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TECHNICAL SESSION: Population diversity of plant pathogenic bacteria

Prevalence of Streptomycin-resistant Erwinia amylovora and Epiphytic Bacteria in Ohio Apple Orchards
Alejandra Jimenez Madrid - The Ohio State University. Melanie Lewis Ivey- Ohio State University

Fire blight, caused by the bacterium Erwinia amylovora (Ea), occurs annually and is the most destructive bacterial disease in Ohio apple orchards. Management of fire blight is challenging and requires an integrated approach to minimize losses. Antibiotics are very effective to reduce Ea populations during bloom. Streptomycin, oxytetracycline, and kasugamycin are registered in the United States (US) for fire blight management. Streptomycin is the most cost-effective antibiotic and is frequently used in Ohio apple orchards to control fire blight. While streptomycin-resistant (SmR) Ea strains have been reported in several northeastern US states, the prevalence and mechanism of SmR in Ohio orchards has not been reported. In 2018, flowers and shoots from 24 orchards with trees known to be infected with Ea were collected and a total of 981 putative Ea isolates were purified and screened for streptomycin resistance using bioassays. From these 918 isolates, 215 were confirmed as Ea using PCR with primers specific to Ea plasmid pEA29. Forty-six percent of the strains were SmR, of which 18% had a minimum inhibitory concentration (MIC) value >500 ug/ml and 26% had MIC values >2500 ug/ml. Streptomycin resistance was observed in 67% of the remaining 766 epiphytic isolates. Overall, SmR Ea or epiphytic bacteria were recovered from 19 or 21 of the 24 orchards, respectively. The molecular mechanism of streptomycin resistance in all the SmR strains is currently under investigation.