See:
Fir, Douglas and True - Armillaria Root Rot
Oak (Quercus spp.) - Armillaria Root Rot
Note In western North America, Armillaria species are common on dead aspen, but less commonly associated with disease of living trees than in eastern North America. Armillaria occurs on the roots of many forest tree species. In forests west of the Cascade Range, this disease is associated with stress conditions while it is most common in eastern forests located in moist habitats. The fungus can live many years as a saprophyte in soil, on infected stumps, roots, and other organic matter. Diagnostic mycelial fans can be observed between the bark and wood of the root crown. Mycelial fans are thick, white layers of fungus that adhere to the inner root bark and/or the wood beneath the bark. A few cultural controls can help avoid or limit the disease.