Mechanistically Compatible Mixtures of Bacterial Antagonists Improve Biological Control of Fire Blight of Pear

Abstract

Mixtures of biological control agents can be superior to individual agents in suppressing plant disease, providing enhanced efficacy and reliability from field to field relative to single biocontrol strains. Nonetheless, the efficacy of combinations of Pseudomonas fluorescens A506, a commercial biological control agent for fire blight of pear, and Pantoea vagans strain C9-1 or Pantoea agglomerans strain Eh252 rarely exceeds that of individual strains. A506 suppresses growth of the pathogen on floral colonization and infection sites through preemptive exclusion. C9-1 and Eh252 produce peptide antibiotics that contribute to disease control. In culture, A506 produces an extracellular protease that degrades the peptide antibiotics of C9-1 and Eh252. We hypothesized that strain A506 diminishes the biological control activity of C9-1 and Eh252, thereby reducing the efficacy of biocontrol mixtures. This hypothesis was tested in five replicated field trials comparing biological control of fire blight using strain A506 and A506 aprX::Tn5, an extracellular protease-deficient mutant, as individuals and combined with C9-1 or Eh252. On average, mixtures containing A506 aprX::Tn5 were superior to those containing the wild-type strain, confirming that the extracellular protease of A506 diminished the biological control activity of C9-1 and Eh252 in situ. Mixtures of A506 aprX::Tn5 and C9-1 or Eh252 were superior to oxytetracycline or single biocontrol strains in suppressing fire blight of pear. These experiments demonstrate that certain biological control agents are mechanistically incompatible, in that one strain interferes with the mechanism by which a second strain suppresses plant disease. Mixtures composed of mechanistically compatible strains of biological control agents can suppress disease more effectively than individual biological control agents.

Improvement of Fitness and Efficacy of a Fire Blight Biocontrol Agent via Nutritional Enhancement Combined with Osmoadaptation

“…This can be due to many reasons, including the performance of the biocontrol agent (33), pathogen aggressiveness (7), type (species and cultivar) and phenological state (prebloom, bloom, harvest, etc.) of the host, or environmental conditions (climate and location) (33). Then, to counteract partially the risk of inconsistent results, we covered a wide range of host conditions using pear and apple as model plant species, different apple (Modi and Golden) and pear (Doyenne du Comice, Abate Fetel, Passe Crassane, and Conference) cultivars, and different plant organs (flowers and immature fruit).…”

Characterization of the Biosynthetic Operon for the Antibacterial Peptide Herbicolin in Pantoea vagans Biocontrol Strain C9-1 and Incidence in Pantoea Species

“…Thirty-four streptomycin-sensitive isolates and 25 streptomycin-resistant isolates of E. amylovora from commercial orchards in the U.S. Pacific Northwest (25) were evaluated for sensitivities to the antibiotics of P. vagans C9-1, using the double diffusion assay on MGA medium described above. L-Histidine (10 mM) was added to the overlay medium in replicate plates for each isolate to suppress antibiosis due to production of pantocin A. Additionally, isolates of Ea153 that were recovered from 20 diseased blossom clusters on pear trees treated twice with P. vagans C9-1 during bloom also were tested (42). Sensitivity assays were repeated twice.…”

Co-inoculum of Beauveria brongniartii and B. bassiana shows in vitro different metabolic behaviour in comparison to single inoculums

“…To increase the biocontrol efficacy, combinations of different antagonist organisms have been utilized: fungal mixtures 6,7 and bacterial/fungal mixtures 5,8–10 or only bacterial mixtures 11,12 . The application of a dual inoculum composed by Beauveria bassiana and Metarhizium flavoviride to control grasshoppers populations was used, for example, to overcome some of the temperature constraints encountered in the use of a single species 13 .…”