Anamorph: not known.
Teleomorph: stromata spreading around branches and often extending for several cm, forming between the cortical parenchyma and the periderm, splitting and pushing aside the outer bark layers. Stromata black, with an outer crust 50-90 µm thick composed of angular thick-walled olivaceous cells 5-10 µm diam, mixed with shrivelled remains of substratum cells. Below the crust, the cells are smaller, lighter in colour and merge into a layer 200-300 µm thick of small, orange-red degraded substratum cells, interspersed with richly branched golden brown hyphae 2-4 µm diam. Stroma surface with patches of black velvety or woolly subiculum, with numerous columnar extensions 200-350 (-600) high and 60-270 µm diam., bluntly conical or truncated, then almost cylindrical or clavate, often ± compressed laterally and elliptical in section. Extensions composed of ± perpendicular, thick-walled and strongly melanized agglutinated hyphal tissue with cells to 18 µm long and 6-12 µm diam.
Ascomata perithecia, 300-400 µm diam., immersed in the stroma with emergent strongly papillate cylindric-conical necks 80-130 µm diam. and to ca 500 µm long. Peridium membranous, 12-20 µm thick above and ca 30 µm thick at the base, made up of several layers of heavily compressed, irregularly polygonal, translucent dark brown cells to 15 µm diam., with an inner hyaline layer of angular tissue. Ostiole lined with hyaline periphyses. Interascal tissue of very thin-walled hyphae 1-1.5 µm diam., soon evanescent to form a mucilaginous mass. Asci numerous, the spore-bearing part 20-28 (-32) x 5-7 µm, clavate, thin-walled, not fissitunicate, with a short to elongate evanescent stalk, the apex truncate with a well-developed cylindrical refractive ring that does not blue in iodine, 8-spored. Ascospores usually arranged biseriately, 7-10 ( -12) x 1.2-1.8 µm, rod-shaped, rounded at both ends, straight or slightly allantoid, hyaline, aseptate, often with two rather indistinct oil droplets, without a gelatinous sheath or appendages.
Not formally assessed, but considered to be Vulnerable/D2 by Evans et al. (2006). It has a scattered and restricted distribution, and its conspicuous nature and likely pathogenicity might indicate that it is not substantially under-recorded. More survey work would be advisable.
The common Diatrype stigma also forms extensive black stromata that push aside the outer bark layer of Betula spp., but the surface of the stroma is almost smooth and dotted with minute sulcate ostioles.
Known only from Betula species, usually developing on/in dead attached branches. The discomycete Dencoeliopsis johnstonii is thought to be restricted to stromata of Xenotypa aterrima, but their nutritional relationship is not clear.
In GB&I, known from several locations in VC17 Surrey and a single site in Northern Ireland. It was at one time present in the Berwick-upon-Tweed area (information extrapolated from a record of Dencoelioipsis johnstonii) but it has not been seen in that area in recent times.
Known from boggy areas of lowland heath.
Pathogenicity has not been researched into, but the species must have an initial endophytic stage and could cause premature mortality of infected branches.