Description: Thallus crustose, endosubstratic or thinly episubstratic, thin, white to pale grey, sometimes with a pale orange tinge, often bordered by red-brown prothalline lines. Apothecia arthonioid, soon erumpent and sessile, round to narrowly ellipsoid or polygonal, rarely lobed, 0.2-0.7(-1.3) mm, with a reddish brown, flat to convex disc often densely covered by a white or red pruina, the marginal parts almost always bright red-pruinose, especially in young apothecia. Proper exciple poorly developed, of branched and anastomosing hyphae often incrusted by orange red granules; epithecium more or less inspersed with orange-red granules and/or colourless crystals, 20-20 µm high, K+ red; hymenium colourless to pale yellow, 60-70(-100) µm high, K-, I+ blue, K/I+ blue; paraphysoids branched and anastomosing above the asci, slightly sinuose, 1-1.5 µm thick, but often brown-walled and to 2.5 µm thick in epithecium; hypothecium colourless to yellowish or pale reddish brown, 25-40 µm high. Asci 8-spored, clavate to broadly clavate, semi-fissitunicate, the lateral walls thin, with a large apical dome, without K/I+ blue tholus structures. Ascospores (2-)4-7-septate, at first hyaline, then brownish, oblong ovoid, much larger at one end with progressively smaller cells, (15-)18-28(-33) x (5-)7-10(-11) µm. Pycnidia rare, immersed, the wall red-brown, K+ olive-grey. Conidia hyaline, simple, bacilliform, 3.5-6 x 0.8-1.2 µm. Photobiont trentepohlioid. Spot tests: red pigments K+ red-violet, dissolving. Chemistry: different unidentified anthraquinones.
Note: a mild-temperate, perhaps holarctic species found on Fraxinus, but also on trees with harder, more acid bark, such as Carpinus, Fagus and even Quercus ilex, in open, humid broad-leaved woodlands, e.g. along rivers; more frequent in the past, especially in northern Italy.
Growth form: Crustose
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)