Kasumin: Field Results for Fire Blight Management and Evaluation of the Potential for Spontaneous Resistance Development in Erwinia amylovora

The aminoglycoside antibiotic kasugamycin (Ks) targets the bacterial ribosome and is particularly active against E. amylovora. The efficacy of Ks formulated as Kasumin 2L (Arysta Corp.; Cary, NC) for control of fire blight was evaluated in six experiments conducted over four field seasons in our experimental orchards in East Lansing, MI. Blossom blight control was statistically equivalent to the industry standard streptomycin in all experiments. We performed a prospective analysis of the potential for kasugamycin resistance (Ks(R)) development in E. amylovora which focused on spontaneous resistance development. In replicated lab experiments, the development of spontaneous resistance in E. amylovora to 250 or 500-ppm Ks was not observed when cells were directly plated on medium containing high concentrations of the antibiotic. However, exposure to increasing concentrations of Ks in media (initial conc. 25 mu g ml(-1)) resulted in the selection of Ks resistance in the E. amylovora strains Ea110, Ea273, and Ea1189. Analysis of mutants indicated they harbored mutations in the kasugamycin target ksgA gene and that all mutants were impacted in relative fitness observable through a reduced growth rate in vitro and decreased virulence in immature pears. Our data confirm the importance of kasugamycin as an alternate antibiotic for fire blight management and lay the groundwork for the development and incorporation of resistance management strategies.