Bull. Soc. Bot. France 23:31. 1876.
Common Name: Yellow Stainer
Cap 5-17 cm broad, convex, expanding to nearly plane; margin incurved, then decurved, yellowing readily when bruised; surface dry, smooth to appressed fibrillose; color variable: white in shaded locations, more typically buff-brown at the disc shading to a white margin; flesh white, firm, thick; odor of phenol, most noticeable in fresh, young material; all parts yellowing in KOH.
Gills, free, close, at first pale, becoming pink, finally blackish-brown.
Stipe 4-14 cm tall, 1-3.5 cm wide, more or less equal, often with a small basal bulb, hollow to stuffed; surface smooth, white, bruising yellow, especially at the base; partial veil membranous, thick, white, upper surface smooth, lower surface typically smooth near the stipe, floccose near the margin, but various combinations of the two are possible, the margin conspicuously thickened with a "double lip".
Spores 5-6 x 4-5.5 µm, elliptical, smooth; spore print chocolate-brown.
Scattered, gregarious, or in broad arcs in a variety of habitats: under conifers, hardwoods, in grass and in gardens; fruiting throughout the year when moisture is available, but most abundant from early fall to mid-winter.
Toxic, causes GI upsets for most people although some individuals claim to eat and relish it. The phenolic odor which is intensified by cooking should be a deterrent.
Agaricus xanthodermus is a mildly toxic species found abundantly in parks and woodlands throughout the Bay Area. To the chagrin of local mycophagists, it and a close related, inedible cousin, Agaricus californicus, are our most common Agaricus species. It is distinguished by a white cap with a pale brown disc, phenolic odor, and rapid yellowing of the cap margin and stipe base when bruised. Unfortunately, in age, both the staining reaction and odor may disappear leading to confusion with other Agaricus species, notably A. arvensis. The latter, a good edible, is not nearly so common. It has a sweet anise odor, more cream to pale yellowish tones in the cap, and usually lacks a bulb at the stipe base. Additionally, while it also bruises yellow, it does so slowly. See "Comments" under Agaricus californicus for how to distinguish it from A. xanthodermus.
Kerrigan, Richard W. (1986). The Agaricales (Gilled Fungi) of California. 6. Agaricaceae. Mad River Press: Eureka, CA. 62 p.
Kerrigan, R.W., Callac, P., Guinberteau, J., Challen, M.P. & Parra, L. (2005). Agaricus section Xanthodermatei: a phylogenetic reconstruction with commentary on taxa. Mycologia 97: 1292-1315.