Timing of Dormant Applications of Vegetable Oil and Ethephon Affect Flower Bud Thinning

Flower thinning is often done to adjust crop load on early ripening or difficult to size peach cultivars to produce fruit with adequate size for the retail market. Pre-bloom flower bud thinning with vegetable oils such as soybean oil have been as effective as hand removal of flowers at bloom in increasing final fruit size. A combination application of soybean oil (e.g., Vegetoil®) and ethephon (i.e., Ethrel) to dormant peach trees have increased flower bud thinning, but results have been inconsistent possibly due to time of application. Peach [Prunus persica (L.) Batsch] cultivars of ‘Cresthaven', and ‘Rubyprince' near Clemson, South Carolina were sprayed at 5 different dates in both Winters 2008-2009 and 2009-2010 with Vegetoil® (VO), an emulsified soybean oil adjuvant (a.i. 93% soybean oil; Drexel Corp.) and Ethrel® (E) (a.i. 21.7% ethephon). The rates were 10% VO combined with 75 ppm ethephon. The control treatment each year was a December application of 3% petroleum-based dormant oil. In 2009, the VO plus E treatments significantly reduced flower bud survival and fruit set in both ‘Rubyprince' and ‘Cresthaven' on all treatment dates when compared to the control. The highest flower bud mortality for both cultivars occurred on the earliest (first) application date (treatment). ‘Cresthaven' full bloom was delayed by the VO plus E treatment on 3 dates, but there was no effect on full bloom date for ‘Rubyprince'. All treatments significantly reduced fruit yield (i.e., overthinned) for both cultivars. However, fruit size was not affected (i.e., not increased) with the thinning treatments. Fruit set and crop load was less than normal in 2009 due to poor pollination weather, so control trees did not have much excess crop to thin off and thus overall fruit size was large. Thinning and fruiting data for 2010 will be presented.