The disease usually starts around a pruning wound or some other injury on a stem. Healthy callus tissue develops and is invaded by the woolly apple aphid. In feeding, these insects create small wounds and injure the surrounding tissue. The fungus then invades and develops in this injured tissue. Winter injury of the tissue is an important factor in the advance of the canker.
Cankers progress farther following severe winters than after mild ones.
In the spring, healthy callus again is formed at the margin of the canker and the cycle is repeated resulting in another ring of dead tissue. In addition to the presence of woolly aphids and low winter temperatures, any factor that produces excess succulent growth, such as high nitrogen fertilization or overirrigation, or that weakens the tree, increases the severity of perennial canker. Bull's eye rot is a postharvest disease that is usually observed when infected fruit are removed from storage. The disease gets its name from the lesions that frequently are comprised of concentric rings of necrotic tissue. Cream colored tufts of spores are sometimes present.