Some species of fungi attack dying or dead wood and act as an agent to break down the waste material; others are more deadly attacking healthy trees and killing them. Root rot (Heterobasidion annosum) is one of the latter and is a preditor of conifers although it can occur on broad-leaved trees as well. It causes a very serious white decaying of the inside of the tree, the brackets are the visible fruiting bodies and can be seen at any time of the year.
A common species, the brackets appear near the base of the trunk (and can be obscurred by other vegetation) all year round and there are usually more than one of them. Being at the base of the tree it looks as though the roots are 'foaming', hence its other common name, root fomes.
This is certainly not one for the frying pan! My book says it smells like emulsion paint!