Agaricus megalocarpus Y. Gui, Zuo Y. Liu, Callac, L.A. Parra & K.D. Hyde.
Index Fungorum number: IF 808756, MycoBank: MB808756
Pileus 16 cm in diameter, up to 0.4 cm thick, plano-convex to applanate at maturity; surface smooth, and brown at the disc, outside the disc covered with small appressed concentrically arranged brown squamules on a grey background densely congregated towards the disc and only slightly sparser at the margin. Lamellae up to 0.7 cm wide, free, crowded, with intercalated lamellulae, brown. Stipe 16x3 (4 at the base) cm, central, clavate to slightly bulbous, with rhizomorphs, surface white, and smooth overall, but somewhat flocculose near the base. Annulus superous, up to 0.7 cm broad, membranous, double, smooth, and white on the upperside, with floccose scales on the underside. Context firm, white (oac909). Odour not recorded. Basidiospores [108/1/1] 7.1–8.5(–11.1) – 4.3–5.3(–6.3) mm [avX = 7.8 ± 0.4 × 4.8 ±0.3, Q = 1.43–1.85, avQ = 1.64 ± 0.09], ellipsoid to elongate ellipsoid, with a prominent oblique apiculus, without apical pore, smooth, thick-walled, brown. Basidia, (10.5–) 16.8–25.2(–27.4) × (5.8–)7.7–10.6 mm, 4-spored, clavate, smooth, hyaline, with sterigmata 2.2 mm on average. Cheilocystidia 10–22.2 × 8.5–16.7 mm (Q = 1.04–1.92), pyriform or subglobose simple or catenulate with intracellular granulate brownish yellow pigment. Pleurocystidia absent. Pileipellis a cutis of hyphae 33.7–97.3 × 4.2–9.4 mm, thin, cylindrical, with intracellular yellowish-brown pigment or rarely hyaline, not constricted at septa. Annulus hyphae, 27.7–82.1 × 3.1–6.4 mm, cylindrical, smooth, not constricted, hyaline. Stipitipellis (below the annulus) a cutis of hyphae, 34.7–55.4 × 3.9–6.3 mm, cylindrical, smooth, with intracellular brownish-yellow pigments. (Fig. 1 i–j; Fig. 2)
Etymology: in reference to its large-sized sporocarps.
Holotype: China, Yunnan Province, Kunming, Qiongzhu Tempt, 5 Oct. 2011, Hao-0608 (HKAS 71717).
Habitat: Solitary in soil of forest dominated by Fagaceae species.
Specimen examined: China, Yunnan Province, Kunming, Qiongzhu Tempt, 5 Oct. 2011, Hao-0608 (HKAS 71717).
Distribution: Known only from highland subtropical Southwest China.
Notes: Agaricus megalocarpus is remarkable by its large size and its pileus surface covered with fine brown squamules. It has simple or catenulate brownish-yellow cheilocystidia formed by pyriform or subglobose elements. The pileipellis has intracellular yellowish-brown pigment. Morphologically, A. megalocarpus is most similar to A. augustus and A. subrufescens, which also have large sporocarps and a pileus surface with yellowish-brown to reddish-brown squamules. However, A. augustus differs in having usually erect scales on the lower part of the stipe, although collections with a glabrous stipe are also known. Agaricus subrufescens can clearly be distinguished by its smaller spores measuring 6 4.3 mm. Species in the section Arvenses developing concentric brownish or ochraceousbrown to grey fibrillose scales towards the centre on the surface of pileus are A. agrocyboides Heinem. & Gooss.-Font., A. magniceps Peck. and A. ochrascens Heinem. & Gooss.-Font. They have clearly a smaller spores than A. megalocarpus since their ranges of spore length and width are 4.5e6.8 mm and 3e4.2 mm respectively. The recently described species A. flocculosipes differs by its stipe, white floccose surface and smaller spores (5.9 3.7 mm).
Fig 1. Macrocharacters of Agaricus guizhouensis, A. longistipes, and A. megalocarpus. (A–E) Macrocharacters of A. guizhouensis. (D) Annulus of A. guizhouensis from above. (E) Annulus of A. guizhouensis from below. (F–H) Macrocharacters of A. longistipes. (IeJ) Macrocharacters of A. megalocarpus. Bars: A–C, F–J = 2 cm. D, E = 1 cm.