After having redefined narrower limits to the genus Hypoxylon Bull. (Ju & Rogers, 1996), Rogers & Ju (1996) reinstated the genus Entoleuca in order to accommodate Hypoxylon mammatum (Wahlenberg) J. H. Miller.  Hypoxylon mammatum was the combination proposed by Miller (1961) for a fungus known to be a pathogen of numerous trees including Alnus, Betula, Populus and Salix, in northern temperate regions of America and Europe. This name was long accepted and used by most mycologists, although Pouzar (1985) suggested it could be transferred to Entoleuca Syd., while Laessøe & Spooner (1994) considered Entoleuca as a synonym of Rosellinia De Not.

Ju and Rogers (1996) excluded H. mammatum from their concept of Hypoxylon basing on the following characters which do not fit it: in H. mammatum the stroma is strongly carbonaceous and lacks granules yielding KOH-extractable pigments, ascal apical apparatus is higher than broad, ascospores have a germ slit on the concave side and bear a cellular appendage when immature and perithecia are embedded in a massive white to grey brown tissue. Moreover, its anamorph is referable to Geniculosporium Chesters & Greenhalgh, a feature which supports its affinities with other Xylariaceous genera such as Nemania S. F. Gray, Kretzschmaria Fr., Rosellinia De Not. and sessile forms of Xylaria Hill ex Schrank rather than with Hypoxylon. On the other hand, basing on molecular studies and phylogenetic analysis, Granmo et al. (1999) assume that affinities of E. mammata rather lie with Hypoxylon sect. Annulata.

After a comparison with the type species E. callimorpha Syd., Rogers & Ju (1996) assessed it would be better accomodated in Entoleuca. A detailed discussion about the relationship of E. mammata (Wahlenberg: Fr.) Rogers & Ju with other Xylariaceous genera is given in their publication, along with the distinctive characters of the genus: erumpent multiperitheciate stromata with a strongly carbonaceous surface, ostioles coarsely papillate, massive white tissue beneath and between perithecia, anamorph in nature on specialized pegs that rupture the overlying bark and a virulent bark pathogen activity.

Entoleuca mammata is widespread in northern America and Europe, with a tendency to be much more frequent in hemiboreal and boreal areas (Granmo et al., 1989, Mathiassen, 1989, Mathiassen, 1993, Petrini & Müller, 1986). Therefore its morphology and biology are well documented, unlike E. callimorpha which is known from only two collections from U. S. A., one of them being immature (Rogers and Ju, 1996). According to these latter authors, E. mammatum differs from E. callimorpha in having larger ascospores (19-)22-28(-33) x 9-10(-12) µm versus (14.5-) 17.5-20.5(-22) x (6.5-) 7-9.5 µm. Fresh collections of E. callimorpha and culturing data would allow a better delimitation of these both species.