Agaricus brunneolutosus Linda J. Chen, Karun. & K.D. Hyde
Facesoffungi number: FoF 02284.
Pileus 5.5–8.5 cm diam, 3–5 mm thick at disc; convex to applanate, or uplifted; surface dry, covered with brown fibrils, densely at disc and radially arranged elsewhere, somewhat sparse towards the margin, on a white to yellowish white background. Margin straight, shortly exceeding the lamellae, often with appendiculate remains of the annulus. Lamellae free, crowded, 3–5 mm broad, with intercalated lamellulae, at first white, then pinkish brown, finally dark brown. Stipe 70–110 × 7–10 (10–12 at base) mm, clavate or tapering upwards, fistulose, surface above the ring smooth, below fibrillose, white, flavescent when bruised. Annulus simple, superous, membranous, upper surface smooth, lower surface fibrillose, white, changing to yellowish with time or when rubbed. Odour of almonds. Context firm, discoloration when cut not recorded.
Spores 3.9–4.7(–5.2) × 2.7–3.3 µm, (x = 4.3 ± 0.22 × 2.9 ± 0.14 µm, Q = 1.32–1.65, Qm = 1.48 ± 0.03, n = 20), ellipsoid, smooth, brown, thick-walled. Basidia 13–18 × 5.5–8 µm, clavate to broadly clavate, hyaline, smooth, 4-spored. Cheilocystidia 17–42 × 9–15 µm, abundant, simple, pyriform to broadly clavate, hyaline or with yellowish pigments, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae of 5–13 µm wide, cylindrical, hyaline or with light yellow pigments, smooth, sometimes constricted at the septa; terminal elements observed cylindrical, 13–25 µm wide, with rounded or attenuate apex. Microchemical reactions — KOH reaction positive, yellow. Schäffer’s reaction positive, reddish-orange on dry specimen. (Figs. 1–2)
Etymology: Refers to the brown-yellow colour of the pileus.
Habitat: Solitary on soil, in forest dominated by Casternopsis and Lithocarpus.
Material examined: CHINA, Yunnan Prov., Mengsong, 7 July 2012, S.C. Karunarathna, MS514 (holotype MFLU16-0976; isotype HMAS279153); Yunnan Prov., Mengsong, 10 July 2012, S.C. Karunarathna, MS541 (MFLU16-0977, HMAS279154).
Notes: Agaricus brunneolutosus is distinguished by its yellowish white pileus, entirely covered with brown fibrils, small spores on average size of 4.3 × 2.9 μm, large cheilocystidia and the pileipellis hyphae with terminal elements 13–25 μm wide. Among the members of A. sect. Minores, few species have spores on average shorter than 5 μm: A. comtulus, A. dulcidulus, A. edmondoi, A. entibigae, A. friesianus, A. matrum, and A. pallens. However, A. comtulus and A. entibigae have wider spores. The remaining taxa usually have pinkish, reddish pink or reddish purple fibrils on the pileus disc, and are white elsewhere. Additionally, they are well separated by molecular data. Agaricus brunneolutosus forms a sister clade with A. fulvoaurantiacus and A. luteofibrillosus, however, the two latter species have larger spores (larger than 5 × 3 μm on average).