Agaricales » ‎Agaricaceae » Agaricus

Agaricus robustulus

Agaricus robustulus Linda J. Chen, Callac, L.A. Parra, K.D. Hyde & De Kesel

Facesoffungi number: FoF 02292; MycoBank MB818056

Pileus 2–6(–8.5) cm diam, 2–3 mm thick at disc; at first parabolic, becoming conico-convex to convex, sometimes with truncated centre, finally applanate; surface dry, with reddish brown or dark golden brown fibrils, densely at disc, soon with pileus expansion, outside the unbroken disc the surface disrupts into triangular scales, concentrically arranged on a dirty white background. Margin incurved, becoming straight when mature, not exceeding the lamellae, often with appendiculate remains of the annulus. Lamellae free, crowded, 4 mm broad, with intercalated lamellulae, subventricose to ventricose, at first white to pink, then light brown, finally dark brown. Stipe 20–40(–95) × 6–10 mm (13–14 mm at base), cylindrical with a bulbous base, fistulose, surface above the ring smooth, below the ring fibrillose, sometimes with camel-coloured appressed scales, white, strongly flavescent when rubbed. Annulus simple, superous, membranous, occasionally somewhat floccose on the below side, white, fragile. Context firm, white, flavescent when cut. Odor of almonds. Spores 5.4–6.2(–6.6) × 3–4 µm, (x = 5.8 ± 0.25 × 3.7 ± 0.16 µm, Q = 1.47–1.74, Qm = 1.56 ± 0.04, n = 20, Asiatic collections), ellipsoid, rarely oblong, smooth, brown, thick-walled; 4.4–6.1 × 3.1–3.6(–3.8) µm, (x = 5.2 ± 0.43 × 3.3 ± 0.18 µm, Q = 1.37–1.79, Qm = 1.56 ± 0.11, n = 30, African collection), ellipsoid, rarely oblong, smooth, brown, thick-walled. Basidia 12–22 × 6–9 µm, clavate to broadly clavate, hyaline, smooth, 4-spored. Cheilocystidia 16–40(–66) × 14–20(–23) µm, simple, ovoid, pyriform or broadly clavate with a thin base, with yellowish pigments, smooth. Pleurocystidia absent. Pileipellis a cutis composed of cylindrical hyphae 4–13(–15) µm diam, not or slightly constricted at the septa, the thicker the more constricted. With greyish brown diffuse internal pigment. One terminal element observed 8 µm wide with progressively at[1]tenuated and rounded apex. Macrochemical reactions — KOH reaction positive, yellow. Schäffer’s reaction positive, reddish on dry specimen. (Figs. 1–2)


Habitat: Solitary or scattered in sandy soil of secondary forest or in park.

Material examined: CHINA, Yunnan Prov., Lincang, Yongde County, 15 July 2012, Q.H. Yu, ZRL2012357 (HMAS273958). – Malaysia, Langkawi Island, 21Apr. 2013, P. Callac,AK075 (KLU); 22Apr. 2013, J. Ha, K Yun & P. Callac, MAR145 (KLU). – Thailand, Chiang Mai Prov., Chiang Mai University, 25 July 2010, J. Guinberteau, CA847 (holotype MFLU16-0973); Chiang Mai Prov., Doi Suthep Pui National Park, 15Aug. 2009, S.C. Karunarathna, NT055 (MFLU).

Notes: Agaricus robustulus is morphologically well characterized by its fleshy sporocarps, the reddish-brown or dark golden brown, fibrillose or squamose pileus, spores with mean of 5.8 × 3.7 μm and simple cheilocystidia. The average spore size of the new species is slightly different between Asian and African collections which can be considered as intraspecific variation. Several species morphologically resemble A. robustulus by having fleshy sporocarps, fibrillose or squamose pileus and variable colour from reddish brown to purplish brown, such as: A. brunneolusA. goossensiae, and A. megalosporusAgaricus goossensiae differs by its larger spores (6.3 × 4.4 μm on average, re-examination of the holotype GF929) and inconspicuous cheilocystidia; A. brunneolus and A. megalosporus are easily separated when their pilei exceed 7 cm diam. Otherwise, the sequence data are crucial for an accurate identification.