684 Thinning and Crop Load Effects on Apple Fruit Growth

Abstract

Chemical thinners can be classified as either blossom thinners or postbloom thinners. Blossom thinners act by inhibit further pollination, pollen germination, or pollen tube growth. At petal fall it is not possible to distinguish between fruit that have been injured by blossom thinners, and those that will persist and continue to grow. The receptacles of blossom thinned fruit do not grow, whereas fruit that has not been treated and that also contain viable seeds, resumes growth within 4 to 6 days, depending upon temperature. Abscission of fruit treated with postbloom thinners does not usually occur until 1.5 to 3 weeks after application. Frequently, it is possible to identify fruit that will abscise and to make an initial assessment of thinning efficacy, within 4 to 6 days following application by measuring fruit growth rate. A reduction in fruit growth by as little as 15% to 20% less than rapidly growing fruit is usually sufficient to assume that the fruit will abscise sometime during the June drop period. The effects of specific chemical thinners on fruit growth and subsequent thinning will be discussed.