Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2010
Publication Date: 12/1/2010
Citation: Stockwell, V., Johnson, K.J., Sugar, D., Loper, J.E. 2010. Control of fire blight by Pseudomonas fluorescens A506 and Pantoea vagans C9-1 applied as single strains and mixed inocula. Phytopathology. 100(12):1330-1339. Interpretive Summary: Fire blight is an important disease of pear, apple, and related plants, which causes significant losses to fruit and nursery growers in the United States and abroad. In the past, the disease was managed by spraying trees with streptomycin, but in many regions of the United States, the fire blight pathogen (a bacterium called Erwinia amylovora) is now resistant to this antibiotic. Consequently, streptomycin sprays are no longer very effective for management of fire blight and alternative control measures are needed. In this study, we evaluated two biological control agents for suppression of fire blight in ten field trials in pear and apple orchards. We learned that one of the biological control agents, Pantoea vagans C9-1, was quite effective in suppressing fire blight, decreasing disease by an average of 42% over all field trials. We mixed P. vagans C9-1 with a second biological control agent, Pseudomonas fluorescens A506, and we were surprised that the mixture did not perform better than either strain alone. We formulated an explanation for this surprising result, and tested it in a companion study. This study was important because it identified a biological control agent (P. vagans C9-1) that was effective in suppressing fire blight in ten replicated field plots, and provided direction for further study to improve the efficacy and reliability of biological control.
Technical Abstract: The biological control agents Pseudomonas fluorescens A506 and Pantoea vagans C9-1 were evaluated individually and in combination for the suppression of fire blight of pear or apple in ten field trials inoculated with the pathogen Erwinia amylovora. The formulation of pathogen inoculum applied to blossoms influenced establishment of the pathogen and the efficacy of biological control. P. vagans C9-1 suppressed fire blight in all five trials in which the pathogen was applied as lyophilized cells but in none of the trials in which the pathogen was applied as freshly-harvested cells. In contrast, P. fluorescens A506 reduced disease significantly in only one trial. A mixture of the two strains also suppressed fire blight, but the magnitude of disease suppression over all field trials (averaging 32%) was less than that attained by C9-1 alone (42%). The two biological control agents did not antagonize one another on blossom surfaces, and application of the mixture of A506 and C9-1 to blossoms resulted in a greater proportion of flowers having detectable populations of at least one bacterial antagonist than the application of individual strains. Therefore, the mixture of A506 and C9-1 provided less disease control than expected based upon the epiphytic population sizes of the antagonists on blossom surfaces. We speculate that the biocontrol mixture was less effective than anticipated due to incompatibility between the mechanisms by which A506 and C9-1 suppress disease.