Q What would cause mushrooms to appear on the side of my mature maple tree?
A Fungus growing on a living tree is a symptom of heart rot. Trees are infected through wounds from lawn mowers, weed-whacker whips, deer rubbing, rodent chewing, frost cracks, broken branches and such. Once inside the tree, the fungi use a variety of enzymes and other chemicals to break down the wood for food.
A tree with signs of heart rot does not need to be immediately removed, but should be examined to determine how structurally sound it is. Be careful with trees located where they could damage people or property if they or their branches were to fall. In natural areas, trees with heart rot serve a valuable function in providing nesting sites for birds and animals, and are best left undisturbed. The stability of an infected tree depends on the type of tree infected, the extent of the decay and whether other defects such as cracks or cavities are present.
The U.S. Forest Service recommends stabilizing or removing a tree if it has signs of heart-rotting fungi and a crack, open wound or other defects, or if there's less than an inch of solid wood for every 6 inches of tree diameter. (The level of internal decay can be difficult to determine if an open cavity is not present.) Professionals can run a variety of tests to assess the stability of the tree; contact a city forester or certified arborist.
To prevent heart rot:
Take care not to wound trees with lawn care equipment.
Install fencing to protect trees from deer rubbing and rodent damage.
Use proper pruning cuts when pruning trees and never leave a branch stub.
If branches are broken during a storm, cut damaged branches with a proper pruning cut below the damaged area. The tree will heal from a smooth pruning cut more quickly than from a jagged rip.
Information from Michelle Grabowski, educator, University of Minnesota Extension.