Mycotaxon 9: 219. 1979.
Common Name: none
Synonym: Collybia butyracea (Bull.: Fr.) Quélet
Cap 3-7(9) cm broad, convex, expanding to plano-convex; margin finely striate, incurved at first, level at maturity, occasionally uplifted and wavy; surface glabrous, lubricous when moist, hygrophanous often unevenly dingy brown to mustard brown, paler towards the margin; context soft, up to 5 mm thick near the disc, rapidly thinning towards margin, white, unchanging; odor fungal, taste mild.
Gills adnexed to notched, sometimes appearing free, close, 7-10 mm broad, white in youth, becoming cream colored, unchanging, edges serrulate in age; lamellulae in 3-4 series.
Stipe 35-60 (70) x 5-10 mm in width, equal or with a clavate base, hollow in age, cartilaginous, readily splitting; surface finely striate, pallid, typically paler than the cap; partial veil absent; whitish tomentum at the base.
Spores (5) 6-7.5 (8) x 3-4 µm, ellipsoid in face view, tear-shaped in profile, slightly inequilateral; smooth, some spores slightly dextrinoid in Melzer’s reagent; spores cream-pink in deposit.
Scattered in small groups in conifer litter, primarily pines, occasional under hardwoods, coastal and montane; fruiting from late fall to mid-winter; common in most years.
Probably edible, but not substantial; culinary experience is lacking.
Two forms of Rhodocollybia butyracea occurs in California, R. butyracea f. asema with a dingy light brown cap described above, and R. butyracea f. butyracea, with a cap that has reddish-brown tones. Both forms occur under pine, but the dingy brown form is the most common. The reddish cap tones of Rhodocollybia butyracea f. butyracea sometimes leads to confusion with the similarly colored Gymnopus dryophilus. Several characters, however, distinguish the two, the most obvious being habitat. Rhodocollybia butyracea (both forms) are typically found with pine, while the aptly named Gymnopus dryophilus is most common under oaks, occasionally venturing under pine. Morphologically the species can be separated by the nature of cap, lubricous in Rhodocollybia butryracea, moist in Gymnopus dryophilus. Additionally, the gill edges are serrulate at maturity in Rhodocollybia butryracea, while entire in Gymnopus dryophilus. Finally, the spores of Rhodocollybia butryracea are sporadically dextrinoid in Melzer’s reagent, the reaction absent in Gymnopilus dryophilus. Some greyish Melanoleuca species found under pine also bear a resemblance to Rhodocollybia butyracea f. asema, but their caps are not lubricous, the gill edges are not serrulate, and the spores are amyloid, not slightly dextrinoid in Melzer’s reagent.
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