Pleuromyces hungaricus V. Papp, Dima & P.-A. Moreau, in Crous et al., Persoonia 40: 311 (2018)
Etymology. Name reflects the country (Hungary) where the species was collected.
Basidiocarp pleurotoid, pileus 5–8 mm diam, flabelliform, yellowish brown, densely covered with fibrillose squamules, turning ferruginous with age, dry, not hygrophanous. Lamellae ventricose, yellowish when young, rusty brown on dry specimens, edge serrulate, slightly darker. Stipe short, up to 1.5 × 3 mm, eccentric or lateral, concolorous with pileus or darker, flocculose-fibrillose, dry, solitary. Context yellow brown, spore deposit not noted. Basidiospores in side view (6.5–)7(–7.5) × (4–)4.5(–5) μm, Q = 1.4–1.8, mean 7.19 × 4.5 μm, Q = 1.6 (n = 30), ovoid to ellipsoid, somewhat amygdaliform in side view, smooth, thick-walled, yellowish brown, often with one or two guttules, apex usually blunt and subpored. Basidia 23–26 × 5–6 μm (excluding sterigma), subclavate, 2- or 4-spored. Cheilocystidia abundant, in clusters on lamella edge, variable in shape (heteromorphous, from slender fusiform to ± lageniform), not incrusted, thin-walled, hyaline, up to 85 μm long. Pleurocystidia not observed. Hymenophoral trama regular, made of slender hyphae 2–4.5 μm wide, pale, mostly smooth. Pileipellis a thin adpressed trichocutis made of 1–2 layers of hyphae 2.5–5.5 μm wide, coarsely incrusted, with cylindrical elements 35–60 μm long, mostly filled with oily droplets, terminal element usually rounded to slightly inflated at apex, thick-walled and smooth at apex. Pileitrama made of cylindrical hyphae 2–4 μm wide, pale, smooth or incrustate-zebrate, sometimes thickened and darker at septa. Clamp connections present at all septa.
Habitat: On large Fagus sylvatica log, in a lowland old growth beech forest (Vértes Mts, Hungary). So far only known from the type locality.
Index Fungorum number: IF824586
Notes: Based on a BLAST search of NCBIs GenBank nucleotide databases, the closest hits using the LSU sequence are Romagnesiella clavus (as Pachylepyrium sp., GenBank HQ832461; Identities = 1 333/1 361 (98 %), 2 gaps (0 %)), Phaeomarasmius fulvidulus (GenBank KF830080; Identities = 1 330/1 361 (98 %), 5 gaps (0 %)), Agrocybe pediades (GenBank DQ110872; Identities = 1 339/1 372 (98 %), 4 gaps (0 %)) and Galerina sp. (GenBank HQ827183; Identities = 1 341/1 374 (98 %), 8 gaps (0 %)). The closest hit by BLAST using the ITS sequence had highest similarity to an ‘uncultured fungus’ which was sequenced from soil in Illinois, USA (GenBank KX195359); the ITS2 of this environmental sample is very similar to Pleuromyces hungaricus (Identities = 323/327 (99 %), 2 gaps (0 %)). The second closest hit using the ITS sequence is Tubaria sp. (GenBank KY462443; Identities = 475/556 (85 %), 29 gaps (5 %)), with low query cover (85 %). Based on a discontinuous megablast search the closest hits by best query cover (100 %) are Flammulaster cf. carpophilus (as Phaeomyces dubiosus, unconfirmed, P.-A. Moreau unpubl. data; GenBank KF830099; Identities = 543/644 (84 %), 42 gaps (6 %)) and Crassisporium carbonicola (as Pachylepyrium carbonicola; GenBank LN714579; Identities = 541/645 (84 %), 39 gaps (6 %)). Pleuromyces hungaricus forms a distinct clade in our phylograms, well separated from other genera of Tubariaceae. Microscopical observations (spores smooth, thick-walled and subpored; pileipellis with coarsely incrusted hyphae) suggest closest affinities with species of Phaeomarasmius and Flammulaster pp. (F. muricatus/F. limulatus), but the weak differentiation of the pileipellis is a distinctive feature for species of these genera. The holotype of Phaeomyces dubiosus was not available for revision but its description shows strong affinities with P. hungaricus; however, distant lamellae and the pileipellis with branching and erected terminal hyphae suggest that it represents a distinct species of still unclear systematic position.