Our 80+ year old white oak (possibly swamp white oak) has a large fungus at its base on the northeast side. It started about 3 years ago, but we broke it off each summer. In 2008 the Des Plaines River flooded our yard, and in 1998 a professional landscaper installed 3 dogwood trees, 2 - 4 bushes, perennials and groundcover over its root zone. What is the fungus and how should we treat it?