Description: Thallus crustose-placodioid, densely white-pruinose, forming 1-4 cm wide, usually orbicular rosettes, sometimes several thalli merging and covering larger surfaces. Lobes radiating, contiguous, slightly convex, rounded and often slightly broader and fan-shaped at ends, with laminal soralia at first orbicular and developing from small bulges, finally bursting and irregular; soredia finely granular, greenish white to grey; lower surface usually pale, attached by medullary hyphae. Upper cortex pseudoparenchymatous, formed by vertically arranged hyphae, filled with minute crystals not dissolving in K; medulla usually white, lax. Apothecia very rare, lecideine, black, up to 1 mm across, laminal. Epithecium granular, brown-black, K-; hymenium colourless, hypothecium brown or brown-black. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-septate, brown, thick-walled, ellipsoid, 9-15 x 4-8 µm, Dirinaria-type, with ontogeny of type B (apical inner wall-thickenings produced before septum formation). Pycnidia laminal, immersed. Conidia hyaline, simple, rod-shaped, 5-8 µm x 0.7-1 µm. Photobiont chlorococcoid. Spot tests: cortex K+ yellow, C-, KC+ pale yellow or yellow-orange, P-; medulla K-, C-, KC-, P-, UV+ dull orange. Chemistry: cortex with atranorin and often chloroatranorin, medulla with diploicin (major), and other minor compounds.
Note: a rather western and southern lichen in Europe, found on a wide variety of substrata including base-rich or eutrophicated bark, calciferous sandstone, and limestone, sometimes also found in underhangs of calcareous rocks protected from rain; rare in northern Italy and along the eastern side of the Peninsula. All records from Piemonte (see Nimis 1993: 277) and Valle d'Aosta (see Piervittori & Isocrono 1999: 119), being dubious, are not accepted here. See also note on D. subcanescens.
Growth form: Crustose placodiomorph
Substrata: bark and rocks
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)