Effect of electrostatic spray parameters on the viability of two bacterial biocontrol agents and their deposition on blueberry flower stigmas

The fungus Monilinia vaccinii-corymbosi infects blueberry flowers via the stigma-style ovary pathway to cause mummy berry disease. Previous laboratory experiments documented considerable activity of stigma-applied biofungicides containing the bacteria Bacillus subtilis and, to a lesser extent, Pseudomonas fluorescens against flower infection by the pathogen. However, adequate and targeted delivery of the biocontrol agents to the stigmatic surfaces of open flowers in the field has remained problematic. Here we consider the application of the biofungicides Serenade AS (containing B. subtilis QST713) and BlightBan A506 (containing P. fluorescens A506) to blueberry flowers by air-assisted electrostatic spraying. In laboratory experiments with typical field-use rates, viability of B. subtilis and P. fluorescens was unaffected by different levels of induction-charging voltage (0–1.2 kV) and atomizing pressure (138–276 kPa) applied to an electrostatic spray-charging nozzle, showing that the bacteria in both formulations readily survived exposure to the intense electrical fields and near-sonic atomizing air shear encountered during electrostatic spraying. Electrostatically charged application significantly (P<0.0001) increased deposition of B. subtilis on the stigmatic surfaces of detached blueberry flower clusters by a factor of 4.5 compared with conventional hydraulic spraying; a similar comparison showed that population densities of P. fluorescens on the stigma were increased by a factor of 2.9, but this effect was not statistically significant (P=0.1487). For Serenade, the increased coverage and/or retention on the flower stigma, along with the excellent bacterial survival, portend well for electrostatic application for mummy berry disease control in the field.