Estimating the Chilling Requirement of Southern Highbush Blueberry Breeding Selections

Currently, the chilling requirements of southern highbush blueberry (Vaccinium corymbosum L. hybrids) selections within the University of Florida (UF) blueberry breeding program are not quantified prior to commercial release.  Due to the primary selection location, many of the cultivars released from the program are estimated to need approximately 300 hours of chilling (0 to 7 °C) for normal growth and development; thus, these selections are not well-adapted to areas that receive less chilling, such as central and southern Florida.  In low chill production areas, blueberry plants are often treated with the dormancy-breaking compound, hydrogen cyanamide, to overcome inadequate chilling and promote uniform budbreak.  However, application rate and timing of this compound are cultivar specific and phytotoxicity can be a problem. Furthermore, this compound is not available for organic production. There is a need for cultivars with a lower chill requirement that do not require hydrogen cyanamide application, as well as cultivars that respond well to hydrogen cyanamide when their chilling requirement is not met.  The purpose of this study was to estimate the chilling requirement of advanced selections in the UF blueberry breeding program with and without treatment with hydrogen cyanamide.  Breeding selections were clonally propagated in 2009 and were planted in blocks at Citra and Windsor, FL, in 2010. Plants in Windsor were treated with hydrogen cyanamide in December 2011, while plants in Citra were not. Three branches from each treatment at each location were collected at approximately 50 chill hour increments.  Branches were then placed in a greenhouse and floral bud development was evaluated biweekly for five weeks.  The floral bud chilling requirement of a genotype was estimated based on a comparison of the rate and total percentage floral bud break for each 50 chill-hour evaluation. Of the 25 genotypes evaluated, 13 had the same chill requirement estimate both with and without hydrogen cyanamide treatment.  Of the 12 remaining genotypes, only four were estimated to have a lower chill requirement when treated with the dormancy breaking compound.   For treatments both with and without hydrogen cyanamide, an estimate of 100 chill hours requirement was the most frequently assigned (12 and 13 genotypes, respectively) followed by 150 chill hours (5 and 6 genotypes, respectively.)  This initial analysis suggested that the majority of the germplasm in the breeding program can successfully flower with a minimum of 150 chill hours and without hydrogen cyanamide application.