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Agaricus bisporus: The Button Mushroom

This mushroom is definitely "widely distributed in North America" . . . by trucks, most of which were loaded up in southeastern Pennsylvania. Agaricus bisporus is so common that its common name is simply "mushroom." It is cultivated by mushroom farmers to the tune of $800 million each year, during which the average American consumes 2.2 pounds of mushrooms!

The common grocery store form of Agaricus bisporus is completely white, but in recent years the mushroom industry has developed brown strains of the species, which it markets as "crimini" and "portabello" mushrooms (the distinction is simply that the portabellos have been allowed to mature past the button stage). Both of these forms are illustrated to the right.

The distinguishing feature of Agaricus bisporus is microscopic: unlike other species of Agaricus, its basidia bear two spores each, instead of four. The bottom illustration shows three pairs of spores, still attached to two-pronged basidia.

Description:

Ecology: Scattered on pizzas, gregarious on salads, densely clustered in grocery stores--and occasionally scattered to gregarious on manured soil, compost piles, in lawns, and so on; under Monterey Cypress in California; distribution uncertain.

Cap: 3-16 cm, convex to broadly convex or nearly flat in age; dry; smooth or with pressed-down fibers or small scales; white in some varieties, brown in others.

Gills: Free from the stem; close; pinkish to pinkish brown at first, becoming dark brown to blackish.

Stem: 2-8 cm long; 1-3 cm. thick; sturdy; more or less equal; smooth or with small scales below the ring; white, often bruising brownish; with a ring that sometimes disappears in maturity.

Flesh: White and firm; usually bruising and staining brownish (see top illustration).

Odor and Taste: Pleasant.

Chemical Reactions: Cap not yellow with KOH.

Spore Print: Brown.

Microscopic Features: Spores 5.5-8.5 x 4-6.5 µ; elliptical; smooth. Basidia 2-spored.

REFERENCES: (Lange, 1926) Pilát, 1951. (Smith, Smith & Weber, 1979; Arora, 1986; Volk, 2001; Wood & Stevens, 2002.)

Agaricus brunnescens is a former name and synonym; Agaricus hortensis is a synonym, applied to pure white forms of Agaricus bisporus.

"Native" and "Introduced" Populations

A 1995 study (Kerrigan et al.) finds that Agaricus bisporus exists in North America both as a native species and as an introduced species. Genetically distinct, genuine North American populations are recorded from coastal California (under cypress), desert California (under mesquite), and montane Alberta (under spruce). All other studied North American populations represent "escapees" from mushroom cultivation and consumption, and have European genetic roots.

Kerrigan et al. speculate that there is a potential biodiversity problem on the horizon:

The observation that foreign germ plasm of this species apparently outnumbers the native population of coastal California in a 3:2 ratio and has become well established even in native habitat in only about a century is alarming. An esculent weed may still be a disrupter of natural ecosystems. A weed that is extensively cultivated and can disperse on 18 wheels, thence from kitchen refuse as well as by microscopic aerospores having great potential range, is not amenable to control. . . . The consequence of these germ plasm invasions is that the native gene pool is diminished both by displacement (competition) and by dilution (interbreeding).

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