Mycena hongfengensis L. N. Liu 2025, sp. nov.

Mycena hongfengensis L. N. Liu sp. nov. Figs 6, 7 Diagnosis. Differs from M. castaneicola in having smooth cheilocystidia, dermatocysts present in the pileipellis, and stipitipellis hyphae. Holotype. China • Hunan Province, Xiangxi Tujia-Miao Autonomous Prefecture, Jishou City, Hongfeng Forest Park, 28°16'23"N, 109°40'45"E, elev. 230 m, 22 April 2024, ZhuXiang Liu, JSU 114 (collection number JD 114). Etymology. Refers to the Hongfeng Forest Park, from where the holotype was collected. Description. Pileus 2–5 mm diam., hemispherical when young, becoming nearly campanulate or plano-convex with age, with a centrally flattened depression, margin smooth, sulcate, translucently striate, pure white (4 A 1), white pubescent. Context pure white, thin, fragile. Lamellae 16–18 reach the stipe, with 1–2 tiers of lamellulae, narrowly free, pure white (4 A 1), concolorous with the sides. Stipe 15–40 × 0.1–0.5 mm, almost equal or slightly expanding towards the base, hollow, white (4 A 1) to greyish-white (4 B 1), pubescent or puberulous, base swollen. With a not well-developed basal disc, covered with white hirsute. Odor and taste not distinctive. Basidiospores (6.2) 6.3–7.6 (7.7) × (3.4) 3.5–4.9 (5.2) μm, Q = 1.4–2.0, Qm = 1.7 ± 0.2, oblong or pip-shaped, hyaline, thin-walled, amyloid. Basidia 8–16 × 4–8 μm, two- and four-spored, clavate, hyaline. Cheilocystidia 11–43 × 5–9 μm, obpyriform, fusiform, ventricose, filiform, with a long neck, up to 25 μm, with an acute and occasionally branched apex, hyaline. Pleurocystidia absent. Pileipellis hyphae 2–13 μm wide, hyphae cylindrical, densely covered with warts and cylindrical excrescences, 1.0–6.0 × 1.0–2.0 μm, with irregularly cylindric to strangulated dermatocysts, 63–200 × 7–20 μm, walls 1.0–2.0 μm, greenish grey (1 C 2). Hyphae of the stipitipellis 1.0–6.0 μm wide, smooth, dermatocysts numerous, clavate to pyriform, 50–320 × 5–20 μm, long, flexuous, filiform, simple, and tapering towards the apex. Clamp connections are absent in the basidia, pileipellis, and stipitipellis hyphae. Habitat. Gregarious on decaying leaves of deciduous trees. Known distribution. Xiangxi Tujia-Miao Autonomous Prefecture, Hunan Province. Additional materials examined. China • Hunan Province, Xiangxi Tujia-Miao Autonomous Prefecture, Jishou City, Hongfeng Forest Park, 28°16'26"N, 109°40'48"E, elev. 255 m, 22 April 2024, ZhuXiang Liu, JSU 121 (collection number JD 121). Notes. Mycena hongfengensis is characterized by its pure white basidiomata, free lamellae, oblong to pip-shaped spores, and the presence of dermatocysts in the pileipellis and stipitipellis. According to the Maas Geesteranus classification, the new species could belong to an uncertain position. When we first found this specimen in the field, we thought it might be a member of either sect. Saccharifera or sect. Amparoina. All species in two sections have a white basidiomata, pubescent pileus and stipe, and stipe with a basic disc (Maas Geesteranus 1983; Desjardin 1995; Na and Bau 2019 b). However, M. hongfengensis can be clearly distinguished from other species through microscopic characteristics. The presence of dermatocysts in the pileipellis and stipitipellis is the most important characteristic for separating M. hongfengensis from species of sect. Sacchariferae and sect. Amparoina. Mycena castaneicola T. Bau & Q. Na is the most similar to the new species in the macroscopic characteristics, but the microscopic features differ significantly. Sequences labeled as Mycena sp., which were extracted from the host Myrica rubra (Lour.) Siebold & Zucc. and originated from Japan, along with the sequence of M. hongfengensis, formed a well-supported lineage in the phylogenetic analysis (BS / BP = 100 / 1.00). We are very fortunate to have collected Mycena sp. in the field. However, we only collected one tiny basidioma; the microscopic structures we observed are not comprehensive. Both taxa have white pileus and pubescent pileus and stipe. The uncertain Mycena species differs in pileipellis, which has “ acanthocysts ” that are spherical or vesicular and covered with spines. These features can be used to easily differentiate the two species.