This unprepossessing crustose lichen has a thin, smear-like, pale greenish-grey to grey-white crustose thallus (becoming greyer on long-storage in the herbarium) dotted with half-immersed, small black perithecia, c. 0.3mm diam., sometimes with a slightly paler zone around the ostiole apex, which are at first covered by a thin thalline layer. The thallus can sometimes appear immersed in the bark. Accurate identification requires critical and careful microscopic examination of the photobiont, asci and ascospores (see ‘similar species’ below).
Microscopic examination to establish the identity of the photobiont (Trentepohlia), the form of the asci (thickened tholus with ocular chamber) and the ascospore-type (fusiform, (3-)7(-(9)-septate) is essential in order to distinguish it from various similar-looking genera, such as Anisomeridium, Celothelium, Porina, Rhaphidicyrtis & Thelenella, and other species of Strigula.
S.stigmatella has been found but once growing epiphytically in Wales, on an old Quercus in a relic fragment of upland broadleaved woodland at Hafod, near Pont-rhyd-y-groes, E of Aberystwyth, in 2000 (Chambers, 2000).