Streptomycin Resistance of Erwinia amylovora Isolated from Apple (Malus domesticus) in Utah
Fire blight, caused by the bacterium Erwinia amylovora, causes devastating losses in multiple fruit and ornamental crops worldwide. It is the most important disease for apple and pear growers in Utah. Currently, the only effective management strategy is the application of streptomycin. In 2006, isolates resistant to streptomycin were detected in an apple orchard in central Utah for the first time. To determine the distribution of resistant isolates and the level of resistance, isolates of E. amylovora were collected between 2006 and 2012 from apple trees across the state and tested for resistance to streptomycin. Each isolate was initially screened against 0, 50, 100, and 1,000 ppm of streptomycin. Selected isolates resistant to 1,000 ppm were exposed to higher concentrations of streptomycin. The majority of resistant isolates were found in Utah Co., the largest apple and pear production area in the state. Resistant isolates tolerated up to 200,000 ppm of streptomycin. The resistance mechanism in all isolates obtained in Utah was identified as a mutation of codon 43 found in the rpsL gene. Accepted for publication 15 August 2013. Published 25 October 2013.