Lecanora praepostera

Thallus rimose, often uneven, effuse, areoles convex and often raised in clusters, white to yellow-white; prothallus zoned white and blue. Apothecia (0.3–) 0.5–1.5 (–1.8) mm diam., dispersed or in small groups, sessile, somewhat constricted below; thalline margin well-developed, crenulate to crenate, persistent, green-grey or as the thallus; disc pale brown to dark brown or black, often piebald, flat to slightly convex, sometimes white-grey pruinose; epithecium colourless or olivaceous, ± interspersed with granules, K+ red (with abundant red needle-like crystals); hymenium 70–90 µm tall; paraphyses 1.5–2 µm diam., unbranched or sparsely branched, anastomosed below, apices 2–3 µm diam., slightly swollen. Asci 50–65 × 16–18 µm, elongate-clavate. Ascospores (8–) 11–15 × (5–) 6–8 µm, ellipsoidal. Conidia 22–31 × ca 1 µm, thread-like to curved. Thallus C–, K+ yellow→red (crystals), Pd+ orange-red, UV– (atranorin, norstictic acid).

A distinctive member of the L. subfusca group, easily recognized by its ecology and K+ yellow→red reaction. See also Glaucomaria swartzii.