Leucoagaricus flavus Sysouph. & Thongkl.
Index Fungorum number: IF559485; Facesoffungi number: FoF 10799
Pileus 15–20 mm diam., umbonate, expanding applanate with low umbo, with straight margin; smooth and pale yellow to pastel yellow (3A3–4) at the umbo, breaking around umbo and with concentrically concolorous fibrillose toward the margin, sometimes peeling and with white to the pale yellow fibrillose background; marginal zone fibrillose, concolorous with dish, sulcate and fringed when mature. Lamellae free, pale yellow to pastel yellow (3A3), ventricose, up to 1.5 mm wide, slightly crowded, with 3lamellulae, with a concolorous eroded edge. Stipe 30–35 × 3–5 mm, cylindrical or slightly tapering to apex, with wide base; covered with white to yellowish-white (4A2) fibrillose or soft hair, on yellowish-white (2A2) background. Annulus cuff-liked, fibrillose, concolorous with pileus covering, attached at the middle zone of the stipe. Context in pileus white, thin, up to 2 mm wide; in stipe hollow, concolorous with surface, hollow and with white fibrils in hollow. Smells distinct, soft, flower, fleshy. Taste not observed. Spore print white.
Basidiospores [50,2,2] l × w = 4.8–5.3 × 3.5–4 μm, avl × avw = 5.0 × 3.8 μm, Q = 1.2–1.4, Qav = 1.3, in side-view broadly ellipsoidal ovoid to ellipsoidal ovoid, in frontal view ovoid to ellipsoid, slightly thick-walled, hyaline, without a germ pore, dextrinoid, congophilous, cyanophilous, metachromatic. Basidia 13.0–16 × 6.0–7.5 μm, clavate, hyaline, thin-walled, 4-spored. Pleulocustidia absent. Cheilocystidia 14–42 × 5–10 μm, clavate to narrowly clavate, hyaline, thin-walled. Pileus covering a cutis made up of cylindrical elements, sometimes with narrowly clavate, 15–41 × 3–7 μm, thin-walled, hyaline to pale yellow, with cylindrical and concolorous hyphae, 3–5 μm wide. Stipe covering same as pileus covering, a cutis made up of cylindrical and hyaline elements, 10–35 × 3–5 μm μm wide, thin-walled, with hyaline hyphae, 2–3.5 μm wide, thin-walled. Clamp connections are absent in all tissues.
Habitat and known distribution: growing solitary, saprotrophic, on humus soil; two samples were found in deciduous and bamboo forests during the rainy season (October to November 2015) of central Laos.
Material examined: LAOS, Bolikhamxay Province, Thaphabath District, Tad Xay Waterfall, 14 Oct 2015, P. Sysouphanthong, PS148 (HNL503375, holotype); Vientiane Capital, Xaythany District, Houay Nhang Preserve Forest, 17 Nov 2015, P. Sysouphanthong, PS201 (HNL503428, paratype).
Notes: Leucoagaricus flavus has pale yellow to pastel yellow basidiomata, free and pale yellow to pastel lamellae, white to the yellowish-white stipe, cuff-liked annulus, ovoid basidiospore, clavate to narrowly clavate cheilocystidia and a cutis pileus- and stipe covering. These characters distinctly distinguish the species from other characters. However, there are some species with yellow to yellowish green color that might be confused with Leucoagaricus flavus. Leucoagaricus orientiflavus Z.W. Ge is most similar to Leucoagaricus flavus by pastel yellow, light yellow, yellow, or yellowish-white fibrillose pileus; yellowish to yellowish-white stipe surface; white to yellowish-white annulus; amygdaliform basidiospores; clavate to narrowly clavate cheilocystidia and a cutis pileus- and stipe covering. However, Leucoagaricus orientiflavus is different from Leucoagaricus flavus on larger basidiomata (30–80 mm diam. of pileus), larger basidiospore (6.0–7.5 × 3.0–4.0 μm) and larger basidia (17–24 × 6–7.5 μm) (Ge 2010). Furthermore, analysis of nrITS sequences showed that Leucoagaricus flavus and Leucoagaricus orientiflavus were clustered, but not identical (Fig. 1). Leucoagaricus houaynhangensis is different in yellowish-green basidiomata with dark grey to black squamules on pileus and with appendiculate margin, basidiospores with a germ pore, and a trichodarm pileus covering (Sysouphanthong et al. 2018). The analysis of nrITS sequences showed they are not related together (Fig. 1).
Fig. 1 Phylogenetic tree based on Maximum Likelihood analysis of nrITS sequences. Bootstrap values ≥ 60% are indicated above the branches, and Bayesian posterior probabilities ≥ 0.95 are placed above after ML bootstrap values. GenBank accession numbers are given after species names. The new sequences are in bold. Agaricus bisporus (J.E. Lange) Imbach, Chlorophyllum molybdites (G. Mey.) Massee and Macrolepiota dolichaula (Berk. & Broome) Pegler & R.W. Rayner were used as outgroups.