How to Manage Pests

Pests in Gardens and Landscapes

Armillaria root rot (Oak root fungus) — Armillaria mellea

Symptoms on trees infected with oak root fungus vary. If the disease develops quickly, leaves tend to be small and chlorotic and drop early. Shoot growth is poor and shoot dieback is common. Trees with slow development of the disease show premature fall coloring of the foliage. Foliage symptoms may be expressed on only one or two scaffolds if the crown is only partially girdled. Dark rhizomorphs (shoestringlike structures) develop and grow along the surface of healthy roots.

Identification

Solutions

If less than half of the crown's circumference is girdled, it may be possible to save the tree. In late spring, remove the soil from around the crown area and cut away all bark and tissue of the crown invaded by the white leathery fungus. Leave uninfected tissue intact and keep the crown exposed for drying. If more than half of the crown is infected, the tree should be removed. Before replanting, allow the soil to dry out. Plant on mounds with roots close to the soil surface, and replant with a resistant species. Maintaining trees with good fertilization and irrigation programs helps preserve their natural resistance to this fungus.

Aboveground symptoms of oak root fungus
Aboveground symptoms of oak root fungus
Rhizomorph (top) with healthy root
Rhizomorph (top) with healthy root

Statewide IPM Program, Agriculture and Natural Resources, University of California
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