Potential modes of action of Pseudomonas fluorescens ZX during biocontrol of blue mold decay on postharvest citrus

BACKGROUND The application of chemical fungicides is currently the main method for the control of postharvest decay of fruits and vegetables. However, public concern has been growing towards the effect of fungicides on food safety, human health and environmental pollution. Thus, interest in microbial biocontrol agent development has grown, such agents being considered both safe and environmentally friendly. Pseudomonas fluorescens is widely distributed in nature, and one of the most valuable biocontrol and plant growth-promoting rhizobacteria. In this study, the efficacy and the potential associated modes of action of P. fluorescens ZX against Penicillium italicum on oranges (Citrus sinensis Osbeck) were investigated. RESULTS The application of P. fluorescens ZX significantly reduced blue mold lesion size and incidence in comparison to the control, where P. fluorescens ZX was effective when applied preventatively but not curatively. In dual cultures, treatment with cell-free autoclaved cultures or culture filtrate had a limited capacity to suppress P. italicum, while P. italicum was inhibited by bacterial fluid and bacterial suspension with living cells in vitro. The P. fluorescens ZX isolate displayed protease, but not chitinase, glucanase or cellulose, activity, and produced siderophores and volatile organic compounds with antifungal abilities. Competition tests showed P. fluorescens ZX could use fructose, sucrose, aspartic acid, threonine, serine, glycine, valine, lysine and proline better than P. italicum. Furthermore, an effective biofilm that peaked after a 24-hour incubation at 30 degrees C was formed by the P. fluorescens ZX isolate. Light microscopy and scanning electron microscopy observations indicate the P. fluorescens ZX isolate could not undergo direct parasitism or hyperparasitism. CONCLUSIONS Competition for nutrients and niches, biofilm formation, inhibition of spore germination and mycelial growth, and production of inhibitory metabolites may play important roles in P. fluorescens ZX antagonism of P. italicum. (c) 2019 Society of Chemical Industry