Effect of flower-applied Serenade biofungicide (Bacillus subtilis) on pollination-related variables in rabbiteye blueberry

Application of Serenade, a commercial biofungicide formulation containing the bacterium Bacillus subtilis, to the stigmatic surface of open blueberry flowers suppresses floral infection by the mummy berry fungus Monilinia vaccinii-corymbosi. The deliberate targeting of the stigma with the biocontrol agent in this pathosystem prompted us to evaluate potential negative impacts on pollination and pollination-related fruit characteristics. Application of Serenade to the stigmatic surface of detached blueberry flowers in the laboratory had no effect (P > 0.05) on the number of pollen tubes entering the style or their growth rates within the stylar canal. There was also no reciprocal effect, i.e., population dynamics of B. subtilis were unaltered by the presence of pollen. Application of the biocontrol product to open flowers, regardless of whether it was done 1 day before or immediately prior to pollination, did not impact fruit set or the number of seeds per berry, but marginally (P = 0.048) affected fruit weight in one of two experimental runs in the greenhouse; fruit weights in the two Serenade timing treatments were significantly different from each other but neither was different from that of the control that received pollen only. In a field experiment in which honey bees were utilized to vector the biocontrol product to open flowers, application of Serenade did not affect fruit weight but significantly reduced fruit set from 49.1 to 38.1% (P = 0.0382) and seed number to about half of that of the untreated control (P = 0.0109). However, fruit weights and seed numbers in the experiment were low even in treatments receiving no Serenade, indicative of poor pollination overall. Taken together, these results indicate that application of Serenade has no inherently adverse effects on pollination and associated fruit characteristics, but caution should be exercised in applying this product in conditions otherwise unfavorable for adequate pollination.