2019 ASHS Annual Conference

Most commercial blackberry (Rubus subgenus Rubus Watson) cultivars require 300 to 900 hours of cumulative chill hours below 7.2°C to induce uniform flower bud break in spring. To achieve high blackberry yields under subtropical climates in Florida, strategies to improve bud break must be developed. Gibberellic acid (GA₃) is a phytohormone known to break dormancy and regulate flowering in many plant species. Our preliminary experiments demonstrated that a single application of GA₃ at 49 g·ha^–1 was effective in inducing blackberry bud break, but its optimum application rate is unknown. A field experiment was conducted in west central FL in the 2017–2018 season to evaluate dose effects of exogenous GA₃ on phenology and yield of ‘Natchez’ blackberry. Plants were sprayed with GA₃ at four application rates (0, 25, 99 or 198 g·ha^–1) at the end of the winter chill period (20 Feb. 2018). Cumulative chill hours below 7.2 °C during the 2017-2018 growing season were 230 hours. The onset of bud break, flowering, and fruit ripening was unaffected by exogenous GA₃. Although increasing GA₃ application rate increased the percentage of bud break from 25% to 58%, it decreased flower number by up to 81% (21.8 vs. 4.2 flowers per cane) as a result of flower abortion. Increasing GA₃ application rate also decreased the number and average size of marketable fruits by up to 62% (287 vs. 109 fruits/plant) and 36% (7.30 vs. 4.65 g/fruit), respectively. The consequent yield loss by GA₃ application was up to 83% (2.04 vs. 0.34 kg/plant). All measured responses were dose-dependent. These results suggest that exogenous GA₃ is effective in inducing blackberry bud break even under insufficient chilling conditions, but its practical use is not recommended for ‘Natchez’ blackberry because of the negative side effects on flower and fruit development.