Since it is spread almost entirely by man, through pruning, it is essentially a shade tree disease. Leaves are dwarfed and sparse in part or all of the tree top. The staining cankers occur on trunks or branches. The first symptom on the yellow or green bark is a brown to black lens-shaped discoloration. Cankers may become 20 to 40 inches long in one year, but usually only two inches wide.
Cankers widen each year, and often coalesce, girdling the tree or branch.
Older cankers shed their darkened, dead bark exposing the wood, which dries, cracks, and turns black. Stain patterns are radial, generally reaching the pith. The fungus sporulates abundantly on newly-killed wood in wet weather from May until October. Avoid injuring the tree. Prune out dead limbs during winter.