Evaluation of Streptomycin, Oxytetracycline, and Copper Resistance ofErwinia amylovoraIsolated 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 μg/ml) or CuSO 4 (0.16 mM). Nevertheless, spontaneous mutants with tolerance to 0.16 mM CuSO 4 were observed at a frequency of 10 −6 -10 −7 mutant colonies per wild type colony in most strains