Discovery of (+)-Methyl Nonactate as a Potential Fungicide against Gummy Stem Blight

Gummy stem blight (GSB), which is caused by Stagonosporopsis cucurbitacearum, threatens pumpkin yields and agriculture. Effective, safe antifungal agents are urgently needed. In this study, fermentation broth supernatant of Streptomyces sp. NEAU-T55 demonstrated considerable antifungal activity against S. cucurbitacearum. Activity-guided isolation identified 2 new and 14 known compounds, with (+)-methyl nonactate (10) determined as the main active ingredient. This compound exhibited strong antifungal activity (EC₅₀ = 0.12 μg mL^-1), outperforming difenoconazole (EC₅₀ = 0.17 μg mL^-1), and achieved 74.1% control efficacy in the pot experiments. Microscopy revealed that (+)-methyl nonactate impeded mycelial growth and induced morphological alterations. Transcriptomic analysis indicated that (+)-methyl nonactate may inhibit acetolactate synthase, thereby disrupting amino acid metabolism and diminishing precursor availability for the tricarboxylic acid cycle. This research represents the first application of (+)-methyl nonactate for GSB control and provides insights into its antifungal mechanisms, laying the groundwork for its potential development as a novel agricultural antibiotic.