Lecanora barkmaniana

Thallus determinate, rimose-verruculose, to 5 cm diam, pale grey, warts 0.1–0.2 mm diam., to 0.1 mm thick, angular to rounded, corticate, surrounded by a pale grey prothallus; soralia initially 0.1–0.3 mm diam., punctiform, soon coalescing to form irregular patches and usually covering most of the centre of the thallus; soredia granular, 15–30 µm diam., in a dense mass, pale yellow to yellowish grey, contrasting in colour with the thallus. Apothecia rare, sessile, but sometimes appearing immersed between the soredia; 0.4–0.7 mm diam., margin strongly crenulate, pale grey to pale yellow, often sorediate; disc concave, pale brown to buff, with some isolated pale grey granules, not pruinose; hymenium colourless to pale brown, 50–75 µm high, hypothecium colourless and copiously filled with chlorococcoid algae and large (15–50 µm) colourless angular packets of crystals. Ascospores 12–15 × 8.5–10 µm, narrowly ellipsoidal. Pycnidia not observed. Thallus C–, K+ yellow, Pd± yellow; thalline margin Pd–, UV+ pale orange (chloratranorin, zeorin and atranorin).

A sorediate member of the Lecanora subfusca group (in its very broad sense), when sterile superficially resembling L. compallens, L. expallens and L. farinaria , but differing in its chemistry. The pale yellow (not yellow-green) tinge of its soredia is diagnostic. Also somewhat resembles Buellia griseovirens which contains norstictic acid, K+ red (crystals) and often has darker grey soralia. The recently discovered Lecanora variolascens (q.v.) is similar but has heavily pruinose disks and paler and delimited soralia.