Compatibility of bacterial antagonists of Erwinia amylovora with antibiotics used to control fire blight.
In field experiments 2 bacterial antagonists, Pseudomonas fluorescens strain A506 and a streptomycin-resistant mutant of E. herbicola strain C9-1 (C9-1S), that suppress fire blight, were sprayed onto apple blossoms at 30% bloom. Water, streptomycin sulfate or oxytetracycline were sprayed onto blossoms 2 and 7 d after the antagonist to determine their effect on the population dynamics of the 2 strains during bloom. Recovery incidences (the proportion of blossoms supporting detectable bacterial populations) and population sizes of the 2 strains on stigmas within individual blossoms were estimated with a dilution plating assay before and after each antibiotic application. Max. recovery of P. fluorescens A506 and E. herbicola C9-1S from blossoms treated subsequently with water ranged from 58 to 100% and 47 to 100%, respectively. The av. population sizes of both strains were 104-106 c.f.u/flower. Streptomycin did not reduce the incidence of recovery or the population size of either antagonist. Oxytetracycline applications made 2 and 7 d after the antagonist applications reduced the incidence of recovery by 23-58% and also reduced the population size of both strains 10-100 fold. When the first oxytetracycline treatment was delayed to 7 d after the application of the antagonists, only a slight reduction in the incidence of recovery and the population size of either antagonist was observed. It is suggested that the population dynamics of both antagonists, and presumably the degree of protection that they provide, need not be adversely affected by the concomitant use of chemical antibiotics within the same season. It is suggested that opt. integration of biological and chemical methods for suppression of fire blight may require that oxytetracycline applications are delayed until after epiphytic populations of antagonists have become established on flowers.