The
thallus of
Astrothelium solitarium is , smooth, shiny, and continuous, covering areas up to in diameter. It is olive-green and less than 0.1 mm thick, surrounded by a dark brown
prothallus line about 0.1 mm wide. This species does not induce
gall formation on the host bark. Ascomata are spherical, measuring 0.3–0.5 mm in diameter, and appear solitary or in groups of 2–4, erumpent from the bark. They are black, partly with irregular thallus coverage, and not in . The wall is black all around, with a thickness of up to about 30
μm.
Ostioles are apical, not fused, flat, and dark brown, often surrounded by a pale brown ring. The is not with oil globules.
Asci contain 8 ascospores, which are
hyaline, 3-septate, , and measure 33–36 by 10–11 μm.
Pycnidia were not observed in this species. In term of standard
chemical spot tests, the thallus and medulla are UV− and K−.
Thin-layer chromatography shows no
secondary substances detected. ==Habitat and distribution==