Fire blight caused by the bacterium Erwinia amylovora, is a commercially significant disease of apple and pear. The use of phage-based biopesticides is an environmentally responsible alternative to chemical pesticides currently in use. Field trails applying both Erwinia spp. phage and Pantoea agglomerans resulted in 6 out of 13 treatments significantly reducing disease incidence. P. agglomerans is a non-pathogenic orchard epiphyte capable of maintaining a replicating phage population prior to pathogen introduction. Optimal efficacy was achieved with a suspension of phage and epiphyte at 10⁸ units/ml each (ratio of 1:1). Treatment efficacies were also not statistically different from the “gold standard” treatment streptomycin. Multiplex real-time quantitative PCR was used to monitor population ecology during treatments. In treatments with significant disease control, the phage population increased at the expense of the P. agglomerans population. Application of pathogen resulted in a continual increase in phage population now at the expense of the pathogen. The reduction in pathogen population correlated with reduced disease incidence. Studies on capsular polysaccharides (EPS), which may play a protective function against phage attack, were also conducted. A difference in E. amylovora isolate susceptibility to phage attack was found to be influence by EPS production. Phages within the family Podoviridae showed a high efficiency of plating on hosts that produced large amounts of EPS, whereas Myoviridae have similar results but on isolates that produced low amounts of EPS. From this, treatments that match efficient lytic activity to its ideal host should increase treatment efficacy, thereby reducing disease incidence.