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Agaricus arvensis: The Horse Mushroom
by Michael Kuo
The Horse Mushroom is a stately and impressive mushroom, recognized by its preference for grassy areas, its cap colors (pale yellow to whitish, often with pressed-down fibers), its sweetish smell, and its "cogwheeled" ring. 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. However, there are at least two genetically, biologically, and morphologically distinct species passing as "Agaricus arvensis"; see the comments below for details.
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 . . .
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.
Flesh: Thick and white throughout; not changing color when exposed, or yellowing slightly; flesh in stem base not yellowing.
Odor and Taste: Odor sweet (reminiscent of anise or almonds) when young and fresh, becoming less distinctive; taste pleasant.
Chemical Reactions: Cap yellow with KOH.
Spore Print: Dark brown.
Microscopic Features: Spores 7-9 x 4.5-6 µ; elliptical; smooth.
REFERENCES: Schaeffer, 1774. (Saccardo, 1887; Kauffman, 1918 [Psalliota]; Smith, Smith & Weber, 1979; Arora, 1986; Phillips, 1991/2005; Lincoff, 1992; Evenson, 1997; Calvo-Bado et al., 2000; McNeil, 2006; Miller & Miller, 2006; Kuo, 2007.) Herb. Kuo 09280605.
A study by Calvo-Bado and collaborators (2000) examined several Agaricus arvensis collections from across the globe for their morphology, ability to mate, and their DNA. Results supported dividing the collections into two groups: in the first group, the caps were pale yellow and developed, by maturity, a broadly bell-shaped cap; in the second group the caps were white and, at maturity, broadly convex (additionally, spores in the first group were slightly longer than spores in the second group). Mating ability and DNA supported both morphological groups, but suggested the possibility that additional species groups may be hiding within the second group.
Further Online Information:
Officially a European species, Agaricus macrosporus may occur in North America. It is very similar to Agaricus arvensis, but grows in association with trees (especially spruce) rather than in treeless areas. It has larger spores (9-13 x 6-7 µ). Future Agaricus publications may officially confirm its presence in North America. MushroomExpert.Com contributor John Plischke collected the illustrated specimens in Pennsylvania; the spores matched the range of Agaricus macrosporus.
Cite this page as:
Kuo, M. (2007, May). Agaricus arvensis: The horse mushroom. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/agaricus_arvensis.html