Ascocarps in vertical section subcuticular. Upper wall 10-15 µm wide near outside edge of ascocarp, 30-50 µm wide near ascocarp opening, comprising mostly dark tissue with no obvious cellular structure. Layer of 10-20 x 3-4 µm, cylindric, hyaline to pale brown cells developing across exposed face of broken upper wall. Lower wall a single layer of brown, slightly thick-walled cells. In upper wall of immature ascocarps near centre of ascocarp is an area of thin-walled, hyaline cells extending from the inside, almost to the outside edge of the wall.

Paraphyses 1-1.5 µm diam., undifferentiated or circinate at apex, extending 10-15 µm beyond asci. Asci 90-135(-160) x 7.5-9 µm, subclavate, tapering to rounded apex, wall not thickened at apex, 8-spored. Ascospores 50-75 x 1.5-2 µm, 0-1 septate, with narrow gelatinous sheath. Pycnidia in vertical section subcuticular, upper wall 3-5 µm wide, comprising dark tissue with no obvious cellular structure. Lower wall 5-10 µm wide, of dark brown, thick-walled cells. Conidiogenous cells lining lower wall, solitary, 6-10 x 1.5-2.5 µm, flask-shaped, proliferating both sympodially and percurrently, wall at conidiogenous loci slightly thickened, remains of thickening visible following proliferation. Conidia 3-4 x 0.8-1 µm, cylindric, 0 septate, hyaline.

ETYMOLOGY: atrum = black; refers to black anamorph pycnidia.

NOTES: L. atrum is macroscopically similar to L. cf. richeae and both species are found on the same hosts, often on the same leaves. L. atrum can be distinguished by its larger, black pycnidia.