Thallus: continuous to fissured, with thin to moderately thick verrucae; margin: definite, often zonate upper surface: whitish gray to greenish gray, margins entire to weakly zoned, epruinose soredia: farinose, restricted to verrucae or coalescing and spreading onto ruptured thallus surface Apothecia: no fertile specimens seen Pycnidia: not seen Spot tests: K- or K+ yellow to red, C-, KC+ rose-violet, P- or + yellow to red, UV- Secondary metabolites: picrolichenic and ± protocetraric acids (both major); ± conprotocetraric and subpicrolichenic acid (both minor). Substrate and ecology: on deciduous trees, such as Aesculus, Quercus or conifers, and also occasionally on siliceous rocks in shady places World distribution: widely distributed in the Northern Hemisphere Sonoran distribution: in coastal areas of southern California and Baja California and scattered in inland mountains up to 1800 m in Arizona and Chihuahua. Notes: Pertusaria amara is characterized by small, but coalescing soralia with farinose soredia, a relatively thin, grayish thallus and the presence of the picrolichenic acid chemosyndrome and thus a bitter taste. It is hardly confused with any other Pertusaria species occurring in western North America, except P. albescens. The distinction of both species is discussed above.