Description: Thallus crustose, episubstratic, white to pale grey, thin to rarely verrucose, sorediate, with or without a thin whitish prothallus. Soralia more or less delimited when young, but soon confluent into a continuous subleprose crust covering the whole thallus, except a c. 1 mm wide marginal zone. Soredia whitish grey, often with a yellowish tinge, rarely greenish yellow, contrasting with the colour of thallus, farinose, 25-40 μm in diam. Apothecia rare, lecanorine, sessile, 0.5-1 mm across, with a yellowish brown to brown, often slightly pruinose disc, and a slightly to strongly crenulate, c. 0.1 mm thick, often partly sorediate thalline margin. Thalline exciple indistinctly corticate, with large crystals visible under polarized light; epithecium brown, granulose; hymenium colourless, 50-75 μm high; paraphyses mostly simple, conglutinated, 1.5-2 μm thick, the apical cells very slightly swollen; hypothecium colourless. Asci (6-)8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, broadly ellipsoid to rarely subglobose, 12-15 x 8.5-10 μm. Photobiont chlorococcoid., Spot tests: thallus K+ yellow, C-, KC- P- or P+ pale yellow, UV- or UV+orange. Chemistry: atranorin, chloratranorin and zeorin (major), sometimes substances of the pulvinic acid complex (in species with yellowish soredia).
Note: usually on roadside trees in sites with strong eutrophication; most common in Western Europe and widespread in the Alps (outside Italy), but rarely collected; to be looked for, especially in Tyrrhenian Italy.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)