Symptoms and Signs
Infected trees have slowly thinning canopies and appear weak. This symptom often develops first on one side of the tree and then progresses over several years to involve the whole tree. The bark and outer wood of the upper roots and crown show discoloration. 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.
In other crops, mushrooms may appear at the base of Armillaria-infected trees during cool, rainy weather in fall, but this is rarely observed with olive trees.
Comments on the Disease
Armillaria root rot is not generally a serious disease of olive trees in California, although it occasionally attacks olives and sometimes can eventually kill trees. It is more prevalent following wet winters. The fungus survives on dead roots in the soil and can survive for decades if not subjected to desiccation.
Armillaria root rot is most likely to be present in soils where oak trees previously grew. Avoid planting olive groves where forest or oak woodlands have recently grown or where there is a history of Armillaria root rot. If trees are infected, the growth of the fungus may be slowed by drying out the crown and upper root area of the tree. No olive rootstocks are known to be resistant, and infected trees cannot be cured.
Although research has not been conducted on olive trees, in other tree crops, exposing an infected crown and upper root area of a tree infected with Armillaria mellea may help restrict the fungus to individual roots and allow the tree to regrow. Remove soil from around the base of the tree to a depth of 9 to 12 inches. Leave the trunk exposed and keep the upper roots and crown area as dry as possible. During winter, provide drainage if necessary so that rain doesn't collect in the hole. Recheck the hole every few years to make sure it has not filled in with leaves, soil, and other matter; the hole must be kept open and the crown and upper roots exposed.