Abstract
Apple black rot, a destructive postharvest disease caused by Alternaria alternata, poses significant economic threats during fruit storage and transportation. However, effective biocontrol bacteria to manage this disease remain limited. In this study, Leuconostoc mesenteroides strain GY-2, isolated from healthy apple fruit surfaces, had a remarkable biocontrol ability on apple black rot. While GY-2 exhibited no direct inhibitory effects in confrontation assays, volatile organic compounds (VOCs) emitted by the strain suppressed colony diameter of A. alternata by 70.8% in dual plate assays, indicating potent fungistatic activity. Notably, these VOCs produced by L. mesenteroides displayed broad-spectrum antifungal properties against multiple apple fungal pathogens. Microscopic analysis revealed that VOC exposure induced structural anomalies in A. alternata hyphae, including surface perforations and protoplast leakage, suggesting membrane integrity disruption. The VOCs produced by strain GY-2 were identified; four compounds had antifungal activities, among them, isoamylol exhibited the highest antifungal activity. Applying bacterial suspensions of strain GY-2 on apple fruit significantly reduced 91.4% of lesion areas of black rot. The strain exhibited robust colonization capacity on fruit surfaces, maintaining viable populations for over 15 days post-application, guaranteeing a sustained disease prevention. Furthermore, GY-2 treatment enhanced systemic resistance in apple fruit, as evidenced by upregulated antioxidant enzymes and defense-related enzymes. Importantly, application of GY-2 did not adversely affect key parameters of fruit quality, including firmness, soluble solids content, or acidity. These findings showed that the bacterial L. mesenteroides GY-2 was a promising biocontrol agent for managing postharvest black rot of apple fruit.