Oxytetracycline and Streptomycin Resistance Genes in Xanthomonas arboricola pv. pruni , the Causal Agent of Bacterial Spot in Peach

pv. ( ) causes bacterial spot, a major worldwide disease of species. Very few chemical management options are available for this disease and frequent applications of oxytetracycline (OTC) in the United States peach orchards have raised concerns about resistance development. During 2017-2020, 430 strains were collected from ten peach orchards in South Carolina. Seven OTC-resistant (OTC ) strains were found in 2017 and 2020 from four orchards about 20-270 km apart. Interestingly, the seven strains were also resistant to streptomycin (STR). Six strains grew on media amended with ≤100 μg/mL OTC, while one strain, R1, grew on ≤250 μg/mL OTC. Genome sequence analysis of four representative OTC strains revealed a 14-20 kb plasmid carrying , , and in each strain. These three genes were transferable to via conjugation, and they were PCR confirmed in all seven OTC strains. When and were cloned and expressed together in a sensitive strain, the transconjugants showed resistance to ≤100 μg/mL OTC. When was cloned and expressed alone in a sensitive strain, the transconjugants showed resistance to ≤250 μg/mL OTC. and expression was inducible by OTC in all six wild-type strains resistant to ≤100 μg/mL OTC. However, in the R1 strain resistant to ≤250 μg/mL OTC, was not expressed, possibly due to the presence of Tn in the gene, and in this case was constitutively expressed. These data suggest that confers OTC resistance in strains, and regulates the level of OTC resistance conferred by . To our knowledge, this is the first report of OTC resistance in plant pathogenic xanthomonads.