Diagnosis. Differs from P. tinctorum by the coarse, pustulate isidia, leaving distinct circular scars when breaking off the thallus surface.
Type: Ecuador. Galápagos: Isla Santiago, along the trail from Bucanero to Jaboncillos, ~ 3 km SE of Bucanero, 0°10′52″S, 90°48′33″W, 362 m alt., transition zone, open woodland of large Burseragraveolens and smaller Psidiumgalapageium trees, grassland and basalt boulders in between, N-exposed, on vertical front of basalt boulder, sunny, wind- and rain-exposed, 22-Mar-2006, Bungartz, F. 4624 (CDS 28711 – holotype!).
Description.Thallus corticolous, lignicolous, less frequently saxicolous; uppersurface whitish gray to ivory, dull, not shiny, epruinose, emaculate, not cracked, but soon conspicuously wrinkled, gnarled granular; abundantly isidiate; isidia laminal, irregularly granular-globose, pustulate, developing from the very irregular central surface and where eroded leaving conspicuous circular scars, isidia not dark tipped, but concolorous with the thallus, eciliate; lobes broad, moderate-sized to large, 0.6–10(–11) mm wide, ± rotund, axils incised, margins eciliate; lowersurface with a broad, pale to deep brown, erhizinate, ~ 3–5 mm wide margin, blackening and sparsely rhizinate towards the thallus center; rhizines short, stout, pale to dark brown or black, mostly simple, rarely sparsely branched, often growing in clusters; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with lecanoric acid [P–, K–, C+/KC+ bright red (C and KC reactions are evanescent and must be examined under continuous observation); UV–].
Etymology. Named for its pustulate isidia that are distinctly different from the otherwise similar P. tinctorum.
Ecology and distribution. Currently known only from the type and presumably endemic, collected on an exposed vertical boulder in the transition zone of Santiago Island.
Notes. The dull surface of this species, soon becoming conspicuously gnarled, developing into granular-globose isidia that break apart into pustulate soredia, is very characteristic. These pustules do not resemble the cylindrical isidia of P. tinctorum, with which P. pustulotinctum shares the same chemistry. Throughout the Neotropics, P. tinctorum may be considered one of the most common species in the genus; the name, however, has nevertheless often been applied far too widely. Some of the morphological variation previously included within P. tinctorum has already been distinguished as distinctly different. Marcelli et al. (2007), for example, described a new species as P. eitenii Marcelli & Benatti. It is not isidiate but forms marginal soralia with granular soredia. Although the soredia of P. eitenii are coarse, they are ontogenetically clearly distinct from the pustulate structures characteristic of P. pustulotinctum. These structures are initially globose isidia, which secondarily break apart. Occasionally the whole structure breaks off, leaving distinct circular scars. These circular scars can also be observed in P. internexum, which, however, has much finer, dark-tipped isidia and a different chemistry.