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

The application of chemical fungicides is currently the main method to control 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, which is 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.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 cellulase 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-h incubation at 30°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.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. This article is protected by copyright. All rights reserved.