Distribution: N - Lomb (Zocchi & al. 1997), Piem (Griselli & al. 2003, Piervittori 2003, Piervittori & al. 1996b), VA (Valcuvia & al. 2000), Emil (Fariselli & al. 2020), Lig (Putortì & al. 1999b). C - Tosc (Tretiach & Ganis 1999, Putortì & al. 1999c, Brunialti & Frati 2010), Umb (Brackel 2015), Laz (Ravera 2006, 2006c). S - Cal (Puntillo 1996), Si (Brackel 2008b, 2008c).
Description: Thallus crustose, episubstratic, coarsely granular-powdery, more or less continuous to cracked, thin to thick, greenish to grey-green, entirely or partially sorediate. Apothecia lecanorine, 0.4-1(-1.5) mm across, sessile, slightly constricted at base, with a pale green-grey, grey-brown or pinkish brown, concave to slightly convex disc, and a thin, persistent, crenulate, partially sorediate thalline margin. Thalline exciple corticate (ecorticate in sorediate parts), 60-100 μm wide laterally; epithecium colourless to pale yellow-brown, with granules dissolving in K; hymenium colourless, 45-75 μm high; paraphyses branched and anastomosing, 1.5-2.5 μm thick, at mid-level, the apical cell to 3 μm wide; hypothecium colourless, 45-55 μm high. Asci 8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, broadly ellipsoid to ovoid, 9-14(-16) x 4-7 μm, thick-walled. Conidia thread-like, often curved, 12-22 μm long. Photobiont chlorococcoid. Spot tests: K- or K+ faintly yellow turning dirty red, C-, KC-, P+ rusty-red, UV+ dull orange. Chemistry: with variable amounts of usnic and fumarprotocetraric acids.
Note: this famous lichen, one of the most resistant against pollution, is very common in western Europe. Most of the Italian records, however, are dubious. An earlier record from Venezia Giulia (see Nimis 1993: 348), as well as that from Friuli by Badin & Nimis (1996) are wrong (vidi!), those from Sicilia by Grillo & al. (1996) and Grillo (1998) should be checked, and are not accepted here. In recent times this species is known with certainly only from geothermic areas of Toscana and Latium with emissions of sulphur dioxyde, where it is fairly abundant, and from Sicily. It is included in the Italian red list of epiphytic lichens as “Data Deficient” (Nascimbene & al. 2013c).
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)