Bull. Torrey Bot. Club 22:203. 1895.
Common Name: none
Cap 4-11 cm broad, cylindrical to convex becoming plane at maturity, sometimes with a broad umbo; surface dry, smooth to fibrillose, white on the cap margin shading to a grayish-brown disc; surface sometimes bruising pale yellow, conspicuously yellow in KOH; odor faintly of phenol, most noticeable in young specimens.
Gills free, close, pallid at first, then bright pink, pinkish-brown and finally blackish-brown.
Stipe 3-7 cm long, 0.4-1.0 cm thick, smooth, equal, to slightly bulbous at base; partial veil membranous, upper surface smooth to finely striate, lower surface minutely fibrillose, forming a well developed median to superior ring, the latter with a distinctive thickened margin.
Spores 5-7.5 x 4-5.5 µm, elliptical, smooth; spore print blackish-brown.
Scattered, clustered or in broad arcs in a variety of habitats, e.g. lawns, gardens, under both hardwoods and conifers, occasionally well-decayed wood chips; fruiting whenever moisture is available but most abundant from late summer in watered areas to early winter.
Mildly toxic. Causes gastrointestinal upsets for most people although some individuals are able to eat it without negative effects.
Agaricus californicus is frequently confused with its somewhat larger cousin A. xanthodermus. Both species have caps with brownish discs and pale margins although Agaricus xanthodermus may be entirely white if developing in deep shade. They are best told apart by odor and bruising reactions. Agaricus californicus has a faint, usually barely discernable phenolic odor while fresh specimens of A. xanthodermus usually have a pronounced phenolic odor. Additionally, fresh mataerial of A. xanthodermus bruise bright yellow on the cap margin and stipe base, while in A. californicus the yellow-bruising is faint, if at all. Agaricus californicus can be separated from other Agaricus species that may have a brownish disc, e.g. A. bisporus by its annulus with a thickened "double-lipped" margin.
Desjardin, D.E., Wood, M.G. & Stevens, F.A. (2015). California Mushrooms: The Comprehensive Identification Guide. Timber Press: Portland, OR. 560 p.
Kerrigan, R.W. (1986). The Agaricales (Gilled Fungi) of California. 6. Agaricaceae. Mad River Press: Eureka, CA. 62 p.
Kerrigan, R.W. (2016). Agaricus of North America. New York Botanical Garden: Bronx, NY. 574 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.
Peck, C.H. (1895). New species of fungi. Bull. Torrey bot. Club 22(5): 198-210. (Protologue)