Phylogeny and taxonomy of Erysiphe spp. on Rhododendron, with a special emphasis on North American species

ABSTRACT

The genus Rhododendron comprises over 1000 evergreen and deciduous species. In the Pacific Northwest Coast region of North America (PNWC), powdery mildews infecting deciduous Rhododendron spp. are well documented but less so on evergreen Rhododendron spp. Infections of both groups of hosts historically have been attributed to Erysiphe azaleae or E. vaccinii. No formal characterizations of powdery mildew fungi infecting either deciduous or evergreen Rhododendron spp. in the PNWC have been completed. The objectives of this study were to identify the powdery mildew pathogens infecting evergreen Rhododendron spp. in the PNWC and to assess the phylogenetic position of these fungi within the Erysiphaceae. To ascertain valid taxonomic conclusions, and to determine whether potential introductions of exotic Rhododendron powdery mildews in North America have occurred, it was necessary to put the new North American phylogenetic data into a worldwide context. Therefore, available phylogenetic data from all Erysiphe spp. on Rhododendron have been included in our analyses.

Based on analyses of numerous new internal transcribed spacer (ITS) and 28S rDNA sequences and already available sequences deposited in GenBank retrieved from evergreen and deciduous Rhododendron spp., the following Erysiphe spp. could be phylogenetically confirmed (all belonging to Erysiphe sect. Microsphaera): Erysiphe azaleae nom. cons. (Oidium ericinum could be verified as a synonym), E. digitata (holotype sequenced), E. izuensis, and E. vaccinii. Erysiphe azaleae and E. vaccinii are epitypified with sequenced specimens, and an ex-neotype sequence has been obtained for Oidium ericinum. Erysiphe rhododendri (Erysiphe sect. Erysiphe), only known from two collections in India (Himalayan region), was not available for phylogentic analyses.

KEYWORDS:

• Deciduous
• Erysiphe
• evergreen
• North America
• phylogeny
• Rhododendron
• systematics

ACKNOWLEDGMENTS

We thank Drs. Frank Dugan and Sanjaya Gyawali for their critical reviews of the manuscript. We also thank Dr. Benjamin Hall, Dr. Dean Glawe, and Mr. Dale Lindsley for their technical assistance; Dr. Mary Berbee for the use of laboratory facilities; and The Washington Park Arboretum, Van Dusen Botanical Garden, University of British Columbia Botanical Garden, and the Rhododendron Species Garden for permission to collect specimens. Sincere thanks are due to the curators of E (Edinburgh, UK) and HAL (Halle, Germany) for allowing us to sequence powdery mildew collection in their keeping. The authors have no conflicts of interest to disclose.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

FUNDING

This project was funded in part by the Northwest Horticultural Society and American Rhododendron Society.

SUPPLEMENTARY MATERIAL

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