Biocontrol yeast Kurtzmaniella quercitrusa BS-AY-S1 controls postharvest green mold of citrus fruit by producing metabolites with antifungal activity

• BS-AY-S1 isolated from the citrus inter-root soil exhibits good colonization; • BS-AY-S1 inhibits the growth of P. digitatum both in vivo and in vitro ; • BS-AY-S1 produces metabolites and VOCs with antifungal activity; • VOCs and fermentation broth from BS-AY-S1 control the green mold in citrus fruit; • A total of 20 VOCs produced by BS-AY-S1 were identified by GC-IMS. Green mold caused by Penicillium digitatum is the leading postharvest disease in citrus fruit. While chemical fungicides are commonly used for management, yeast-based biological control strategy offers a promising and eco-friendly alternative. In this study, yeast strains were isolated from citrus orchards and the strain with the best biocontrol efficacy was screened to investigate its mechanisms for controlling postharvest green mold in citrus fruit. In total, 30 yeast strains were isolated from fruits, leaves and soil of two citrus orchards. Subsequently, 8 yeast strains tested in vivo effectively inhibited green mold in citrus fruit, with Kurtzmaniella quercitrusa BS-AY-S1 achieving the best biocontrol efficacy by reducing the disease incidence (DI) by 58.33 % and lesion diameter (LD) by 73.36 % compared to the control on the 6th day. BS-AY-S1 exhibited excellent colonization ability, laying the foundation for its biocontrol efficacy. Furthermore, the antifungal properties of BS-AY-S1 metabolites were evaluated. The fermentation supernatant of BS-AY-S1 effectively inhibited the growth of P. digitatum and controlled green mold in citrus fruit. Additionally, the volatile organic compounds (VOCs) from BS-AY-S1 markedly reduced the spore germination, spore production and mycelial growth of P. digitatum , and slowing down the progression of green mold in citrus fruit. In total, 20 VOCs, including alcohols, aldehydes, esters, ethers and ketones produced by BS-AY-S1, were identified by gas chromatography-ion mobility spectrometry (GC-IMS). In conclusion, BS-AY-S1 demonstrated potential for controlling postharvest green mold in citrus fruit, with its mechanisms of action including the ability to colonize citrus fruit wounds and produce antifungal metabolites and VOCs.