Oak root fungus (Armillaria root rot)—Armillaria
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.
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
Rhizomorph (top) with healthy root