Pertusaria pustulata (Ach.) Duby
Thallus crustose, thin, continuous or minutely cracked, pale grey-yellowish, pale grey-greenish, whitish with dispersed more or less flattenned tubercules. Apothecia 0.5-1 mm diam., on top of tubercules usually 1 or 2-3 per tubercule or on fertile warts, emerging from the thallus surface through a simple linear slit or branched or stellate slits, discs blackish or dark brown, more or less visible at the bottom of slits. Ascii 2-spored. Ascospores broadly ellipsoid with double wall, 50-140 x 25-45 µm. Photobiont: chlorococcoid. C+ pale yellow, K+ yellow, KC+ yellow, Pd+ yellow to orange. Rather uncommon, on smooth or slightly rugose bark of alignment trees in gardens and parks.
Cf. Pertusaria hymenea of which the thallus is K-, apothecia more numerous with more apparent apothecia at the bottom of fertile warts, and above all with ascii 8-spored. See also Pertusaria leioplaca of which ascii is 4-spored and the thallus C-, covered with crystals.
N.B. Frequently parasitized species.
N.B. Do not confuse in the field with Thelotrema lepadinum whose ascoma has a different structure.