What causes there are mushrooms growing at the bottom of my tree?
Wounds, years earlier. Mower and string-trimmer scars on the flare, roots cut for a driveway or a utility trench, soil or mulch piled over the root collar during construction, and old stumps of the same species left in the ground nearby are the usual entry points. In central Indiana the common actors are Armillaria (honey mushrooms in the turf, spreading root to root), Ganoderma (the hard shelf on the trunk), and Kretzschmaria (the black crust). Each arrives long after the injury that let it in, which is why homeowners never connect the two.
What you'd see
Clusters of honey-colored mushrooms in the lawn ring around the trunk in fall; a hard shelf-like bracket fused to the trunk flare, often brown on top and white underneath and bruising when scratched; a black or white crust in the bark seams right at ground level; a flare that looks sunk or column-like going into the ground instead of spreading out; a dull thud instead of a sharp crack when you rap the base with a hammer handle. The crown may look completely fine — thinning is a late sign here, not an early one.
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