How to Manage Pests
UC Pest Management Guidelines
Armillaria Root Rot (Oak Root Fungus)
Pathogen: Armillaria mellea
(Reviewed 8/06, updated 3/09)
In this Guideline:
Roots infected with Armillaria mellea have white to yellowish fan-shaped mycelial mats between the bark and the wood. Dark brown to black rhizomorphs sometimes can be seen on the root surface. Apple rootstocks have some resistance to Armillaria, but if the level of inoculum is high in the soil and the strain of Armillaria is highly virulent, or the tree is already weakened by some other factor, the fungus can overcome this resistance.
The fungus survives on dead roots.
There is no truly effective control for Armillaria root rot. Exposing an infected crown and upper root area of a tree infected with Armillaria mellea will stop the development of the fungus into the crown area and allow the tree to regrow. In spring, remove soil from around the base of the tree to a depth of 9 to 12 inches. Leave the trunk exposed for the remainder of the growing season. During the spring, summer, and fall, keep the upper roots and crown area as dry as possible.
UC IPM Pest Management Guidelines:
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:B. L. Teviotdale, Kearney Agricultural Center, Parlier