Most commonly, trees are infected by rhizomorphs or by root contact with a diseased individual. Rhizomorphs grow out through the soil from an established infection and penetrate directly into the root surface of uninfected trees to spread the disease. When roots from a healthy tree touch an infected root or stump a new infection results. Armillaria fre quently occurs with other root diseases such as laminated root rot and Annosus root disease. Armillaria itself is a predisposing factor for bark beetle attack.
Young trees seem to be more susceptible than older trees of the same species.
This is probably more true on the west side rather than the east side of the Cascades. Armillaria often occurs with other root diseases and is a predisposing factor for bark beetle attack. For that reason, trees killed by bark beetles should be examined for signs of Armillaria or other root diseases, as that may affect choices about which species to replant and silvicultural methods to use. Armillaria can remain viable in stumps for 50 years. Chemical treatments have not been shown to be cost-effective.