Effect on Yield and Vegetative Growth of Prohexadione Calcium Treatments to Southern Highbush Blueberry "Emerald" and "Star"

Summer pruning or hedging is performed postharvest for southern highbush (SHB) blueberry to control height and stimulate new growth for next season’s production. Hedging is usually finished by mid-May to early June, leaving ample growing season to set reproductive buds and allow for growth, which is often excessively vigorous. The objective of this study was to reduce vigorous regrowth with the gibberellin biosynthesis inhibitor prohexadione-Ca (commercial formulation Apogee, BASF Research Triangle, NC). The product was applied to the foliage of ‘Emerald' and ‘Star’ post-hedging in 2014. On each date, 24 July and 5 Aug, a solution of 122 ppm active ingredient (415 g 935.3 l^-1 ha^-1 label rate) was applied. Shoots were measured and fruiting buds counted during dormancy and whole bush yield were collected at color change of the fruit in 2015. Shoot growth in ‘Emerald’ showed a numerical (14%) reduction in shoot length compared to the untreated plants. Bud count and yield were not significantly affected in either variety. However, late summer observations showed reduced shoot elongation, which suggested that prohexadione-Ca affected both ‘Star’ and ‘Emerald’ shortly after application; however, the impact was short-lived as vigorous growth resumed in the fall.  In sub-tropical climates, late fall growth may be exposing succulent shoots to freezing events, which can damage the plant and cause open wounds that are points for disease infection. This work suggests prohexadione-Ca at two applications were not sufficient for growth suppression and there is need to identify cultivar specific response.