The bark of a tree consists of inner and outer bark . Phloem cells make up the inner bark. The phloem tissue transports the carbohydrates produced in the leaves and other substances throughout the tree. The outer bark performs the critical function of protecting all inner tissues from injury and infection. Cankers caused by pathogens .
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.