The genus Buellia Sensu lato in the greater sonoran desert region: Saxicolous species with one-septate ascospores containing xanthones

Six species of saxicolous Buellia s.l. containing xanthones are reported from the Greater Sonoran Desert Region: Buellia concinna, B. halonia, B. mamillana, B. prospersa, B. subaethalea, and B. trachyspora. All species have a pale yellow to greenish yellow thallus characterized by the presence of xanthones. Nevertheless, they all show distinct characters and may consequently only be distantly related. Buellia concinna, a taxon previously reported only from Europe, is the valid name for the North American B. semitensis. Buellia subaethalea, first described by Bouly de Lesdain, but subsequently largely ignored, is reported for the first time from the region. Buellia trachyspora, a subtropical species barely reaching the Sonoran Region, is distinguished from B. mamillana.