Physcia americana
Physcia americana photo by Ray Showman
DIAGNOSTIC FEATURES: Small size; light gray with very white circular soralia clustered toward center of thallus; cortex and medulla K+ yellow; compare with Dirinaria frostii which has undersurface black and is found on rocks and P. caesia, on rock, maculate, with undersurface fibrous.
ECOLOGY: Widespread in the eastern US; common in Ohio, usually found on ash or white oak, more rarely on rock.