Common Name: button mushroom
Cap 5-12 broad, convex at first with incurved margin, becoming plane in age; surface dry, with light brown, usually innate scales over a pallid ground color; flesh thick, white, typically bruising pinkish-brown to orange brown but not changing color in KOH; odor and taste mild.
Gills free, pinkish-brown, becoming purple-brown, finally blackish-brown.
Stipe 2-5 cm tall, 1.5-2.5 cm thick, more or less equal to slightly bulbous at base; white bruising slowly brown, smooth; veil white, cottony-membranous, forming a medial to superior ring.
Spores 6-8.5 x 4.5-6 µm, elliptical, smooth. Spore print blackish-brown.
Solitary, scattered to gregarious near manure piles, in grass or duff under conifers, especially Monterey cypress; fruiting all months of the year when moisture is available except mid-winter.
Edible and very good.
Agaricus bisporus is well known to mycophagists as the common "button mushroom" of commerce. In the San Francisco Bay Area both wild forms and escapees from mushroom farms occur. Interestingly, both types look similar and can not be distinguished without biochemical analysis. Agaricus bisporus, though not as distinctive as other Agaricus species, can be recognized by the following combination of characters: relatively short stature, cap with pale brown appressed scales, flesh which bruises slowly orange-brown to reddish- brown, but does not stain yellow in KOH, a well developed ring, smooth stipe, and a preference for fruiting with Monterey cypress. Agaricus californicus and A. xanthodermus also occur under Monterey Cypress but the latter bruises bright yellow on the cap margin and stipe base, while the former, though similar in stature, has a distinctive double-lipped ring (best seen in fresh, young material), and stains yellow in KOH. Agaricus bisporus is sometimes also confused with A. campestris, the Meadow Mushroom, but the latter grows in grass, usually away from trees, lacks the brown cap scales of A. bisporus, has an evanescent ring, and a tapered stipe base.