Thallus: foliose, moderately to loosely adnate, ± circular, 4-10 cm diam., fragile and easily breaking apart, irregularly lobate lobes: irregular, 3-10 mm wide, sinuous in the axils, continguous to slightly imbricate; apices: rotund; margin: entire, eciliate upper surface: pale yellowish greenish or bluish gray (whitish gray in herbarium) with pale yellow patches where the cortex has disintegrated, dull to shiny in younger portions, smooth but becoming strongly rugulose submarginally, in part white pruinose soredia: granular, arising from coarse dactyls which break open apically forming pustules; isidia: absent medulla: pale yellow to yellow-orange with a ± continuous algal layer lower surface: black, smooth to rugulose, moderately to sparsely rhizinate; rhizines: simple to sparsely furcated, with a narrow, brown, erhizinate marginal zone Apothecia: not seen Pycnidia: not seen Spot tests: cortex: K+ yellow, C-, KC-, P-; medulla: K-, C-, KC-, P- or pigmented area turning darker yellow in K Secondary metabolites: cortex with atranorin (major), chloroatranorin (minor); usnic acid (minor/trace), medulla with secalonic acid A (minor), secalonic acid B (minor), secalonic acid C (minor), butlerin F (minor), unknown secalonic acid derivatives (minor). Substrate and ecology: on acidic rocks (including rhyolite) in moist to somewhat open oak-conifer forests World and Sonoran distribution: along the Sierra Madre Occidental from SW Chihuahua to SW Durango adjacent Sinaloa. Notes: Parmotrema xanthopustulatum closely resembles the Venezuelan species, Parmotrema enteroxanthum Hale, as both have a yellow medulla and develop similar pustules or open dactyls with an intense yellow or yellow-orange interior. However, P. enteroxanthum differs in developing lobulate-dissected margins, in being sparingly ciliate and in containing medullary salazinic acid whereas P. xanthopustulatum has entire margins, lacks lobules, cilia and salazinic acid. Parmotrema xanthopustulatum could also be confused with Parmotrema flavomedullosum Hale, as the latter species has a much more robust, coriaceous thallus, coarse, irregular, submarginal soralia rather than open dactyls and contains medullary gyrophoric acid.