Abstract
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 Pseudomonas is known for its metabolic versatility and genetic plasticity, encompassing pathogens as well as antagonists. We characterized strain Pseudomonas orientalis F9, an isolate from apple flowers in a Swiss orchard, and determined its antagonistic activity against several phytopathogenic bacteria, in particular Erwinia amylovora, the causal agent of fire blight. P. orientalis F9 displayed antagonistic activity against a broad suite of phytopathogenic bacteria in the in vitro tests. The promising results from this analysis led to an ex vivo assay with E. amylovora CFBP1430Rif and P. orientalis 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 Pythium ultimum as pathogen, F9 showed results comparable to the known antagonist P. protegens CHA0.
Highlights
Pseudomonas is a diverse genus, including pathogens, tritagonists and mutualists that are impacting on larger hosts (Silby et al, 2011; Freimoser et al, 2015)
We evaluated the antagonistic activity of P. orientalis F9 against several plant pathogens in vitro and against the fire blight pathogen, E. amylovora, in vitro and ex vivo
Pseudomonas orientalis F9 was isolated in spring 2014 from Boskoop apple flowers in a Swiss orchard during a screen for novel bacterial biocontrol agents of E. amylovora
Summary
Pseudomonas is a diverse genus, including pathogens, tritagonists (microorganisms of unknown ecological function) and mutualists that are impacting on larger hosts (Silby et al, 2011; Freimoser et al, 2015). Strains of the species Pseudomonas fluorescens are biocontrol agents and include P. fluorescens A506, and P. fluorescens EPS62e that are known for their antagonistic activity against Erwinia amylovora, the causal agent of bacterial fire blight in Rosaceae The most effective control against fire blight is the application of antibiotics, e.g., streptomycin, during the flowering period. The application of antibiotics is limited due to regulatory restrictions and the rise and spread of multiple resistant pathogens (Chiou and Jones, 1995; Manulis et al, 1998; McGhee et al, 2011). In light of the restricted regulations and public concerns over the application of antibiotics and pesticides, the intensified use of beneficial and antagonistic organisms in agriculture is a much favorable alternative (Johnson and Stockwell, 2000; Seibold et al, 2004; Born et al, 2017)
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