The Use of 1-Aminocyclopropane-1-carboxylic Acid and Sysstem-Cal™ for Post-bloom Thinning of Apples

Application of blossom thinners is an alternative option to post-bloom thinners but they present a risk of overthinking in the areas that weather is less predictable. Availability of the existing post-bloom thinners and the prospects for additional thinners appear to be limited. Thus, we have been searching for new post-bloom thinners or thinners supplements that may be useful at different physiological stages of fruit development.  In Study 1, we used various rates of ACC or Ethrel on ‘Gala’ and ‘Fuji’. In Study 2, various combinations of Sysstem-Cal™ with and without MaxCel® were used in ‘Gala’ apple.  Application of ACC at the rate of 250 ppm or 350 ppm significantly reduced fruit set, increased fruit size, and improved fruit quality, while ACC at 150 ppm or Ethrel at 300 ppm did not result in sufficient thinning in ‘Gala’ apple.  In ‘Fuji’, ACC at rates of 300 or 450 ppm showed effective fruit thinning and resulted in larger fruit with better quality attributes.  In Study 2, Control trees had numerically the highest fruit set while trees with MaxCel® alone applied at 1% at 5-10 mm stage and those receiving Sevin and NAA had the lowest fruit set in ‘Gala’ apple. While there were numerical improvements, there were no statistically significant differences in fruit set within Sysstem-Cal™ treatments and between the Sysstem-Cal™ treatments and Control. It seems that ACC is a promising post-bloom thinner for apple, especially when used at the 20 mm stage where few thinners are effective.  This naturally occurring phytohormone deserves to be further studied under different climate conditions and in different cultivars of apples.