What You Should Know
Rhodocollybia butyracea (Bull.) Lennox is a very common leaf litter decayer, and one of the most frequent agarics involved in humus formation in both coniferous and deciduous forests of boreal and temperate regions. It is characterized by its dark-brown, hygrophanous pileus, white to whitish-dirty adnate gills and concolorous, striate, spongy stipe. It belongs to Omphalotaceae, a family that includes many saprobic litter and deadwood species that play an important role in nutrient cycling of forest soil. Molecular analyses includes R. butyracea in the Marasmioid clade (Matheny et al., 2006) within the order Agaricales.
This fungus gets its name from the greasy feel of the cap. The cap is 3-7cm across and has a distinct umbo (central bump). The Collybia's are called 'toughshanks' because of their fibrous and flexible stem, which has no ring or volva. Stem slightly swollen towards base, the base of stem covered in wooly hairs. Spore print white or pale pink.
Rhodocollybia butyracea is edible, but not substantial; culinary experience is lacking.
Other names: Butter Cap, Greasy Toughshank.
Rhodocollybia butyracea Mushroom Identification
Saprobic; decomposing the litter of conifers—especially pines; late summer and fall (also winter and spring in warmer climates); fairly widely distributed in North America.
2–6 cm across; convex, becoming broadly convex or nearly flat; moist, with a greasy feel, when fresh; bald; reddish-brown to brown, fading to cinnamon.
Narrowly attached to the stem or nearly free from it; close; short-gills frequent; whitish; often developing finely jagged edges.
3–8 cm long; and 1–2 cm thick; usually slightly to moderately club-shaped; moist or dry; bald; whitish to buff above; colored like the cap below; often with a whitish dusting when fresh; becoming hollow; basal mycelium white.
Whitish; unchanging when sliced.
KOH gray on cap surface.
Whitish or, with a thick print, faintly pinkish.
Spores smooth; 8–10 x 4.5–6 µm; ellipsoid; smooth; hyaline in KOH; at least a few (often many) dextrinoid. Basidia 4-spored. Pleurocystidia not found. Cheilocystidia inconspicuous; subclavate to subcylindric; often lobed or with projections; up to 40 µm long. Pileipellis a cutis; elements 2.5–7.5 µm wide, smooth or brownish-encrusted, clamped at septa.
Rhodocollybia butyracea Look-Alikes
Rhodocollybia prolixa (syn. Collybia distorta)
Is a much less common mushroom of conifer forests; it has a dry, red-brown cap and a swollen base below a fibrous and often slightly twisted stem.
Is usually paler and has brown-spotted gills.
Rhodocollybia butyracea Taxonomy and Etymology
When Jean Baptiste Francois Pierre Bulliard described this species in 1792 he gave it the scientific name Agaricus butyraceus. The Butter Cap was transferred to the genus Rhodocollybia in 1979 by American mycologist J W Lennox, since when its preferred scientific name has been Rhodocollybia butyracea; however, many field guides continued to list it as Collybia butyracea until quite recently.
Synonyms of Rhodocollybia butyracea include Agaricus butyraceus Bull., Agaricus leiopus Pers., Agaricus asemus (Fr.) Fr., Collybia butyracea (Bull.) P. Kumm., Collybia butyracea var. butyracea (Bull.) P. Kumm., Collybia asema (Fr.) Gillet, and Collybia butyracea var. asema (Fr.) Quél.
The pinkish tinge to the gills provides a clue to the genus name Rhodocollybia, as the prefix Rhod- means pink (as in Rhododendron). The second part of the generic name -collybia is also Latin and means a small coin. The nickname Pink Penny comes to mind, therefore, although as I just made it up it is perhaps now best forgotten.
As you might expect, the specific epithet butyracea simply means buttery.
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Photo 2 - Author: Bill Sheehan (Creative Commons Attribution-Share Alike 4.0 International)
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Photo 4 - Author: Dominicus Johannes Bergsma (Creative Commons Attribution-Share Alike 4.0 International)