Thallus: foliose, tightly adnate to loosely appressed, up to 8 cm in diam.; lobate lobes: sublinear, contiguous, plane, 2-6 mm wide, apices truncate to rounded upper surface: light bluish gray to greenish gray, turning buff in the herbarium with age; lobe margins usually tinged brown, sometimes shiny, rarely with white maculae; smooth to rugulose or sometimes ridged and foveolate, occasionally also scrobiculate, never pruinose pseudocyphellae: rare to frequent, white, rounded to at least partly linear or branching; soon developing into soredia, reaching soredia: granular to very coarsely granular, capitate to irregular, white to greenish white (rarely discoloring to gray around the soralia), often originating from laminal pseudocyphellae or cortical cracks, but also partly marginal and sometimes crescent-shaped; up to 1 mm diam., but joining into larger, linear or irregularly shaped structures (rarely developing as stacked, corticate "isidioid" structures) lower surface: white to pale tan or light grayish tan, sometimes darker toward the margin, rhizines, simple to weakly forked, thin, concolorous with the lower surface Apothecia: rare, laminal, up to 4 mm wide; disc: dark rusty-brown; exciple: gray, concolorous with the thallus and often with numerous pseudocyphellae and/or soredia asci: Lecanora-type, clavate, 8-spored ascospores: ovoid to ellipsoid, 12-16 x 6-9 µm, wall 1 µm wide Pycnidia: occasionally present, immersed to elevated, 0.1-0.2 mm wide, black conidia: filiform, 6.5-14 x 1 µm Spot tests: upper cortex K+ yellow, C-; medulla K-, C+ red Secondary metabolites: upper cortex with atranorin (most frequently scored as "minor" or less commonly in "trace" amounts), medulla with lecanoric acid (major). Substrate and ecology: widespread on bark and wood of various angiosperms (Acer, Alnus, Bursera, Lycium, Prosopis, Prunus, Quercus, Robinia, Sabal), conifers (Abies, Juniperus, Picea, Pinus, Pseudotsuga) and on rocks from 240-3200 m World distribution: probably cosmopolitan, known from North and Central America, Europe and Asia Sonoran distribution: widespread in most of the region including Arizona, southern California (including the Channel Islands), and Durango, Sinaloa, Sonora, Chihuahua, and at higher elevations in Baja California and Baja California Sur. Notes: Most specimens from North America previously called Punctelia subrudecta (Nyl.) Krog belong here. Aptroot (2003) provides notes and a key for distinguishing P. perreticulata from the European species, P. subrudecta and P. ulophylla. The morphology of the soredia in this taxon is highly variable throughout the greater Sonoran region. Specimens from Arizona and southern California tend to have finely granular soredia, produced commonly both on lobe margins and on the laminal surface. In some Mexican material, especially from the Sierra Madre Occidental, the soredia are primarily laminal, often coarsely granular and sometimes interspersed with stacked, corticate, "isidioid" structures. I (Egan) noted several specimens in which typical, laminal, sorediate patches occurred on one side of a thallus, while corticate, stacked "isidioid" structures could be found at other locations with intermediate morphologies scattered throughout. One specimen from Sonora, Mexico, with these "isidia-like" structures, at the United States National Herbarium (US) was annotated as P. punctilla. I measured filiform conidia of 8-10 µm on this specimen, well within the range for P. perreticulata, and not the shorter, unciform type indicated for P. punctilla (Adler 1997). This specimen appears to represent one extreme end of the range of variation of soredia found in P. perreticulata.