Apple Postbloom thinning:

contributed by Dr. Kathleen Willemsen
WSU-Tree Fruit Research & Extension Center

NAD (Amid-ThinW). For use on varieties other than Delicious. Application on Delicious can cause abnormally small (pygmy) fruit. Apply 7 to 14 days past full bloom with a surfactant (wetting agent). Without a surfactant, higher concentrations are needed. When the weather is cool after bloom, delay applications until the largest fruit are 2-3 mm in diameter and forecasted temperatures after spraying are above 50 F, and preferably above 65 F.
To increase the amount of thinning, combine NAD (Amid-Thin W) and carbaryl in a single spray. (See section on carbaryl.) The effect is greater than when these materials are applied separately.

NAA. Apply NAA 15 to 25 days past full bloom with a surfactant (wetting agent). Use a higher concentration if applied without a surfactant. During cool springs delay application as necessary until the largest fruits are 10 to 15 mm in diameter and forecasted daytime temperatures are likely to rise above 50 F within the next 5 days.
To increase the amount of thinning, combine with carbaryl (see section on carbaryl). The effect is greater than when these materials are applied separately.

Carbaryl (Sevin). Carbaryl is an insecticide that can be applied to apple as a chemical thinner any time from 10 to 25 days after full bloom or when fruit are 3 to 20 mm in diameter. Rate, use and time of application depend on the variety, fruit set, and degree of thinning desired, presence of bees in or near the orchard, and the potential effect on mites.
Because carbaryl is effective up to 25 days after full bloom, it has been used to treat trees not adequately thinning with other materials.
NOTE: When the weather is cool after bloom, delay applications of carbaryl until the largest fruit is 10-15 mm in diameter and forecasted temperatures after spraying are 70 F or higher. Unless the daytime temperature rises above 70 F within 5 days after application, the result can be poor thinning and small seedless fruit.
Carbaryl may be applied with NAD (Amid-Thin W) or NAA in an earlier application. Their use in combination is more effective in thinning than these same materials used separately. For best response, apply the combination of carbaryl + NAD earlier (5 to 10 days after bloom) than with carbaryl alone, when the largest fruit is 3 to 5 mm in diameter. Overthinning can occur if daytime temperatures are above 90 F.
Caution: Carbaryl (Sevin WP) is highly toxic to bees. Use Sevin XLR Plus or Sevin 4F which are the least hazardous formulations to bees. If blooms are present, apply when bees are not foraging. If you use Sevin WP, be sure to remove blooms in the cover crop.
Carbaryl can also be highly toxic to predatory mites and the rust mites on which they feed. The hazard is greatest in orchard where carbaryl has not been used extensively and therefore where little resistance has developed. Reduce the hazard of injury to mites by directing sprays towards the top of trees, by keeping sprays off the trunk and lower limbs, applying carbaryl earlier in the season and limiting the total number of applications per season.

Accel. 6-benzyladenine (6-BA), a synthetic cytokinin, is currently marketed as Accel. Its use as a chemical thinning agent, as well as a fruit size enhancing material, is currently recommended on varieties such as Gala, Empire, Rome Beauty, Fuji and Delicious. The effects on fruit size and thinning for Delicious and Fuji have been positive, though not consistent, under Washington growing conditions. In Washington, it is recommended that Accel be used in combination with carbaryl as the thinning effect of Accel alone is too weak, particularly with Delicious and Fuji, at current recommended rates. Combination sprays of Accel and NAA may increase the occurrence of pygmy fruits, especially with Delicious.

To increase fruit size, the manufacturer's recommendation states that Accel should be applied between petal fall and 10 to 12 mm fruit diameter. Two or 3 applications of 10 to 15 grams active ingredient/acre, applied 5 to 10 days apart, is suggested. For thinning and sizing, 1 to 2 applications of Accel should be applied between petal fall and 10 to 12 mm fruit diameter at a rate of 20 to 30 grams a.i./acre per application. Accel should be tank-mixed with carbaryl to maximize the thinning effects. Best results are obtained when warm temperatures (greater than 60 F) occur during and after a period of several days following application. Optimum temperatures for response are between 70 and 80 F. It is best to apply Accel under slow drying conditions to enhance uptake.

An airblast sprayer should be used for Accel applications to ensure complete coverage. Spray volumes of 100 to 200 gallons per acre should be adequate for most orchard spacings and tree row volumes. For a thinning response, a minimum of 25 ppm of Accel is recommended per application (see product conversions in table below). As with most plant growth regulators, dilute sprays are recommended, with two-thirds of the spray directed into the upper third of the tree canopy.  See table below.

