Pseudomonas orientalis F9: A Potent Antagonist against Phytopathogens with Phytotoxic Effect in the Apple Flower

In light of public concerns over the use of pesticides and antibiotics in plant protection and the subsequent selection for spread of resistant bacteria in the environment, it is inevitable to broaden our knowledge about viable alternatives, such as natural antagonists and their mode of action. The genus is known for its metabolic versatility and genetic plasticity, encompassing pathogens as well as antagonists. We characterized strain F9, an isolate from apple flowers in a Swiss orchard, and determined its antagonistic activity against several phytopathogenic bacteria, in particular , the causal agent of fire blight. F9 displayed antagonistic activity against a broad suite of phytopathogenic bacteria in the tests. The promising results from this analysis led to an assay with CFBP1430 and F9 infected detached apple flowers. F9 diminished the fire blight pathogen in the flowers but also revealed phytotoxic traits. The experimental results were discussed in light of the complete genome sequence of F9, which revealed the strain to carry phenazine genes. Phenazines are known to contribute to antagonistic activity of bacterial strains against soil pathogens. When tested in the cress assay with as pathogen, F9 showed results comparable to the known antagonist CHA0.