Hymenagaricus siamensis J. Kumla, W. Phonrob, N. Suwannar & S., Lumyong sp. nov. Figure 2 MycoBank number: MB846482 Diagnosis:—Distinguishable from other Hymenagaricus species by pseudoparenchymatous tissue of the pileus squamules, and from H. nigroviolaceus by its larger basidiospores and smaller pileipellis cells. Etymology:—‘ siamensis ’ refers to the former name of the country “ Thailand ” where the holotype was collected. Holotype:— THAILAND, Chiang Mai Province, Muang District, Chiang Mai University, 18°48´03´´N 98°57´20´´E, elevation 333 m, on soil, 29 April 2020, J. Kumla & N. Suwannarach, SDBR-CMUNK1508. Gene sequences (from holotype):—OP836301 (ITS) and OP836385 (nrLSU). Description:— Basidiomata small. Pileus 22‒32 mm in diameter, paraboloid when young, hemispherical to convex with age; surface dry, brownish grey (9D3), pinkish white (8A2) to dull red (8B3), sometimes brownish grey (8D2), covered by brownish orange, to brown (6C6 to 6E8) plate-like scales, with minute scattered squamules towards the margin. Lamellae, l = 3–4 series, 1.5–2 mm wide, free, pinkish white (7A2) when young, dull red (8B3) with age, up to 5 mm wide, close, with 1‒2 tiers of lamellulae. Stipe 22‒30 × 2.5‒3 mm, central, cylindrical with or without bulbous base, hollow; surface white (8A1) becoming brown (6E5) when bruised, covered with white fine squamules or fibrils. Annulus membranous, whitish, superior, attached to upper third of stipe. Context up to 5 mm thick at pileus center, white (8A1) becoming pastel red (10A4) when bruised. Spore print dark brown (8F7). Odor and teste not determined. Basidiospores 6.5‒8.0 × 4.0‒5.0 μm, Q = 1.4‒1.88, Q m = 1.53 ± 0.11, ellipsoid to elongate, yellowish brown to brown, smooth, inamyloid, slightly thick-walled. Basidia 12‒20 × 7‒10 μm, clavate, hyaline, 4-spored; sterigmata up to 3 μm long. Pleurocystidia absent. Cheilocystidia abundant, 20.0‒42.0 × 9.0‒15.0 μm, varying in shape from subcylindrical to clavate, sometimes flexuose, hyaline, thin-walled. Hymenophoral trama regular to subregular, made up of hyaline, thin-walled hyphae, 4.0‒12.0 μm in diameter. Subhymenium layer composed of thin-walled, inflated, globose to subglobose cells, 6.0‒24.0 μm in diameter. Pileipellis a cutis of radially parallel, thin-walled hyphae, hyaline, 4.0‒10.0 μm wide. Squamules on pileal surface, pseudoparenchymatous tissues, subglobose to globose, thin-walled, with pale-brown to brown parietal and intracellular pigments, 4.0‒12.0 μm in diameter in chains. Caulocystidia absent. Stipitipellis made up of parallel, hyaline, thin-walled, 3.0‒7.0 μm wide hyphae. Stipititrama made up of parallel, hyaline, thin-walled, 8.0‒20.0 μm wide hyphae. Clamp connections absent in all tissues. Ecology and distribution:—Fruiting solitary or gregarious on soil in grassland. Known only from the type locality in northern Thailand. Additional specimens examined:— THAILAND, Chiang Mai Province, Muang District, Chiang Mai University, 18°48´1´´N 98°57´19´´E, elevation 350 m, on soil, 15 May 2022, W. Phonrob, SDBR-CMUWP038, gene sequences OP837533 (ITS) and OP836600 (nrLSU). Note:—The pseudoparenchymatous structure of the pileal squamules in H. siamensis was similar to those of H. nigroviolaceus Heinem. However, the smaller size of the pseudoparenchymatous cells (4‒12 μm in diameter) in H. siamensis differs from H. nigroviolaceus (12‒16 μm in diameter) (Heinemann 1985). A phylogenetic analysis of the combined ITS and nrLSU sequences confirmed that H. siamensis formed a monophyletic clade and formed a sister taxon to the African taxon, H. ardosiaecolor. However, the shorter spores (5.8‒6.5 μm long) of H. ardosiaecolor clearly separate it from H. siamensis (7.0‒8.0 μm long) (Heinemann 1956; Tibuhwa & Mwanga 2014). A pairwise nucleotide comparison of ITS data indicated that H. siamensis differed from H. ardosiaecolor by 1.97% (15/759 bp).

Published as part of Kumla, Jaturong, Suwannarach, Nakarin, Khuna, Surapong, Phonrob, Wiphawanee & Lumyong, Saisamorn, 2023, Hymenagaricus siamensis (Agaricaceae, Agaricales), a novel macrofungus from northern Thailand, pp. 135-144 in Phytotaxa 585 (2) on pages 139-140, DOI: 10.11646/phytotaxa.585.2.4, http://zenodo.org/record/7681598