Rhizocarpon santessonii Timdal

Thallus cracked to areolate, consisting of scattered or confluent patches in the thallus of Tremolecia atrata; discrete patches usually up to 5 mm diam.; hypothallus indistinct or lacking; areolae up to 0.5 mm diam., medium grey when young, later becoming greyish yellow, dull, irregularly rounded to angular, plane to weakly convex; medulla KI–. – Apothecia up to 0.7 mm diam., black, epruinose, orbicular to angular, plane or becoming more or less convex and immarginate; excipulum brownish black, K+ red; hypothecium brown, K–; hymenium colourless; epihymenium brownish black, K+ red; no crystals or granules in the apothecia; ascospores 8 per ascus, 1-septate, remaining colourless or becoming unevenly brown with age, 12–15 × 6–8 µm. – Conidiomata not seen.

Unknown yellow pigment, probably related to rhizocarpic acid, in the upper cortex; spot tests: PD–, K–, C–.

Parasitic in the thallus of Tremolecia atrata, mainly on rocks rich in iron minerals. In the boreal zone and in mountains. Sparingly collected.

The greyish yellow thallus, the 1-septate, long colourless ascospores, and the obligate parasitism on Tremolecia atrata is diagnostic. The poorly known R. parvum is also described from this host species, but differs, e.g., in being bright yellow (rhizocarpic acid), strongly amyloid in the medulla, and in forming evenly dark brown ascospores.