Mushrooms and conks are the reproductive bodies of fungi. Most mushroom fungi live on decaying matter. When conditions are favorable, the fungi produce mushrooms and conks (hard, woody growths that protrude from tree trunks) containing spores that are spread by the wind. A number of different mushroom fungi decay the heartwood of living trees. Most of these grow only in older wood, which they enter through wounds. Mushrooms or conks usually appear annually in the dead portions of the trees. Some conks may remain attached to the wood for years. Armillaria mellea, a fungus that causes a disease called armillaria root rot, mushroom root rot, oak root fungus, or shoestring root rot, invades healthy roots. In the fall or winter, mushrooms appear around the base of the plant, growing on the infected roots.
By the time conks or mushrooms appear on the trunk, it is too late to do anything about the wood rot. Inspect the tree to determine the extent of decay (contact a professional arborist if necessary). Trees or branches with extensive decay should be removed. Keep plants vigorous by fertilizing and watering regularly. (For more information about fertilizing and watering, see Nitrogen and About Watering.) The spread of armillaria root rot may be inhibited if it is found in only part of the roots.