Hebeloma humosum - A non-current Hebeloma species

According to our data Hebeloma humosum is not the name of a current valid species of Hebeloma. Our database has records for the following similar names

Full name: Hebeloma humosum S. Imai (1938)
Published: Journal of the Faculty of Agriculture, Hokkaido Imperila University 43 (2): 230 (1938)
Taxonomic status: This is a Agrocybe.
Notes: Current name: Agrocybe humosa (S. Imai) Beker & U. Eberh. Mycol. Prog. 21: 458 (2022)

Types: JAPAN: Hokkaido, Ishikari, Sapporo (approx. 43.1714°N, 141.3155°E, alt. approx. 5 m a.s.l.) on decomposed litter, wood in garden, 25 Jun. 1935, S. Imai (Lectotype. herbarium acc. no. SAPA10000033 (No. 384), HJB1000376). Lectotype designated by Eberhardt et al., Mycol. Prog. 21: (2022) page 458 (MBT10001751).

Original diagnosis: Gregarium vel subcaespitosum. Pileo 3-7 cm. lato, hemisphaerico vel gibboso, dein subplano, laevi, submelleo, raro hygrophano, margine nudo; carne albida, sapore subacri, odore subfarinaceo; lamellis sinuatis vel emarginatis, eonfertis, primo griseis, dein cinnamomeis, acie albo-fimbriatis; stipite 2.5-6 em. longo, 3-10 mm. crasso, aequali, basi incrassato, albido vel subconcolori, fibrilloso, apiee subfurfuraceo, cavo; vela nullo; sporis in eumulo cinnamomeis, ellipsoideis, 10-1l.5 x 7-8 µ, asperulis; eystidiis subfusoideo-ventricosis.

English translation: Gregarious or subcaespitose. Pileus 3-7 cm broad, hemispherical or lumpy, then subapplanate, smooth, pale honey, rarely hygrophanous, with smooth margin; context whitish, taste slightly acrid, smell almost mealy; lamellae sinuate or emarginate, crowded, at first grey, then cinnamon, edge white, fimbriate; stipe 2.5-6 cm long, 3-10 mm thick, cylindraceous with enlarged base, whitish or subconcolorous, fibrillose, apex slightly scurfy, fistulose; veil none; spores in mass cinnamon, ellipsoid, 10-11.5 x 7-8 μ, slightly roughened; cystidia subfusoid-ventricose.

Commentary: There are two collections (SAPA 10000033 and SAPA 10000034) at SAPA, marked as syntypes. The material selected as lectotype fits the original description by Imai (1938) who gives the spore size as 10–1 l.5 × 7–8 μm. This matches most closely with our measurements for SAPA 10000033 (10.1–13.3 × 6.9–9.5 μm, av. 11.1 × 7.7 μm, av. Q 1.44) for which we also have an ITS sequence. The other collection, SAPA 10000034, also appears to be an Agrocybe, with elliptical, smooth, pale brown, thick-walled spores, with a conspicuous germ pore, on av. 13.3 × 8.9 μm (11.8–14.7 × 8.0–10.1 μm), with av. spore Q 1.50. The cheilocystidia are ventricose; the pleurocystidia are similar. The spores of A. smithii (Watling and Bigelow 1983) and of the type of A. flexuosipes (Eberhardt et al., in press) are around the same size (11–13.5 × 6.5–8 μm respectively 12.1 × 7.7 μm) as the spores of A. humosa. Whereas the spores of the former two species are described as (indistinctly) ornamented, the spores observed in the lectotype of A. humosa are smooth. The spores of A. putaminum are described as 10–13 × 5–7 μm, thus somewhat narrower than in A. humosa, but also smooth (Maire 1913). Both morphology (without annulus, yellowish tones in pileus, spore size, and prominent germ pore) and sequence data suggest that A. humosa is closely related with A. smithii, A. putaminum, or other related species (Nauta 2005). Another closely related species is A. flexuosipes (Eberhardt et al., in press). However, none of these species matches the type of A. humosa sufficiently closely for synonymization. Japanese name: Ki-wakafusa-take (Imai 1938).