First Report of Powdery Mildew on <i>Vitis bryoniifolia</i> Caused by <i>Erysiphe necator</i> in China

Grapes, belonging to the genus <i>Vitis</i>, are one of the world's most economically valuable fruit crops, which are widely used as the source of wine, raisins, and juice. The genus <i>Vitis</i> contains about 60 species mainly distributed in Asia, Europe, and North America to northern South America (Wan et al. 2013). In May 2022, severe powdery mildew symptoms were found on wild <i>Vitis bryoniifolia</i> plants at Guizhou Normal University, Guiyang, China. The incidence observed was approximately 85% among 50 <i>V. bryoniifolia</i> plants. Infected leaves appear white powdery patches, eventually leading to chlorosis to necrosis. Mycelia occurred on adaxial and abaxial leaf surfaces, petioles and young shoots. Upon microscopic observation hyphae were hyaline and 3.5-6 μm wide. Hyphal appressoria were solitary and lobed to multilobed. Conidiophores were erect, straight to somewhat flexuous and 80-130 µm long (n = 30). Foot cells were subcylindrical to curved-sinuous at the base, followed by 2-4 cells. Conidia formed singly (occasionally 2-6 in a chain) and were ellipsoid to ovoid in shape with dimensions of 22.5-38 × 12-19.5 µm (n = 50). No fibrosin bodies were observed on conidia. Based on these morphological characteristics, the powdery mildew fungus strongly resembled <i>Erysiphe necator</i> (Braun and Takamasu 2000; Zheng et al. 1987) that has been shown to cause powdery mildew on <i>Vitis</i> (Gadoury et al. 2012; Gaforio et al. 2011; Qiu et al. 2015). To confirm the identification, the ribosomal DNA internal transcribed spacer (ITS) and the ribosomal large subunit (LSU) region were amplified and sequenced using the ITS1/ITS4 primer pair (White et al. 1990) and the NL1/NL4 primer pair (Ziemiecki et al. 1990), respectively. The spliced 1250-bp ITS-LSU sequence (GenBank accession no. PP188565) shared 99.84-100% identity with ITS-LSU sequences of <i>E. necator</i> (LC028995, LC028996, ON073862, LC777882, and OM033353). Based on the phylogenetic analysis of the combined ITS-LSU dataset of <i>Erysiphe</i> species (Takamasu et al. 2015), PP188565 was grouped in a clade with <i>E. necator</i> strains MUMH530, MUMHs141, and VPRI19719. To perform pathogenicity analysis, leaves of three healthy, potted 1-year-old <i>V. bryoniifolia</i> plants were inoculated by gently pressing with diseased leaves. Three non-inoculated healthy plants served as controls. All plants were incubated in a greenhouse at 25 ± 2°C with 80% relative humidity. Powdery mildew symptoms, similar to field conditions were observed on inoculated plants 13 days after inoculation, whereas control plants remained symptomless. Fungus reisolated from inoculated <i>V. bryoniifolia</i> plants was morphologically identical to that on originally diseased plants, and the spliced ITS-LSU sequence of reisolated fungus shared 100% identity with PP188565, fulfilling Koch's postulates. Based on the morphological and molecular characterizations, the powdery mildew fungus was identified as <i>E. necator</i>. To our knowledge, this is the first report of powdery mildew caused by <i>E. necator</i> on <i>V. bryoniifolia</i> in China. This work further expands the host range of <i>Erysiphe necator</i> on <i>Vitis</i> species.