Compatibility of bacterial antagonists of Erwinia amylovora with antibiotics used to control fire blight

In field experiments, two bacterial antagonists that suppress fire blight, Pseudomonas fluorescens strain A506 and a streptomycin-resistant mutant of Erwinia herbicola strain C9-1 (C9-1S), were sprayed onto apple blossoms at 30% bloom, Water, streptomycin sulfate, or oxytetracycline were sprayed onto blossoms 2 and 7 days after the antagonist applications to determine the effect of these chemicals on the population dynamics of P. fluorescens strain A506 and E. herbicola strain C9-1S during bloom. Incidences of recovery (the proportion of blossoms supporting detectable bacterial populations) and population sizes of P. fluorescens strain A506 and E. herbicola strain C9-1S on stigmas within individual blossoms were estimated with a dilution plating assay before and after each antibiotic application. Maximum incidences of recovery of P. fluorescens strain A506 and E. herbicola strain C9-1S from blossoms treated subsequently with water ranged from 58 to 100% and 47 to 100%, respectively; average population sizes of both strains were 10(4) to 10(6) CFU/flower. Streptomycin did not reduce the incidence of recovery or the population size of either antagonist. Oxytetracycline applications made 2 and 7 days after the antagonist applications reduced the incidence of recovery by 23 to 58% and also reduced the population size of both P. fluorescens strain A506 and E. herbicola strain C9-1S by 10- to 100-fold. In contrast, when the first oxytetracycline treatment was delayed to 7 days after the application of the antagonists, only a slight reduction in the incidence of recovery and the population size of either antagonist was observed. The population dynamics of P. fluorescens strain A506 and E. herbicola strain C9-1S, and presumably the degree of protection that they provide, need not be adversely affected by the concomitant usage of chemical antibiotics within the same season. Optimal integration of biological and chemical methods for suppression of fire blight, however, may require that oxytetracycline applications be delayed until after epiphytic populations of antagonists have become established on flowers.