2018 ASHS Annual Conference

Chemical and/or manual defoliation of apple trees is a common commercial practice in the apple nurseries before digging. Premature defoliation by hand is time-consuming and therefore, costly, and can cause damage to young trees. Hence, leaf removal is greatly facilitated by chemical treatments and this has long been desired by nurseryman. In order to find alternative chemicals or chemical combinations that would provide the most effective leaf senescence and defoliation of apple nursery trees, a two-year study was conducted at an on-farm nursery located in the Western New York. Four chemicals and their combinations were applied to nursery trees of ‘Gala’/B.9 and ‘SweeTango’/B9: copper, urea, a commercial formulation of abscisic acid – ABA (Protone^®, Valent BioSciences, Libertyville, IL) and organosilicone surfactant (Silwet^® L-77; Helena Chemical Co.), in the fall of 2016 and 2017. Rates and timing of application were tested. Timing of application was set according to a projected date determined by the nurseryman of when trees would be harvested for subsequent cold storage. Percentage of defoliation (%) was assessed weekly after treatment application in both years. In the following spring of each year trees were re-assessed for any type of bud injury that the products applied could have caused and for initial growth. Temperature was one of the major driving factor that influenced the efficacy of the defoliants. Warm temperatures (above 55^oF) during and after spray application (at least 3 to 4 days) are required for better defoliation. In both years, the maximum defoliation of some treatments was observed in one to three weeks after application. In some cases, a frost event is desired for previously treated trees to complete defoliation. ABA has shown great efficacy in promoting leaf senescence and defoliation. However, previous studies have shown that the application of ABA itself is not sufficient for full defoliation of young trees. Therefore, the addition of quelated copper, Silwet or urea in the ABA tank provided substantial increase of defoliation. Based on the two-year experiment, we have found several promising alternatives, however, the best option that has shown the most consistent results over the two years was, so far, the combination of ABA + CuEDTA + Silwet. Some choices will depend on how much defoliation is desired (50 to 100% defoliation) and the additional benefits the combination may provide. Cost-benefit will be probably favorable.