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

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.

杜鹃花属（Rhododendron）包含超过1000种常绿与落叶物种。在北美太平洋西北沿岸地区（Pacific Northwest Coast region of North America, PNWC），侵染落叶杜鹃花属（Rhododendron spp.）的白粉菌已有充分文献记载，但针对该地区常绿杜鹃花属物种的相关研究则相对稀缺。此前学界普遍认为，两类宿主的白粉病均由杜鹃花白粉菌（Erysiphe azaleae）或越橘白粉菌（E. vaccinii）引起，但目前尚无针对PNWC地区侵染落叶或常绿杜鹃花属的白粉菌开展正式分类学鉴定的研究。
本研究的目标为：明确PNWC地区侵染常绿杜鹃花属物种的白粉菌病原菌种类，并解析此类真菌在白粉菌科（Erysiphaceae）中的系统发育位置。为确保分类学结论的严谨性，同时明确北美地区是否存在外来杜鹃花白粉菌的引入事件，需将北美地区新获得的系统发育数据置于全球框架下进行分析。因此，本研究整合了所有已发表的、来自杜鹃花属宿主的白粉菌系统发育数据。
基于对大量新获得的内转录间隔区（internal transcribed spacer, ITS）序列与28S核糖体DNA（28S rDNA）序列，以及从基因银行（GenBank）中调取的、来自常绿与落叶杜鹃花属物种的已有序列开展的系统发育分析，本研究经系统发育确认了以下4个隶属于小钩丝壳组（Erysiphe sect. Microsphaera）的白粉菌物种：具备合法保留名的杜鹃花白粉菌（Erysiphe azaleae nom. cons.，经证实欧石南粉孢（Oidium ericinum）为其同物异名）、掌状白粉菌（E. digitata，已对主模式标本完成测序）、伊豆白粉菌（E. izuensis）以及越橘白粉菌（E. vaccinii）。本研究为杜鹃花白粉菌与越橘白粉菌指定了带有测序信息的新模式标本，并为欧石南粉孢获得了新后模式序列。仅在印度喜马拉雅地区有两份采集记录的杜鹃白粉菌（Erysiphe rhododendri，隶属于白粉菌组Erysiphe sect. Erysiphe）因未获得可用样本，未能纳入本次系统发育分析。