may appear resin-soaked. Advanced decay consists of a yellow, stringy type of rot. Signs of the fungus may include: white mycelial fans under and within bark of roots and lower bole, rhizomorphs (black shoestring-like structures) on the surface of dead or infected roots, or golden-yellow mushrooms that grow from infected material in the autumn.

Damage: In coastal forests, armillaria root disease causes obvious mortality on trees to about age 25. In the interior, Armillaria kills trees of all ages. It spreads primarily by new host root contact with infected roots or rhizomorphs. The probability of infection is influenced by host species, age and vigour of host, site factors (e.g., infection is higher in mesic than dry or wet sites), and presence of inoculum. Some stand management activities have the potential to increase inoculum. Juvenile trees die quickly and show little species-specific resistance, while mature trees die more slowly and exhibit greater host specificity.

Can Be Confused With: Symptoms are similar to other root diseases, bark beetles, and abiotic problems. If mature, standing dead trees exhibit boring dust or bark beetle galleries, check for signs of root disease. However, only armillaria root disease will produce fan-shaped mycelial sheets under the bark. There are other Armillaria species (e.g., A. sinapina) that act as weak pathogens of stressed trees, or as true saprophytes (non-pathogenic). These secondary invaders can be differentiated from A. ostoyae in several ways. They appear on standing dead or down trees, their mycelial fans do not grow in the bark, and they often produce abundant rhizomorphs. A. ostoyae rhizomorphs branch dichotomously (forked) while many saprophytic species branch monopodially (right-angled). 

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BC Ministry of Forests
Forest Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136

Last updated February 18, 2002