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Agaricus amicosus

[ Basidiomycota > Agaricales > Agaricaceae > Agaricus . . . ]

by Michael Kuo

Agaricus amicosus is a common fungal feature of Colorado's high-elevation conifers forests; it is particularly common under Engelmann spruce and subalpine fir during the late summer monsoon season. Physical features separating Agaricus amicosus from other Rocky Mountain Agaricus species include the light to medium brown, appressed fibers on the cap, the fairly large size, and the red-staining flesh.


Ecology: Saprobic; growing scattered or gregariously in piled litter of Engelmann spruce and subalpine fir; summer and fall; documented from north-central Colorado to northern New Mexico; perhaps to be expected throughout the Rockies. The illustrated and described collections are from Colorado.

Cap: 6–14 cm across; convex at first, becoming broadly convex or nearly flat; dry; whitish to tan underneath appressed brown fibrils that remain tightly affixed or, rarely, become aggregated into fine scales; often bruising reddish to pinkish brown on the margin.

Gills: Free from the stem; close or crowded; pink when young; later dark chocolate brown to blackish; covered when in the button stage with a whitish to brownish partial veil.

Stem: 4–14 cm long; 2–4 cm thick; more or less equal above a slightly to moderately swollen base; with a skirtlike, whitish to brownish ring; whitish; bruising reddish to pinkish brown; bald or slightly fibrillose.

Flesh: White; changing to reddish or brownish red when sliced.

Odor and Taste: Odor not distinctive, or sweet and fruity; taste similar.

Chemical Reactions: KOH negative on cap surface.

Spore Print: Dark brown.

Microscopic Features: Spores: 5.5–7 x 4.5-5 µm; broadly ellipsoid, with a fairly prominent apiculus; smooth; thick-walled; pale to dark brown in KOH. Basidia 4-sterigmate. Cheilocystidia basidiole-like; 20–30 x 5–10 µm; clavate to cylindric; smooth; thin-walled; hyaline in KOH. Pleurocystidia not found. Pileipellis a cutis; elements 2.5–5 µm wide, smooth, yellow-brown in KOH.

REFERENCES: Kerrigan, 1989. (Kerrigan, 1989; Evenson, 2015; Cripps, Evenson & Kuo, 2016; Kerrigan, 2016.) Herb. Kuo 08100704. Herb. DBG RMNP 2009-059; RMNP 2012 5022-34, 5025-12.

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Agaricus amicosus

Agaricus amicosus

Agaricus amicosus

Agaricus amicosus

Agaricus amicosus

Agaricus amicosus

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Kuo, M. (2017, November). Agaricus amicosus. Retrieved from the MushroomExpert.Com Web site: