Thallus subcuticular, algiferous, in round patches, to 1.5 mm diam., margins effigurate and crenulate, with one or a few and often a number of pycnidia, sometimes confluent, usually dispersed over a 5 mm broadzone at the margin of the under surface of the leaf, wrinkled or with small depressions, pale greenish-grey with a slight bluish tinge or sordid grey (dead specimens white), prothallus absent. Perithecia 0.2-0.35 mm diam., convex, black, shining, naked, base immersed. Paraphyses branched and anastomosing c. 1 µm thick. Asci ± cylindrical 45-70 × 7-10 µm, when mature rather thin-walled. Ascospores uniseriate or biseriate, 1- or 3-septate, ± fusiform, 12-17 × 4-5 µm. Pycnidia often numerous, two different types being found, one to 0.15 mm diam., convex, black, shining with bacillar macroconidia, 1-septate, 12-13 × 3.5-4 µm, the other to 0.03 mm diam., convex, black, shining, microconidia fusiform, simple, 4 × 1.5 µm.