Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: irregular, rarely orbicular, up to 3 cm diam., gray to gray brown, usually with distinctly darker lobe tips lobes: variable width, but rarely exceeding 1 mm in width, eciliate upper surface: whitish gray to cream-colored, mostly without pruina and maculation, margins of the lobes often paler, sorediate soredia: in marginal soralia that are mostly at the lobe tips and lip-shaped upper cortex: paraplectenchymatous medulla: white lower cortex: prosoplectenchymatous lower surface: pale, with white to black rhizines Apothecia: uncommon, not seen in Sonoran material, elsewhere up to 2 mm diam.; margin: usually sorediate ascospores: brown, 1-septate, Physcia-type 16-24 (-28) x 6-10 µm Pycnidia: not seen in material from the area, elsewhere rare conidia: subcylindrical, 4-6 x 1 µm Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow; medulla K-, C-, KC-, P- Secondary metabolite: upper cortex with atranorin. Substrate and ecology: growing on acid rocks in open situations World distribution: widely distributed species in boreal and temperate areas of the Northern Hemisphere. It is also known from the southern cold temperate part of South America Sonoran distribution: known from montane to upper montane habitats in Arizona, southern California and Sonora. Notes: The lip-shaped soralia, the usually nonpruinose thallus with K- medulla and the prosoplectenchymatous lower cortex are the main characters of the species. In the cold temperate areas, where it mainly grows, no other species is known which could be confused with P. dubia, but in warm temperate areas it might be confused with P. tribacia, which has a shiny upper cortex and paraplectechymatous lower cortex.