Diploicia canescens
Thallus forming placodioid rosettes to 6 cm diam, often smaller and coalescing. Outer lobes radiating, thickly pruinose, closely contiguous, convex, pleated, 0.5-1 mm, wider at the margins, glaucous white. Lobe ends incurved, white-grey, verruculose and cracked-areolate towards the centre; soralia mainly laminal in irregular patches at first then coalescing; soredia green- white sometimes developing grey tips, finely granular.
Anamorph: pycnidial. Pycnidia more common on new lobes of fertile specimens, c. 0.04-0.50 mm diam., narrowly ‘U’-shaped in section, c. 0.24 mm tall, 60-70 µm wide; wall colourless; dark brown or green-black around the ostiole only. Conidia simple, rod-shaped 5-8 × 0.7 µm.
Teleomorph: ascomata apothecial. Apothecia 0.3-1 mm diam. locally common near the coast, rare inland; margin (true exciple) black with a marginal zone of crystals. Ascospores (8.5-)9-12(-15) × (4-)5.5-7(-7.5) µm.
Chemistry: Thallus C–, K+ yellow, KC+ yellow, Pd–, UV± dull orange (atranorin, chloroatranorin, diploicin and a xanthone).
Assessed as of Least Concern by Woods & Coppins (2012).
The neat rosettes with very distinct placodioid lobes which have a pleated appearance are easily recognised.
Resembles Solenopsora candicans, which is not sorediate, has a thalline exciple, 1-septate, colourless ascospores, different chemistry and is restricted to hard limestone. Caloplaca teicholyta is somewhat similar but smaller overall and with flatter marginal lobes.
Throughout lowland British Isles, but rarer and increasingly restricted to coastal districts in N. Britain.
On dry rocks and stonework, and dry bark in nutrient-enriched or calcareous habitats.