Cankers of Hardwood Deciduous Trees Plant Disease Facts Cooperative Extension - The Pennsylvania State University Cankers of Hardwood Deciduous Trees Gary W. Moorman Plant Pathologist Localized areas of dead bark and underlying wood on twigs, branches and trunks are called cankers. Cankers can be caused either by living organisms, including fungi and bacteria, or by nonliving things such as excessive low or high temperature or hail. Many fungi that cause cankers normally inhabit the surface of the tree, gain entrance through natural or man-made wounds, and only cause disease when the tree is under stress. However, some fungi aggressively attack trees and cause cankers.
Infection occurs during the host's dormant season.
During the growing season, host callus tissue walls off the canker and prevents further spread. Although annual cankers do not persist, continued stress makes it likely that more cankers will form and other more serious diseases may develop. Perennial cankers are seldom lethal to the tree but do weaken its structure and detract from its appearance. Wounds and branch stubs are invaded by the fungus during the tree's dormant period. The host forms callus around the infection site during the growing season but the fungus invades more tissue the following dormant period.