Synonyms: Arthonia astroidea var. galactitella (Nyl.) H. Olivier; Arthonia epipastoides var. galactitella Nyl.; Arthonia oleandri Rabenh. in Bagl.
Distribution: C - Sar (Sundin 1999, Rizzi & al. 2011, Di Nuzzo & al. 2022).
Description: Thallus crustose, not lichenized, mostly endosubstratic and inapparent or forming greyish white patches. Apothecia arthonioid, rounded to subangular or rarely elongate, erumpent, 0.1-0.7 mm wide, 50-60(-100) µm tall, flat to slightly convex, dark brown to black, but often appearing grey-pruinose, being covered by remnants of the bark. Proper exciple poorly developed, olivaceous brown, with 2-3 µm thick hyphae, 10-25 µm wide; epithecium olivaceous brown, 5-15(-20) µm high; hymenium colourless or pale olive-green, 30-70 µm high, with granules, I+ blue turning wine-red, K/I+ blue; paraphysoids 0.5-1 µm thick, with 1-1.5 µm wide, olivaceous brown tips without distinct pigment caps; hypothecium colourless to pale olive, thin, 5-10(-20) µm high. Asci 8-spored, globose to broadly clavate, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, the apex with a K/I+ blue ring-structure, Arthonia-type. Ascospores 3(-4)-septate, often constricted at septa, with more or less equal loculi, narrowly ovoid to narrowly ellipsoid, 13-20 x 4-6 µm, with an epispore. Pycnidia c. 0.1 mm wide, the wall olivaceous brown. Conidia bacilliform, straight, (2-)3-6(-8) x c. 1 µm. Photobiont absent. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: thallus without lichen substances.
Note: a mainly Mediterranean-Atlantic, European species growing on the smooth bark of broad-leaved trees and shrubs. Perhaps non-lichenised. The synonymy with A. griseoalba (Nimis 1993: 82), and hence the earlier record from Lombardia, are not certain (see Sundin 1999).