Peach flower buds thinned with dormant applications of Vegetoil adjuvant plus ethephon

Early removal of flower buds, flowers or small fruitlets increases peach fruit size and value at harvest. Due to the scarcity of labeled or consistent thinning chemicals, peach growers generally wait approximately 30 days after full bloom before hand-thinning fruit, which limits potential fruit size. Experiments were conducted near Clemson, South Carolina to determine the efficacy of combining Vegetoil (VO), an emulsified soybean oil adjuvant (93% soybean oil), with Ethephon (Ethrel a.i. 21.7%) for pre-bloom flower bud thinning of peach cultivars. ‘Contender’, ‘Cresthaven’ and ‘Rubyprince’ peach were sprayed in January 2006, 2007 and 2008 with 10% VO except for ‘Rubyprince’ in 2006 (8% VO). Ethephon concentrations used varied by year and were 25, 50, 75, 100, or 150 ppm. Dormant oil (DO) at 3% and VO at 10% were the control treatments in 2006+2008 and 2007 respectively. VO (10%) plus Ethephon at 100 and/or 150 ppm significantly delayed bloom in ‘Contender’ and ‘Cresthaven’ in 2006 and in ‘Cresthaven’ and ‘Rubyprince’ in 2007. In 2006 and 2008, VO (10%) significantly reduced the number of flower buds alive at bloom for ‘Contender’, ‘Cresthaven’, and Rubyprince (only 2008), and the addition of Ethephon to the VO spray significantly increased flower bud mortality when compared to the 10% VO application. There were no differences among the ‘Rubyprince’ treatments in 2006, but the addition of ethephon slightly increased flower bud death in 2007 and significantly more in 2008. Basal flower buds on marked shoots had significantly lower survival than the distal or terminal flower buds in the VO treatments in all years and all cultivars except ‘Cresthaven’ in 2006. Generally, ‘Rubyprince’ is easier to flower bud thin with soybean oil than ‘Cresthaven’, but the opposite was observed in this study when using the Vegetoil® soybean oil product.