Spores of wood-decay fungi germinate and grow into a tree’s vascular system or wood through wounds created when branches are pruned incorrectly or break during severe weather.
The appearance of odd-shaped “growths” at the base of your tree, along its trunk, at right, or on its branches are the "fruiting bodies" of these fungi and indicate that they are well-established, and may have caused large areas of decay inside of the tree.
Unfortunately, by the time these growths appear, left, the decay is often so widespread that large portions of affected trees must be removed - or the entire tree may need to be taken down.
Just remember that these fruiting bodies are different than the various gray, green, fuzzy and/or smooth "lichens" that are found on the trunks and branches of almost all trees and large shrubs. Lichens, below, do not harm the plants on which they're growing and, therefore, are no cause for concern.