Floral Colonization Dynamics and Specificity of Aureobasidium pullulans Strains Used to Suppress Fire Blight of Pome Fruit.

Aureobasidium pullulans is used as a biocontrol agent for fire blight protection in organic apple and pear production. We assessed colonization of pome flowers by A. pullulans in orchards located near Corvallis, OR and Wenatchee, WA. Blossom Protect, a mix of A. pullulans strains CF10 and CF40, and its citrate-based companion, Buffer Protect, were sprayed at 70% bloom. Later in bloom, the population size of putative A. pullulans on flowers was estimated by dilution plating; plate scrapings of putative A. pullulans were then sampled and subjected to a PCR analysis. Sequenced PCR amplicons of the internal transcribed spacer region and the elongase gene confirmed the presence of A. pullulans, whereas a multiplex PCR with primers specific to CF10 and CF40 was used to determine the presence of the introduced strains. At Corvallis, a wet spring environment, A. pullulans, was recovered from most (>90%) Bartlett pear and Golden Delicious apple flowers sampled from experimental trees, regardless of whether the trees were treated with Blossom Protect. Nevertheless, population size estimates of A. pullulans on the flowers were correlated with the number of times Blossom Protect was sprayed on the trees. At Wenatchee, an arid spring environment, A. pullulans was detected on most flowers from trees treated with Blossom Protect, but only on a minority of flowers from nontreated controls. In both locations, the combined incidence of strains CF10 and CF40 on flowers averaged 89% on Blossom Protect-treated trees, but only 27% on adjacent, nontreated trees. During subsequent trials, the efficacy of Blossom Protect for fire blight control was compared with alternative yeast isolates, with each applied with Buffer Protect; local isolates of A. pullulans and Cryptococcus neoformans and a postharvest biocontrol strain of Cystofilobasidium infirmominiatum were used All yeast strains suppressed fire blight to a degree; however, in each of four trials, the level of suppression was highest with Blossom Protect, and it was significantly superior (P ≤ 0.05) to other yeast isolates in two of the trials. Because A. pullulans strains CF10 and CF40 were detected primarily on flowers on trees treated with Blossom Protect, and because they were detected much less frequently on nearby nontreated tress, we recommend treating every tree row with Blossom Protect at least once for organic fire blight suppression.