Rust fungi on rhododendrons in Asia: Diaphanopellisforrestii gen. et sp. nov., new species of Caeoma, and expanded descriptions of Chrysomyxa dietelii and C. succinea.

Many rust fungi (Uredinales) that infect rhododendrons are difficult to identify because of similar spore size and overall morphology. As part of a morphological study of rusts in the genus Chrysomyxa, herbarium specimens of Asian rhododendron rusts were examined by light and scanning electron microscopy. They were compared with similar taxa from Europe and North America. Revised and illustrated descriptions are provided for the uredinia and telia of Chrysomyxa dietelii and Chrysomyxa succinea; details of the conspicuous uredinial peridium of both species are described for the first time. A new genus and species, Diaphanopellis forrestii, is proposed to accommodate a rust fungus with uredinia covered by a peridium of ornamented cells (Aecidium-type) and teliospores enclosed in transparent outer sheaths. This species includes the previously described anamorphs Aecidium rhododendri and A. sino-rhododendri. Three new anamorphic species with unique urediniospore morphology also are described: Caeoma clemensii from Philippines, Caeoma spinulospora from Tibet, and Caeoma yunnanensis from Yunnan, China. For morphological and nomenclatural reasons Uredo rhododendri ('rhododendronis') is renamed as Caeoma dumeticola and Uredo rhododendri-capitati is transferred to Caeoma. A key to Asian rhododendron rusts that form uredinia is provided. In general morphological groups of rhododendron rusts correlate with the subgenera of Rhododendron on which they occur, suggesting coevolution of these parasites with their hosts.