Biocontrol Efficiency of Leuconostoc mesenteroides GY-2 Against Postharvest Black Rot Caused by Alternaria alternata and the Mechanisms of Action

Apple black rot, a destructive postharvest disease caused by <i>Alternaria alternata</i>, poses significant economic threats during fruit storage and transportation. However, effective biocontrol bacteria to manage this disease remain limited. In this study, <i>Leuconostoc mesenteroides</i> 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 <i>A. alternata</i> by 70.8% in dual plate assays, indicating potent fungistatic activity. Notably, these VOCs produced by <i>L. mesenteroides</i> displayed broad-spectrum antifungal properties against multiple apple fungal pathogens. Microscopic analysis revealed that VOC exposure induced structural anomalies in <i>A. alternata</i> 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 <i>L. mesenteroides</i> GY-2 was a promising biocontrol agent for managing postharvest black rot of apple fruit.