Summary:
Collybia tuberosa is distinguished by its small size, whitish to buff cap, and growth on mushrooms from an appleseed-like sclerotium.

Cap:
0.3-1cm across, convex to flat or centrally depressed; "whitish to buff, sometimes with a darker (yellowish to brownish or pinkish-buff) center", dry, smooth, (Arora), 0.2-0.7(1)cm across, obtusely convex to pulvinate [cushion-shaped] with an incurved margin when young, becoming flat-convex or flat, with downcurved or straight margin, sometimes shallowly depressed on the disc; subhygrophanous, pinkish buff on disc, whitish elsewhere, becoming more or less whitish overall when old; surface dry to moist, bald to pubescent, occasionally plicate-striate [pleated-striate] at margin, (Halling), 0.2-1.2cm across, convex becoming flat, with shallow umbo in some immature specimens, margin remaining slightly inrolled or becoming straight, often somewhat centrally depressed when old; subhygrophanous, white to pallid, center tinged pinkish buff, drying slightly darker and more pinkish; moist-watery, often drier and dull, faintly pubescent, especially under lens, margin often crenate [scalloped] and plicate-striate [pleated-striate] in older specimens, fraying then as well, (Lennox)

Flesh:
very thin; white, (Arora), thin; whitish to light buff, (Halling), very thin, rather soft when moist, tougher when dry; colored as cap surface, (Lennox)

Gills:
adnate to adnexed, close or crowded; white or rarely tinged pinkish, (Arora), adnate, somewhat subdecurrent when old, close to subdistant, thin, narrow to moderately broad; whitish to pinkish buff; edges even, (Halling), broadly adnate, appearing almost subdecurrent if cap margin raised, subdistant, 20 reaching stem, thickish, rather broad; "whitish, occasionally tinged faintly pinkish", (Lennox)

Stem:
1-3cm long and up to 0.1cm wide, often arising from small orange-brown to reddish brown to blackish, appleseed-like "tuber", equal; white or tinged brown; dry, minutely downy, (Arora), 1-5cm long and up to 0.1(0.2)cm wide, generally equal, often flexuous [wavy], fibrous-pliant, stuffed becoming hollow, arising from dark red-brown sclerotium, 0.3-1.2cm x 0.2-0.5cm, shaped lacrymoid to fusoid-ventricose, occasionally nearly ellipsoid or nearly spherical; stem whitish to pinkish buff, somewhat darker after handling; dry, furfuraceous to pruinose at top, pubescent to strigose at the base, surface becoming wrinkled or furrowed when old, (Halling), 1.3-2.5cm x 0.05-0.1cm, equal, solid or more rarely narrowly tubular, rather tough, never really pliant; at first concolorous, darkening slightly when old and with handling, then pinkish; surface powdery-pruinose at top, with a scant white tomentum lower down and nearly strigose at base; arising from a prominent sclerotium, 0.4-1.3cm 0.2-0.9cm, dark mahogany brown to light orange-brown, often mottled ferruginous when lighter-colored, often resembling an apple seed, white to cream within, becoming longitudinally furrowed when old or dry, surface shiny, (Lennox)

Odor:
none (Halling), not distinctive (Phillips), sweet or fungoid, (Lennox)

Taste:
none (Halling), not distinctive (Phillips), none (Lennox)

Microscopic spores:
spores 3-6 x 2-3 microns, elliptic, smooth, inamyloid, (Arora), spores 4.2-6.2 x 2.8-3.5 microns, oboval to elliptic or cylindric in face view, elliptic to lacrymoid [tear-shaped] in side view, smooth, inamyloid, acyanophilic; basidia 15.4-21 x 3.5-5 ┬╡m, clavate to cylindric, not siderophilic; pleurocystidia absent, cheilocystidia scattered to infrequent, inconspicuous, 17.5-31.5 microns long, "cylindric contorted, subclavate to irregularly diverticulate"; no pileocystidia, cap cuticle "a thin layer of more or less radially oriented, repent, cylindric and somewhat subgelatinous hyphae", cells 2-5 microns in diameter, "smooth, thin-walled, occasionally diverticulate"; clamp connections present in all tissues, (Halling), spores 5-6.5 x 3-3.5 microns, ovate to cylindric, smooth, inamyloid, colorless, thin-walled; basidia 4-spored, 19-27 x 4.5-5.5 microns, (Lennox)

Spore deposit:
white (Arora)

Notes:
It has been found at least in WA and ID, (Lennox), and NL, AK, MT, NC, NY, VA, Finland, Sweden, Switzerland, and United Kingdom, (Hughes). There are collections from BC at the University of British Columbia. Collybia tuberosa is also found elsewhere in North America and Europe and in Asia.

EDIBILITY

no (Phillips)

Synonyms and Alternate Names:
Microcollybia tuberosa (Bull.: Fr.) Lennox