Plants most susceptible to Armillaria root rot are those under stress from prolonged drought, repeated insect defoliation, root injury, or recent transplanting.
Armillaria survives well in dead roots and stumps and in the soil as long, brown, shoestring-like structures. These rhizomorphs (root-like structures) can grow for several feet through soil from stumps to nearby trees and from tree to tree. This is very unusual for a fungus since most fungi remain relatively dormant in soil unless a host plant is very near. The rhizomorph invades a tree at the buttress or upper part of the root system. Just under the bark, obvious white fans of fungal growth form. A honey-brown colored mushroom develops in the autumn on the roots or buttress of infected trees. The spores liberated from the mushroom are windblown to other areas.
Top growth of the infected tree slows, branch dieback occurs, and roots rot. Trees may appear to die quickly. White fans of fungal growth are found when bark is peeled off infected trunks near the soil line. Conifers have abundant resin flow from the trunk at the soil line.
Encourage Good Tree Vigor By Reducing Stress Including:
- Control insects that cause defoliation.
- Irrigate to relieve drought.
- Protect plants against injuries to the lower trunk and upper portion of the root system.
- Do not replace an Armillaria-killed plant with another woody species.
Rhizomorphs of Armillaria
Armillaria mycelial fan.
Prepared by Gary W. Moorman, Professor of Plant Pathology