Abstract: Penicillium digitatum, P. italicum, Geotrichum citri-aurantii and Colletotrichum gloeosporioides are the major pathogens infecting postharvest citrus fruit. It is necessary to develop effective safe and economical agent for control of these diseases. Thus, we investigated the antagonism of Bacillus velezensis HY19, a self-isolated biocontrol bacterium, against the four major pathogens, and their control efficacies against C. gloeosporioides infection on postharvest citrus fruit by microbial culture and fruit wound infection experiment. The results showed B.velezensis HY19 produced siderophores, chitinase and antifungal substances in soluble and volatile forms. Subsequently, both fungus-free fermentation broth and volatile organic compound produced from B. velezensis HY19 significantly increased O₂^- but decreased proteins in the pathogen hyphae, leading to the antagonism against P. digitatum, P. italicum, G. citri-aurantii and C. gloeosporioides. The fermentation broth also significantly increased the activities of antioxidant enzymes in citrus peels, which might benefit the elimination of reactive oxygen species and alleviate the damage to fruit tissues and cells caused by pathogens. The control efficacy of ^{B. velezensis} HY19 fermentation broth plus prochloraz at half of the recommended dosage (79%) against the rate of wound infection by ^{C. gloeosporioides} was equivalent to that of prochloraz at the recommended dosage (76%), but greater than ^{B. velezensis} HY19 fermentation broth (65%) and prochloraz at half of the recommended dosage (59%). Therefore, the mechanisms of ^{B. velezensis} HY19 against ^{C. gloeosporioides} infection of wounds on postharvest citrus fruit could be iron competition with pathogen fungi, degradation of fungal cell walls, production of antibacterial substances, destruction of cell structure, inhibition on protein synthesis, and induction of systematic resistance in fruit..

Key words: citrus fruit, biological control, B. velezensis