An Overview of Streptomycin Resistance in Erwinia amylovora from Ohio Apple Orchards

Streptomycin is the primary method used to control apple blossom infections caused by Erwinia amylovora in the United States (US). Improper use of streptomycin can contribute to resistance development in E. amylovora and other epiphytic bacteria in the apple tree. Streptomycin resistant (SmR) E. amylovora strains have been reported in several apple production states in the US, however, the prevalence and genetic basis of E. amylovora resistance in Ohio (OH) orchards is reported for the first time in this study. While most strains isolated from OH were sensitive to streptomycin, 7.8% had a resistant phenotype with a minimum inhibitory concentration (MIC) of 500 or 2500 µg/ml. Resistance was conferred by the linked strA-strB genes on the non-conjugative plasmid pEA29. Only one strain (MIC=2500 µg/ml) did not have the linked strA-strB genes or a mutation in codon 43 of rspL. Epiphytic bacteria, including other species of Erwinia and species of Pantoea, with a resistant phenotype were also recovered from apple blossoms. Epiphytic isolates (44%) were resistant up to 500 µg/ml and 56% up to 2500 µg/ml. The genetic basis for resistance of a subset of epiphytic bacteria (n=54) was confirmed to be through the presence of the plasmid transmitted gene pair, strA/strB.