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

## TL;DR

This study shows that Leuconostoc mesenteroides GY-2 effectively controls apple black rot by suppressing the fungus and enhancing fruit resistance without harming quality.

## Contribution

The study identifies L. mesenteroides GY-2 as a novel biocontrol agent with broad-spectrum antifungal VOCs and long-term colonization ability.

## Key findings

- VOCs from GY-2 reduced A. alternata colony diameter by 70.8% and caused structural damage to hyphae.
- Application of GY-2 reduced black rot lesion areas by 91.4% and maintained fruit quality parameters.
- The strain enhanced apple fruit resistance by upregulating antioxidant and defense-related enzymes.

## 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.