Pertusaria amara (Ach.) Nyl.
Lepra amara (Ach.) Hafellner var. amara
Thallus usually rather thick, well delimited but not well-zoned by prothallus or inconspicuously, widespreading on tree trunks (20 x 50 cm), markedly visible from far, smooth to verrucose, whitish, pale greyish, more or less covered with globose soralia, 0.5-1.5 mm diam., sometimes distributed in concentric lines, whitish, hosting concolourous farinose soredia, typically extremely bitter (scratch a moist finger on the soralia, taste, note that the bitterness is very persistant as for quinine). Apothecia very rare. Ascii 1-spored. Ascopsores 130-150 x 40-50 µm. Photobiont: Clorococcoid. Soralia : C-, K-, KC+ purple, evanescent, Pd- or seldom faint red-orange. Common, highly variable but easy to recognize using the finger assay, on bark of deciduous trees, seldom coniferous, in woodlands and also on isolated trees. N.B. Some authors consider the taxon growing on rocks as Pertusaria amara var. flotowiana, (see the file under this name). Not to be confused with Pertusaria albescens of which the soralia are flat and not bitter, and with Pertusaria hemispherica which is C+ red.*