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

Agaricus praeclaresquamosus A.E. Freeman
flat-top Agaricus

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

Introduction to the Macrofungi

© Jim Riley  Email the photographer   (Photo ID #15591)

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Distribution of Agaricus praeclaresquamosus
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Species Information

Distinguishing characters are size; minute blackish-brown scales on cap; free, close gills that pallid to pinkish when young; bald stem that stains yellow when the end of the base cut; thick rubbery double ring; and phenolic odor. The online Index Fungorum, accessed January 2, 2005, gives this as a synonym of Agaricus moelleri Wasser, but as Kerrigan(2) discusses in 2005, the uncertainties about the correct name(s) for the west coast species are not yet resolved. Until clarity is achieved, Agaricus 'praeclaresquamosus' is used here because several recent popular field guides use that name (an exception is Trudell(4)). Agaricus placomyces refers to a similar species common in eastern North America. Agaricus 'praeclaresquamosus' is common in the Pacific Northwest, including at least BC, WA, OR, and CA., CHEMICAL REACTIONS cap surface staining yellow in KOH (Arora), bright yellow 3% KOH, yellow to orange 15% KOH, (Isaacs)
5-25cm across, at first convex to somewhat marshmallow-shaped, then broadly convex or flat; "covered with flattened inky-gray to grayish-brown or brown fibrils or fibrillose scales (at least at center) on a whitish background, but often developing reddish or pinkish stains in wet weather", in one form bruising yellow; dry, (Arora), 5-11(20)cm across, at first cylindric-truncate to cuboidal, later convex or often angular-truncate, finally flat or rarely uplifted; dark brown fibrils and scales, background whitish to buff; dry, innately fibrillose, becoming appressed-squamulose [appressed - fine-scaly], fine scales about 0.1-0.4cm long, pointed, (Kerrigan(1))
thick; "white, unchanging or staining slightly yellow when bruised and then slowly discoloring brownish or vinaceous"; in extreme base of stem usually but not always staining bright yellow when cut, (Arora), up to 1.5(3)cm thick, white, in cap unchanging immediately after sectioning but several minutes later developing vinaceous tints where flesh has been crushed, in stem white, and when cut basal 0.05-0.2cm usually quickly becoming bright yellow, otherwise stem flesh initially unchanging, several minutes later yellow tones fading, then vinaceous areas appearing especially in lower part, (Kerrigan(1)), the base of the stem where bruised quickly becomes bright yellow and this color then slowly changes to reddish-brown, the same colors appear more slowly and less intensely in the upper stem and cap, except that the yellow first becomes red and then brown, in the cap it may finally change to purplish red, (Hotson)
free at maturity, close; at first pallid, then grayish or light pink turning reddish brown to chocolate brown and finally blackish brown, (Arora), free, close, up to 0.5(1.2)cm broad; at first pallid, later somewhat pinkish, finally dark blackish brown, (Kerrigan(1))
7-18cm x 1-3(4)cm, equal or widened below or sometimes tapering to a point if growing in clusters, stuffed or hollow; white but often discoloring reddish brown to dingy brown either when old or upon handling, extreme base of stem often bruises bright yellow when nicked like Agaricus xanthoderma; smooth, without scales, (Arora), 8-12(15)cm x 1-2.7(4.5)cm, equal to slightly bulbous, "base moderately shallowly rooted in soil"; white, unchanging except for vinaceous tones that appear long after bruising; bald, (Kerrigan(1))
membranous, white, thick, felt-like, somewhat rubbery, often splitting at the margin, rupturing to form a persistent, superior, skirt-like or intermediate ring, (Arora), veils separating early from cap margin, forming thick, tough, pendant to rarely intermediate, subapical, white ring, sometimes with an upper boundary, "upper and lower surfaces smooth, tending to remain entire, separating only to form a slight marginal groove", or occasionally some short, rudimentary, radial fissures present in universal veil, margin thick, up to 0.6(1)cm, (Kerrigan(1))
phenolic odor of crushed flesh especially in base of stem, (Arora), mild or phenolic, often strongly so, (Isaacs), phenolic (Kerrigan(1)), unpleasant, phenolic, ink-like, especially when flesh is crushed or cooked, (Phillips)
unpleasant metallic (Arora), mild, almond, or oily-disagreeable, (Isaacs), similar to odor (Phillips)
Microscopic spores:
spores 4-6.5 x 3-4.5 microns, elliptic, smooth, (Arora), spores (3.4)4.7-5.7(6.8) x (3.0)3.2-4.0(4.8) microns, (more robust fruiting bodies seem to have slightly broader spores than their slender counterparts), elliptic, dark brown, hilar appendix somewhat prominent, germ pore not evident; basidia 4-spored, about 20 x 7-8 microns, clavate, sterigmata 2-3 microns; cheilocystidia scattered, 18-23 x 7-9 microns clavate to broadly clavate, gill margin primarily narrow hyphae oriented parallel to margin and almost sterile, (Kerrigan(1))
Spore deposit:
chocolate brown (Arora)

Habitat / Range

single or in groups or clusters on ground in woods or under trees, especially along roadsides and paths, (Arora), single, gregarious, or in thick fleshy clusters, growing under both conifers and hardwoods, occasionally in lawns, (Isaacs), single or gregarious and often in a row, frequently in compacted soil along roads and paths, most commonly under Sequoia, occasionally under other trees, (Kerrigan(1) for California),, summer, fall, winter

Taxonomic and Nomenclatural Links

Additional Range and Status Information Links


poisonous to many, with vomiting and diarrhea (Arora)

Additional Photo Sources

Related Databases

Species References

Kerrigan(1), Arora(1)*, Phillips(1)*, Kibby(1)*, Trudell(4)* (as A. moelleri), Courtecuisse(1)*, Sept(1)*, Hotson(1) (as Agaricus placomyces), Isaacs(1) (as Agaricus placomyces var. placomyces), Murrill(7) (as A. placomyces), Kerrigan(2) (discussion under

References for the fungi

General References