Identification of a Reduced Rate Combination of a Plant Growth Inhibitor with a Plant Defense Inducer for the Management of the Shoot Blight Phase of Fire Blight

The secondary shoot blight phase of fire blight is a critical component of disease epidemics in apples, pears, and other Rosaceae family plants with infection occurring at the tips of vigorously growing branches. Shoot blight infections are exacerbated in modern high-density apple plantings, where growers emphasize maximizing tree growth to recapture planting costs and increase yields of high-quality fruit. The overarching goal of this study was to develop new strategies for shoot blight management that do not impact the growth and yield of young apple trees. 'Gala' apple trees of various ages were inoculated with the fire blight pathogen <i>Erwinia amylovora</i>. Being treated with a combination of reduced rate mixtures of prohexadione calcium (ProCa; 6-12× rate reduction) with acibenzolar-<i>S</i>-methyl (ASM; 2× reduction) resulted in a significant decrease in shoot blight incidence and severity without significant impacts on branch growth. The systemic spread of <i>E. amylovora</i> was significantly reduced in trees sprayed with these lower-rate mixtures. Comparable rates of either treatment alone were not as effective in reducing lesion length. A transcriptomic analysis revealed a synergistic effect in which the expression of marker genes associated with systemic acquired resistance was higher in apple trees sprayed with the low-rate mixture of ProCa + ASM than with either compound alone. We conclude that the combination of ProCa + ASM at reduced rates is an effective treatment for the shoot blight phase of fire blight without impacting horticultural practices associated with high-density apple production.