Description
This plant disease, also known as mushroom root rot, honey mushroom, oak root fungus, and shoestring fungus, is caused by a soil-borne fungus (Armillaria mellea) that rots the roots of a wide range of plants. Many diseased ornamental and fruit trees die slowly over a period of years. However, these plants may die suddenly, especially if they are under stress. The fungus spreads short distances (under a foot) through the soil by thick fungal strands (rhizomorphs) that penetrate the roots of nearby susceptible plants. Infection is most severe in heavy, poorly drained soil. Once the fungus enters the plant, it produces a white, fan-shaped mat between the bark and the wood that decays the roots and lower trunk. As the roots die, the top of the plant slowly starves. If the crown is girdled by the fungus, the top wilts and dies quickly. In the fall, mushrooms--the spore-producing fruiting bodies of the fungus--often appear around the base of the infected plant. A different fungus (Clitocybe tabescens), also known as mushroom root rot, causes similar and often identical damage to woody trees and shrubs in the Southeast.
Control
Remove badly infected trees, including the stumps and, if possible, the roots. You can usually save mildly infected plants, or at least prolong their lives. Remove the soil from around the rotted parts of the roots and trunk. Cut out the diseased tissue and allow the healthy wood to air through the summer. When temperatures drop toward freezing, cover the exposed roots with loose soil. Replace dying plants with armillaria-resistant plants. For a list, see Plants Resistant to Armillaria Root Rot.
