Tree fungi come in many different shapes and sizes, and not all types of fungi that are found on a tree are parasitic. Still, fungi are responsible for many tree diseases and some of the most virulent types of tree diseases are invisible to the naked eye.
Shelf fungi (sometimes called bracket fungi) can be found growing on live trees. They resemble shelves or brackets and they can remain for years on a tree and take on a woody form. Over time, this fungi growth can be detrimental to the tree.
Sometimes a fungus organism can be very small and barely visible. Often these micro-organisms can be the most destructive. A rust is one such kind of parasitic fungus that is capable of infecting many trees in an outbreak.
Perhaps the most destructive plant disease has been the chestnut blight fungus, another microscopic fungus that infects the chestnut leaves, stem and branches, but not the root system. As a result the forest is filled with many chestnut saplings, but the tree is attacked by the virulent fungus before it can produce seeds.
Dutch Elm Disease
This destructive disease of the stately American elm is caused by a small fungus that is transported by the bark beetle. Although not as the destructive as the chestnut blight ( there are some elm survivors), the American elm can no longer grow in the forest as a competitive species.
The Tree As a Microclimate
In some places like the crotch of a branch or a divided tree trunk, a small microclimate and living environment will develop, which can support the growth of mushrooms and other types of fungi. This fungal growth more often than not is not destructive to the tree.