Summary:
Tricholoma portentosum is distinguished from the grayish dry-capped Tricholomas by its viscid cap. Other features include a streaked or radially fibrillose cap that is pale gray to dark gray, brownish gray, or purplish gray, or with olive tints; adnexed or notched, white gills that become grayish or pale yellow; a dry stem that is white or sometimes tinged yellow; a mild to farinaceous or cucumbery odor and taste; a white spore deposit; and elliptic, smooth, inamyloid spores. |Siegel(2) note that Tricholoma portentosum is a European name and species they note for California may end up being called Tricholoma avellaneifolium, which was described from the Cascade foothills of Oregon. [That species has the current name Melanoleuca avellaneifolia Murrill in the online Species Fungorum, accessed September 25, 2018, but was also called Tricholoma avellaneifolium by Murrill and Tricholoma portentosum var. avellaneifolium by A.H. Smith.]

Cap:
4-12cm across, convex to obtusely umbonate or flat, or with uplifted margin when old; pale gray to dark gray, brownish gray, or purplish gray, (center sometimes nearly black and margin often paler), yellow tints occasionally present also, especially when old; "viscid when moist, smooth but with a streaked or radially fibrillose appearance", (Arora), 5-12cm across, broadly convex, becoming flat "with a wavy margin in age, sometimes splitting in age, umbonate"; dark gray to dark grayish brown, "occasionally with lilac or grayish yellow tones resulting from dark radiating fibrils and streaks of dark gray color over a pale yellow to nearly white background"; "viscid becoming dry, appressed-fibrillose", (Shanks), 5-10(12)cm across, bell-shaped - convex becoming expanded, with obtuse umbo, margin incurved for a long time, acute; "dark gray-brown with yellowish, greenish, or somewhat violet tones, as well as black, radial, sometimes raised fibrils"; smooth, slightly lubricous, (Breitenbach)

Flesh:
fairly thick; white or tinged gray, (Arora), thin; whitish, watery gray or with pale yellow color, especially near cap surface, in stem white to pale grayish, and yellow near stem surface, (Shanks), thin; white to watery gray, somewhat yellowing in center of cap, flesh in cortex of stem sometimes yellowing, (Breitenbach)

Gills:
adnexed or notched, fairly close; "at first white, becoming grayish or sometimes pale yellow in age", (Arora), "sinuate to strongly notched, close"; white at first, developing pale yellow tones when old that may be visible only on face, occasionally some gray also present, (Shanks), slightly notched and broadly adnate, 40-44 gills reaching stem, broad, thickish, 3 subgills between neighboring gills; white; edges smooth, (Breitenbach)

Stem:
5-10cm x 1-2.5cm, more or less equal, firm; white or sometimes tinged yellow; dry, smooth, (Arora), 6.5-15cm x 1.5-2.8cm, equal with abruptly narrowing or somewhat bulbous base, often twisted and contorted, solid; white to pale yellow; dry, silky-fibrillose, (Shanks), 5-8(10)cm x 1-3cm, equal, firm, solid; white, spotting yellow-brownish to olive-brownish; longitudinally fibrillose, (Breitenbach)

Veil:
absent (Arora)

Odor:
mild to farinaceous, (Arora), not distinct to mildly farinaceous, (Shanks), slightly farinaceous (Breitenbach)

Taste:
mild to farinaceous, (Arora), not distinct to mildly farinaceous, (Shanks), mild, farinaceous, (Breitenbach), mealy, cucumber-like, (Kibby)

Microscopic spores:
spores 5-7 x 3-5 microns, elliptic, smooth, [presumably inamyloid], (Arora), spores 5.3-8.6 x 3.0-5.8 microns, elliptic, smooth, inamyloid; basidia 4-spored, 28-38 x 6.2-7.2 microns, clavate, occasionally thick-walled; pleurocystidia and cheilocystidia absent; clamp connections absent, (Shanks), spores 5.1-6.9 x 3.9-5.3 microns, nearly round to broadly elliptic, smooth; basidia (2)4-spored, 27-35 x 6-9.5 microns, clavate; pleurocystidia and cheilocystidia absent; clamps absent on cap cuticle and absent on basidia, (Breitenbach)

Spore deposit:
white (Arora, Breitenbach)

Notes:
It was reported from WA by O''Dell(1) and included for CA by Shanks(2). A collection from BC was deposited by Paul Kroeger at University of British Columbia. There are collections from WA and OR at Oregon State University. Bessette(5) show photographs from ID, NY, and WV. There is a collection from PQ at the University of British Columbia and a collection from MT at the University of Washington. It is found throughout North America (Kibby). Breitenbach(3) gives the distribution as Europe, North America, Asia, and North Africa.

EDIBILITY

excellent, but be sure of identification, (Arora)