Evaluation of Streptomycin, Oxytetracycline, and Copper Resistance of Erwinia amylovora Isolated from Pear Orchards in Washington State. Joyce E. Loper, Agricultural Research Service, U.S. Department of Agriculture, Horticultural Crops Research Laboratory, 3420 N.W. Orchard Avenue, Corvallis, OR 97330. Marcella D. Henkels, Rodney G. Roberts, Gary G. Grove, Michael J. Willet, and Timothy J. Smith. Agricultural Research Service, U.S. Department of Agriculture, Horticultural Crops Research Laboratory, 3420 N.W. Orchard Avenue, Corvallis, OR 97330; Agricultural Research Service, U.S. Department of Agriculture, Tree Fruit Research Laboratory, 1104 N. Western Avenue, Wenatchee, WA 98801; Washington State University, Tree Fruit Research and Extension Center, 1100 N. Western Avenue, Wenatchee, WA 98801; Yakima County Cooperative Extension, 233 Courthouse, Yakima, WA 98901; and Chelan-Douglas-Okanogan Counties Cooperative Extension, 400 Washington Street, Wenatchee, WA 98801. Plant Dis. 75:287-290. Accepted for publication 11 September 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/PD-75-0287.

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 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 colony in most strains.