Powdered Speckled Shield Lichen

The thallus of Punctelia subrudecta is foliose and usually forms loosened rosettes several centimetres across. Lobes are rather narrow (mostly 2–4 mm wide), flat to slightly ridged, and display a pale grey-green colour that darkens when wet. The upper surface is smooth and lacks the shallow pits (scrobiculae) that give P. perreticulata its reticulate look; it is also free of the frost-like bloom (pruina) that dusts the lobe tips of P. jeckeri. Minute white pores (pseudocyphellae) are scattered but never abundant. Vegetative reproduction dominates in P. subrudecta. Powdery propagules called soredia—tiny balls of algal cells wrapped in fungal threads—break through the cortex in discrete patches known as soralia. In P. subrudecta the soralia first appear along lobe edges and secondary lobes; older parts of the thallus may also develop scattered laminal soralia, but the soredia remain fine and never coarsen into isidioid granules. The lower surface is characteristically pale off-white to light tan (never black) and bears sparse, similarly pale anchoring rhizines, a combination that helps separate the species from dark-undersided members of the genus. Microscopically the pycnidia produce unciform (gently hook-shaped) conidia only 4–6 μm long—one of the most reliable diagnostic characters, because related North-American taxa have straight, much longer conidia. …

Punctelia subrudecta is predominantly corticolous, forming tightly attached rosettes on the bark of trees in temperate and Mediterranean woodlands; specimens examined from the Iberian Peninsula were taken almost exclusively from trunks and branches, with only occasional collections on siliceous rock faces that mimic bark microclimates. Populations recorded in Atlantic oak forests and in the drier, eastern Mediterranean mountains show that the species tolerates a broad moisture gradient, provided atmospheric pollution remains low. Thalli from heavily trafficked coastal sites in eastern Spain show pink central necrosis and marginal bleaching, symptoms the authors link to chronic oxidant and nitrogen pollution. Polish herbarium data show the species colonises a wide range of phorophytes, most frequently oak (Quercus, 26 % of records) but also alder (Alnus, 13 %), European ash (Fraxinus excelsior, 11 %), small-leaved linden (Tilia cordata, 11 %), willow (Salix, 10 %) and a suite of other broad-leaved hosts; rare saxicolous thalli have even been found on wood and concrete. Sequence-verified collections confirm an essentially Old-World range stretching from Fennoscandia south to the Mediterranean, eastwards into Kenya and at least parts of China and Australasia; records from the Americas are now considered misidentifications. …