2018 ASHS Annual Conference

Most current blackberry (Rubus subgenus Rubus Watson) cultivars are adapted to temperate climates, requiring accumulation of 300 to 900 hours below 7.2°C to overcome the endodormancy of floricanes. To achieve high blackberry yields under subtropical climates in Florida, bud break must be artificially induced. Gibberellic acid (GA₃) is known to break dormancy and regulate flowering in many plant species, but its effectiveness in blackberry is unknown. Field experiments were conducted in the 2015-16 and 2016-17 seasons to evaluate the effects of foliar spray of GA₃ on phenology and yield of three floricane-fruiting blackberry cultivars, ‘Natchez’, ‘Navaho’ and ‘Ouachita’. The experimental orchard was established in Balm, FL in 2013. Plants were sprayed with GA₃ at 0 or 49 g·ha^-1 on 24 Dec. 2015 and 27 Jan. 2017. Cumulative chilling hours below 7.2 °C during the growing season were 130 and 165 hours for the 2016-17 and 2015-16 seasons, respectively. The onset of bud break was advanced by GA₃ by 70 to 80 days in the 2015-16 season, and by 14 to 34 days in the 2016-17 season. Fruit earliness was also improved remarkably in ‘Navaho’ (10 June vs. 23 May ) and ‘Ouachita’ (11 June vs. 26 May) in the 2016-17 season. Significant yield increases by GA₃ were observed in both seasons, although some cultivars showed inconsistent responses. In the 2015-16 seasons, GA₃ increased yield of ‘Ouachita’ by 116%, but it had no significant effects on other two cultivars. In the 2016-17 season, the GA₃ x cultivar interaction was not significant, and yield increase by GA₃ averaging across three cultivars was 23%. These yield increases were due mostly to improved fruit set, as fruit yield showed a higher correlation with fruit number than with berry weight. By contrast, GA₃ reduced average berry weight by 13% in the 2015-16 season. In all cultivars, total soluble solid content was unaffected by GA₃ in both seasons. These results suggest that foliar spray of GA₃ is effective in promoting early blackberry bud break and fruit earliness, particularly for ‘Navaho’ and ‘Ouachita’ with relatively high chilling requirements, and that it can increase fruit set and yield with minimal negative impact on fruit quality. This strategy may improve the adaptability of existing blackberry cultivars with high chilling requirements to Florida’s warm climate, and expand the blackberry production area.