Control of postharvest green mold of citrus fruit by application of endophytic Paenibacillus polymyxa strain SG-6

A bacterial strain SG-6, isolated as an endophyte from the root tissue of Sophora tonkinensis, was identified as Paenibacillus polymyxa based on morphology, 16S rDNA gene analysis and Biolog tests. The strain was found to be highly effective in reducing postharvest green mold decay of citrus fruit caused by Penicillium digitatum. In an in vitro assay by dual-culture of strain SG-6 and P. digitatum on potato dextrose agar plates, the bacterium significantly inhibited growth of P. digitatum. Conidial germination of the pathogen was greatly inhibited in the presence of the living bacterial cell suspensions. In tests of in vivo activity, the unwashed cell suspension of strain SG-6 was more effective than the washed cell suspension and culture filtrate. Green mold control improved when the period between treatment with strain SG-6 and inoculation with the pathogen was increased. The concentration of the antagonist also had a significant effect on biocontrol effectiveness. At the concentration of 1 × 109 cells mL−1 of strain SG-6, the disease incidence and lesion diameter declined to 18.3% and 5.5 mm, respectively, which were significantly lower than control after 5 d of incubation at 25 °C. Population of strain SG-6 increased rapidly in citrus fruit wounds when inoculated at 25 °C but remained static at 6 °C. The bacterium also significantly reduced the natural development of green mold decay of citrus fruit following storage at 25 °C for 4 weeks or at 6 °C for 4 weeks followed by 25 °C for 2 weeks. Furthermore, it did not impair the quality parameters, including weight loss, firmness, total soluble solids, ascorbic acid, and titratable acidity. To our knowledge, this is the first report about antagonistic P. polymyxa strain SG-6 as a promising biocontrol agent against postharvest green mold of citrus fruit.

► A novel antagonistic bacterial strain SG-6 was identified as Paenibacillus polymyxa.
► The isolate significantly reduced postharvest green mold decay of citrus fruit.
► This particular strain also reduced natural decay development of citrus fruit.
► The bacterium did not impair the quality parameters of fruit.
► P. polymyxa can therefore be used to control postharvest green mold.