Leaf spot on cucumber caused by Stemphylium vesicarium newly reported in China

Leaf spot is one of the common diseases of Cucurbitaceae crops, including cucumber, muskmelon, and pumpkin. Between August and October 2021, cucumber leaves exhibiting disease symptoms were observed in farmland around Qingdao city, Shandong province. The corresponding diseased leaves were collected to isolate and characterize the causal agent of cucumber leaf spot. Multilocus phylogenetic analysis (internal transcribed spacer regions 1 and 2 and intervening 5.8S nrDNA, calmodulin, and glyceraldehyde-3-phosphate dehydrogenase) and morphological characteristics revealed that the fungal isolates belonged to Stemphylium vesicarium. The symptoms of brown to black lesions appeared on all artificially inoculated cucumber leaves, which were highly similar to those observed in the field. The pathogen was re-isolated from the corresponding diseased leaves to fulfill Koch's postulates. Therefore, S. vesicarium was confirmed as the pathogen of cucumber leaf spot in this study. To our knowledge, this is the first report of S. vesicarium causing leaf spot on cucumber in China.