Pluteus saisamorniae N. Wannathes, J. Kumla & N. Suwannarach
Pileus 25–30 mm diam., plano-convex with subumbonate, surface dull, dry, fibrillose, fully covered by dark brown minute squamules, margin not striate; disc greyish brown (6F3-4), margin light brown (6D7-8) or paler. Context thin, soft, pinkish white. Lamellae free, close, with 2 series of lamellulae, ventricose, broad (5 mm), pinkish white (9A2), with light brown lamellar edge. Stipe 28–30 _ 2.5–3 mm cylindrical, central, fistulose, surface dull, dry, minutely dotted brownish toward the base, greyish orange (5B5) overall. Basidiospores (5–)6–7 × 4.5–6 µm (x = 6.20 _ 0.47 _ 5.05 _ 0.38, Q = 1.08–1.30, q = 1.23 _ 0.05, s = 2) subglobose to broadly ellipsoid, smooth, hyaline, inamyloid, thickwalled. Basidia 16–19 × 4–5 µm, clavate to cylindrical, thin-walled, 4-spored, unclamped. Lamellar edge sterile. Cheilocystidia common, 28–44 × 6–8 µm, fusiform to narrowly fusiform, hyaline to pale yellow, inamyloid, thin- to thick-walled (up to 1.5 _m). Pleurocystidia common, 17–54 × 10–20 µm, fusiform, lageniform to broadly lageniform, hyaline to pale yellow, inamyloid, thick-walled. Pileipellis a trichohymeniderm, with terminal cell 30–46 × 4–7 µm, narrowly fusoid, cylindrical, acute or round toward the apex, with pale yellow cytoplasmic pigment, thin-walled. Pileus trama interwoven, hyphae 5–16 µm diam., cylindrical, inflated, hyaline, inamyloid, non-gelatinous, thin-walled. Lamellar trama convergent, hyphae 3–10 µm diam., cylindrical, hyaline, inamyloid, non-gelatinous, thin-walled, with numerous refractive oleiferous hyphae interspersed. Stipitipellis a cutis with caulocystidia, hyphae 5–18 µm diam., cylindrical, pale yellow to hyaline, inamylid, non-incrustrated, non-gelatinous, thin-walled. Caulocystidia numerous, group in clusters, fusiform with acute apex, clavate to cylindrical, brown, thin-walled. Clamp connections absent in the examined tissues.
Habit, habitat, and known distribution: Solitary on decaying wood in bamboo thickets, deciduous dipterocarp-oak forests in Thailand.
Additional specimen examined: THAILAND, Phitsanulok Province, Pibulsongkram Ratjabhat University, herbal garden beside Faculty of Science and Technology, on decaying wood, September, 2020, NW 1567.
Notes: Pluteus saisamorniae is characterized by relatively small basidiomata that appear plano-convex with a broad umbo pileus, fully covered with dark brown minute squamules, light brown lamellar edges, greyish orange stipe covered with brown granules, subglobose to broadly ellipsoid basidiospores with mean dimensions of 6.2 × 5.1 µm, abundant with cheilo- and pleurocystidia displaying fusiform to lageniform shape, a trichohymeniderm pileipellis with fusiform to cylindrical terminal cells, clustered brown caulocystidia, and the absence of a clamp connection.
Pluteus granularis Peck, the species described as being from the USA, is macromorphologically similar to P. saisamorniae since they both form a relatively small, brown granular surface of basidiomata. However, they are noticeably different according to their microscopic morphology. As such, P. granularis forms elongate basidiospores with mean dimensions of 6.5 × 4.8 µm, possesses apical mucilage or with forked appendages as pleurocystidia, and displays thin-walled broadly rounded apex cheilocystidia. A phylogenetic study indicated that the P. granularis SF 20 clade was associated with P. saisamorniae with
notable distances. The new species is also morphologically similar to P. umbrosus and P. umbrosoides in terms of the color and surface of the pileus. It has lamellar edges with a dark color and the basidiospores are similar in size and shape. However, both species differ by having larger basidiomata as well as having distinctly longer terminal cells of both the pileipellis (65–180 µm long) and caulocystidia (74–230 µm long) Pluteus podospilloides shares some morphological features such as color, shape, and surface of the pileus. and stipe. It also has dark brown gill edges but differs by having smaller basidiomata with a pileus of 9–12 mm, a stipe of 10–13 × 1–1.5 mm, smaller basidoispores with mean dimensions of 4.8 × 4.4 µm and with a hymeniderm to epithelium pileipellis. According to the phylogenetic analyses of ITS, P. saisamorniae is closely related to P. pantherinus Courtec. & M. Uchida, which has been described from Japan. However, the latter species is distinct by its larger basidiomata (pileus 50–60 mm diam. and stipe 60–80 × 5 mm), as well as by appearing fibrous and irregularly dotted with large and small white spots on the ocher background of the pileus, with white stipe and globose to subglobose basidiospores of 5.5–6.5 × 5–6 µm.