A revision of Rhododendron section Pentanthera

Abstract

Rhododendron sect. Pentanthera G. Don (Ericaceae) comprises a group of closely related, highly ornamental plants which are commonly called ‘azaleas'. Thirteen of the fifteen species recognized in this section are indigenous to North America. One species (R. molle) is native to Japan and China, and one species (R. luteum) is indigenous to the Caucasus region. Phylogenetic analysis of the species within the section indicates that R. molle is the sister to the rest of the section. It is the sole member of R. subsect. Sinensia. The remaining species form a monophyletic group recognized as R. subsect. Pentanthera. Within this subsection the presence of a blotch on the upper corolla lobe defines two primarily orange to red-flowered groups. The first group has a Tertiary Period disjunct distribution and comprises R. luteum, R. austrinum and R. occidentale. The second group is indigenous to eastern North America and comprises R. calendulaceum, R. cumberlandense, R. flammeum, R. prunifolium and R. alabamense. In both groups the cladistically basal species has white flowers with a yellow blotch on the upper corolla lobe (R. occidentale, R. alabamense, respectively). The pink to white early flowering species R. canescens, R. periclymenoides and R. prinophyllum do not form a monophyletic group.Phenetic analyses indicate that the eastern Asian taxon, R. molle, is best recognized as one species with two geographical subspecies; Rhododendron prinophyllum is quite distinct from R. canescens and R. periclymenoides. The latter two species are very similar morphologically, but their similarities are due to the retention of primitive characters and they should be recognized as distinct species. Rhododendron calendulaceum can be distinguished from R. cumberlandense using a combination of morphological and phenological characters. The various taxa previously segregated out of R. viscosum are merely extreme forms of a widespread and variable species and are not given any formal rank. No subspecific taxa are recognized for R. occidentale. Distribution maps, keys to the species, species descriptions and specimen citations are included.