INFLUENCE OF POTENTIALLY NEW POST-BLOOM THINNERS ON APPLE FRUIT THINNING

Availability of the existing post-bloom thinners for apple (Malus ×domestica Borkh.) and the prospects for additional thinners appear to be limited. Application of blossom thinners may present a risk of over-thinning in areas with unpredictable weather. Therefore, we have been searching for new post-bloom thinners or thinner supplements that may be useful at different physiological stages of fruit development. In Study 1 various combinations of Sysstem-CalTM (SCal) with and without 6-benzyladenine (6-BA) (MaxCel^®) were used in ‘Gala’ apples. Control trees had numerically the highest fruit set while trees with 6-BA alone applied at the rate of 200 mg/L at 5-10 mm stage and those receiving carbaryl (Sevin 4F) and naphthaleneacetic acid (NAA) had the lowest fruit set in ‘Buckeye Gala’ apple. While there were numerical improvements, there were no statistically significant differences in fruit set within SCal treatments and between the SCal treatments and control. In Study 2, we used various rates of the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) or the ethylene releasing agent 2-chloroethyl phosphonic acid (Ethrel) on ‘Gala’ and ‘Fuji’ in 2013. In Study 2, Application of ACC at the rate of 250 mg/L or 350 mg/L significantly reduced fruit set while ACC at 150 mg/L or Ethrel at 300 mg/L did not result in sufficient thinning in ‘Gala’ apples. It seems that ACC is a promising post-bloom thinner for apples, especially when used at the 20 mm stage where few presently-used thinners are effective. This naturally occurring phytohormone deserves to be further studied under different climate conditions and in different cultivars of apples.