Identification, characterization, and fungicide response of fungal pathogens causing postharvest diseases of watermelon (Citrullus lanatus) in Northern Thailand

Watermelon is an economically important crop widely cultivated in Thailand, but its postharvest quality is increasingly threatened by fungal diseases. Between 2024 and 2025, anthracnose and fruit rot symptoms were observed on postharvest watermelon fruits in Northern Thailand. This study aimed to isolate, identify, and confirm the pathogenicity of associated fungal pathogens. Fifteen fungal strains were recovered and identified using morphological characteristics combined with multi-locus phylogenetic analyses. Species of Colletotrichum were determined as causal agents of anthracnose, whereas members of Fusarium and Neocosmospora were associated with fruit rot. Pathogenicity tests confirmed their ability to cause disease, and all pathogens were successfully re-isolated, fulfilling Koch’s postulates. In the fungicide response tests, copper hydroxide inhibited all fungal species except C. truncatum and F. sulawesiense, demonstrating broad-spectrum activity. Colletotrichum truncatum was completely inhibited by benalaxyl-M (4%) + mancozeb (65%), cyproconazole, and difenoconazole, while F. sulawesiense was inhibited only by copper oxychloride. To our knowledge, this study provides the first global report of anthracnose caused by C. siamense and fruit rot caused by F. hainanense, F. sulawesiense, N. ferruginea, and N. suttoniana on watermelon. It is also the first report in Thailand of anthracnose caused by C. chlorophyti and C. truncatum and fruit rot caused by F. oxysporum. These findings enhance disease diagnosis and support the development of more effective postharvest disease management strategies in watermelon production systems.