Punctelia borreri (Sm.) Krog

    Thallus foliose, rosette-forming, 5-10 cm diam, tightly appressed to the substratum, lobes 5-10 mm wide, apices rounded, upper surface smooth and not pruinose, grey-bluish when dry, grey-greenish to greenish when moist, pseudocyphella numerous, evenly distributed, irregular in shape and punctiform, whitish, snow-white and markedly contrasting with the cortex (star-spangled like), from where arise farinose soralia, white-greyish, also present at lobe apices although less numerous there; lower surface dark brown, blackish, notably towards the center, somewhat paler at margin, with simple rhizines, back (contradictory according to some authors). Apothecia usually absent or rare, stipitate and bowl-shaped, 0.3-0.6 mm diam., disc brown-red, margin concolourous to the thallus, rough to slightly sorediate.  Photobiont: Trebouxioid. Cortex : K+ yellow ; medulla : C+ pink, K-, KC+ pink, Pd-, UV-. Common, corticolous, on deciduous trees in well-lit and clear areas notably on ornament trees along roads, occasionally on rocks near the seashore. May be confused with Punctelia subrudecta of which medulla is C+ carmine and lower surface brown instead of black, paler towards the center; with Punctelia reddenda that is medulla C- and with Punctelia jeckeri of which lobes are pruinose and soralia marginal. N.B. In case the thallus is yellowish see Flavopunctelia flaventior. N.B. Macroscopic characteristics are highly variable (lower surface, rhizine colour notably) and sometimes contradictory according to authors. There contribution to identification is therefore limited. In order to separate Punctelia borreri from Punctelia subrudecta, we suggest, for P. borreri, to prioritize the chemical spot tests and some other traits: bright whitish pseudocyphella, whitish and scarce soralia. In addition, numerous intermediate specimen may be found. N.B. Not to be confused with some forms of Parmelia sulcata with medulla and soralia K+ red-orange and Pd+ orange.*