E-Flora BC: Electronic Atlas of the Flora of British Columbia

Agaricus arvensis Schaeff.
horse mushroom

Species account author: Ian Gibson.
Extracted from Matchmaker: Mushrooms of the Pacific Northwest.

Introduction to the Macrofungi

© Adolf Ceska  Email the photographer   (Photo ID #19189)

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Distribution of Agaricus arvensis
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Species Information

Features include large size; dry whitish to buff or yellowish cap that is smooth or sometimes cracks into small scales, and usually bruises yellow if rubbed; thick flesh that is unchanging or yellowing slightly when crushed; free, close gills that are pallid then grayish (rarely if ever pinkish) and then chocolate-brown or blackish brown from spores; stout stem that is smooth or with cottony scales in lower part; ring whose lower surface in conspicuously radially cracked; odor that is anise or almond when young and musty when old; and growth usually in grass. According to Chariton(1) there are two forms in the Pacific Northwest: 1) the mainland form tall and slender with a thin, pliant ring that bruises yellowish if rubbed hard, and 2) the island form squat with a thick ring, bruising quickly. Isaacs also enumerates two forms: one, found on the San Juan Islands in northern Washington, has small spores 6.5-7.7 x 4.7-5.3 microns, and the other, collected in the Blaine area and in Seattle, has spores 7.7-9.5 x 5-5.9 microns. Lange has considered Agaricus arvensis conspecific with Agaricus silvicola and intermediate forms are sometimes found in WA. A. arvensis has been found at least in BC (in Redhead), WA (Hotson, Isaacs), and CA (Kerrigan). There are collections from OR at Oregon State University. Breitenbach gives distribution as North America, Europe, Asia, North Africa, and Australia., CHEMICAL REACTIONS cap surface staining yellow in KOH, (Arora), staining brick-red with Henry's reagent (unlike xanthoderma)
8-20cm across, "convex with flattened disc, especially when young; white to dull brassy yellow with age or when bruised"; smooth to floccose-scaly, (Phillips), (7)10-20cm across, convex becoming semi-flat; white to buff, becoming yellow, especially if desiccated; dry, bald to appressed-fibrillose, or if desiccated becoming squamulose [finely scaly] to squamose [scaly] or rimose [cracked], (Kerrigan), (4)7-20cm across, oval or convex becoming broadly convex or flat; "white to creamy, buff, or yellowish (especially toward center), usually bruising yellow if rubbed (especially when young)"; "dry, smooth or sometimes cracking into small scales, especially at the center"; margin sometimes with hanging veil remnants, (Arora), (7)10-20cm across (Chariton)
firm; white, when cut turning slightly buff to yellowish, (Phillips), up to 2.5cm thick, firm when young; "white, unchanging or yellowing slightly", in stem "white, unchanging or becoming faintly yellow", (Kerrigan), thick, firm; white, unchanging or bruising slightly yellow when crushed, (Arora)
free, crowded; white to grayish (not pink), then dark brown, (Phillips), "free, close, up to 1.2cm broad"; "at first white, rarely if ever with a subsequent pink stage", later persistently grayish, finally dark blackish brown, edge somewhat pallid, (Kerrigan), free when mature, close; "pallid becoming grayish (rarely pinkish)", then chocolate brown or darker, (Arora)
6-15cm x 1-2.5cm, equal to club-shaped; white; smooth to slightly floccose-scaly below ring, (Phillips), 10-17cm x 2-3cm, equal or slightly club-shaped to slightly bulbous, stuffed-hollow, cavity about 0.5cm broad; white, becoming yellowish when old; bald in upper part, often with floccose patches in lower part, (Kerrigan), 5-12(17)cm x 1-3cm, equal or widened slightly in lower part, stuffed or hollow; "white, sometimes bruising or aging yellowish" but extreme base not bruising yellow when cut; smooth or with small cottony scales below ring, (Arora), 10-17cm x 2-3cm (Chariton)
ring large, thick, pendant, upper side smooth, underside with cogwheel-like veil remnants, (Phillips), veils forming broadly flaring, subapical, white ring, partial veil entire, 0.1-0.2cm thick, smooth above, universal veil friable, 0.2-0.5cm thick, characteristically forming cogwheel or breaking up into white patches which may become yellow to orange or buff, "also leaving a few small warts or larger patches" on lower part of stem, (Kerrigan), "membranous, white or tinged yellow, with cottony patches on underside that often split to form a starlike or cogwheel pattern", forming fragile, superior, skirt-like ring, (Arora), double (Hotson)
pleasant, almond-like, (Phillips), of almonds when young, becoming musty and urine-like when old, (Kerrigan), sweet like anise or almond when young, often somewhat musty when old, (Arora), anise (Miller)
pleasant, almond-like, (Phillips), mild, nutty, (Breitenbach), mild (Miller)
Microscopic spores:
spores 7-9 x 4.5-6 microns, ovate, (Phillips), spores (5.3)7.4-8.0(9.4) x (4.9)5.3-5.6(6.8) microns, elliptic, dark brown, hilar appendix not prominent, germ pore not evident; basidia 4-spored, 19-23 x 8.5-9.5 microns, clavate to ventricose; cheilocystidia abundant, forming a continuous band on gill margin, 7-35 x 3-16.5 microns, "often globose to subglobose and catenulate, or cylindric, oblong, or pyriform", (Kerrigan), spores 7-8.5 x 5-6 microns, elliptic, smooth, (Arora), pleurocystidia not seen (Breitenbach)
Spore deposit:
deep brown (Phillips), chocolate brown (Arora), purple brown (Schalkwijk-Barendsen), purplish brown to blackish brown (Miller)

Habitat / Range

gregarious, often in fairy rings, in lawns and other grassy places, (Kerrigan), single, scattered or in groups in grass, (Arora), sometimes in woodland clearings (Kibby), fields and meadows, June to October (Phillips), summer, fall

Taxonomic and Nomenclatural Links

Additional Range and Status Information Links


yes, (Phillips), occasional people stomach upset

Additional Photo Sources

Related Databases

Species References

Kerrigan(1), Phillips(1)*, Arora(1)*, Miller(14)*, Kibby(1)*, Schalkwijk-Barendsen(1)*, Lincoff(1)*, Courtecuisse(1)*, Bessette(2)*, Hotson(1), MykoWeb(1), Chariton(1), Redhead(5), Isaacs(1), Breitenbach(4)*, Buczacki(1)*, Bacon(1)*, AroraPocket*

References for the fungi

General References