Fire blight, a bacterial disease of rosaceous plants caused by Erwinia amylovora, is one of the most important diseases affecting commercial apple production worldwide. Antibiotics, applied at bloom to protect against blossom infection, are the most effective means of management but raise concern due to the potential for antibiotic resistance in both the pathogen population and nontarget organisms. In addition, most fire blight outbreaks in New York State often emerge in late June to July as shoot blight, calling into question the role of blossom infections and the antibiotic applications made to manage them. Prohexadione-calcium (PhCa) is a gibberellic acid inhibitor used post-bloom to control shoot vigor and to manage shoot blight. However, the magnitude of shoot blight management is directly related to the suppression of shoot growth, which is undesirable, especially in young orchards during establishment years. PhCa is believed to control shoot blight by thickening cell walls in cortical parenchyma, preventing invasion of host tissues by E. amylovora. We hypothesize that PhCa applied pre-bloom could similarly prevent invasion of blossom pedicels following infection, leading to reduced disease incidence. We evaluated novel pre-bloom PhCa programs for their effects on disease management (blossom and shoot blight) as well as their impact on shoot growth for three years in a mature ‘Gala’ orchard in New York. In all three years of the study, all PhCa programs resulted in less than 27% incidence (71% control) of blossom blight and less than 13% incidence (77% control) of shoot blight with minimal effect on tree growth. Inclusion of a biopesticide during bloom further reduced the incidence of blossom blight in one year of three. Using light microscopy, we found that cell walls in the cortical parenchyma of fruitlet pedicels on trees receiving pre-bloom PhCa applications were significantly thicker than those of untreated trees 40 days after full bloom and inoculation. Overall, we found that pre-bloom applications of PhCa had utility in reducing blossom blight and shoot blight with minimal impacts on tree growth. These pre-bloom programs would fit with standard production practices and may contribute toward the development of fire blight management programs without the use of antibiotics.
