Life habit: lichenized Thallus: effuse, rimose, cracked, 0.1-0.3 mm thick photobiont: Trentepohlia Ascomata: rounded or angular, immersed to erumpent or sessile, up to 1 mm wide disc: pale gray due to white pruina of calcium oxalate, underneath reddish brown; peripheric hyphal structures: with crystals between hyphae epihymenium: with pale brown gelatinous matrix, c. 15 µm (pruina on top ca. 20 µm) thick, sometimes with airfilled holes; hyphal tips not distinctly thickened, anastomosing, widely spaced and pale reddish brown, to 2 µm thick hymenium: hyaline to pale reddish brown, 45-60 µm tall; paraphysoids: branched and anastomosed, c. 1.5 µm wide; subhymenium: hyaline, 90-150 µm thick asci: clavate, of Arthonia type, 40-55 x 17-25 µm, 8-spored ascospores: persistently hyaline, 3-5-septate, narrowly obovate, 14-17 x 6-8 µm, septal ontogeny bi-directional, lacking a distinct epispore Pycnidia: not observed Chemical reactions: thallus and ascomata C- or C+ red; thallus and subhymenium I+ pale blue; ascomatal gels I+ blue rapidly turning red, KI+ blue; ascus tholus KI- Secondary metabolite: arthoniaic acid (and calcium oxalate crystals often present in ascomata and thallus). Substrate and ecology: on young twigs or on isolated, old broad-leafed trees, more common in humid areas, but avoiding direct rainfall and in habitats with somewhat lower humidities World distribution: widespread in the Mediterranean area parts of Europe and north Africa, and North America Sonoran distribution: southern California (Channel Islands and mainland), Baja California, and Baja California Sur. Notes: Arthonia pruinata is perhaps the most common Arthonia species in the Sonoran Desert area. It is also rather variable in its morphology. Its thallus color may vary from pale yellow to white. Arthonia pruinata, as accepted here, also containing morphs with a C- reaction, because no clear morphological characters of the mycobiont distinguish these forms and because the C+ red reaction can be variable or restricted to the apothecia or their surrounding thallus parts. Also specimens determined as A. gyalectoides or A. ochrolutea belong here. Arthonia nivea, with a more white appearance and lacking a C+ reaction, is known from the Galapagos Islands and is very similar to this species. The delimitation of other species with C- thalli, which key out here as well, such as Arthonia antillarum and the recently described Arthonia subantillarum, also needs further clarification. Arthonia antillarum is a subtropical to tropical species from coastal habitats on smooth bark of palms.