Pertusaria flavida (DC) J. R. Laundon

    Thallus crustose, thin, more or less verrucose, cracked, areolate, areoles convex, yellowish, yellow-greenish, pale grey-yellowish (rather similar to Lecanora sulphurea), seldom with a grey prothallus, covered with minute globose to cylindrical isidia, giving rise although hardly ever to pale yellow soralia. Apothecia very rare, 1 to 5 per wart, punctiform, brown, exciple thick. Ascii 4-8-spored. Ascospores narrowly fusiform, 60-100 x 25-40 µm. Thallus: C+ orange, K-, KC+ orange, Pd-, UV+ orange. Rare in our prospection area, on bark of alignment and ornmental trees, notably old oaks (Quercus) and  beech (Fagus). A special attention should be paid to the following C+ orange species that are difficult to distinguish: Lecanora expallens of which the thallus is sorediate, more yellow-greenish and UV-; Pyrrhospora quernea  usually more brown-yellow and fertile; Ochrolechia inversa  = Lecanora alboflavida which is sorediate.