Pathogenic organisms cause many types of cankers in trees. Cankers may appear as sunken areas or lesions in the bark. Other types appear as round, elliptical, or long, narrow 'target' cankers with surrounding ridges of callus tissue. Some types of cankers may be accompanied by gum formation or bleeding. The pathogens that form annual and perennial cankers are relatively weak.
Trees wall off annual cankers in a year's time.
The host and pathogen reach a standoff in the perennial cankers, whereby each year the tree grows and walls off the invader during the growing season, but during the dormant season the pathogen invades new tissue. In contrast, some types of canker-causing organisms are very strong pathogens that infect, girdle, and kill trees in a relatively short period of time. When a tree is girdled, the cambium is killed in a complete circle around the trunk. Girdled trees eventually die because the transport of photosynthates from the leaves to lower regions is interrupted. Frost cracks and sun scald .