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, very variable in size, up to 5 cm diam., ± closely adnate lobes: very variable in width, up to 2 mm, but in specimens on twigs and rocks not exceeding 1 mm, lobes sometimes imbricate, sometimes distinctly separated, saxicolous material with convex lobes; tips: usually truncate, eciliate upper surface: white to dark gray, sometimes with white dots (c. P. aipolia), with or without pruina, some specimens on rocks with shiny upper surface and darker gray margins, sorediate soredia: mainly in laminal, crateriform to almost capitate soralia, usually orbicular, starting as cracks in the upper cortex; in some specimens predominantly marginal soralia which may develop into rounded patches at angles of lobes upper cortex: paraplectenchymatous medulla: white lower cortex: a type between proso- and paraplectenchymatous lower surface: white to pale brownish gray, with white to dark gray rhizines Apothecia: rare, up to 1.5 mm diam.; margins: sometimes with sorediate ascospores: brown, 1-septate, Pachysporaria-type, (16-) 19-24.5 (-26) x (7.5-) 8.5-10.5 (-11) µm Pycnidia: rare Spot tests: cortex and medulla K+ yellow, C-, KC-, P+ yellow Secondary metabolites: upper cortex with atranorin; medulla with atranorin, zeorin and sometimes unidentified substances. Substrate and ecology: growing on rocks in open situations World distribution: known from South America and East Africa, but may also occur in most tropical and subtropical areas Sonoran distribution: scattered localities at coastal to intermediate elevations from Arizona, southern California, Baja California, Baja California Sur, Chihuahua, Sonora and Sinaloa. Notes: Physcia poncinsii similar to forms of P. caesia, but there are distinct differences in the lobes. P. poncinsii has convex lobes with truncate tips but P. caesia has flat to convex lobes with rounded tips. P. poncinsii also is somewhat similar to P. nashii which is, however, smaller, has narrower lobes and different chemistry (no zeorin).