A total of 323 strains of Pseudomonas syringae pv. syringae were collected from six pear orchards in the Mid-Columbia region of Oregon and Washington from 1989 through 1992. Of all strains of P s. pv. syringae, 8, 25, 75, and 99% did not grow on modified casitone-yeast extract-glycerol medium amended with 0.25, 0.5, 1.0, and 2.0 mM CuSO 4 , respectively. Strains of P. s. pv. syringae resistant to 50, 100, 250, and 500 μg of oxytetracycline per ml were recovered from six, three, two, and one of the six orchards, respectively. Nevertheless, over 70 and 90% of the strains in all six orchards were sensitive to 50 and 250 μg of oxytetracycline per ml, respectively. Strains of P. s. pv. syringae resistant to 50 and 500 μg of streptomycin per ml were found in six and four of six orchards, respectively. Twenty-five strains were resistant to both copper (1 mM) and streptomycin (100 μg/ml), and three of those were resistant to copper, streptomycin, and oxytetracycline (250 μg/ml). To our knowledge, this is the first report demonstrating resistance of strains of P s. pv. syringae to copper, streptomycin, and oxytetracycline. Resistance correlated positively to the antibiotic spray programs in the orchards.
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R. A. Spotts (1995) studied this question.