Ascomata subcuticular. In vertical section upper wall of unopened ascomata up to 50-60 µm thick. Inner and outer parts of the wall comprise dense, black tissue with no obvious cellular structure, otherwise mostly of dark brown, angular cells. Along the future line of opening, in the inner part of the wall, is a group of thinner-walled, paler cells. In opened ascomata the upper wall is 50-70 µm thick near the opening slit, becoming abruptly thinner about half way to the base of the wall, comprising mostly brown to pale brown, thick-walled, angular cells, but with a patch of dense, black tissue on the inner edge of the wall near the ascomatal opening, and along outermost 5-10 lam of the wall. Exposed face of the broken upper wall is lined with a compact layer of cylindric, hyaline, thin-walled cells. Lower wall 5 µm thick, of 1-2 layers of brown to pale brown, angular cells.

Paraphyses 1.0-1.5 µm diam., circinate at apices, extending 10-15 µm beyond asci. Asci 120-160 x 11-14 µm, clavate-stipitate, tapering to small, truncate apex, sometimes with tiny apical pore, 8spored, spores confined to upper half of ascus. Ascospores 35-45 x 3-4 µm (average 41.2 x 3.1 µm), in face view cylindrical-bifusiform, rounded at apex, tapering near base, 1-1.5 µm wide at central constriction, 0-septate, surrounded by gelatinous sheath.

Conidiomata subcuticular. In vertical section lenticular in shape, upper wall 5 µm thick, of brown material with no visible cellular structure. Lower wall of 2-3 layers of brown to dark brown, thick-walled, angular cells. Lower wall is lined with hyaline, thin-walled cells, on which the conidiogenous cells are held. Conidiogenous cells 7-20 x 2-3 µm, flask-shaped, with a broad base and narrow apex, sympodial proliferation, often with more than one conidium held at apex. Conidia 3-4 x 1 µm, cylindric, straight, 0-septate, hyaline.

ETYMOLOGY: albo = white, rubrum = red; refers to the white or bright red zone of cells lining the ascomatal opening.

H. alborubrum can be difficult to distinguish from two other species often found on the same leaves, H. campanulatum and H. carinatum. The most distinctive macroscopic feature of H. alborubrum is the zone of bright, white or red cells lining the opening slit. Microscopically this species has distinctly bifusiform ascospores, while the other two species have ascospores which are only slightly, if at all, constricted. The anamorph of H. alborubrum also differs in having conidiogenous cells which proliferate sympodially rather than percurrently, and in lacking any wall thickening at the conidiogenous locus.

See also notes under H. carinatum and H. campanulatum.