Trees, as they live longer and grow larger, become more and more likely to support fungi growth on the outer surface of the tree. Presence of a fungus on the bark does not necessarily mean the tree will be under duress. There are many types of fungal diseases, however, that can lead to a premature death of the tree.
Benign or Malignent
Not all fungal growth on the bark of trees is detrimental to the host. Since just underneath the bark lies some very important living tissue, however, any fungus that penetrates the outer protective layer is likely to be highly damaging to the health or life of the tree. While there are many types of fungi that form colourful layers of growth, only a few are able to penetrate the outer cambrium layer and feed off the cells of the phloem and xylem bundles, which comprise the vascular system of the tree.
Cankers in the bark are often a sign of a fungal invasion to the tree. Fortunately, not all cankers develop into a destructive disease, but the presence of a shallow lesion in the bark is always a cause for concern. Cankers can form when the tree has been physically damaged or if the plant is under environmental stress due to drought or heat. Often certain species of canker-forming fungi will only attack specific typs of trees. For example, sugar maple trees are sometimes attacked by a fungus (Steganosporium) that causes a damaging canker.
Mechanical damage to the main trunk of the tree can become an ideal place for fungi to find a home. When the interior parts of a tree are exposed to the elements, the likelihood of a fungi infection increases, as do the chances that the presence of fungi will harm the tree. It matters little if the injury has natural causes or if it was man made. Some problem areas, however, like the jagged end of a broken branch, can be improved by cutting the exposed stub flush with the trunk of the tree. A flat surface area will greatly reduce the chance of infection.
Lichens are considered to be unique biological organisms, created by a symbiotic relationship of algae and fungi. They can grow profusely on rocks, tree trunks and man-made structures. This fascinating group of plants is not considered to be parasitic to the living plant. Lichens usually resemble scales or flakes, but there is one species, commonly called Old Man's Beard, that hangs down from tree branches in a way similar to Spanish moss.
Bracket or shelf fungi are large, exterior growths on trees. They have small, numerous pores on the bottom side and a hard, smooth top. Some shelf fungi attack the tree when it is still alive, while others only grow after the tree has died and fallen to the ground. Since bracket fungi always grow with the pores facing toward the ground, it is easy to tell if the fungus starting growing when the tree was standing upright. Because the long filaments of these types of fungi often penetrate to the heart of the living tree, shelf fungi can be very damaging to the life of a tree.
A fungus infection can follow the presence of insects. There are several that feed on the bark and in their wake leave a slightly altered outer layer that can be a fertile substrate for fungi to grow. Beech bark disease is a prime example of this situation. First an insect scale attacks the bark and then the fungus, Nectria coccinea, invades.