The presence of mushroom clusters (annual / perennial fruiting body) located at the base of any tree typically indicates either root or butt rot / decay. However, most wood-decaying fungi are slow moving (i.e., they steadily, but very slowly impact
Management: An excellent resource that I found discussing Root and Butt Rot Pathogens of Oak (Quercus spp.) is from Dr. Nicholas Brazee, Umass Amherst:
The passage below is quoted directly from the Umass Amherst publication listed above:
Management of oaks infected with root and butt rot fungi can be extremely difficult. First, the occurrence of a wood-decaying fungal pathogen must be confirmed. In many cases, the only confirmation is the presence of an annual or perennial fruiting body (mushroom or conk) growing directly from the trunk or from a lateral root close to the base. Regular lawn mowing often destroys fruiting bodies growing from nearby roots before they can mature and be properly identified. In addition, there are numerous beneficial mycorrhizal fungi that may produce annual fruiting bodies at the base of oaks, creating confusion. Regular scouting from mid-July through mid-October is required to find and identify a potential pathogen. Minimizing mechanical wounds that may serve as a potential infection point is essential to limit the introduction of these fungi. These wounds may be created by lawnmowers and weed trimmers, automobiles, or careless placement of tools and equipment. Maintaining a large mulch ring around the base of landscape trees can help protect them from basal wounding.
An important point to consider is that wood-decaying fungi grow very slowly. It can take these fungi many years to decades to cause a defect large enough to cause uprooting or stem failure. The rate of decay varies with tree species, fungal pathogen and occurrence (roots or lower trunk). The presence of fruiting bodies from certain pathogens (e.g. Grifola frondosa and Ganoderma sessile) does not always indicate extensive decay is present, whereas others (e.g. Ganoderma applanatum and Laetiporus spp.) often signify extensive decay has taken place. Accurate identification is critical. Fungicides are of little use against wood-decaying fungi because the pathogen lives within the roots and/or lower trunk, often in the heartwood, making it very difficult to contact.