Commonly, an infected tree will show a gradual decline, evidenced by small, yellowish leaves, reduced growth, and dieback of branches. Trees which are chronically infected may wilt suddenly during periods of stress, such as drought. The white mycelial mats are often shaped like fans with veined margins. These mats may extend upward several feet from the base of the tree for and may be associated with decay of the plant. On dead and decaying wood these mycelial mats may glow in the dark, indicating the presence of the fungus.
Rhizomorphs have a white core and black outer layer.
As rhizomorphs grow through the soil, they branch and penetrate roots, causing new infections. Mushrooms of armillaria may develop annually in the fall, during wet periods. Although spores from these mushrooms can infect recently wounded plants, they apparently do not play an important role in the spread of the disease. It is important to note that many other fungi also produce mushrooms on or near decaying wood. Chad Behrendt Infection often develops after trees have been weakened by freezing, drought, or other environmental stresses.