Agaricus luteofibrillosus M.Q. He, Linda J. Chen & R.L. Zhao
Index Fungorum number 570234, Facesoffungi number. FoF xxx
Pileus 3–10 cm diam, 4–6 mm thick at disc, at first parabolic, then hemispherical or plano-convex, finally applanate or planoconcave, occasionally with a slightly depressed centre; surface dry, initially and uniformly covered with appressed fibrils of a brownish-orange tone and more densely at disc, with pileus expansion, the disc remains unbroken, disrupting into subtle squamules or triangular scales appressed or upturned elsewhere, on a yellowish-white background, flavescent when rubbed. Margin incurved or straight, not exceeding the lamellae, often with appendiculate remains of the annulus. Lamellae free, crowded, 3–7 mm broad, with intercalated lamellulae, first white, then brownish orange, finally dark brown. Stipe 40–120 × 3–15 (5–25 at base) mm, abruptly bulbous, or rounded with rhizomorphs, fistulose, surface above the ring smooth, below the ring fibrillose woolly of a brownish-orange colour, yellowish discoloration when rubbed. Annulus simple, superous, thick when young, with cortinate fibrils connected with stipe, membranous when mature, fragile, smooth on both surfaces, white, sometimes with brownish-orange tinge towards to the margin. Odour of almonds. Context firm, white, discolouring slightly yellowish when cut.
Spores (4.7–)5.1–5.9(–6) × 2.8–3.5(–3.8) µm, (x = 5.4 ± 0.22 × 3.2 ± 0.19 μm, Q = 1.44–1.91, Qm = 1.72 ± 0.01, n = 20), ellipsoid, smooth, brown, thick-walled. Basidia 16–20 × 6–8 µm, clavate to broadly clavate, hyaline, smooth, 4-spored, rarely 2-spored. Cheilocystidia 16–22(–30) × 8–15 µm, abundant, simple or sometimes in short chains (in this case, elements measuring 8–11 × 5–9 µm), globose, pyriform, or sphaeropedunculate, rarely clavate, hyaline, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae of 5–12.5 µm diam, cylindrical, hyaline or with light yellow pigment, smooth, occasionally constricted at the septa. Macrochemical reactions — KOH reaction positive, bright yellow. Schäffer’s reaction positive, bright orange. (Figs. 1–2)
Habitat: Caespitose or gregarious on soil, in Fagaceae and Pinaceae mixed forest.
Material examined: THAILAND, Chiang Rai Prov., Doi Mae Salong, 22 June 2015, J.Z. Sun, LD201501 (MFLU16-0981); Chiang Mai Prov., Tong Jown, 3 Aug. 2005, R.L. Zhao, ZRL2110 (BBH19490, HMAS279140); Chiang Mai Prov., Pathummikaram Temple, 8 June 2006, Tim, ZRL3039 (BBH19545, HMAS279155); Chiang Mai Prov., Doi Suthep, 20 June 2010, K. Wisitras sameewong, NTT037.
Notes: Agaricus luteofibrillosus is a species recently described from China. It is morphologically characterized by its yellowish white pileus surface covered with brownish orange squamules or triangular scales and the stipe with concolour fibrils. Our collections match well with the original diagnosis, except for their slightly smaller spores (5.8 × 3.4 μm on average), which can be considered as intraspecific variation. This is the first record of A. luteofibrillosus from Thailand.