Thinning Apple Fruit

Apple trees typically produce more flowers and fruit than are needed to produce a full crop of marketable fruit. Many of the excess fruitlets will drop shortly after petal fall, or later, during June drop. In a good crop year, the remaining crop load will still be too large for individual fruit to develop marketable size. Additionally, heavy crop loads inhibit the ability of trees to develop blossom buds for the following year, resulting in biennial bearing. Thinning the crop will maximize fruit size and quality and allow for adequate flower bud initiation for the following year.

Fruit size is determined by the total cell number per fruit.  In apples, cell division ceases by about 30 days after full bloom. Therefore, final fruit size is influenced greatly within the first month after bloom. Likewise, initiation of apple flower buds for the subsequent year’s crop also occurs within the first month after bloom. To optimize both fruit size and return bloom, excess fruit must be removed during this period. Chemical thinning preferentially removes small, weak fruit.

Determining Crop Load

The following questions will help you evaluate whether your crop needs to be thinned.  Remember, it’s better to be conservative when applying thinning materials.  It’s possible to take more fruit off but not to put fruit back on.

  • How many seeds are present?  When fruitlets are 3-5 mm, cut open a few and count the seeds.  Fruitlets with fewer  than five seeds are more likely to drop naturally and will be easier to thin than fruitlets with more than five seeds.
  • What color are the seeds?  Tan or brown seed color at this time of the season indicates that the seeds are not viable, whereas viable seeds will be white to yellow.  Fruitlets with fewer viable seeds are more likely to drop naturally, and are also more sensitive to chemical thinners. In some cultivars, the color of the pedicel (stem) is also an early indicator of whether or not the fruitlets will persist beyond June drop.  Red color in the pedicel indicates that the fruitlet will likely not persist.
  • Does the tree have too many apples?  If fruit clusters are within 6-8 inches of each other and if there are more than two fruitlets developing in each cluster, there are too many apples on the tree.
  • What was the crop load like last year?  Trees will thin more easily in the year following a heavy crop.
  • What was bee activity like in the orchard?  Were pollination conditions good or less than ideal.  Remember that bees don’t like to work in cloudy, rainy, windy weather any more than you do.

PGR Products for Thinning Apples

There are a number of PGR-based materials available for fruit thinning. The best material to use will depend on the cultivar, the condition of the trees, and time of application.  

  • ABA (ProTone) is a natural plant hormone.
  • ACC (Accede) is a natural precursor that stimulates the release of ethylene within the plant.
  • Carbaryl (Sevin) is an insecticide that has thinning action.  
  • Benzyladenine (MaxCel, Exilis) contains a synthetic analog of the cytokinin plant hormone.  
  • Ethephon (Ethrel, Verve) releases ethylene.
  • Naphthalene acetic acid (NAA, Fruitone L, Refine) is a synthetic auxin growth regulator.  
  • Naphthalene acetamide (NAD, Amid-thin) is also a synthetic auxin growth regulator.  
Most have a timing recommendation based on the size of the king fruit. Since fruit diameter tends to vary with time of day, measure the fruit at the same time each day beginning at petal fall to determine optimum timing. See table below.

Rate Timing Effectiveness Compatability Notes
ABA (ProTone)
100 to 500 ppm in 100 gal/acre 1 or 2 applications 7 to 10 days apart, between 5 and 20 mm king fruit diameter Not been tested in Utah Works synergistically with other thinning compounds Best if followed by 2-3 days of overcast conditions with temperatures in the mid-70s.
ACC (AccedeM)
200 to 400 ppm in 100 gal/acre From full bloom to 25 mm king fruit diameter. Most active at 15 to 20 mm. Not tested in Utah   Apply in cool conditions. Don’t use on injured or stressed plants
Carbaryl (Sevin)
¼ to ½ lb per 100 gallons of water Within 28 days after petal fall

If cool weather persists, instead apply when king fruits are 10-15 mm in diameter		 Very effective

Produces larger fruit than NAD or NAA		 NAA or NAD will improve results

Can also be mixed with BA		 May harm bees or beneficial insects; use the XLR formulation, lowest rate, and apply in evening, after petal fall
Benzyladenine (BA or Maxcel)
75-200 ppm When king fruits are 5-10 mm in diameter (generally 7-21 days after full bloom) Use at 70-75°F for peak effectiveness Works best in combination with NAA or Sevin High temperatures within 8 hr of application will increase thinning

Apply at night for greatest drying time
Ethephon (Ethrel, Verve)
1.5 to 6 pints per acre 10 to 20 days after full bloom Higher rates for difficult to thin varieties Carbaryl, decrease amount of NAA by half, for hard-to-thin cultivars Promotes return bloom beyond the thinning effect. Can be used on non-bearing trees to increase return bloom but may require different timing and rates.
Naphthalene acetic acid (NAA)*
10-15 ppm When king fruits are 8-12 mm in diameter Very effective and potent, use with caution

Use at 70-75°F for peak effectiveness		 Carbaryl, decrease amount of NAA by half, for hard-to-thin cultivars Use only on cultivars that mature after Sept. 1

Fruit may not size up as well as carbaryl or BA thinners, but NAA helps with return bloom
Naphthalene acetamide (NAD)
35-50 ppm Between late bloom and petal fall (4-8 days after full bloom)

Applications after petal fall result in poor thinning		   Carbaryl For cultivars that mature before Sept. 1

On Red Delicious, it can cause excessive pygmy fruit
*Additional notes on NAA:
  • Weak trees and young trees are more sensitive to NAA.
  • Shaded limbs tend to over thin.
  • Light rain or dew within a few hours of application will increase uptake and thinning action.
  • For some hard-to-thin cultivars, a combination of NAA and carbaryl will increase thinning. For this combination, the concentration of NAA should be decreased by half.
  • NAA should not be combined with other PGRs such as Maxcel or Promalin.  Studies with Red Delicious in Michigan indicated that applications of NAA and either Maxcel or Promalin within the same season resulted in excessive “pygmy” fruit.

Effect of Sunlight on Thinning Agents

A growing amount of evidence suggests that PGRs sensitivity is related to the carbohydrate balance of the tree, as many act to temporarily inhibit photosynthesis. Carbohydrate balance is related to photosynthesis (supply) and respiration associated with plant growth (demand). Conditions that increase photosynthesis (sunny days) or that decrease respiration (cool nights) increase carbohydrates status. Conversely, low photosynthesis and/or high respiration lead to carbohydrate deficits. Trees with an excess of carbohydrates are much harder to thin than those with a carbohydrate deficit. There is an online tool available to predict tree carbohydrate status based on projected crop load (% of flowering spurs), stage of development (days after bloom) and on weather conditions (max and min temperature and light levels). Select the closest weather station to your orchard (only Utah and Idaho stations are currently available), enter a bloom date and estimated bloom density. The tool will then calculate current and forecasted carbohydrate status based on past and predicted weather conditions and make a recommendation for rate of PGR thinner.