APPLE FRUIT THINNING


Chris Williams and Richard P. Marini
Department of Horticulture
Virginia Tech
Blacksburg, VA

This page was prepared mainly by the senior author as part of a project for ENT 4987, Arthropod Management in Fruit Crops.

Updated 30 March 2002





Introduction
It is important to thin apple trees to achieve optimum fruit growth and enhance tree vigor. Proper thinning also reduces limb breakage. An overloaded apple tree will produce many small, low value fruit, whereas a tree that only has several fruit is not producing to its maximum potential. Insufficient thinning may also lead to alternate bearing, an event where an extremely heavy crop will not produce any fruit the following year. Many different factors can affect the success of the chosen thinning practices. Combinations of several chemicals, such as Vydate, Sevin, NAA, and Accel often work the best. However, there is no single outstanding combination with which to thin. Some treatments may require several applications. With this in mind, thinning apple trees becomes more of an art than a science. With a heavy crop load, it may be required to remove 70-80% of the fruit. Although several different methods of physical or chemical mechanisms can be utilized to accomplish the same result when thinning, the following information will concentrate on chemical tools available for thinning.
 

Timing
The timing of the application of the chemical thinner is an important component in the success of the treatment. Thinning can be done anytime between full bloom and when the fruit reach as great as 25-30 mm, depending on the various chemicals that are being used. Sevin and Vydate, applied to `Redchief Delicious' in this example, thin fruit when applied at full bloom through 20 mm. These chemicals are effective over a greater range than most people realize. NAA is effective from petal fall to 19 mm, but pygmy fruits may develop on `Delicious' varieties when fruit is greater than 9 mm. Consideration should also be given to possible toxicity to bees, since the application could be made around bloom. As a general rule, apple trees need 35-45 leaves to produce one fruit. Thinning should be done to hit this "target" mark. Overthinning will result in fewer apples and reduced tree vigor; underthinning will lead to small fruit.

Measuring fruit diameter is often very time consuming. Therefore, an additional model has been provided by Dr. Rich Marini:

To estimate fruit diameter:

1. Randomly pick 100 apples per block (that look as though they will set).
2. Retain stems on fruit.
3. Record the weight of the fruits.
4. If necessary, convert the weight to grams (multiply # of ounces by 28.35 to get grams)
5. Look at the chart to estimate average fruit diameter in mm.

For example, say you pick 100 fruit that weigh 17 ounces.

  1. convert to grams: 17 ounces X 28.35 = 481.95 grams
  2. using the chart, estimate average fruit size (481.95 grams = approximately 20 mm)
*For reference, when average fruit diameter is 10 mm, 100 fruits will weigh about 90 grams or 3.2 ounces. When average fruit diameter is 12 mm, 100 fruits will weigh about 170 grams or 6.1 ounces.
 
 





Chemicals*
 

  • Naphthylacetamide (NAD) -- Amid-Thin, usually applied 4-8 days after full bloom. Should not be used on `Delicious' after petal fall because pygmy fruit may be induced. Applied at 50 ppm.

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  • Vydate -- oxymyl; a carbamate insecticide used for controlling aphids, leafhoppers, and leafminers is also effective as a chemical thinner. Vydate is recommended at 2 to 4 pints per acre when fruit is 8 to 25 mm. Do not apply superior oil with Vydate because fruits may develop a dark dull red color at harvest.

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  • Sevin -- carbaryl; apply at 14-21 days after full bloom. Sevin is toxic to bees and can also result in increased mite populations because it is toxic to mite predators. Recommended for use with other chemicals. Use at 1.5 lbai/a. Addition of 1qt/100 gal of superior oil will promote additional thinning, but may cause russetting on varieties that are prone to russett.

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  • Accel -- plant growth regulator, relatively new to market. Accel is a cytokinin, a hormone involved in cell division, therefore it may increase fruit size on some varieties (Empire, McIntosh, Golden Delicious, and Paulared). Two applications may be needed, and application should be made when fruit size is around 10 mm. Thinning rates may improve when applied at petal fall Maximum rate is 30 grams ai/a, or 53.4 fluid ounces of Accel per acre per application. Use between 50 and 200 gallons of water per acre.

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  • NAA -- Naphthaleneacetic acid, applied at 14 to 18 days after full bloom (10-12 mm). May produce pygmy fruits on spur strains of `Delicious'. Used at 2.5 ppm. Often used in conjunction with other chemicals to produce optimum effectiveness. Application of NAA when fruit are less than 10 mm will cause more thinning and will induce fewer pygmy fruit. Chemicals used alone typically do not have the same level of effectiveness as chemicals that are used in combination with each other.

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    *Always follow the instructions provided on the label.
     

    Factors Affecting Thinning Response

    Absorbtion of chemical thinner by leaf is necessary. Dry, sunny conditions for more than a week increase cutin on the leaf and reduce absorbtion. Extended cool periods, including freezing conditions, increase absorbtion.

    Drying time. Slow drying time increases absorbtion and response. Low humidity, wind, and low spray volume can result in increased drying time, which thus leads to reduced absorbtion and reduced thinning effectiveness. Warm temperatures with high humidity are favorable for absorbtion.

    Shade. The combination of 2-3 days of cloudy weather with chemical thinners increase effect of treatment.

    Temperature. Effectiveness of thinners seems to be related to temperature at time of application. Optimum temperatures are 70-75 degrees Fahrenheit. Increased response occurs above 85 degrees Fahrenheit.

    Tree Vigor. Low tree vigor increases competition between fruits and leads to increased thinning. Low tree vigor can be caused by poor fertilization, drought, or excessive moisture, to name a few.

    Age of Tree. Young trees will thin more readily than older trees.
     

    Conclusions
    Thinning apple trees can be very complex, due to the many variables that can play a part of the relative effectiveness of treatment. Different combinations of several chemicals usually work best. Consideration should be given to time of application, stage of growth, bee safety, handler safety, cost, and past historical data that may be useful in predicting the amount of thinning required.

    **For more information regarding fruit thinning, try the following sites:

    Washington State

    Jerry Hull, Michigan State University

    Virtual Orchard and guide





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