Agaricus luteopallidus Linda J. Chen, Karun., R.L. Zhao & K.D. Hyde
Index Fungorum number 818053, Facesoffungi number. FoF 02289.
Pileus 3–6 cm diam, 2.5–3 mm thick at disc, conico-truncatewhen young, then convex to hemispherical, finally applanate; surface dry, with pallid yellow to light brownish yellow fibrils, densely at disc, with pileus expansion, outside the unbroken disc, the surface disrupts into finely triangular scales, on a white background; turning yellowish when rubbed. Margin straight, not exceeding the lamellae, often with appendiculate remains of the annulus. Lamellae free, crowded, ventricose, 3–4 mm broad, with intercalated lamellulae, first pink, then pinkish brown, finally dark brown. Stipe 65–95 × 5–11 mm, cylindrical or with slightly bulbous base, with numerous rhizomorphs, fistulose, surface above the ring smooth, below the ring fibrillose, white, strongly flavescent when bruised or by handing. Annulus simple, superous, cortinate when young, membranous when mature, fragile, white. Context firm, white, unchanging when cut. Odour of almonds. Spores (4.5–)5–6 × (3–)3.2–4 µm, (x = 5.4 ± 0.36 × 3.6 ± 0.3 µm, Q = 1.38–1.83, Qm = 1.52 ± 0.02, n = 20), ellipsoid, smooth, brown, thick-walled. Basidia 13–20 × 5.5–7 µm, clavate to broadly clavate, hyaline, smooth, 4-spored, rarely 2-spored. Cheilocystidia 14–28 × 10–22 µm, abundant, simple, rarely in short chains, globose to pyriform or sphaeropedunculate, rarely clavate, with yellowish pigments, smooth. Pleurocystidia absent. Pileipellis a cutis composed of hyphae of 4–11 µm diam, cylindrical, with yellowish membranous pigments, smooth, at times slightly constricted at the septa. Microchemical reactions — KOH reaction positive, bright yellow. Schäffer’s reaction positive, bright reddish-orange. (Figs. 1– 2)
Etymology: Refers to the pallid yellow colour of the pileus.
Habitat: Solitary, scattered or gregarious on soil, in grassland or rotted litter.
Material examined: Thailand, Chiang Rai Prov., Parnae Lao Park, 2 Aug. 2006, R.L. Zhao, ZRL3088 (holotype BBH19604; isotype HMAS279147); Chiang Mai Prov., Doi Suthep, 29 June 2010, P. Sysouphanthong, NTF26; Chiang Rai Prov., Mae Fah Luang University park, 3Aug. 2010, S.C. Karunarathna, NTS-CR01 (MFLU10-0674); 5 Mar. 2011, S.C. Karunarathna, SCK099 (MFLU11-1285); 5 June 2011, S.C. Karunarathna, SCK120 (MFLU11-1287); 5 July 2011, S.C. Karunarathna, SCK121 (MFLU11-1287); Chiang Mai Prov., MRC, 13 May 2011, S.C. Karunarathna, SCK138 (MFLU11-1296); Chiang Rai Prov., Mae Fah Luang University, 20 July 2012, J. Chen, LD2012113 (MFLU12-0950); 21 July 2012, J. Chen, LD2012120 (MFLU12-0956).
Notes — Agaricus luteopallidus is well characterized by having a pileus surface covered with pale yellow to light brownish yellow fibrils or triangular squamules, spores 5.4 × 3.6 μm on average, and the simple cheilocystidia containing yellowish pigments. In general, several species resemble A. luteopallidus by having a pileus surface with yellowish tinge, and later covered with fibrillose scales, such as A. xantholepis, A. azoetes and A. luteoflocculosus. According to the original diagnosis, A. xantholepis, which has been considered as a synonym of A. brunneolus, exhibits a distinctively bulbous base up to 15 mm broad and has smaller spores, 4–5.5 × 3 μm. Agaricus azoetes was originally described from Hawaii and can be easily distinguished from A. luteopallidus by its smaller sporocarps not exceeding 4.5 cm, wider basidiospores (5.7 × 4.3 μm on average), lacking of cheilocystidia and the arid habitats. Agaricus luteoflocculosus differs from the new species by the floccose on the lower side of the annulus and fibrillose woolly scales on the stipe surface. According to the phylogenetic results, A. luteopallidus is closely related to A. flavopileatus. The latter differs at 6 positions in ITS sequences, and more than 20 positions in tef-1α sequences.