Description: Thallus crustose, continuous to rimose, ecorticate, pale yellowish to yellowish green, often surrounded by a thin, whitish to bluish black prothallus, forming small, usually <2 cm wide patches, soon dissolving into diffuse, pale sulphur yellow or yellow-green soralia. Soredia loosely aggregated, farinose, 10-25(-35) µm in diam. Apothecia rare, biatorine, 0.3-0.7 mm across, with a concave to convex, pale yellow. grey-violet or bluish black, often white-pruinose disc, and a concolorous or paler, at first raised and distinct, finally often excluded, usually non-sorediate proper margin. Excipular hyphae with globose to ellipsoid lumina, the outer rim of exciple and epithecium with small colourless crystals soluble in K, and varying amounts of a green pigment (K- , N+ purple), apothecia otherwise colourless and without crystals; hymenium 40- 50 µm high, I+ blue; paraphyses simple or sparingly branched in upper part, 1.0-1.5 µm thick at mid-level, the apical cells clavate or not thickened. Asci 8-spored, cylindrical-clavate, the tholus with a conical, blunt, a lacking or indistinct ocular chamber, and a distinctly conical axial body, the remainder of tholus I+ dark blue, darkest around the axial body. Ascospores (0-)1(-3)-septate, hyaline, ellipsoid, straight or slightly curved, 8-14 x 2.5-4 µm. Pycnidia rare, immersed, with a gaping ostiole, colourless except for a narrow bluish black zone around the ostiole (the pigment blue-green, K-, N+ purple). Conidia pyriform, 2-2.5 x l.5-2 µm. Photobiont chlorococcoid. Spot tests: thallus K- or K+ faintly yellow, C-, KC-, P+ red, UV-. Chemistry: thallus with fumarprotocetraric acid (major), plus traces of atranorin and protocetraric acid.
Note: on both broad-leaved and coniferous trees in forests and open woodlands in areas with an oceanic climate; widespread in Western Europe, with a few records from the northern rim of the Alps; to be looked for in Italy.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)