Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.

Thallus: orbicular to irregular, up to 3 cm across, usually firmly adnate also at the lobe tips lobes: overlapping to separate, up to 1 mm wide, ± truncate, eciliate upper surface: shining to dull, whitish gray to pale brownish gray, occasionally with a white pruina, sorediate soredia: in laminal, crateriform soralia that sometimes are ± capitate, starting as cracks in the upper cortex upper cortex: paraplectenchymatous medulla: white lower cortex: paraplectenchymatous, the lower part dark brown to black lower surface: black except for the paler tips, with black rhizines Apothecia: not seen in Sonoran material, elsewhere rare, sessile; margin: often sorediate ascospores: brown, 1-septate, Pachysporaria-type, 21-27 x 9-12 µm Pycnidia: not seen in Sonoran material, elsewhere rare, immersed conidia: subcylindrical, 4-6 x 1 µm Spot tests: upper cortex and medulla K+ yellow, C-, KC-, P+ yellow Secondary metabolites: upper cortex with atranorin; medulla with atranorin and zeorin. Substrate and ecology: growing on rocks in ± open situations World distribution: SW North America, Mexico, East Africa and South America Sonoran distribution: a species from moist canyons at intermediate elevations at scattered localities SE Arizona south through the Sierra Madre Occidental in Chihuahua, Sonora and Sinaloa and the southern mountains of Baja California Sur. Notes: The black lower surface, the crateriform to capitate soralia and the convex lobes makes P. erumpens easily separated from other Physcia species in the area. From its close relative, P. sorediosa, it is separated by the type of soralia as P. sorediosa has marginal, small and delimited soralia.