Spray thinning strategies for ‘Red Delicious’ apple using naphthalene acetic acid and ethephon

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Abstract
In a fruit thinning trial, conducted at the Grove Research Station in Southern Tasmania on ‘Red Delicious’ apple trees, an unsprayed control, hand-thinning and a naphthalene acetic acid (NAA) spray were compared with ethephon alone, ethephon followed by NAA, and NAA followed by ethephon spray combinations. All trees treated with NAA were over-thinned, whether sprayed with NAA alone or in combination with ethephon. Full-bloom (FB) ethephon treatments at concentrations of 400 and 800 ppm thinned most acceptably. Ethephon sprays of 100 and 200 ppm under-thinned and the combination sprays over-thinned. Mean fruit weight was excellent throughout the trial, with the target size being approached even in the unthinned controls. Some thinning treatments produced fruit significantly larger than the controls. Fruit was reddened when thinned by NAA alone or in combination with ethephon, but not as consistently by ethephon alone. There was no significant yellowing of fruit caused by any ethephon, NAA or combination sprays when compared to hand-thinned fruit. Mean shoot length was unaffected by any treatments. The ethephon-NAA and NAA-ethephon combinations used in this trial can so easily over-thin ‘Red Delicious’ in Tasmania that they cannot be recommended. Ethephon used alone was the safest and most predictable alternative to hand thinning.

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