Overwintering Capability of Xanthomonas arboricola pv. pruni Strains with Different Bactericidal Sensitivities on Peach Trees.

Abstract

Xanthomonas arboricola pv. pruni (XAP) causes bacterial spot of peach and is primarily managed in commercial South Carolina peach orchards with copper and oxytetracycline applications. The pathogen overwinters in peach twigs and forms spring cankers which generate primary inoculum for early-season leaf and fruitlet infections. Copper-tolerant and oxytetracycline-resistant strains have recently been reported, but their overwintering capability in the absence of selection pressure is unknown. Similarly, the impact of infection during different phenological stages on XAP overwintering is unknown. To investigate these factors, peach twigs were sprayed with strains of XAP with different sensitivities to copper and oxytetracycline in an experimental orchard during bud set (June), after harvest (August), and during leaf drop (October) in the fall. Viable XAP from peach buds was quantified using qPCR with or without propidium monoazide (PMA) treatment. Results indicated that bud cankers were a major inoculum reservoir for XAP, and the overwintered XAP densities in buds were significantly and positively correlated with spring canker ratings (R² = ~0.6, p < 0.001). Inoculation of peach twigs at all three time points yielded overwintering XAP populations and canker symptoms in spring, with the highest amount of canker and XAP from the leaf drop [42.7% canker incidence, 31,258 CFU/bud] and after harvest [30.4% canker incidence, 8,511 CFU/bud] inoculations. There were no significant differences in overwintering capability of XAP strains in the absence of copper or oxytetracycline sprays, suggesting that these oxytetracycline-resistant and copper-tolerant strains may persist and accumulate in peach orchards across seasons.