Prevalence and persistence of oxytetracycline-resistant and copper-tolerant Xanthomonas arboricola pv. pruni in peach orchards of the southeastern USA

Xanthomonas arboricola pv. pruni (XAP) causes bacterial spot of peach. Current disease management in the southeastern USA mainly relies on routine applications of copper and oxytetracycline (OTC), but copper-tolerant and OTC-resistant XAP have been reported in South Carolina (SC) peach orchards. To study the prevalence and persistence of copper-tolerant and OTC-resistant XAP, a survey was conducted across seven SC and Georgia peach orchards/farms annually during 2021 to 2024. Of the 1,741 XAP isolates collected, 4.4% were copper sensitive; 24.3%, 65.5%, and 5.8% were copper-tolerant up to 150 (LCT150), 175 (LCT175), and 200 µg/ml of copper sulfate pentahydrate, respectively. All the OTC-resistant isolates (24.9% of the population) came from the three conventional SC orchards and contained tetC. In the same orchards, trees with OTC-resistant isolates had significantly higher bacterial spot incidence and severity on both fruit and leaves than the other trees, while higher bacterial leaf spot incidence was observed on trees with LCT175 vs. LCT150 XAP, suggesting potential negative impact of OTC resistance and copper tolerance on the chemical spray programs. In each orchard, bacterial spot incidence, severity, and defoliation increased over time within each season, but the percentage of the resistant/tolerant XAP population remained similar. Regardless of the spray programs utilized, copper-tolerant and OTC-resistant XAP populations have been consistently recovered from the sampled trees across years. Season-long chemical spray programs are still valuable for this disease, but the prevalence and persistence of copper-tolerant and OTC-resistant XAP emphasizes the need for resistance management and development of novel disease management strategies.