Disease Cycle

The fungi causing the disease overwinter in cankers and dead twigs. Small, black, fruiting bodies appear on the smooth bark covering diseased areas of dead wood and begin to produce spores once temperatures are above freezing. Wet weather washes the spores from the fruiting structures. Infections do not usually occur when trees are growing vigorously; most occur during early spring, fall, and winter.

Healthy bark or buds are not attacked by the fungus. Cold-injured buds or wood and pruning cuts are the most important sites of infection. The fungus also can penetrate brown rot cankers, oriental fruit moth damage, sunscald wounds, hail injury, leaf scars, and mechanical wounds. Once established in the wood, the fungus forms a canker by invading the surrounding healthy tissue.