First Report of Erysiphe alphitoides Causing Powdery Mildew of Cocculus orbiculatus in China

Cocculus orbiculatus (L.) DC. (Menispermaceae) is a vine traditionally used as a medicinal herb in Asia and grows primarily in wet tropical biomes (POWO 2022). In late April 2022, typical symptoms of powdery mildew were observed on leaves of C. orbiculatus on the campus of Nanjing Forestry University, China. Approximately 90% of the plants were infected. Superficial mycelia and conidia were amphigenous on the leaves, pale yellow, and severe infections caused necrotic discoloration of the leaves. Infected leaves were collected to identify the pathogen. Hyphae were hyaline and branched. Conidiophores were solidary, unbranched, straight, cylindrical, smooth, hyaline, 69.3 ± 11.1 × 7.9 ± 0.6 µm, (n = 50). Foot cells were mostly cylindrical, straight, rarely curved, smooth, hyaline, 53.2 ± 6.2 × 7.5 ± 0.4 µm, (n = 50). Appressoria were lobulate, solitary or in opposite pairs, hyaline to pale yellow. Conidia were single, ellipsoid, oval or doliform, hyaline or pale yellow, 38.6 ± 2.3 × 20.9 ± 0.8 µm, (n = 50). Conidial germ tubes developed at a subterminal position. No chasmothecia were observed. Representative specimens were deposited in the NJFU Herbarium (NF50000010). Based on these morphological characteristics, this fungus (MFJ 1-1) was provisionally identified as Erysiphe alphitoides (Takamatsu et al. 2007). To verify the identification of the pathogen, mycelia and conidia were obtained from diseased leaves and genomic DNA of the fungus (MFJ 1-1) was extracted. The internal transcribed spacer region (ITS) and large subunit (LSU) gene were amplified with primers ITS1/ITS4 and LR0R/LR5, respectively (White et al. 1990, Rehner and Samuels 1994). The sequences were deposited in GenBank (ON612134 for ITS, ON620080 for LSU). BLAST results showed that the ITS and LSU sequences were highly similar to E. alphitoides [ITS: KF734882, identities = 632/633 (99%) LSU: MK357414, identities = 890/893 (99%)]. Phylogenetic analyses with the concatenated sequences using Bayesian inference and maximum likelihood placed the isolate in the clade of Erysiphe alphitoides. Pathogenicity was confirmed by gently pressing the infected leaves onto five leaves per plant, and three healthy plants were inoculated. Three uninoculated plants served as controls. The plants were placed in a growth chamber with a 16 h photoperiod at 22 ± 2°C, 70% of relative humidity. Symptoms developed 10 days after inoculation, whereas the control leaves remained symptomless. The powdery mildew developing on the inoculated plants was identified to be E. alphitoides based on morphological characters and ITS sequences. This fungus has a worldwide distribution and a broad host range. Recently, Ipomoea obscura (Pan et al. 2020) and Aegle marmelos (Banerjee et al. 2020) have been found to be additional hosts. To our knowledge, this is the first report of powdery mildew caused by E. alphitoides on C. orbiculatus in the world. This finding provides crucial information for developing effective strategies to monitor and manage this disease.