The Effects of Exogenous Bioregulators and Environment on Regular Cropping of Apple

Abstract

The use of exogenous plant bioregulators or plant hormones to adjust crop load in apple ( Malus × domestica Borkh.) and promote regular cropping remains challenging to both researchers and producers. Responses to these hormones are sensitive to the rate and timing of application, to physiological status of the tree, orchard system, variety, rootstock, and a myriad of cultural practices and environmental factors. Of the environmental factors, temperature plays the most important role in determining response and efficacy of a given material. All classes of plant bioregulators have been used over the past 30 to 40 years as postbloom chemical thinning materials. Most of the standard postbloom thinning programs involve application of a synthetic auxin, such as naphthalene acetic acid (NAA) in combination with carbaryl (Sevin), a commonly used insecticide. The mode of action of these two compounds is not clearly understood. Gibberellins generally have not been effective thinning materials for apple because of their negative impact on return bloom. Ethylene-releasing compounds have been used successfully as postbloom thinning materials. Cytokinins, particularly synthetic sources such as 6-benzyladenine (6-BA), have been shown to effectively thin fruit and enhance fruit size on many commercial varieties. The rate and timing of 6-BA applications are critical to obtain desirable thinning and fruit size responses. The use of these different bioregulators is essential for regular cropping of apple, particularly for spur `Delicious', `Fuji' and other varieties that are difficult to thin chemically and which are prone to severe alternate bearing. The focus of this discussion is the use of these bioregulators in commercial apple production areas in the United States.