First Report of Powdery Mildew of <i>Cocculus orbiculatus</i> var. <i>mollis</i> Caused by <i>Erysiphe pseudolonicerae</i> in China

The genus <i>Cocculus</i> comprises diverse medicinal plants in China, including two species <i>C. laurifolius</i> DC. and <i>C. orbiculatus</i> (L.) DC., and one variety <i>C. orbiculatus</i> (L.) DC. var. <i>mollis</i> (Kunming Institute of Botany, Chinese Academy of Sciences. 1983). In January 2023, severe powdery mildew symptoms were found on <i>C. orbiculatus</i> var. <i>mollis</i> at the Huaxi Campus (26°23'04'' N, 106°37'57'' E) of Guizhou Normal University, Guiyang, China. The incidence observed was ~95% among 80 <i>C. orbiculatus</i> var. <i>mollis</i> plants. Infected leaves presented chlorotic to necrotic areas. Mycelium grew mainly on the adaxial surface of leaves. Hyphae were hyaline and 4-10 μm wide. Hyphal appressoria were lobed to multilobed, and solitary or in opposite pairs. Conidiophores were erect, straight to somewhat flexuous and 55-130 µm long (n = 40). Foot cells were subcylindrical to curved-sinuous at the base, followed by 2-4 cells. Conidia formed singly and were ellipsoid to ovoid in shape with dimensions of 28.8-54 × 13.3-17 µm (n = 50). Based on these morphological characteristics, the powdery mildew fungus (named as GZCO-1) was provisionally identified as an <i>Erysiphe</i> species (Bradshaw et al. 2023). To identify the species, the ITS and LSU regions were amplified using the primer pairs PM10/PM28R (Bradshaw and Tobin 2020); the CAM, GAPDH, GS, and RPB2 region were amplified using the primer pairs PMCAM1/PMCAM4R, PMGAPDH1/PMGAPDH3R, GSPM2/GSPM3R, and PMRpb2_4/ PMRpb2_6R, respectively (Bradshaw et al. 2022); the TUB region was amplified using the primers BTF5b/BTR7a (Ellingham et al. 2019). The obtained 1364-bp ITS-LSU sequence (GenBank accession no. PQ818141), 282-bp CAM sequence (PQ822234), 316-bp GAPDH sequence (PQ822231), 468-bp GS sequence (PQ822235), 763-bp RPB2 sequence (PQ822232), and 733-bp TUB sequence (PQ822233) were deposited in GenBank. Using a phylogenetic tree based on the concatenated ITS-LSU, CAM, GAPDH, GS, RPB2, and TUB sequences (Bradshaw et al. 2023), the isolate GZCO-1 was grouped in the same clade as <i>E. pseudolonicerae</i>. To perform pathogenicity tests, leaves of three healthy potted <i>C. orbiculatus</i> var. <i>mollis</i> plants were inoculated by gently pressing with diseased leaves. Non-inoculated plants served as controls. All plants were incubated in a greenhouse at 25 ± 2°C with 80% relative humidity. Similar powdery mildew symptoms appeared on inoculated plants 12 days after inoculation, whereas no symptoms were observed on control plants. The fungus from inoculated <i>C. orbiculatus</i> var. <i>mollis</i> plants was morphologically identical to that on originally diseased plants. In addition, ITS-LSU sequences of the fungus from the artificially inoculated plants shared 100% identity with PQ818141. Based on the morphological and molecular characterization, the powdery mildew fungus was identified as <i>E. pseudolonicerae</i>. This powdery mildew on this host has previously been reported from China (Bradshaw et al. 2024), however this is the first report supported by sequence analyses.