Summary:
Features include 1) cushion-shaped fruitbodies on willow wood, 2) fruitbody color purplish gray becoming black, the surface with conspicuous perithecial mounds, 3) dull reddish brown granules beneath the surface and between perithecia, with KOH-extractable pigments greenish olivaceous, and 4) spores that are elliptic, nearly equilateral, and brown, with a continuous to discontinuous germ slit much less than spore-length, and with dotted ornamentation on the entire epispore. Miller, J.H.(1) considered H. macrosporum P. Karst. to be a synonym of H. mammatum (Wahlenb.) P. Karst. [== Entoleuca mammata (Wahlenb.) J.D. Rogers & Y.-M. Ju].

Microscopic:
SPORES 22-31 x (7.5) 8.5-11 microns, elliptic, "nearly equilateral, with narrowly rounded ends, sometimes with one or both ends broadened", "brown to dark brown, unicellular", "with straight, continuous to discontinuous germ slit much less than spore-length", "perispore indehiscent in 10% KOH", epispore smooth, finely dotted beneath epispore; ASCUS 190-260 x 13-16 microns in total, spore-bearing part 130-180 microns long, the stem 50-90 mm long, with apical ring lightly blueing in Melzer''s reagent, discoid, 0.18-0.2 microns high and 0.5-0.6 microns broad; PERITHECIUM 100-400 microns in diameter and 300-400 microns high, spherical to obovoid, (Ju)

Notes:
Hypoxylon macrosporum has been found in BC, WY, Russia, and Switzerland, (Ju).

Synonyms and Alternate Names:
Hyphoderma laeta P. Karst.
Hypoxylon vogesiacum var. macrosporum J.H. Mill.