Forest Pests: Insects, Diseases & Other Damage Agents

Armillariella mellea (formerly Armillaria mellea), the cause of shoestring root rot, often kills trees previously stressed by drought, insects, other pathogens, or unfavorable environment. However, under some circumstances the fungus, or certain strains of the fungus, can kill vigorous trees. Both hardwoods and conifers are attacked.

The symptoms are similar to those caused by other root-rotting organisms. The leaves or needles may become yellow or smaller than normal, the crown may become thin, and branches may die one at a time as the tree gradually declines. Often, however, death is sudden. Trees may die singly or in groups. Infected conifers often produce resin at their bases. Infected hardwoods may appear healthy except for a cracking or splitting of the bark near the ground line. White fans of fungal tissue are produced under the bark, usually at root collar level, although they may extend up to 6 feet or more. Honey-colored mushrooms with yellowish-white gills on the undersurface are sometimes produced at the base of the tree in the fall. Black shoestring-like structures (rhizomorphs) may be found around the roots and in the soil.

Armillariella spores can germinate on dead wood, and the fungus acts as a wood rotter. Live roots become infected when they come in contact with decaying wood or the rhizomorphs. The rhizomorphs may grow out for several feet through the soil from a large food source, such as an old stump. Young pines planted on an area from which hardwoods have been logged are especially likely to be killed. The soil can remain infested for many years.

Dave Powell, USDA Forest Service	Sign; mycelial fan under bark of dying tree USDA Forest Service - Rocky Mountain Region Archives, USDA Forest Service
Damage; rhizomorphs Robert L. Anderson, USDA Forest Service

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