Agaricus flavopileatus Linda J. Chen, Karun. & Callac
Index Fungorum number 818051, Facesoffungi number: FoF 02287.
Pileus 4–6 cm diam, 3–4 mm thick at disc; at first parabolic, then hemispherical to plano-convex, truncate or slightly depressed at disc, finally applanate; surface dry, covered withgreyish yellow to yellow ochre fibrils or squamules, densely at disc and radially or concentrically arranged elsewhere, on a white to yellowish white background; sometimes squamules are not uniformly distributed on pileus surface. Margin straight, shortly exceeding the lamellae, often with appendiculate remains of the annulus. Lamellae free, crowded, 3–4 mm broad, with intercalated lamellulae, pink to brown, finally chocolate brown. Stipe 30–75 × 4–12 mm, clavate or tapering upwards, with rhizomorphs, fistulose, surface above the ring smooth, below the ring fibrillose, white, strongly flavescent when bruised. Annulus single, membranous, superous, white, upper surface smooth, lower surface fibrillose. Context firm, white. Odour of almonds.
Spores 4.6–5.2(–5.3) × (2.6–)2.7–3.3(–3.4) µm,(x = 4.8 ± 0.13 × 2.9 ± 0.15 µm, Q = 1.42–1.87, Qm = 1.65 ± 0.1, n = 20), ellipsoid, smooth, brown, thick-walled. Basidia 11–17 × 4–6 µm, clavate, hyaline, smooth, 4-spored. Cheilocystidia 14–28 × 5–18 µm, abundant, simple, pyriform to broadly clavate, or sphaeropedunculate, with yellowish pigments, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae of 4–14 µm wide, cylindrical, hyaline or with yellowish-brown pigments, smooth, sometimes constricted at the septa. Macrochemical reactions — KOH reaction positive, yellow. Schäffer’s reaction positive, reddish-orange on dry specimen. (Figs. 2–3)
Etymology: The epithet ‘flavopileatus’ refers to the yellow pileus.
Habitat: Solitary or scattered on soil, in forest.
Material examined: CHINA, Yunnan Prov., Mengsong, 21 July 2012, S.C. Karunarathna, MS596 (holotype MFLU16-0984); Yunnan Prov., Mengsong, 22 July 2012, S.C. Karunarathna, MS603 (MFLU16-0983, HMAS279150).
Notes: Agaricus flavopileatus is morphologically well-characterized by the white to yellowish-white pileus, radially or concentrically covered with greyish-yellow to yellowish-brown fibrils or squamules, small spores and the simple, pyriform, broadly clavate, or sphaeropedunculate cheilocystidia. Comparing with other members of A. sect. Minores, which sometimes also have a yellowish to ochre pileus surface, A. flavopileatus can be distinguished as follows: A. azoetes and A. pseudolutosus have larger spores, with an average of 6.37 × 4.78 μm and 5.7 × 4.3 μm, respectively; A. comtulus has wider spores, on average 4.87 × 3.55 μm , and phylogenetically, it is quite distant from A. flavopileatus, A. luteoflocculosus differs in having larger spores (5.95 × 4.1 μm on average), the lower side of the annulus is floccose and stipe surface has fibrillose woolly scales.
Fig. 1 Maximum Clade Credibility tree of Agaricus resulting from analysis of ITS, LSU, tef-1α sequence data with the outgroup Heinemannomyces sp. Posterior probabilities which are equal and above 80 % are indicated. The 95 % highest posterior density of divergence time estimation are marked by horizontal bars. Species sampled from GMS and Europe are in red and blue, respectively; new taxa are in bold.