Species delimitation in Rhododendron subsect. Caroliniana (Ericaceae) has been disputed in the past with one or three species accepted. Here we report a fourth species, Rh. smokianum, a narrow endemic from the Great Smoky Mountains (Tennessee/North Carolina, USA). It is characterized by short-tubed corollas and small, compact growth. We support our conclusion by phylogenetic analyses of ITS and plastid trnL-F sequence data. The new species is a specialist of open mafic rock slopes and maintains morphological and phenological differences to the related species even in cultivation.

The nuclear ribosomal ITS-region (hereafter ITS) and the plastid trnL intron, trnL 3´ exon and trnL-F spacer (hereafter trnL-F region) were amplified using primers ITS A (Blattner 1999) and ITS4 (White et al. 1990) for ITS, and the trnL-F region with primers c and f and sometimes including internal primers d and e (Taberlet et al. 1991). Sequences were generated by Sanger sequencing at commercial sequencing companies. Sequences were manually aligned in Phyde v.0.9971 (Müller et al. 2010).