Agaricus albosquamosus L.J. Chen, K.D. Hyde & R.L. Zhao.
Pileus 36 mm in diam. when young, 65–77 mm in diam., 7–7.5 mm thick at the disc when mature; convex to hemisphaerical, then becoming applanate-convex, finally applanate, occasionally with a slightly depressed centre; surface dry, fibrillose, with fibrillose squamules near the margin, white; in high humidity, the pileus has a reddish tone, margin exceeding to slightly appendiculate. Lamellae free, crowded, intercalated with lamellulae, 5–6 mmbroad, at first white, pink, then pinkish brown, finally dark brown. Stipe 98 × 8–12 mm, cylindrical with slightly bulbous, hollow, white, surface, smooth above the annulus and fibrillose below the annulus, not staining on touching. Annulus superous, simple, membranous, thickening towards the edge, upper side surface smooth, lower side surface fibrillose, floccose near the margin. Context firm, on cutting white in both pileus and stipe, not discolouring on exposure. Odour of almonds. Macrochemical reactions: KOH reaction no discolouration, Schäffer’s reaction negative. Basidiospores 5.1–6 × 3.4–3.9 μm, [x = 5.5 ± 0.27 × 3.7 ± 0.16, Q= 1.32–1.64, Qm= 1.47 ± 0.05, n = 20], ellipsoid, brown, smooth, thick-walled. Basidia 15–18× 7–8.5 μm, clavate to broadly clavate, hyaline, smooth, 4-spored or rarely 2-spored, without germ pore. Cheilocystidia 15.5–25 × 9–16.5 μm, abundant, simple, or sometimes in short chains, globose to pyriform or sphaeropedunculate, hyaline, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae 3–9 μm in diam., cylindrical, hyaline, smooth, and at times slightly constricted at the septa. (Fig. 2 e-h; Fig. 3)
Etymology: the epithet Balbosquamosus^ refers to the white and squamose pileus of this species.
Holotype: THAILAND, Chiang Mai Prov., Mae Sa National Park, 16 September 2012, collector Jie Chen, LD2012192 (MFLU12-1017, holotype; HMAS273987, isotype).
Habitat: scattered or in groups, in soil under bamboo woods or in a forest clearing.
Specimen examined: THAILAND, Chiang Mai Prov., Mae Sa National Park, 16 September 2012, collector Jie Chen, LD2012192 (MFLU12-1017, holotype; HMAS273987, isotype). THAILAND, Chiang Mai Prov., 3 km down the road from Tharnthong Lodge, 4 June 2012, collector Jie Chen, LD201235 (MFLU12-0879).
Notes: this distinct species is characterized by pure white, but squamose pileus, which is different from all other species in this section.
Fig. 1 Phylogeny of Agaricus subgenus Spissicaules generated from Bayesian analysis of ITS sequences, rooted with A. campestris. Bootstrap support (BS) values > 50 % and Bayesian posterior probability (PP) values > 80 % (or >50% in some main clades) are given at the internodes (PP/BS).