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Agaricus kriegeri

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

by Michael Kuo

This species was just named in 2016 by Agaricus expert Richard Kerrigan, in his monumental treatment of Agaricus of North America. According to Kerrigan the species has previously been folded into Agaricus placomyces, beginning with Louis Krieger's early 20th-century treatments. However, Krieger's "placomyces" had a somewhat stockier stem, a darker and scalier cap, and a more rubbery ring—along with sphaeropedunculate cheilocystidia and, we now know, different ITS sequences. To honor Krieger's contributions to North American Agaricus studies, Kerrigan named the species Agaricus kriegeri.

Identifying features for Agaricus kriegeri include the many brown, fibrillose scales on the cap; the fairly thick stem with a gradually swollen base; the rubbery ring with a thick edge; and sphaeropedunculate cheilocystidia. The flesh in the stem base may turn slightly yellow when sliced, or it may not.

Thanks to Cecily Franklin for collecting, documenting, and preserving some of the illustrated and described specimens; her collection is deposited in The Herbarium of Michael Kuo.


Ecology: Saprobic; growing gregariously under trees (including hemlocks, maples, and pines), usually in urban settings; summer and fall; possibly distributed throughout eastern North America. The illustrated and described collections are from Pennsylvania and Illinois.

Cap: 4–9 cm; convex or subconic at first, expanding to broadly convex; dry; when in the "button stage" covered with a fine, grayish brown, fibrillose covering that soon begins to be stretched into fibrillose scales; by maturity pale brownish underneath fine grayish brown scales that are more crowded over the center; the margin not lined, yellowing very slightly when rubbed repeatedly—or not yellowing.

Gills: Free from the stem or nearly so; crowded; short-gills frequent; pink, becoming brown and then dark brown in maturity; covered with a white partial veil when in the button stage.

Stem: 4–12 cm long; 1–2.5 cm thick; equal above a gradually swollen base; more or less bald; with a thick, rubbery, white ring that has a thickened, felty edge; whitish; bruising slightly yellow, or not bruising; basal mycelium white.

Flesh: White; firm; unchanging when sliced, or becoming yellowish to yellow in stem base.

Odor: Phenolic, or not distinctive.

Chemical Reactions: KOH yellow on cap surface.

Spore Print: Dark brown.

Microscopic Features: Spores: 5–7 x 3–4 µm; ellipsoid; smooth; thick-walled; brown in KOH, with a pale, contrasting apiculus; brown in Melzer's. Basidia 4-sterigmate. Cheilocystidia 20–32 x 8–12 µm; widely cylindric to sphaeropedunculate; smooth; thin-walled; hyaline in KOH; soon collapsing. Pleurocystidia not found. Pileipellis a cutis; elements 2.5–10 µm wide, smooth or slightly encrusted with pigment, brownish to golden brown in KOH.

REFERENCES: Kerrigan, 2016. (Krieger, 1935; Kerrigan, 2016.) Herb. Kuo 09270802, 06071701.

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Agaricus kriegeri

Agaricus kriegeri

Agaricus kriegeri

Agaricus kriegeri

Agaricus kriegeri

Agaricus kriegeri

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Kuo, M. (2018, January). Agaricus kriegeri. Retrieved from the MushroomExpert.Com Web site: