Buellia griseovirens

Thallus ± immersed and thin to thick, smooth to wrinkled or ± rimose-cracked, grey-white, pale grey-brown to greenish grey; a grey- black prothallus sometimes evident (especially on smooth bark); soralia mostly discrete, 0.15–0.4 mm diam., often crowded, a few becoming confluent, pale green-grey to ash-grey, often with a blue tinge but pale yellow when abraded; soredia 15–26 µm diam., pigmented hyphae brown, K–, N–; medulla I–. Apothecia have been very rare, but their occurence may be increasing, 0.4–1 mm diam., superficial; disc flat to convex, black, not pruinose; exciple K+ yellow; epithecium brown; hymenium 110–120 µm tall, without oil droplets; hypothecium brown. Ascospores (13–) 15–28 × 7–13 µm, submuriform, grey to brown. Thallus and soralia C± yellow, K+ yellow→red (crystals), KC–, Pd+ yellow or yellow- orange, UV– (atranorin, norstictic acid and traces of other substances of the stictic acid complex).

A common and variable species. The K reaction is often weak and production of red crystals should be checked in microscope preparations. The C+ yellow reaction is unexplained but often characteristic. Often confused with Mycoblastus caesius (UV+ bright white), Violella fucata (K– & Pd+ orange-red) and Xylographa vitiligo (K+ yellow, Pd+orange). Buellia arborea (K+ yellow & Pd± yellow) is similar when sterile.

Fertile thalli have become much more frequent in the New Forest, Hampshire and recent years, and should be looked out for elsewhere.