Summary:
Tricholoma populinum is characterized by its reddish brown, viscid cap, the association with cottonwood or poplar, usually in sandy soil, and a tendency to grow in densely packed masses or long arcs. The whitish gills develop reddish brown spots and stains: they are typically close, and typically adnexed or notched. The stem is whitish and also develops reddish brown stains. Odor and taste are farinaceous. Turner(1) documented the use of this mushroom for food by some First Nations people in the Pacific Northwest, and the related customs and knowledge.

Cap:
5-16cm across, convex with inrolled margin becoming flat or with uplifted margin; dull reddish cinnamon to pale dingy reddish brown, margin usually paler or whitish; "viscid when moist, then dry, often radially streaked or with watery spots", smooth, (Arora), 6-10cm across, flat-convex to wavy-convex; '"sayal brown" to "fawn color", lightening toward margin, often whitish-dusty or chalky appearing in places'; viscid when moist, bald, often reticulate - innately-fibrillose on disc, (Ovrebo), 8-12cm across, convex with inrolled margin; rosy brown, paler toward margin; viscid, bald, occasionally with watery spots near margin, (Shanks)

Flesh:
firm, thick; white, (Arora), thick; whitish, buff, in stem whitish-buff, (Ovrebo), white, rosy near cap surface, white in stem, (Shanks)

Gills:
"typically adnexed or notched, close"; white, developing reddish brown spots and stains, especially on edges, (Arora), ''sinuate to broadly sinuate, broad''; ''whitish and becoming darker'', eventually discoloring brown in patches on edge and face, (Ovrebo), sinuate; "white, discoloring reddish brown", (Shanks)

Stem:
2.5-7.5cm x 1-3cm, equal or widening downwards, solid, firm; dull whitish, developing dingy reddish brown stains when old or after handling, (Arora), 5-7.5cm x 1.5-2cm, narrowing toward base, ''often flattened, twisted and bent''; white especially toward top, ''discoloring brown in patches or all over, yellowing toward base''; bald with occasionally dark fibrils projecting, area near top slightly pruinose, (Ovrebo), 5-10 x 1-3cm, equal or narrowing towards base, solid; whitish at first, colored as cap when mature, sometimes yellowish at base, (Shanks)

Veil:
absent (Arora)

Odor:
strongly farinaceous (Arora), farinaceous (Ovrebo, Shanks)

Taste:
strongly farinaceous (Arora), farinaceous (Ovrebo, Shanks)

Microscopic spores:
spores 5-6 x 3.5-4 microns, elliptic, smooth, [presumably inamyloid], (Arora), spores 6.8-7.5 x 3.3-4 microns, elliptic to oblong in face view, bean-shaped to subfusiform [nearly spindle-shaped] in side view, smooth, inamyloid, guttulate; basidia 27.8-32 x 6-6.8 microns, clavate, colorless in KOH, yellow in Melzer''s; pleurocystidia and cheilocystidia absent; clamp connections absent, (Ovrebo), spores 5.5-6.5 x 3.5-4 microns, elliptic, smooth, inamyloid, colorless; basidia 4-spored 25-35 x 6-7 microns, clavate; pleurocystidia and cheilocystidia absent, (Shanks)

Spore deposit:
white (Arora, Ovrebo)

Notes:
It occurs at least in BC (Turner), WA (Ammirati, Ovrebo), OR (in Arora), ID (Ovrebo), CA (Shanks, Arora), MT (Cripps), NM (Arora), and AK (Bessette).

EDIBILITY

yes but be sure associated with cottonwood and is not in the Tricholoma pessundatum group (Arora)