Bacterial spot, caused by Xanthomonas arbicola pv. pruni, is considered an economically important disease of peach and other stone fruit. In peaches, bacterial spot can affect leaves, twigs, and fruit. The disease can cause slight to nearly complete defoliation of infected trees. When significant defoliation occurs early in the season, fruit size can be reduced. In more severe cases, fruit may become infected. Small water-soaked spots can develop on the fruit at any time during fruit development. Early infection is the most concerning because unsightly cratering or pitting on the fruit can occur. Copper based fungicides are the main product used to manage bacterial spot in peaches and are associated with the potential for elevated phytotoxicity. This damage to the leaves resembles the actual disease and can cause defoliation in severe cases. Bacteriophage technology employs viruses that specifically infect bacteria offering a targeted approach to managing bacterial spot without the risk of phytotoxicity. The efficacy of these products have not been widely tested in the southeastern United States. The objective of this study was to determine the optimal timing during the growing season for applying a bacteriophage product in Central Alabama to achieve control of bacterial spot that is comparable to or better than a traditional copper-based spray program. A study was conducted at the Chilton Research and Extension Center in Clanton, AL. Treatments were applied to single tree replications and each treatment was replicated four times. The study followed a randomized complete block design. The control treatment (standard protocol) consisted of a copper-based product applied at each phenological stage. Bacteriophage treatments consisted of applications of the bacteriophage at select phenological stages in place of the copper-based product. Treatments were applied weekly. The entire canopy of each treatment tree was rated for incidence of bacterial spot every two weeks. At harvest, fruit were separated by degree of bacterial spot infection, counted, and weighed. Applications of bacteriophage during specific phenological stages—such as from pink to open bloom, delayed dormant to early bud swell, and from petal fall to early shuck split—were associated with the lowest incidence of foliar bacterial spot. Across all bacteriophage treatments, most fruit exhibited only mild symptoms and remained marketable. These findings suggest that targeted application timing may improve bacteriophage efficacy, though additional studies are needed to validate and optimize these strategies.
