Biocontrol of Postharvest Soft Rot in Kiwifruit by Antagonistic Fungi from Kiwifruit Tissues and Rhizosphere Soil: Screening, Identification, and Mechanisms

Postharvest soft rot is a major constraint in kiwifruit production, causing substantial economic losses and raising food safety concerns. Therefore, the development of sustainable biocontrol alternatives to conventional chemical treatments is urgently needed. In this study, thirty-seven fungi were isolated from healthy kiwifruit tissues and rhizosphere soil and screened for their biocontrol potential. Three highly effective antagonistic strains were ultimately identified: Mucor circinelloides N2-1-1, Aspergillus niger Pb-2-2, and Trichoderma hamatum Nd-1-1. In vitro antagonism assays showed that A. niger Pb-2-2 exhibited the strongest and broadest-spectrum inhibitory activity against four postharvest pathogens, namely Fusarium sp., Alternaria sp., Botryosphaeriaceae sp., and Phomopsis sp. In contrast, in vivo protection assays demonstrated that T. hamatum Nd-1-1 almost provided the highest control efficacy against all four pathogens in kiwifruit fruit. Fermentation broth protection experiments further revealed that A. niger Pb-2-2 produced stable and broad-spectrum antimicrobial metabolites. Although M. circinelloides N2-1-1 showed the weakest antagonistic activity among the three fungi, this is the first report indicating that M. circinelloides has antagonistic activity against phytopathogenic fungi. Physiological analyses indicated that all three antagonistic fungi alleviated the inhibitory effects of pathogens on superoxide dismutase, catalase, and ascorbate peroxidase activities, helped maintain reactive oxygen species homeostasis, and activated the phenylpropanoid pathway, thereby enhancing disease resistance in kiwifruit. These findings provide a theoretical basis and promising microbial resources for the biological control of postharvest soft rot in kiwifruit.