Biocontrol of Penicillium digitatum on Postharvest Citrus Fruits by Pseudomonas fluorescens

The effectiveness of the bacteria antagonist Pseudomonas fluorescens to control green mold caused by Penicillium digitatum on oranges (Citrus sinensis Osbeck, cv. Jincheng) and the possible modes of action were evaluated. Whether in vitro or in vivo, treatments with cell-free autoclaved cultures or culture filtrate had limited capacity to suppress P. digitatum, while P. digitatum was significantly inhibited by bacterial fluid (P. fluorescens in the nutrient broth liquid medium) and bacterial suspension (P. fluorescens in sterile distilled water) with living cells. There was a positive relationship between the concentration of P. fluorescens in bacterial suspension and its biological efficacy. In addition, P. fluorescens was effective when applied preventatively but not when applied curatively. In the inoculated wounds, the population of P. fluorescens was an approximately 28- and 34-fold increase after being incubated at 20°C for 8 d and at 4°C for 16 d, respectively, and P. digitatum could effectively stimulate the growth and reproduction of P. fluorescens. Moreover, P. fluorescens was able to inhibit spore germination and germ tube elongation of P. digitatum as well as induce resistance on citrus peel by increasing the chitinase (CHI) activity and advancing the activities peaks of β-1,3-glucanase (GLU), peroxidase (POD), and phenylalanine ammonia lyase (PAL). All of these results support the potential application of P. fluorescens against green mold on postharvest citrus