Marasmius maoershanensis T. Bau & W. N. Hou 2026, sp. nov.

Marasmius maoershanensis T. Bau & W. N. Hou sp. nov. Figs 5 G – J, 13 Diagnosis. It is characterized by a grayish-white pileus with a black central depression and shallow or indistinct grooves, lacrymoid basidiospores, and Rotalis - type cheilocystidia and pileipellis cells. Etymology. “maoershanensis” refers to the type locality, Mao’er Mountain, Shangzhi City, Harbin City, Heilongjiang Province, China. Description. Basidiomata small. Pileus 0.3–0.6 cm diam., initially subconical, becoming plano-convex to nearly plane; center with a distinct umbilicate depression; depression margin grayish-white (RAL 7044), slightly raised and forming a ring; depression interior black (RAL 9004); surface initially dark gray (RAL 7045), becoming pale gray (RAL 7035), roughened, with shallow or indistinct grooves; margin straight to slightly upturned. Context white (RAL 9003), very thin. Lamellae free, with a collar-like structure, sparse, equal, narrow, 1–2 mm broad, L = 12–15, white (RAL 9003). Stipe 1–2.4 cm long, 0.5 mm thick, filiform; surface black (RAL 9004), nearly smooth; base directly attached to habitat with black (RAL 9004) rhizomorphs, some rhizomorph apices dark gray (RAL 7047). Basidiospores (5.2) 5.8–7.2 (7.5) × (2.6) 2.8–4 (4.2) µm, Q = (1.61) 1.61–2.34 (2.5) [1.87 ± 0.22], lacrymoid, thin-walled, hyaline, inamyloid. Basidia 15–23 × 4.6–6.9 µm, short-clavate, smooth, thin-walled, hyaline, 4 - (rarely 2 -) sterigmate. Basidioles 16–20 × 5–6.8 µm, subfusiform, with acute apex, smooth, thin-walled, hyaline. Cheilocystidia Rotalis - type, 11–17 × 5.3–14 µm; main body spherical to short-clavate, hyaline; apex bearing 6–12 short spines, spines 3.5–4.5 µm long, pale brown. Pleurocystidia not observed. Lamellar trama interwoven; hyphae 3.8–5.4 µm wide, thin-walled, hyaline, weakly dextrinoid. Pileipellis hymeniform, composed of Rotalis - type cells, 12–20 × 9–15 µm; main body subglobose, hyaline to brown; apex bearing 7–10 small spines, spines 1.4–2.7 µm long, yellowish-brown. Pileus trama composed of interwoven, smooth, thin-walled, hyaline, dextrinoid hyphae 3–3.9 µm wide. Stipitipellis composed of smooth, thick-walled, dark brown hyphae 4–4.8 µm wide. Clamp connections present in all tissues. Typus. CHINA • Heilongjiang Province, Harbin City, Shangzhi City, Mao’er Mountain, on rotten leaves in broad-leaved forests, 5 July 2025, 45°18'46"N, 127°33'18"E, alt. 300 m, W. N. Hou, FJAU 69415 (H 25070510), Holotype. Additional materials examined. CHINA • Inner Mongolia Autonomous Region, Tongliao City, Horqin Left Wing Rear Banner, Daqinggou National Nature Reserve, on rotten leaves in broad-leaved forests, 16 July 2023, 42°48'06"N, 122°10'28"E, alt. 229 m, T. Bau & Q. R. Liu, FJAU 69416 (R 23779). Inner Mongolia Autonomous Region, Tongliao City, Horqin Left Wing Rear Banner, Daqinggou National Nature Reserve, on rotten leaves in broad-leaved forests, 16 July 2023, 42°48'02"N, 122°10'36"E, alt. 222 m, T. Bau & W. N. Hou, FJAU 69414 (H 230790). Jilin Province, Changchun City, Jingyuetan National Forest Park, on rotten leaves in broad-leaved forest, 12 July 2026, 43°45'31"N, 125°28'11"E, alt. 309 m, W. N. Hou, FJAU 69457 (H 26070802). Jilin Province, Changchun City, Jingyuetan National Forest Park, on rotten leaves in broad-leaved forest, 12 July 2026, 43°45'31"N, 125°28'10"E, alt. 309 m, W. Sun, FJAU 69458 (SW 26070801). Notes. Ma. maoershanensis and Ma. capillaris Morgan both present a convex pileus with an umbilicate depression. However, the pileus of the present species is uniformly pale gray to grayish-white, whereas that of Ma. capillaris is pale brown with a grayish-white central area. Furthermore, the grooves on the pileus surface of this species are shallow or indistinct, differing from the prominent, deep grooves of Ma. capillaris (Harper et al. 1883; Gilliam 1976; Mustafabayli and Aghayeva 2019). This species is closely related to Ma. tubulatus and Ma. wisteriae. It differs from Ma. tubulatus, which has a pileus distinctly radially sulcate toward the center and pruinose, a stipe surface with fine longitudinal striations, and larger basidiospores (7–9 × 3–4 µm) (Petch 1947). Ma. wisteriae can be readily distinguished by its brownish-orange to whitish pileus (Antonín et al. 2014 b). In addition, the two species exhibit clear differences in micromorphology. Ma. wisteriae produces clavate to fusiform basidiospores up to 14 μm in length, whereas the present species has lacrymoid basidiospores with a maximum length of only 7.5 μm. This species can be readily confused with Ma. rotula (Scop.) Fr., which differs by possessing 12 prominent grooves on the pileus surface (Fries 1838).