Isolation and identification of three antagonistic strains against kiwifruit soft rot, study on antibacterial mechanism, and preparation of composite biocontrol agents.

Kiwifruit is highly susceptible to soft rot after harvest, resulting in huge economic losses. Traditional chemical control methods tend to cause pesticide residues, induce pathogen resistance, and give rise to environmental pollution. Therefore, it is urgent to develop a safe and efficient new control technology. In this study, the pathogen causing kiwifruit soft rot and three antagonistic strains were isolated separately. The pathogen was identified as Botryosphaeria dothidea, which causes soft rot in kiwifruit by degrading pectic substances in the fruit cell wall. The three antagonistic strains were identified as Hanseniaspora pseudoguilliermondii, Bacillus altitudinis and Kosakonia cowanii, designated as strain Y3, strain J6 and strain K9, respectively. Their antifungal mechanisms were then investigated. It was found that strain Y3 inhibited pathogens mainly through volatile organic compounds (VOCs), strain J6 through liquid fermentation metabolites, and strain K9 through mycoparasitism. Studies on their growth characteristics showed that all three strains had short growth cycles, low production costs and high yields. Furthermore, no mutual antagonism was observed among the antagonistic strains, and they could co-evolve with pathogens, making them ideal materials for formulating microbial agents.Finally, the antagonistic strains were prepared into a composite microbial agent and applied to kiwifruit storage. The results showed that preventive spraying completely inhibited the occurrence of soft rot, and remedial spraying after pathogen inoculation also significantly delayed lesion expansion, demonstrating remarkable control efficacy and providing a safe and efficient green solution for postharvest soft rot of kiwifruit.