Fungi and bacteria cause most diseases of tree bark by infecting open wounds on the branches or trunks of the trees. A common symptom is a canker, an area of dead bark that spreads, according to “Cankers on Trees” by Cornell University. Other symptoms of disease include galls, knots or burls.
Target-Shaped Cankers - Eutypella, Nectria and Strumella are some types of fungi that cause target-shaped cankers. The infection grows in the bark of the branches or trunk while the tree is dormant. During the growing season, the tree forms rolls of healthy callus tissue around the edge of the infected area. This process is repeated yearly and creates an area that resembles a target. The cankers can girdle and kill branches. If target cankers girdle the trunk, the whole tree will decline and die.
Diffuse Cankers - Diffuse cankers are shallow with discoloured edges. The fungi grow rapidly and are usually lethal, because the tree has no time to defend itself, according to "Cankers on Trees" by Cornell University. Affected limbs and branches should be removed and destroyed. If the trunk is affected, the whole tree needs to be removed and destroyed. Cytospora and Hypoxylon are two fungi that cause diffuse cankers.
Bleeding Cankers - Bleeding cankers are areas of infected bark that leak reddish-brown to black sap through the bark, rather than out of cracks or holes in the bark. They usually affect the lower 6 feet of tree trunks, although they can spread all the way up the trunk, according to "Bleeding Cankers" by Berkeley University. Infected trees decline and die. Armillaria, Inonotus and Phytophthora fungi, as well as some bacteria, cause bleeding cankers.
Galls, Knots and Burls - Tumuor-like growths on the limbs and branches of trees are referred to as galls, knots or burls. Although numerous galls can affect a tree, the disease does not spread easily from one tree to another, according to "Diseases of Bark" by Cornell University. The wood under a gall is weaker than the surrounding wood and is more likely to break in high winds. Affected limbs and branches should be pruned out.