COMPATIBILITY OF BACTERIAL ANTAGONISTS OF ERWINIA-AMYLOVORA WITH ANTIBIOTICS USED TO CONTROL FIRE BLIGHT

In field experiments, two bacterial antagonists that suppress fire bli ght, Pseudomonas fluorescens strain A506 and a streptomycin-resistant mutant of Erwinia herbicola strain C9-1 (C9-1S), were sprayed onto app le blossoms at 30% bloom, Water, streptomycin sulfate, or oxytetracycl ine were sprayed onto blossoms 2 and 7 days after the antagonist appli cations to determine the effect of these chemicals on the population d ynamics of P. fluorescens strain A506 and E. herbicola strain C9-1S du ring bloom. Incidences of recovery (the proportion of blossoms support ing detectable bacterial populations) and population sizes of P. fluor escens strain A506 and E. herbicola strain C9-1S on stigmas within ind ividual blossoms were estimated with a dilution plating assay before a nd after each antibiotic application. Maximum incidences of recovery o f P. fluorescens strain A506 and E. herbicola strain C9-1S from blosso ms treated subsequently with water ranged from 58 to 100% and 47 to 10 0%, respectively; average population sizes of both strains were 10(4) to 10(6) CFU/flower. Streptomycin did not reduce the incidence of reco very or the population size of either antagonist. Oxytetracycline appl ications made 2 and 7 days after the antagonist applications reduced t he 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 treatm ent was delayed to 7 days after the application of the antagonists, on ly 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 affe cted by the concomitant usage of chemical antibiotics within the same season. Optimal integration of biological and chemical methods for sup pression of fire blight, however, may require that oxytetracycline app lications be delayed until after epiphytic populations of antagonists have become established on flowers.