EVALUATION OF STREPTOMYCIN, OXYTETRACYCLINE, AND COPPER RESISTANCE OF ERWINIA-AMYLOVORA ISOLATED FROM PEAR ORCHARDS IN WASHINGTON-STATE

One hundred and thirty-eight pathogenic strains of E. amylovora were isolated from fire blight cankers of pear trees from 44 orchards in the major pear-growing regions of Washington State. Ninety-eight strains, isolated from 38 of the orchards sampled, were resistant to streptomycin (1 mg/ml). Streptomycin-resistant strains of E. amylovora were ubiquitous in all of the major pear-growing regions of Washington. None of the strains tested were resistant to oxytetracycline (25-mu-g/ml) or CuSO4 (0.16 mM). Nevertheless, spontaneous mutants with tolerance to 0.16 mM CuSO4 were observed at a frequency of 10(-6)-10(-7) mutant colonies per wild type polonyony in most strains.