Effect of Chemical Thinning Sprays on Apple Fruit Set

Abstract

Apple growers of different regions need different chemical fruit-thinning responses for thinning trees of different tree ages, cultural conditions, rootstocks, climates, and amounts of fruit removal desired. In this research, a range of chemical thinning responses was achieved by combinations of thinning materials or addition of potentiating agents. Superior oil, certain organic phosphates, and a light-absorbing agent (ferbam, a fungicide) increased the thinning of carbaryl. In addition, combinations of 50 or 200 ml 6-BA/liter + carbaryl + oil defruited `Campbell Redchief Delicious'/M.111 trees, and 50 ml 6-BA/liter alone over-thinned in one year (however, oil or 6-BA has been shown previously to cause russet in `Golden Delicious'). Carbaryl 50 WP and the 4L carbaryl formulations were equally effective for thinning `Golden Delicious', `Stayman', and `Redspur Delicious', and did not affect fruit russet. Three days of cloudy weather is typical at least once in most seasons in the eastern United States during the fruit set period. Two days of artificial polypropylene shading (92%) (which was nearly equivalent to 3 days of cloudy weather) caused more thinning of `Golden Delicious' and `Stayman' than carbaryl or 10 mg NAA/liter + Tween. Shading reduced viable seed numbers about 50% for `Golden Delicious' in fruit remaining at harvest, but chemical thinning agents (NAA or carbaryl) did not affect viable seed number.