Screening of antagonistic yeasts against citrus blue mold, biocontrol mechanisms of Aureobasidium pullulans JM3-C and synergistic effects of ammonium sulfate

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Citrus blue mold, caused by Penicillium italicum , is a major postharvest disease that leads to significant economic losses. Although antagonistic yeasts represent a promising biocontrol strategy, their standalone efficacy is often insufficient for commercial application. This study aimed to screen antagonistic yeasts, elucidate the biocontrol mechanisms of a selected strain, and evaluate its synergistic combination with ammonium sulfate (AS). From 372 environmental isolates screened, 21 exhibited in vitro antifungal activity. Strain JM3-C demonstrated exceptional inhibition of P . italicum both in vitro and in vivo , and was identified as Aureobasidium pullulans via molecular (26S rDNA) and morphological analyses. Its biocontrol mechanisms involved direct antagonism through the production of antifungal volatile organic compounds (VOCs) and extracellular lytic enzymes (protease, gelatinase, and pectinase), coupled with competitive exclusion characterized by the inhibition of spore germination, robust biofilm formation, strong hyphal adhesion, and efficient wound colonization. Crucially, the addition of AS significantly enhanced biocontrol efficacy. AS acted synergistically to amplify the antifungal activity of yeast VOCs, boost spore germination inhibition, and promote biofilm formation, while also improving yeast population dynamics and persistence in fruit wounds. Consequently, the combined treatment of A . pullulans JM3-C and AS provided superior protection against blue mold during 28 days of storage, outperforming both the yeast applied alone and conventional fungicides (imazalil and prochloraz). This study highlights the combination of A . pullulans JM3-C and AS as a highly effective, sustainable alternative to chemical fungicides for managing postharvest citrus decay.