What You Should Know
This mushroom can cause quite a little confusion. Its gills remain white for quite a long time (as compared to its close relative, Pluteus cervinus), and it is more likely to be found growing from woodchips, buried wood in urban areas (especially where trees have been taken out within a year or two), woody mulch, or sawdust, appearing terrestrial rather than popping out of a stump.
However, the gills do eventually turn Pluteus-pink, and once they do, the same factors that were confusing at first become useful features for identification-coupled with the fact that the cap is usually (but not always!) whitish to very pale brown, with fine brown scales over the center.
For certain identification, however, microscopic features should be checked, since cervinus-like collections of Pluteus petastus (and petasatus-like collections of Pluteus cervinus) are not uncommon.
Pluteus petatus has slightly smaller spores than Pluteus cervinus, and its cheilocystidia are much less frequent. Additionally, and perhaps more easily assessed, Pluteus petasatus features abundant, fusiform "intermediate cystidia" that have not yet developed the apical prongs of mature pleurocystidia.
Other names: Fawn mushroom.
Pluteus petasatus Mushroom Identification
5-13.5 cm broad, convex to convex-umbonate, expanding to nearly plane, at maturity the disc sometimes depressed or raised; the margin in youth, incurved, then decurved, finally plane; surface sticky when moist, otherwise dry, glabrous when young, often shiny, cream-colored with pale innate, brown fibrils concentrated at the disc, the latter occasionally becoming fibrillose to squamulose; in dry weather the cap sometimes squamulose overall; context white, unchanging, soft, up 1.5 cm thick, thin near margin; odor and taste of radish.
Gills free, crowded, broad, pallid, becoming cream to pinkish, finally salmon pink; not marginate; lamellulae up to four-seried.
Stipe 5-9 cm long, 1-1.5 cm thick, straight, the apex and base often enlarged, fleshy, solid at maturity; surface longitudinally striate at the apex, white, elsewhere smooth to slightly wrinkled, in age brownish fibrils sometimes developing at the base; partial veil absent.
Spore print salmon-pink.
Clustered, less commonly solitary, on rotting chips, woody debris or logs; fruiting in late summer in watered areas or after the fall rains.
Spores 5–8 x 3.5–6 µm; ellipsoid; smooth; hyaline and uniguttulate in KOH; inamyloid. Cheilocystidia scattered and infrequent; clavate to sphaeropedunculate; hyaline in KOH; thin-walled. Intermediate cystidia often abundant; fusiform, without apical projections; thick-walled; hyaline in KOH. Pleurocystidia 50–90 x 10–25 µm; lageniform with a flared apex; with 2–4 apical hooks; hooks entire; thick-walled; hyaline with faintly ochraceous walls in KOH. Pileipellis a cutis or slightly gelatinized cutis; elements 4–7 µm wide, hyaline or brownish in KOH, smooth, septate, not clamped; terminal cells cylindric with rounded apices.
Photo 1 - Author: Jerzy Opioła (Creative Commons Attribution-Share Alike 4.0 International)
Photo 2 - Author: Strobilomyces (Creative Commons Attribution-Share Alike 3.0 Unported)
Photo 3 - Author: Strobilomyces (Creative Commons Attribution-Share Alike 3.0 Unported)
Photo 4 - Author: LurkinLizard (Creative Commons Attribution-Share Alike 3.0 Unported)
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