Pseudomonas orientalis F9: A potent antagonist against phytopathogens with phytotoxic effect in the apple flower

© 2018 Zengerer, Schmid, Bieri, Müller, Remus-Emsermann, Ahrens and Pelludat. 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 CFBP1430Rifand 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

Exploring the phyllosphere bacterial community for improving tree crops protection

Plants are able to interact with plentiful bacteria resulting in a number of positive or negative outcomes for plant health. The ecological balance between pathogens and beneficial bacteria could be strategically disturbed and manipulated for improving host plant protection. As bacterial communities present in the phyllosphere of herbaceous plants have been largely studied, a number of biocontrol agents for controlling host diseases are already identified and used with promising results. A few studies on the use of phyllosphere biocontrol agents on woody crop tree plants have revealed encouraging results toward a future where plant disease control could be attained without the application of chemical compounds. In addition to the use of biocontrol agents, disease suppression can be achieved by the manipulation of microbial communities through plant management practices. In this review, an overview of the available knowledge on phyllosphere bacterial communities of woody tree crop species is provided, giving special emphasis to the structural differences of bacterial communities living on and within important tree crop species. Studies and challenges on the application and/or manipulation of these bacteria under in planta conditions are discussed, disclosing new sustainable ways for dealing with woody crop diseases.This work was partially funded by European Structural and Investment Funds in the FEDER component, through the Operational Competitiveness and Internationalization Programme (COMPETE 2020); and national funds, through the FCT – Portuguese Foundation for Science and Technology under the project POCI-01-0145-FEDER-031133. J.D. Mina thanks FCT, POPH-QREN and FSE for PhD grant SFRH/BD/105341/2014