Rust fungi on rhododendrons in Asia:Diaphanopellis forrestiigen. et sp. nov., new species ofCaeoma,and expanded descriptions ofChrysomyxa dieteliiandC. succinea

Abstract

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.

A new rust species of Diaphanopellis on Rhododendron oreodoxa from Southern China

“…The uredinia were surrounded by a conspicuous peridium with square and ornamented cells, the outter surface of peridium was deeply convave and smooth, while the inner surface was densely warted. This taxon clearly differs from D. forrestii (Crane 2005) in its urediniospore surface structure and smaller-size teliospores. Moreover, the morphology of uredinia peridium is different in these two species.…”

“…It is distinctly different from Chrysomyxa -the common pathogen infecting Rhododendron, which produces teliospores covered by hyaline sheath and uredinial anamorph in Aecidium rather than Caeoma (Crane 2005).…”

“…This suggested that the fungus on P. purpurea is the aecial state of the new species Diaphanopellis purpurea. The aecial stage of D. forrestii was not discovered (Crane 2005). Molecular comparisons and inoculations to obtain its aecial states would facilitate further investigations of the two species.…”

“…Diaphanopellis is characterized by the teliospores enclosed in hyaline sheaths, and the uredinia surrounded by a peridium with ornamented cells (Barclay 1891, Balfour-Browne 1955, Crane 2005. Most rusts infecting Rhododendron belong to the genus Chrysomyxa, which are morphologically different from Diaphanopellis species in having uredinial peridium and distinct teliospores.…”

“…Rust genus Diaphanopellis was established by Crane with the type species Diaphanopellis forrestii P. E. Crane occurring on Rhododendron selense Franch (Crane 2005, Kirk et al 2008. Diaphanopellis is characterized by the teliospores enclosed in hyaline sheaths, and the uredinia surrounded by a peridium with ornamented cells (Barclay 1891, Balfour-Browne 1955, Crane 2005.…”