Identification of the Causal Pathogen of Leaf Spot Disease in Watermelon Grown in Two Vinyl Greenhouses in Haman, Korea

The area dedicated to watermelon (Citrullus lanatus) cultivation in Korea spans approximately 14,000 hectares, with greenhouse cultivation accounting for about 25-30% of this total. Notably, winter watermelon cultivation is considered a high-income crop, and Haman-gun in Gyeongsangnam-do is a prominent production area, contributing approximately 70% of the total output. Between 2024 and 2025, leaf spots began to appear on young leaves in two greenhouses located on the eastern side of 37 greenhouses in Haman. The symptoms gradually progressed from spots to concentric circles, resulting in a disease that affected more than 70% of the watermelons in these two greenhouses. The causal pathogen was identified both morphologically and molecularly as Cercospora cf. citrullina. The pathogen exhibited very slow growth, and spore development was also slow under potato dextrose agar and cornmeal agar media. Pathogenicity assays confirmed that the isolates were the causal agents of leaf spot disease, and the fungus was reisolated from the infected leaves, thus fulfilling Koch’s postulates. Our findings will contribute to the development of effective control strategies for this disease, including methods for cultural and chemical controls in the future.