Agaricus fulvoaurantiacus Linda J. Chen & Karun.
Index Fungorum number 818052, Facesoffungi number: FoF 02288.
Pileus 3.7–7 cm diam, 3–5 mm thick at disc, at first parabolic, then convex or plano-convex, finally applanate; surface dry, with light brownish yellow to brownish orange fibrils, densely at disc and radially arranged elsewhere, or sometimes squamose with appressed squamules or thick scales, against a white background. Margin incurved, shortly exceeding the lamellae, often with appendiculate remains of the annulus. Lamellae free, crowded, 2–5 mm broad, with intercalated lamellulae, first white, then pinkish brown, finally dark brown. Stipe 50–70 × 6–8 mm (11 mm at base), clavate, with numerous rhizomorphs, fistulose, surface above the ring smooth, below the ring with light yellowish brown appressed fibrillose scales, white, strongly flavescent when bruised. Annulus simple, superous, membranous, white, upper surface smooth, lower surface decorated with tiny yellowish flakes, connected with the stipe by cortinate fibrils. Odour of almonds. Context firm, white, flavescent when cut.
Spores (5.2–)5.6–6.1 × 3.5–4.1 µm, (x = 5.8 ± 0.22 × 3.8 ± 0.18 µm, Q = 1.26–1.73, Qm = 1.51 ± 0.01, n = 20), ellipsoid, smooth, brown, thick-walled. Basidia 16–18 × 6–8 µm, clavate to broadly clavate, hyaline, smooth, 4-spored, rarely 2-spored. Cheilocystidia (12–)17–30 × 9–13 µm, abundant, simple, pyriform, broadly clavate or sphaeropedunculate, hyaline or with yellowish pigments, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae of 6–12 µm diam, cylindrical, hyaline or with light yellow pigments, smooth, rarely constricted at the septa. Macrochemical reactions — KOH reaction positive, bright yellow. Schäffer’s reaction positive, bright orange. (Figs. 1–2)
Etymology: Refers to the tawny orange colour of the pileus.
Habitat: Solitary or gregarious on soil, in forest or tea plantations.
Material examined: CHINA, Yunnan Prov., Mengsong, 23 June 2012, S.C. Karunarathna, MS316 (MFLU16-0974, HMAS279151); Yunnan Prov., Mengsong, 11 July 2012, S.C. Karunarathna, MS549 (MFLU16-0978, HMAS279152); Yunnan Prov., Mengsong, 3 July 2014, J. Chen, LD201404 (holotype MFLU16-0980; isotype HMAS279149).
Notes: Agaricus fulvoaurantiacus is well characterized by a pileus surface covered with light brownish yellow to brownish orange fibrils or fibrillose squamules, concolorous fibrillose scales on the lower stipe surface, an annulus with tiny yellowish flakes on the lower surface, spores on average 5.8 × 3.8 μm, and the simple cheilocystidia, hyaline or containing yellowish pigments. Generally speaking, A. fulvoaurantiacus is very similar to A. luteofibrillosus by having the same appearance of pileus and stipe. However, A. luteofibrillosus has narrower spores (5.8 × 3.2 μm on average) and different cheilocystidia which are sometimes in short chains (see A. luteofibrillosus below) or septate at the base. According to the phylogenetic analyses, they are closely related. Indeed, A. fulvoaurantiacus differs at four positions in ITS sequences, one position in LSU (except MS316 which is heteromorphic (C and T) at this position), and six positions in tef-1α sequences. Macromorphologically, A. luteoflocculosus roughly resembles A. fulvoaurantiacus by having the bright yellow fibrillose scales on both pileus and stipe surface. However, it differs by its smaller spores (5.1 × 3.7 μm on average) and the habitat on rotting seaweed of the species Fucus vesiculosus on the seashore.