Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2010
Publication Date: 1/3/2011
Citation: Stockwell, V., Johnson, K.J., Sugar, D., Loper, J.E. 2011. Mechanistically compatible mixtures of bacterial antagonists improve biological control of fire blight of pear. Phytopathology. 101(1):113-123. Interpretive Summary: Biological control is a promising and environmentally-friendly approach for the management of plant diseases in the future, but unexpected variations in its efficacy from field to field have prevented it from being used widely in agriculture to date. In this study, we tried to solve this problem by combining biological control agents for the suppression of fire blight disease. We had discovered earlier that one of the biological control bacteria, Pseudomonas fluorescens A506, produced an enzyme that degrades the antibiotic produced by the second bacteria, Pantoea agglomerans Eh252. Because that antibiotic is key to the biological control of fire blight, its degradation could make the biocontrol ineffective. In this study, we combined a derivative of A506 that did not produce the enzyme with Pantoea agglomerans Eh252, and we tested this combination for suppression of fire blight in five pear orchards. The mixture of the two strains was extremely effective in all five field trials. This study is important for two reasons. First, the mixture of biocontrol strains provided excellent control of fire blight consistently in all field experiments. Its efficacy was similar to streptomycin, which is the best chemical control for this disease. In many pear-growing regions, the fire blight pathogen Erwinia amylovora is now resistant to streptomycin, so new control measures are needed. Secondly, this study demonstrates that mixtures of biological control agents can be very effective if the strains are compatible ecologically and mechanistically. The deployment of these mixtures of biocontrol strains is expected to provide a useful and predictable disease management tool for pome fruit growers and nursery operators, especially in the many areas of the United States where streptomycin has lost effectiveness due to resistance of the pathogen.
Technical 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 wildtype 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.