Preliminry examination of thinning strategies on young 'Jonagold' and 'Pink Lady' apples

Strategies were examined for thinning 5-year-old apple trees of 'Jonagold' and 'Pink Lady' with chemical thinners. Both cultivars were left unthinned (control), hand-thinned at 20 days after full bloom (DAFB) or chemically thinned. The chemical treatments consisted of full bloom (FB) sprays of naphthalene acetic acid (NAA) at 2.5, 5 or 10 mg/L or ethephon at 25, 50 or 100 mg/L or combination treatments of 2.5, 5 or 10 mg NAA/L at FB followed by 50 mg ethephon/L at 7 DAFB. In addition 5 mg NAA/L or 50 mg ethephon/L were followed by 150 mg CyLex/L (6-benzyladenine, Abbott Laboratories) at 20 DAFB. Neither NAA, ethephon alone or NAA followed by ethephon were effective at the 2 lower concentrations. The highest level of NAA with ethephon did some thinning of 'Pink Lady' but overthinned 'Jonagold'. The CyLex combination treatments were the most effective. Combining NAA and CyLex did not thin enough to increase fruit size in 'Pink Lady' and slightly overthinned 'Jonagold'. The most effective treatment of ethephon and CyLex removed about half the crop load compared with the control for both cultivars. This resulted in a 29% increase in fruit weight and a 48% increase in fruit size for 'Jonagold'. 'Pink Lady' increases were less, being 16% for fruit weight and 38% for fruit size. All mean fruit weight and size outcomes for the ethephon-CyLex treatment exceeded provisional targets. The ethephon-CyLex combination had important fruit quality outcomes other than fruit weight and size. Fruit soluble solids were increased by this treatment by 10% for 'Jonagold' and 6% for 'Pink Lady' and fruit firmness by 11% for 'Jonagold' and 5% for 'Pink Lady'. The overall results for the ethephon-CyLex combination were most encouraging for both cultivars. Further work is required to substantiate these results.