Lecanora campestris (Schaer.) Hue
Thallus crustose, usually circular, to 10 cm diam., more or less markedly verrucose, areolate, areoles dispersed of contiguous, sometimes rather thin and tenuous (in that case, apothecia seem isolated or in small clusters scattered on the substatum), dull whitish, pale greyish, pale grey-greenish, greyish, pale grey-bluish, prothallus whitish, fimbriate, sometimes inconspicuous (N.B. on horizontal surfaces the thallus is often partially covered with algae and potentially turns to olivaceous or brown-greenish). Apothecia numerous, most often clustered towards the center, sessile, 0.5-1.5 mm diam., disc flat to slightly convex, smooth, brown-red, dark brown-red, brown-orange sometimes tinged greenish, often blackish when old, exciple determinate, whitish, smooth and more or less flexuose or contorted. Ascii 8-spored, 45-65 x 14-20 µm according to our measures. Ascospores simple, broadly ellipsoid, 10-15 x 6-9 µm according to literature, 11-15 x 6-9 µm according to our measures. Photobiont: Trebouxia. C-, K+ yellow, KC+ yellow, Pd+ very pale yellow and often hardly visible. Not especially maritime but common, on slightly acid rocks or on walls near the seashore, sometimes on cement, concrete, asphalt. Often confused with Lecanora horiza, which is K-, mainly corticolous but may also be found on old walls. On calcareous rocks, a white thallus ecoform may be found: Lecanora campestris f. alba; in urban areas on horizontal cement, grounds and surfaces, there is another ecoform with an effuse thallus often covered with algae and dust and with numerous and scattered black apothecia: Lecanora campestris forme des surfaces horizontales en ciment. See also Lecanora campestris forme sur ardoise very nitrophilous ecoform of ammonia-rich environments. As a curiosity, this very nitrophilous species can even colonize PVC garden furniture in rural areas! See: Lecanora campestris forme sur mobilier de jardin en PVC.