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

The genus <i>Rhododendron</i> comprises over 1000 evergreen and deciduous species. In the Pacific Northwest Coast region of North America (PNWC), powdery mildews infecting deciduous <i>Rhododendron</i> spp. are well documented but less so on evergreen <i>Rhododendron</i> spp. Infections of both groups of hosts historically have been attributed to <i>Erysiphe azaleae</i> or <i>E. vaccinii</i>. No formal characterizations of powdery mildew fungi infecting either deciduous or evergreen <i>Rhododendron</i> spp. in the PNWC have been completed. The objectives of this study were to identify the powdery mildew pathogens infecting evergreen <i>Rhododendron</i> 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 <i>Rhododendron</i> 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 <i>Erysiphe</i> spp. on <i>Rhododendron</i> 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 <i>Rhododendron</i> spp., the following <i>Erysiphe</i> spp. could be phylogenetically confirmed (all belonging to <i>Erysiphe</i> sect. <i>Microsphaera</i>): <i>Erysiphe azaleae</i> nom. cons. (<i>Oidium ericinum</i> could be verified as a synonym), <i>E. digitata</i> (holotype sequenced), <i>E. izuensis</i>, and <i>E. vaccinii. Erysiphe azaleae</i> and <i>E. vaccinii</i> are epitypified with sequenced specimens, and an ex-neotype sequence has been obtained for <i>Oidium ericinum. Erysiphe rhododendri</i> (<i>Erysiphe</i> sect. <i>Erysiphe</i>), only known from two collections in India (Himalayan region), was not available for phylogentic analyses.