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 the individual fruit to develop adequate size. Also, heavy crop loads inhibit the ability of the tree 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.
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 varieties, 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.
- Are there too many apples on the tree? 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 foul weather any more than you do.
There are currently four materials available for fruit thinning: carbaryl (Sevin), benzyladenine (Maxcel), naphthalene acetic acid (NAA), and Naphthalene acetamide (NAD). The best material to use will depend on the cultivar, the condition of the trees, and time of application.
Carbaryl is a very effective thinning agent for apples. Trees thinned with carbaryl will have larger fruit than those thinned with NAD or NAA. However, carbaryl is an insecticide that is toxic to bees, and to beneficial insects. To minimize danger to these essential beneficial insects, use the “XLR” formulation, use the lowest possible rate, spray only after dark and after petal fall when bees are not active in the orchard.
Carbaryl should be applied within 28 days after petal fall, after the crop load has been determined. If the weather has been cool, delay application until the central “King” fruit are 10 to 15 mm in diameter. Apply ¼ to ½ lb of carbaryl per 100 gallons of water. For increased thinning action, add NAA or NAD as a tank mix.
BENZYLADENINE (BA OR MAXEL)
Benzyladenine is a synthetic cytokinin. Cytokinins are natural plant hormones involved in regulating cell division. The effect of BA is to increase fruit size beyond that expected by reducing fruit number. It should be applied when king fruitlets are 5 to 10 mm in diameter. Depending on cultivar and weather conditions, this typically occurs within 7 to 21 days after full bloom.
Apply a dilute spray of 75 to 200 ppm at night to maximize drying time. High temperatures within 8 hours of application will increase the thinning response. BA is most effective when air temperatures are between 70°F and 75°F.
BA can be tank mixed with carbaryl to improve the thinning action. Usually ¼ lb of carbaryl is sufficient as a tank mix with BA. Do not tank mix with other pesticides.
NAPHTHALENE ACETIC ACID (NAA)
NAA is a synthetic auxin growth regulator and is recommended for cultivars that mature after September 1. Fruit thinned with NAA do not show as strong of a positive size response as those thinned with carbaryl or BA. However, properly-timed NAA application effectively stimulates return bloom.
NAA should be used when the king fruit are 8 to 12 mm in diameter. 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. Apply NAA at a rate of 10 to 15 ppm for most fall to winter cultivars. NAA is a very active, potent material and should be used with caution. Consider the following factors before applying NAA for fruit thinning:
- Weak trees and young trees are more sensitive to NAA.
- Shaded limbs tend to over thin.
- For best results, apply when temperatures are between 70° and 75°F.
- 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.
NAPHTHALENE ACETAMIDE (NAD)
NAD is another synthetic auxin growth regulator. It is best for summer apples that mature before September 1. Apply NAD between late bloom and petal fall (4-8 days after full bloom). Applications after petal fall can result in poor thinning. NAD is applied at a rate of 35 to 50 ppm for most summer cultivars. Do not use NAD on Red Delicious as it can cause excessive pygmy fruit.