What the fruiting bodies tell you
Wood decay is one of the most common tree diseases. The fungi that cause it produce mushrooms near the base or hard shelf-like conks on trunks and branches. The WSU publication says a conk generally means rot several feet above and below it, and that more or larger conks point to more decay.
Decay weakens wood over time and makes a tree more likely to break in wind. When it sits in the trunk or roots of a tree near a house, street or play area, it becomes a safety question.
What mushrooms do not prove
The same publication cautions that the absence of fruiting bodies does not mean a tree is free of decay, and that some fungi fruit only after extensive decay or rarely. Decay in structural roots is especially important and hard to see, because few decay fungi fruit directly from roots.
A mushroom, then, is a prompt for an inspection and not a verdict. Photograph it and note where it is.
What an arborist checks
The arborist looks at the base, the root flare, the trunk for cavities, cracks and lean, and the crown for dieback, then decides whether the tree can stay, needs pruning, cabling or removal. Our page on root and trunk decay covers what the checks involve.
We do not promise a tree is sound from a photo. If it is leaning, cracking or standing over a structure, treat it as urgent and keep clear. Call 775.232.8243, or see warning signs before a tree fails.
Mushrooms can also appear after wet weather and vanish in days, so note the date you saw them. A tree with fruiting bodies that also has dead branches, cracks or a lean should be treated as a higher priority than one with a single mushroom and a full crown.