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

Agaricus deardorffensis group
flat-top agaricus

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

Introduction to the Macrofungi

© Michael Beug  Email the photographer   (Photo ID #89710)

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Distribution of Agaricus deardorffensis group
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Species Information

This group has been known as the Agaricus moelleri group or the Agaricus praeclaresquamosus group. A. moelleri and its synonym A. praeclaresquamosus are not known to occur in the Pacific Northwest - the group has been renamed according to one of its Pacific Northwest members. Distinguishing characters of this group in the Pacific Northwest are relatively large size; minute blackish-brown to grayish 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, phenolic odor, and yellow KOH reaction. |Kerrigan(3) talks about ''a polyphyletic array of several similar species (characteristically having fairly tall, robust proportions, dark gray to dark brown appressed-fibrillose pileus squamules, inelastic annulus, yellow-staining basal stipe context, and sylvan habitat) well represented in the West Coast region, and to some extent in the eastern United States and northward, that have been known collectively and now informally and archaically as "A. praeclaresquamosus." [italicized]''. |Kerrigan(3) has described Agaricus deardorffensis and Agaricus buckmacadooi in this group (see separate entries) but there are probably other undescribed taxa, including some mushrooms in California that are more slender (less robust) than these two and have narrower spores averaging 3.4 microns in width.

Members of this group have been found at least in BC, WA, OR, and CA.
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 xanthodermus; 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 metallic (Arora), mild, almond, or oily-disagreeable, (Isaacs)
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

Genetic information (NCBI Taxonomy Database)
Taxonomic Information from the World Flora Online
Index Fungorium
Taxonomic reference: Memoirs of the New York Botanical Garden 114: 154. 2016; Agaricus moelleri Wasser Nov. sist. Niz. Rast. 13: 77. 1976 group; Agaricus praeclaresquamosus A.E. Freeman Mycotaxon 8: 90. 1979 group

Additional Range and Status Information Links


poisonous to many, with vomiting and diarrhea (Arora)

Additional Photo Sources

Related Databases

Species References

Kerrigan(1)* (as A. praeclaresquamosus), Arora(1)* (as A. praeclaresquamosus), Trudell(4)* (as Agaricus moelleri), Sept(1)* (as A. praeclaresquamosus), Hotson(1) (as Agaricus placomyces), Isaacs(1) (as Agaricus placomyces var. placomyces), Murrill(7) (as A. placomyces), Kerrigan(2) (discussion under Agaricus moelleri, A. placomyces, A. sp. RWK 1937), AroraPocket* (as A. praeclaresquamosus), Kerrigan(3)

References for the fungi

General References