This well known mushroom is easily recognized by the way it grows on wood in shelf-like clusters; its relatively large size; its whitish gills that run down a stubby, nearly-absent stem; and its whitish to lilac spore print. It fruits in fall and winter across North America, and has a brownish cap. A number of very similar species are closely related, and the broad term “Oyster Mushroom” applies equally to Pleurotus ostreatus, Pleurotus pulmonarius (which is often paler, and appears in the summer), and Pleurotus populinus (which is found on the wood of quaking aspen).
I think oyster mushrooms have a peculiar smell, but I would be hard pressed to describe it. An “oyster mushroom smell” is about all I can come up with, but the not-unpleasant odor seems fairly distinctive in the mushroom world. Because they are large and grow on trees, oyster mushrooms can be seen from the car–like, when it rains hard for days on end and you just have to get out.
Ecology: Saprobic; growing in shelf-like clusters on dead logs and living trees (primarily hardwoods, but sometimes on conifers); causing a white rot; fall, winter, and early spring; common; widely distributed in North America. Oyster mushrooms kill nematodes and bacteria with impunity; see George Barron’s micro-pictures, beginning here. For fungal fans, this is the equivalent of watching lions kill gazelles on the Nature Channel.
Cap: 4-15 cm; convex, becoming flat or somewhat depressed; kidney-shaped to fan-shaped, or nearly circular if growing on the tops of logs; somewhat greasy when young and fresh; smooth; pale brown to dark brown; the margin inrolled when young, later wavy, never lined.
Gills: Running down the stem; close; whitish or with a gray tinge, sometimes yellowish in age; often filled with black beetles, in my collecting areas.
Stem: Usually rudimentary and lateral (or absent) when the mushroom is growing from the side of a log or tree. When it grows on the tops of logs or branches, or at an angle, however, it may develop a substantial and thick stem that is dry and slightly hairy near the base.
Flesh: Thick; white.
Odor and Taste Odor distinctive but hard to describe (see above); taste mild.
Spore Print: Whitish to grayish or lilac. Be sure to check out George Barron’s photo and essay on what can only be called the Mother of All Spore Prints, produced by an oyster mushroom.
Microscopic Features: Spores 8-10.5 x 3-3.5 µ; smooth; cylindrical to narrowly kidney-shaped. These are the measurements given by Petersen & Krisai-Greilhuber (1994) for an epitype collection of Pleurotus ostreatus, and they match measurements supported by various mating studies. Field guides quote a large range of measurements, conflating Pleurotus ostreatus with other members of the species complex.
source : mushroomexpert