Full name: Hebeloma fragilipes Romagn. (1965)
Published: Bull. Trimestriel Soc. Mycol. France 81 (3): 341 (1929) ["1965"]
Taxonomic status: Heterotypic synonym of H. album
Notes: A proposal to conserve the name H. fragilipes against the earlier name H. domardianum,was made in Beker et al., Taxon 64: 383 (2015) and ratified by the Nomenclatural Committee (May, 2017). The name Hebeloma fragilipes has been widely used for this taxon, which appears under this name in many field guides and taxonomic texts. Hebeloma domardianum (Hebelomina domardiana) with white (hyaline) spores is atypical for this taxon.

Types: FRANCE: Pinede du Mont-Po.nr Chantilly (Oise) (approx. 49.19°N, 2.17°E, alt. approx. 120 m a.s.l.) on calcareous, grassy, mossy soil under Betula sp., 7 Sep. 1963, H. Romagnesi (R63-179) (Holotype. herbarium acc. no. PC 0090764, HJB1000031).

Original diagnosis: Pileo 2,3–5 cm, simili pileo H. crustuliniformis cum figura tum colore pallida, sed tenuiore, fragiliore, viscidulo. Stipite 2,5–7 cm x 1–7 mm, tam fragili ut e terra extractus semper frangatur, intus canaliculato, cylindrato, saepe torto, paulum inferne leviter incrassato, candido, leviter deorum fuscescente, fibrilloso, in apice farinoso. Carne fragilissima, pallide brunnea, odore rapaceo, sapore amarissimo. Lamellis stipatis, 4–6 mm solum latis, adnatis sinuatis, segmentiformibus vel linearibus, pallide bruneolis, deinde argillaceis, acie pruinosa ac plorante. Sporis in curnulo umbrinis, amygdaliformibus, 9–11,5 x 5,5–6,2 μm, pallide luteis, manifeste verrucosis. Pilis acierum claviformibus, brevioribus quam apud crustuliniforme, 40–50 x 5–8 μm (ad apicem). Epicute ex hyphis filiformibus, clavulatis (x 2,5–3,5 µ. in superiore parte) constante, cute hyphis cylindratis, 4–6 μm latis, et subcute specie pseudoparenchymatica manifestissima, pigmento pallido membranas colorante. – Sub betulis, locis graminosis vel muscosis.

English translation: Pileus 2.3–5 cm, similar to that of Hebeloma crustuliniforme in stature and pale colour, but smaller and more fragile, slightly viscid. Stipe 2.5–7 cm × 1–7 mm, so fragile that it is difficult to pick it out of the soil without breaking it, inner part hollow, cylindrical, often twisted, towards base slightly broadening, white, towards base slightly browning, fibrillose, mealy at apex. Context very brittle, pale brown; smell raphanoid; taste very bitter. Lamellae crowded, only 4–6 mm broad, adnate-sinuate, segmentiform or linear, pale brownish, then clay-coloured with pruinose, weeping edge. Spores in mass umber brown, amygdaloid, 9–11.5 × 5.5–6.2 μm, pale yellow, strongly verrucose. Marginal hairs claviform, short, like in H. crustuliniforme, 40–50 × 5–8 μm (at apex). Epicutis made of filiform hyphae, clavuliform (x 2.5–3.5 μm at the upper part) constant, cutis hyphae cylindrical, 4–6 μm broad, and subcutis distinctly pseudoparenchymatical with pale coloured membranal pigment. Under Betula in grassy and mossy places.

Commentary: Given the shape of its cheilocystidia (primarily clavate-lageniform) and the spores mainly O3 and D2, Hebeloma fragilipes belongs to H. sect. Denudata, subsect. Clepsydroida. This species most likely corresponds to ICG22 of Aanen & Kuyper (2004). As explained above, no ICG number was assigned by Aanen & Kuyper (1999) as the related strain never formed dikarya; hence, the authors were reluctant to assume that it represented an ICG. It appears that in their studies they had only one collection of this taxon. Within this subsection, the dextrinoidity, usually D2 (weakly but distinctly dextrinoid), places H. fragilipes in the group of four species with H. cavipes, H. ingratum and H. pseudofragilipes. It is worth noting that all four of these species (along with H. hiemale) regularly exhibit median-thickening in the cheilocystidia walls; Hebeloma fragilipes appears to most strongly exhibit this character. Also note that H. cavipes and H. pseudofragilipes often have a majority of spores D3, a character that does not seem to appear in either H. fragilipes or H. ingratum. Hebeloma fragilipes usually has a very pale pileus, at most with cream or yellow tones, which separates it from H. cavipes and H. ingratum, both of which tend to have more brown in the pileus, particularly at the centre. This taxon is very close, both morphologically and molecularly, to H. pseudofragilipes but it may be distinguished from this latter species through consideration of the cheilocystidium averages: If A/B is less than 1.4 and A is at most 6.5 and B is greater than 5 then it is H. pseudofragilipes, and if any of these conditions fail then it is H. fragilipes, i.e. the apex of the cheilocystidium within H. fragilipes is usually more swollen and the basal part of the cheilocystidium less swollen giving a larger ratio between the two. For Hebeloma fragilipes this ratio is always greater than 1.25. However, in cases of doubt only sequences of V6, V9, RPB2 or Tef1a will permit a clear decision. Romagnesi focused on the fragile stipe that this species often (but not always) exhibits and the occasional median thickening of the cheilocystidium wall (which does appear to be a consistent character of the cheilocystidium, but only present here and there). Macroscopically, this species may be mistaken for a small member of the H. crustuliniforme complex or a large H. helodes. However, macroscopically, it does not have the floccose stipe that is present in these other species and, microscopically, the cheilocystidium shape will normally readily separate H. fragilipes from these taxa. We suspect that Hebeloma fragilipes has in the past been confused with the other species that exhibit median thickening of the cheilocystidium wall and that collections labelled H. fragilipes may equally represent H. pseudofragilipes, which appears more common than H. fragilipes, or H. ingratum, with a usually darker pileus and smaller spores or, perhaps, also paler collections of H. cavipes. We have studied the holotype of Hebeloma domardianum. At the time when we combined Hebelomina domardianum into Hebeloma, we had been unable to locate the type material and believed it to be lost. Since that time, with help from the staff at MPU, we were able to find the holotype (‘hidden’ among Tricholoma). Unfortunately, we have been unsuccessful in our attempts to obtain PCR products from this material. However, based on a morphological study of this material we are confident that this material represents Hebeloma fragilipes. Since the basionym for Hebeloma domardianum was published in 1935, many years before Hebeloma fragilipes, this name should take precedence. But the material of Hebeloma domardianum is hardly representative of this taxon and the name Hebeloma fragilipes is now well established. Consequently we proposed that the name Hebeloma fragilipes be conserved. However, we have discovered that H. album (Peck) is an older name for this taxon (and for H. domardianum) and consequently the fact that this name was conserved against H. domardianum is no longer relevant.