Improved biological control of fire blight of pear and apple by introduction of antagonistic bacteria into unopened flowers

Fire blight, caused by the bacterium Erwinia amylovora, is a very destructive disease of pome fruit trees worldwide. Even with an integrated program of chemical and biological control combined with sanitation and orchard management, fire blight is very difficult to control when environmental conditions favor disease. Strategies that enhance potential biological control agents would be very useful to controlling fire blight and reducing antibiotic resistance in the environment. Application of Pseudomonas fluorescens strain A506 (in the form of the commercial bio-pesticide BlightBan A506) with the organo-silicon surfactant Breakthru resulted in a higher proportion of colonized flowers of both 'Bartlett' pear and 'Pink Lady' apple, especially early in the growing season than when the bacterium was applied in water alone. The proportion of flowers colonized with strain A506 throughout the spring generally increased with increasing concentrations of surfactant in which the bacteria were applied in a single application at the time of first bloom. Importantly, most flowers emerging from trees that were inoculated with strain A506 only a single time at "first bloom"' with BlightBan A506 in 0.2% or 0.5% Breakthru were as high as or higher throughout the main bloom and into delayed bloom than that on trees that received weekly applications of the same amount of BlightBan A506. The most effective colonization of flowers from the single early-season spray application of antagonist with surfactant occurred in orchards in which most of the flowers, while not yet open, were poised to open; less colonization occurred in orchards that were sprayed at an earlier stage of bloom. No russeting of fruit was observed in any plot when surfactant was applied before substantial bloom had occurred. The number of applications of strain A506 apparently can be reduced by applying the antagonistic bacterium early in the season with a penetrating surfactant.