Apple Fruit Thinning

Contents:

• Introduction
• Thinning Factors
• Thinning Chemicals
• Monitoring Response
• Links to Thinning Information
• Literature Cited

I. Introduction

Under most conditions, apple trees will set more fruit than needed for a full crop. Most apple cultivars will retain this heavy set of fruit throughout the growing season resulting in small, poorly colored, low quality fruit. Thinning is the removal of a portion of the crop before it matures on the tree to increase the marketability of the remaining fruit and to reduce the biennial bearing tendency of the tree.

HISTORY

In the early 1900's, growers and researchers searched for ways to break the biennial bearing habit that most apple varieties possess. Applications of nitrogenous manures along with different pruning techniques were tried with little success. For a while, thrip infestations were thought to disrupt the physiological balance of the trees and cause alternate cropping. Many growers just accepted the biennial bearing phenomena and tried to get one half of their trees of each cultivar into heavy bearing.

Russel and Pickering (1919) were the first to recognize that the biennial bearing habit of apples could be controlled if flowers were removed at bloom time (Faust, 1989). This practice, however, was slow to gain acceptance due to the intensive labor requirement and cost. During the 1920's, blossoms were thinned with small hand scissors and other small tools to ease the hand labor requirement (Edgerton, 1973). Folger in 1921 reported, "Once a grower has carefully thinned his fruit for a few years, he needs no further proof of the fact that this is one of the most important and profitable of all orchard operations".

It became apparent that thinning not only promoted annual bearing, but also increased fruit size and color, increased tree vigor, and reduced limb breakage and winter injury. Because of these added benefits, thinning practices became more common but still remained expensive.

Bagenal et al. (1925) recognized that lime sulfur sprays used for controlling pests also induced fruit drop. In the early 1930's, Auchter and Roberts were trying to find a chemical that would entirely defruit apple trees that would not pay their way (Childers, 1983). After this initial venture in total crop removal, emphasis shifted toward finding spray materials that would remove a portion of the fruit leaving enough to produce a full crop and ensure adequate return bloom for the following season. Auchter and Roberts found a tar distillate chemical that burned flowers but caused some injury to vegetative tissues. In 1940, the first commercial bloom thinner was introduced, sodium dinitro cresylate (DNOC) and related dinitro compounds.

In 1941, Burkholder and McCown reported success using the hormone type materials, particularly naphthaleneacetic acid (NAA) and naphthaleneacetamide (NAD or NAAm) (Faust, 1989). NAA was used originally to delay pre harvest drop in orchards and was later found to be an effective thinner. Considerable attention was given to the timing of NAA sprays and excellent thinning results were obtained when sprays were applied as late as 2 to 3 weeks after bloom.

In 1958, a new insecticide,Carbaryl (Sevin), was tested on apples. The first application was generally made 10 to 21 days after full bloom which resulted in reduced fruit set (Williams, Edgerton, 1981). After several experiments, Sevin was found to be another highly effective post bloom thinner. Another chemical, ethephon, was added to the arsenal in the late 1960's. Ethephon is generally used late in the thinning season when prior thinning applications have not worked. Vydate and Accel are two more recent post bloom chemical thinners that are now available to include in a chemical thinning scheme.

Post bloom thinners have become popular in part because the risk of spring frosts is reduced. Growers now have a number of thinning materials of which to choose from. Often, two or more chemical thinners are combined to increase thinning strength on hard to thin cultivars. Also surfactants and oil can be added to chemical thinners in some circumstances to enhance thinning. Occasionally hand thinning is still used to touch up chemical thinning applications especially on the more valuable fresh market cultivars.

II. Thinning Factors

Every thinning situation is unique. Blocks of trees are different, varieties are different and every year is different. The causes of variation in response to chemical thinners is not well understood. Some of the important factors to consider are presented in Table 1.

Table 1. Conditions affecting ease of fruit thinning with chemicals¹.

III. Thinning Chemicals

The current recommendations from the Virginia and West Virginia 1996 Spray Bulletin for Commercial Tree Fruit Growers (the entire 1996 Spray Bulletin is available as a PDF file) should be used as a starting point for planning spray applications. Table lists the chemicals currently labeled for use as apple thinners.

Table 2. Fruit thinning chemicals for apple.

IV. Monitoring Response

Careful monitoring before and during the thinning season is essential to a successful program. Before a thinning application is made the grower should be aware of the orchard conditions as well as the factors affecting chemical thinning response. Thinning results are affected by fruit set, climate, weather, tree age and vigor, and orchard management practices.

A few trees in each treatment should remain unsprayed to use as a comparison against sprayed trees. This will help distinguish the effectiveness of the thinning program. Since fruit trees naturally shed some fruitlets during the thinning season, it is sometimes difficult to distinguish natural drop from a chemical thinning response. A few days after a thinning spray is applied, fruit can be measured to determine if it is growing. On warm days, fruit can increase approximately 1 mm per day in size. Fruit that is not increasing in size is not growing and will eventually drop. If most of the fruit is continuing to grow, an additional spray application may be necessary. Some cultivars in some years will require 2 or 3 applications to achieve optimum results.

After chemical thinning, some limbs may still retain too much fruit or be unevenly spaced. Hand thinning can further reduce the crop and help poor coloring cultivars. Hand thinning may increase fruit size but will have little effect on return bloom.

V. Links to Thinning Information

Notice: These links access information from other geographic regions. Their comments and recomendations may not be applicable to the Mid-Atlantic region.

WSU-TFREC Thin-It Expert thinning program. (a protoype of an expert system for thinning recomendations)

The Virtual Orchard (source for information of interest to orchardists)

Growth Regulator Uses in Apples (table of PGR uses in apples from Cornell)

Thinning advice from MSU Extension.

VI. Literature Cited

Childers, N.F. 1983. Modern Fruit Science. 9:94.

Faust, M. 1989. Physiology of Temperate Zone Fruit Trees. 1:222.

Folger, J.C. 1921. The Commercial Apple Industry of North America. 1:278-283.

Williams, M.W., Edgerton, L.J. 1981 , Fruit Thinning of Apples and Pears with Chemicals.