Ethephon (Ethrel). Ethephon may be applied with NAD (Amid-Thin W) to increase fruit thinning and to promote greater return bloom. Also see section on other programs for apple. Caution: Ethephon applied to Delicious before 3 weeks after bloom can affect fruit shape by reducing calyx end development.

Golden Delicious and Fuji are some of the most difficult varieties to adequately thin. Regulating cropping from one season to another is also a serious problem. Whenever a high percentage of the spurs and lateral buds bloom in a single season (snowball bloom), severe alternate bearing can develop the following year. Snowball bloom often develops the year after a light bloom or a severe frost.
To overcome this problem and maintain annual production, a series of sprays is usually required. Start with a bloom thinner, if available, followed by NAD (Amid-Thin W) and an application of carbaryl at 80% petal fall or 3 to 5 mm fruit size. Even greater thinning will occur with a third spray by combining NAA and carbaryl when the fruit are 10 to 15 mm in diameter. When the desired effect is both greater thinning and increased return bloom, ethephon can be used in combination with NAD and carbaryl.

Spur-type Delicious is generally more difficult to thin than non-spur Delicious. When a high percentage of spurs bloom in a single season, adequate thinning is more likely to be obtained by starting with a bloom thinner, if available, followed by a carbaryl or NAA spray, or by a combination of carbaryl plus NAA at a fruit size of 10 to 15 mm diameter. Caution is advised with petal fall sprays of Sevin XLR to Bisbee, RedChief and Vallee Spur as fruit damage and deformation may occur.

Postbloom Thinning--Delicious and Braeburn

Sevin XLR and Sevin 4F are the preferred formulations for reduced hazard to bees. If blooms are present in cover crop, apply while bees are not foraging. If you use Sevin WP, be sure to remove weed blooms in the cover crop. Apply carbaryl any time from 3 to 25 days after full bloom. Apply NAA 15 to 25 days after full bloom. Do not apply carbaryl more than twice during the postbloom period to manage mite populations and to prevent excessive toxicity to beneficial predators.

Spur type Delicious respond to a combination spray of carbaryl and NAA. Apply anytime 15 to 25 days after full bloom when the largest fruit is 10 mm in diameter or larger. When daytime temperatures exceed 90 F, overthinning can occur. Good thinning can be obtained under hot conditions with lower rates of chemicals. Conversely, temperatures below 60 F are too cool for effective thinning.

POSTBLOOM--Spray to run-off. Based on 200 gallons per acre (for average-sized trees). Avoid killing bees on blooming cover crops. See Hazards to Bees and Plants.

*A special local needs registration has been granted for this use under Section 24(c), FIFRA.
**Use surfactant according to manufacturer's recommendations, but not more than 1 pint per 100 gallons of spray.

NOTE: For easier and more accurate measurement of small amounts of liquids, dilute them first. For example, make a 1/10th dilution by putting 1 part in 9 parts water. From this stock solution, measure out and use 10 times the amount shown in the table.

POSTBLOOM THINNING: Golden Delicious, Granny Smith, Gala, Fuji and Jonagold

Apply carbaryl at 3 to 25 days, NAD 3 to 14 days and NAA 15 to 25 days after full bloom, when the largest fruit is 10 to 15 mm in diameter. For greater thinning, use a combination of carbaryl + NAD 3 to 14 days after full bloom. For more response apply carbaryl + NAA when largest fruit is 10 to 15 mm in diameter. Overthinning can occur if daytime temperatures exceed 90 F.

Combination sprays of NAD + ethephon give good thinning and return bloom.

* A special local needs registration has been granted for this use in Washington under Section 24(c), FIFRA

** Use surfactant according to manufacturer's recommendations, but not more than 1 pints per 100 gallons of spray solution.

POSTBLOOM THINNING: Winesap

Apply carbaryl once any time from 3 to 14 days after full bloom, NAD 3 to 14 days after full bloom or NAA 15 to 25 days after full bloom.

* Use surfactant according to manufacturer's recommendations, but not more than 1 pint per 100 gallons of spray.

** A special local need registration has been granted for this use in Washington under Section 24(c), FIFRA.

Apply carbaryl any time from 3 to 25 days after full bloom or NAA 15 to 25 days after full bloom. NAA is less effective on Jonathan and Rome Beauty.

* Use surfactant according to manufacturer's recommendations, but not more than 1 pint per 100 gallons of spray solution.

** A special local need registration has been granted for this use in Washington under Section 24(c), FIFRA.

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Tree Fruit Research and Extension Center
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Wenatchee WA 98801

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May 2, 2000