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Copper tolerance in Xanthomonas arboricola pv. pruni in South Carolina peach orchards.

Bacterial spot of peach, caused by Xanthomonas arboricola pv. pruni (Xap), causes yield loss every year in southeastern United States peach orchards. Management is mainly driven by season-long applications of copper-based products, site location, and choice of cultivar. Although tolerance to copper has not been reported in Xap in the United States, adaptation of populations due to frequent use is a concern. We collected Xap from shoot cankers, leaves, and fruit of cv. O'Henry over two years from three conventional farms and one organic farm in South Carolina, one orchard per farm. The four farms had been using copper extensively for years to control bacterial spot. Xap was isolated from four canker types (bud canker, tip canker, non-concentric canker, and concentric canker) in early spring ('bud break'), as well as from leaf and fruit tissues later in the season at phenological stages 'pit hardening' and 'final swell'. Xap was most frequently isolated from cankers of the organic farm (24% of the cankers) and most isolates (45%) came from bud cankers. Xap isolates were assessed for sensitivity to copper using minimal glucose yeast agar and nutrient agar amended with 38 µg/ml or 51 µg/ml of Cu2+. Two phenotypes of copper-tolerance in Xap were discovered: low copper tolerance (LCT: growth up to 38 µg/ml Cu2+) and high copper tolerance (HCT: growth up to 51 µg/ml Cu2+). A total of 26 (23 LCT and 3 HCT) out of 165 isolates in 2018 and 32 (20 LCT and 12 HCT) out of 133 isolates in 2019 were tolerant to copper. Peach leaves on potted trees were sprayed with copper rates typically applied at 'delayed dormancy' (high rate; 2,397 µg/ml Cu2+), at 'shuck split' (medium rate; 599 µg/ml Cu2+), and during 'summer cover sprays' (low rate; 120 µg/ml Cu2+) and subsequently inoculated with sensitive, LCT and HCT strains. Results indicated that the low and medium rates of copper reduced bacterial spot incidence caused by the sensitive strain but not by the LCT and HCT strains. This study confirms existence of Xap tolerance to copper in commercial peach orchards in the southeastern United States and suggests its contribution to bacterial spot development under current management practices.