[1] Found on the Galápagos Islands, it was formally described s a new species in 2019 by lichenologists Frank Bungartz and Adriano Spielmann.

The type specimen was collected from Santiago Island at an altitude of 362 m (1,188 ft), where it was found growing on an exposed vertical boulder.

The species epithet refers to its isidia, that are pustulate and readily distinguishable from those of the closely related P. tinctorum.

The lower surface has a broad, pale to deep brown, rhizine-free margin that is about 3–5 mm wide and blackens and sparsely rhizinate towards the thallus centre.

The rhizines are short, stout, and pale to dark brown or black, mostly simple, and rarely sparsely branched, often growing in clusters.