Submitted to: Proceedings of the Apple Plant Growth Regulator Meeting
Publication Type: Proceedings
Publication Acceptance Date: March 8, 1996
Publication Date: N/A
Interpretive Summary: Apple trees require thinning of the young fruitlets to reduce the crop load thereby improving the size of the remaining fruits and to reduce the alternate bearing associated with many apple cultivars. Because the window of opportunity for effective thinning is limited and labor is not available to perform the task by hand, commercial fruit growers rely on plant growth regulator (PGR) sprays for apple thinning. The auxin, naphthaleneacetic acid (NAA) is one of the oldest and most widely used PGRs used for thinning apples. The response to chemical thinners is influenced by many factors including the environment, the tree, and the chemical used. Techniques are constantly refined as new information is obtained. This report brings together information on the use of NAA as a chemical thinner for apples. The information is presented at a comprehensive fruit school for fruit growers and is available in a published proceedings. This publication serves as a valuable reference for growers, extension fruit specialists, and researchers.
Technical Abstract: Naphthaleneacetic acid (NAA) is one of the most widely used chemical plant growth regulator (PGR) sprays used for thinning young apple fruitlets. The mode of action is uncertain, but when applied at rates of 2.5 to 20 ppm NAA is an effective thinner for most apple cultivars. Local recommendations must be developed for this chemical since response varies with cultivar, environment, and condition of the tree. NAA must be absorbed to be effective. The abaxil leaf surface is the primary site for absorption. The effect of NAA is concentration dependent. Concentrate sprays are effective, but the margin of error is much greater than when using full dilute sprays. Growers are encouraged to use tree-row-volume when determining proper gallonage to apply. A surfactant will improve coverage and reduces the concentration for effective response. NAA is effective on most apple cultivars, but should not be used on spur Delicious because of pygmy fruit formation. NAA can be combined with other thinners but should not be used with the new material Accel A table is provided which describes conditions that result in easy or difficult thinning.