Growers' reports of disease-control failures following the application of copper and streptomycin bactericides led to collecting and testing strains of Pseudomonas syringae for resistance to copper and streptomycin. A comparison of strains isolated from 25 species of diseased woody plants in the Willamette Valley, Oregon, in 1992 and 1993 was made with strains collected in 1982 and 1983 from 30 diseased woody plant species in Oregon and Washington. On differential media supplemented with copper sulfate or streptomycin sulfate, the growth of 467 isolates recovered in 1992 and 1993 was determined. Twenty-four percent of the isolates were found to be copper-resistant (Cu r ) and streptomycin-sensitive (Sm s ), 6% were Cu s Sm r , 24% were Cu r Sm r , and 46% were Cu S Sm S at the concentrations tested. Of 192 strains isolated in 1982 and 1983, 25% were Cu r Sm s , 7% were Cu s Sm r , none were Cu r Sm r , and 68% were Cu s Sm s . In DNA colony hybridizations with digoxigenin-labeled probes, the copABCD probe from P. syringae pv. tomato hybridized with 10% of the Cu r strains isolated in 1992 and 1993, and the copy probe from P. syringae pv. syringae hybridized with a different 6% of the Cu r strains isolated in 1992 and 1993. Neither probe hybridized with any Cu r strains isolated in 1982 and 1983. A DNA probe encoding the streptomycin resistance determinant strA-strB from P. syringae pv. syringae hybridized with 98% of the strains that grew on King's medium B with 100 μg of streptomycin sulfate per ml (KBS) and 4% of the strains that did not. This is the first report of copABCD, cop J , and strA-strB homologues in strains of P. syringae from Pacific Northwest nurseries. The emergence of strains resistant to both copper and streptomycin shows that growers need to explore new methods to control Pseudomonas diseases.
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H. J. Scheck (1996) studied this question.