EPIDEMIOLOGY AND GENETIC-ANALYSIS OF STREPTOMYCIN-RESISTANT ERWINIA-AMYLOVORA FROM MICHIGAN AND EVALUATION OF OXYTETRACYCLINE FOR CONTROL

Streptomycin-resistant strains of Erwinia amylovora were isolated from 14 of 63 Michigan apple orchards surveyed during 1991-1993. Orchards with resistant strains were located in three geographically distinct areas. The genes encoding streptomycin resistance, strA and strB, were usually located on DNA homologous to transposon Tn5393 and to the self-transmissible plasmid pEa34. Ten percent of the resistant strains from one orchard contained DNA homologous to Tn5393 on the chromosome or a resident plasmid, pEA29. Plasmids homologous to pEa34 but lacking Tn5393 were rare in streptomycin-sensitive isolates, appearing in only three adjacent orchards 127 km from the nearest orchard with resistant strains containing pEa34. A streptomycin resistance mechanism unrelated to strA and strB was detected in all resistant isolates from two adjacent orchards and in 30% of the resistant isolates from a distant orchard, but not in resistant isolates from other orchards. Strains with the alternate mechanism were resistant to higher concentrations of streptomycin than were strains with strA-strB. In laboratory tests, streptomycin reduced populations of a streptomycin-sensitive strain of E. amylovora applied to the stigmata of apple flowers, but it did not affect populations of a streptomycin-resistant strain. Populations of both strains remained static on oxytetracycline-sprayed flowers and increased on water-sprayed flowers. Streptomycin was superior to oxytetracycline in reducing the incidence of blight on blossoms inoculated with a streptomycin-sensitive strain, but oxytetracycline was superior to streptomycin in reducing the incidence of blight on blossoms inoculated with a streptomycin-resistant strain.