Survey of Erwinia amylovora, Causal Agent of Fire Blight, From Apple and Pear Orchards in Utah for Streptomycin Resistance

Fire blight caused by the bacterium Erwinia amylovora results in millions of dollar in losses worldwide. It is the most important disease problem for apple and pear growers in Utah. Currently the only effective management strategy is application of the antibiotic streptomycin. In 2006, resistant isolates were detected in an apple orchard in Utah County. To determine the distribution of resistant isolates, samples collected in 2006, 2007, 2010 and 2011 from orchards across Utah were tested for resistance to streptomycin. Isolates were screened at 0, 100 and 1000ppm of streptomycin. Bacteria were spread on LB agar. A hole was punched from the agar and the streptomycin solution was pipetted into the well. After 24 hours a bacteria-free zone around the well was observed for sensitive isolates but not for resistant isolates. The majority of resistant isolates were found in Utah County where most orchards are located. The resistance in most Utah isolates is caused by a mutation in the rpsL gene but at least one isolate acquired a plasmid containing streptomycin resistance genes