Revision of powdery mildews (Ascomycota, Erysiphaceae) on Rosa in China: unexpected taxonomic complexity with phytopathological implications

Powdery mildew is a persistent disease affecting the cultivation of <i>Rosa</i>, a genus of substantial horticultural and economic value worldwide. Despite more than a century of study, the true diversity of powdery mildews infecting roses has remained unclear, largely due to the long-standing and overly broad application of the name <i>Podosphaera pannosa</i>. To reassess this system, we conducted an extensive investigation of powdery mildew specimens infecting <i>Rosa</i>. A total of 112 collections were examined, including recently gathered material from 23 provinces, historical types, representative specimens from the Herbarium Mycologicum Academiae Sinicae (HMAS), China, and a neotype specimen from Germany. Morphological observations combined with phylogenetic analyses (ITS, 28S, and IGS rDNA) resolved several long-standing taxonomic problems and revealed unexpected diversity within the <i>rose</i> powdery mildew complex. Molecular data from <i>Erysiphe rosae</i> provide the first phylogenetic evidence supporting the synonymy of <i>Medusosphaera</i> with <i>Erysiphe</i>. <i>Sphaerotheca rosae</i>, previously treated as a synonym of <i>P. pannosa</i>, is reinstated as a distinct species as <i>Podosphaera rosae</i> <b>comb. nov</b>., and a previously unrecognized lineage is described as <i>Podosphaera rosae-xanthinae</i> <b>sp. nov</b>. In addition, earlier varieties of <i>E. simulans</i> are shown to lack diagnostic morphological or genetic characters and are no longer supported. Taken together, these results demonstrate that powdery mildews on <i>Rosa</i> represent a complex of five species across two genera, structured by host phylogeny. Clear patterns of host preference and distribution indicate a history of co-evolution and ecological differentiation driven by host availability. This study fundamentally revises our understanding of rose powdery mildews, revealing a level of taxonomic and evolutionary complexity much greater than previously recognized and highlighting <i>Rosa</i> as a key host lineage in the diversification of the <i>Erysiphaceae</i>.