Armillaria Root Rot (Oak Root Fungus)

Affected trees often show a general decline in vigor a year or more before the entire tree collapses. Trees often die in circular areas within an orchard; the circular area expands each year as the fungus grows along roots of infected trees to roots of adjacent healthy trees. Tree death usually occurs in late spring.

Aboveground symptoms can be easily confused with Phytophthora root rot or any other root problem. To diagnose Armilllaria root rot, inspect roots and crown area. 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. All stone fruit rootstocks are susceptible to Armillaria root rot.

The fungus survives on and in dead roots. Armillaria mellea forms resistant structures called rhizomorphs that can survive in the soil for several years in the absence of a host.

Generally, once an apricot tree becomes infected with Armillaria mellea, it cannot be saved and should be removed. Currently available fumigants are not recommended because they lack the ability to penetrate infected roots and do not adequately control this pathogen in the soil.

Marianna 2624 is more resistant to Armillaria mellea than other apricot rootstocks, but is not immune. If the disease is caught early enough, excavating the soil around the base of the tree down to the first layer of lateral roots may delay the progress of the disease from progressing further. This aeration prevents the fungus from gaining access to the crown of the tree.