Evaluation of a contact sterilant as a niche-clearing method to enhance the colonization of apple flowers and efficacy of Aureobasidium pullulans in the biological control of fire blight

Abstract

Due to rapid expansion of organic apple production and issues with antibiotic resistance in conventional production, there is a need for novel strategies to protect orchards from outbreaks of fire blight caused by the bacterial pathogen Erwinia amylovora. The biological control material, Blossom Protect (consisting of two strains of the fungus Aureobasidium pullulans), has shown promising results in some apple growing regions but has shown reduced efficacy in humid apple-growing regions such as the Midwestern and Eastern United States. In an attempt to increase both flower colonization by A. pullulans and disease control efficacy, we applied a contact sterilant to clear the flower niche of native microbial populations 4 hr prior to application of Blossom Protect. Results from four years of field testing showed that application of the contact sterilant resulted in a significant reduction in populations of bacteria, fungi, and native yeast populations; however, all of these microbial populations recovered after 24 h. Examination of fire blight incidence revealed that use of the contact sterilant did not significantly impact disease control. Two applications of Blossom Protect, at 70–80% bloom and full bloom, exhibited the best efficacy in disease control over four years. In addition, we observed marginal differences in the marketability of the final fruit between treatments, each with less than 8% deemed unmarketable due to russet. Our results indicate that niche-clearing of apple flowers did not enhance flower colonization by A. pullulans, nor produce an increase in biological control efficacy. However, our current experimental results (2014–2018) do indicate consistency and an increase in efficacy of Blossom Protect compared to results from previous years.