Summary:
Features include 1) a dry cap streaked with gray fibrils, soon radially cracked exposing white flesh, 2) adnate, close, broad gills that are pale yellow, 3) a white to pale yellow stem that is naked except for fibrillose striations, and 4) microscopic characters. |A.H. Smith differentiated Tricholomopsis fallax from Tricholomopsis platyphylla by the yellow color in the stem cortex (as opposed to pallid in T. platyphylla) and microscopically by "the yellow pigment in the pilocystidia, the lack of diversity in the shape of the cheilocystidia, and the yellow pigment in the gills and cortex of the stipe", (Smith(42)). |However, molecular research by Hughes(3) found that their collections of specimens collected as Megacollybia platyphylla from western North America were close to Tricholoma fallax and recombined that species as Megacollybia fallax, noting that the cheilocystidia of T. fallax are just as variable as other taxa in the Megacollybia platyphylla group. Not only did they not find true Megacollybia platyphylla in North America, but collections from eastern North America were not the same species either.

Cap:
4-7cm across, 3-5cm high, obtuse when young, margin incurved, remaining unexpanded or becoming broadly convex to nearly flat; dry and more or less streaked with gray fibrils, soon radially rimose [cracked] around edge exposing white flesh, margin lobed and lacerate when old, (Smith), 4-7(12)cm across, obtuse when young with incurved margin, remaining unexpanded or becoming broadly convex to nearly flat; "dark gray-brown on disc, outward streaked with gray to sordid deep gray-olive fibrils"; dry, soon radially rimose [cracked], with wide gaps exposing white context, margin lobed, lacerate [torn] when old, (Hughes)

Flesh:
thin but fairly tough; white in cap, yellowish in stem with paler pith, (Smith), thin but fairly tough; white, unchanging on bruising; stem "yellowish in the cortex, the pith paler", (Hughes)

Gills:
"adnate, close, broad"; "pallid in button stages, but becoming pale yellow before the spores mature, finally creamy ochraceous", not changing color when bruised; edges even to slightly eroded, (Smith), adnate, close, ventricose (up to 2cm broad); off-white when young, becoming pale yellow, finally creamy ochraceous, edges the same color; transversely veined, occasionally interveined, gill edges even to slightly eroded, color not changing when bruised, (Hughes)

Stem:
8-11cm x 1-1.7cm, "equal or nearly so, solid"; white to whitish yellow but paler than gills, bald except for fibrillose striations, (Smith), 4-11cm x 1-1.7(3)cm, equal or narrowing slightly downward, solid; "surface white to whitish yellow but paler than the gills"; bald except for fibrillose striations; rhizomorphs sparse, thick (up to 0.4cm in diameter), white, round in cross-section, (Hughes)

Odor:
not distinctive (Smith, Hughes)

Taste:
not distinctive (Smith, Hughes)

Microscopic spores:
spores 6-7.5 x 5-6 microns, elliptic to ovate in face view, in side view with a suprahilar depression, smooth, inamyloid, thin-walled; basidia 4-spored, 40-47 x 6-7 microns, narrowly clavate; pleurocystidia none; cheilocystidia not prominent, 28-35 x 9-12 microns, clavate, walls thin or slightly thickened, colorless in KOH; cap cuticle not differentiated as such but the surface with numerous pileocystidia 28-70 x 9-20 microns, subcylindric to clavate, their content yellow in water, dingy yellowish in KOH, wall thin and smooth; clamp connections regularly present, (Smith), spores (5.6)6-8.5(10) x (4.0)5-7(8.0) microns, nearly round, elliptic to narrowly ovate, flattened adaxially, inamyloid, thin-walled; basidia (2)4-spored, (30)36-47(52) x (6)8-11 microns, basidioles 28-48 x 4-10 microns cheilocystidia occasionally protruding beyond basidia but not generally, 25-55(80) x (6)9-16 microns, "clavate to broadly clavate, occasionally with small apical outgrowths, firmwalled, occasionally secondarily transversely septate, conspicuously clamped", colorless; cap cuticle not differentiated as such but the surface with numerous terminal cells 28-99 x 8-20 microns, narrowly clavate to clavate, firm-walled to thick-walled (wall never more than 1 micron thick and then irregular over inner surface), moderately cytoplasmically pigmented, occasionally transversely septate (with no clamp), conspicuously clamped, rarely with some minute residual debris or slimy sheath, (Hughes)

Spore deposit:
dull white (Smith)

Notes:
Megacollybia fallax was described from ID (Smith(42)). It has been reported also from BC, WA, OR, and AZ, with collections (and molecular sequences) examined from AZ, ID (including holotype), and WA, (Hughes(3)). Their molecular study of the Megacollybia platyphylla group found that their western North American collections were Megacollybia fallax. Their collections of Megacollybia platyphylla were not North American, but they assigned other North American collections to three new species extending as far west as Missouri and Texas (see Megacollybia platyphylla group).

EDIBILITY

Synonyms and Alternate Names:
Tricholomopsis fallax A.H. Sm.