"Fungus decays heartwood and sapwood, causing a white, flaky rot. Infections
occur through open wounds and decay is most extreme when wounds are large. A
cluster of shelflike mushrooms, each 2-8 inches wide, is produced annually
and can indicate localized decay or heart rot that extends 10 feet in either
direction. The mushrooms are smooth on the upper surface with gills that
characteristically extend down along the stalk on the lower surface. "
I would not be surprised if Oyster mushrooms has actually different
subspecies, some of which are not parasitic at all. This is the situation,
for example, with Honey mushroom (Armillaria mellea). It turns out there are
more than a dozen of species, which are hard to distinguish from each other.
Some of these species were found to be purely saprophytic, others are very
virulent.
Andriy
[NAFEX] Danger to fruit trees from oyster mushrooms,
Andriy Palamarchuk, 05/11/2010