( Basidiomycetes > Agaricales > Agaricaceae > Agaricus . . . )

Taxonomy in Transition: ... > Agaricales > Lepiotoid Clade (J&V, 1998)

by Michael Kuo

The Horse Mushroom is a stately and impressive mushroom, recognized by its preference for grassy areas, its cap colors (pale yellow at first; later whitish with pale, pressed-down fibers), its sweetish smell, and its "cogwheeled" ring (enlarge the bottom two illustrations). The cap will often bruise yellow if rubbed, and the flesh will sometimes turn yellowish on exposure to air--but the flesh in the base of the stem is not yellow, which helps distinguish it from other species.

Agaricus arvensis is a good edible--but some people are apparently adversely affected by it. Beginners should be very careful not to confuse it with Chlorophyllum molybdites or the deadly Amanita thiersii (or any other Amanita, for that matter).

I have no idea why they call it the "horse mushroom," other than because it often grows in fields, and sometimes fields have horses in them. Seems kind of lame to me. I think they should call it the "Make Fun of the Foreigner Mushroom" instead, since my most memorable experience with the species involves me attempting to back away from a grill in Finland, where my Finnish girlfriend's Finnish father had just finished igniting the coals with a Finnish match . . . I backed away fast, and stumbled over a patch of very large Agaricus arvensis mushrooms. I landed flat on my American butt, with a little piece of Horse Mushroom on my face, and received a chorus of raucous laughter from my girlfriend's family. They had quite a few things to say about the sieniammattimies ("mushroom expert") and his expertise . . .

Description:

Ecology: Saprobic; growing alone, scattered, or gregariously in grassy places (lawns, fields, and so on); summer and fall (also spring in California); widely distributed in North America.

Cap: 7-20 cm; convex at first, often with a somewhat flattened center; later broadly convex or flat; dry; white or pale yellowish when young; developing pressed-down fibers or scales (especially in dry weather); bruising yellow; the margin often with hanging partial veil remnants.

Gills: Free from the stem; crowded; whitish at first, becoming brown (without a pink stage).

Stem: 5-15 cm long; 1-3 cm thick; equal, or slightly bulbous; smooth, or with scales below the ring; white; sometimes bruising yellow; with a persistent, large ring that is often "cogwheeled" on the lower surface (see the bottom two illustrations).

Flesh: Thick and white throughout; not changing color when exposed, or yellowing slightly; flesh in stem base not yellowing.

Chemical Reactions: Cap yellow with KOH.

Spore Print: Dark brown.

• Pileus

Cap 6-21 cm broad, convex, expanding to plano-convex; margin incurved, decurved at maturity; surface smooth to finely scaled, cracking in dry weather; disc yellowish-buff, shading to a cream-colored margin, bruising yellow slowly, especially when young; flesh white, unchanging or yellowing slightly, thick, firm, but soft in age; odor and taste of anise when fresh.

• Lamellae

Gills close, free, pale at first, then blackish brown at maturity.

• Stipe

Stipe 4-14 cm tall, 1-3.5 cm thick, equal to tapering to an enlarged base, stuffed; surface smooth at the apex, white to cream, sometimes with scattered scales below, occasionally yellowing slowly; veil membranous, upper surface smooth, lower surface, with cream to buff, cottony patches arranged in a gear-tooth pattern, forming a membranous, superior, skirt-like ring.

• Spores

Spores 6.5-9.0 X 4.5-6 µm, elliptical, smooth; spore print chocolate-brown.

• Habitat

In arcs and rings in grassy areas, e.g. playing fields, pastures; fruiting from late fall to early winter.

• Edibility

Edible Excellent, though not as meaty as Agaricus augustus or A. bitorquis.

• Comments