Description: Thallus crustose, thin, corticate, grey, often with a bluish tinge. Soralia often (but not always) present, grey-green to blue-grey, circular and typically crater-like, erose, 0.1-0.3 mm diam., deriving from the rupture of convex vesicles and delimited by a raised thalline rim, with granular soredia. Apothecia biatorine, 0.3-0.5(-0.8) mm across, sessile, with a flat to convex, epruinose, brown disc and a thin, usually finally excluded, paler brown margin; an ephemeral thalline margin is often found at the base of young apothecia. Proper exciple brownish in outer part, colourless within, K-; epithecium almost colourless to pale brown, without crystals, K-; hymenium colourless, K/I+ blue; paraphyses lax, septate, simple to forked, 1.5 μm wide at base, the apical cells 3-4(-6) μm wide; hypothecium colourless. Asci 8-spored, clavate, functionally unitunicate, apically thickened with a broad internal beak, the inner part of apex and external cap I+ blue, Teloschistes-type. Ascospores 2-celled, polarilocular, hyaline, ellipsoid, 10-14 x 4-6(-8) μm, the equatorial thickening (“septum”) 4-8 μm. Photobiont chlorococcoid. Spot tests: thallus and soredia K-, C-, KC-, P-, UV-. Chemistry: thallus and apothecia without lichen substances.
Note: a temperate, perhaps holarctic species found on isolated deciduous trees, probably extinct in most of the Po-plain, but elsewhere not uncommon in orchards. For further details see Arvidsson & Martinsson (1993).
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)