The first step in making the right call on thinning is accurate assessment of initial set.
Small fruit do not have a strong market and a reduced return bloom following a heavy crop can affect overall profitability.
Current thinking suggests that the susceptibility of fruit to chemical thinners is affected by the carbohydrate status of the tree. When carbohydrates are in abundance it is more difficult to thin. Conditions such as cloudy weather and high nighttime temperatures, adversely affect the carbohydrate reserves, and make trees easier to thin.
Weather conditions during the 2 days before and the 4 days after the application of the growth regulator thinners (NAA, NAD, 6BA) are thought to be more important than actual fruit size at time of application. If faced with applying a thinner when weather conditions are cold because the fruit is at the ideal size, delay the application until more favorable weather is expected. This suggestion applies to the range of fruit size between 5 mm and 17 mm. Likewise thinners applied when daytime high temperatures exceed 85o F may thin excessively, so the application should be postponed until temperatures moderate.
Along with the effects of weather, certain materials can be more effective at different fruit sizes. NAA materials and carbamates, carbaryl and Vydate, can be effective from petal fall to fruit sizes of 17 mm. NAA materials, however, should not be applied to Spur Delicious fruit when the size is above 9 mm in diameter. Applications above 9 mm in diameter may result in excessive production of pygmy fruit. NAD can also stimulate the formation of pygmy fruit and should not be used on Delicious or Fuji.
Certain materials should not be mixed together on certain varieties. Do not make applications of 6BA and NAA materials to the same trees of Delicious or Fuji. The combination of these two chemistries can result in excessive formation of pygmy fruit.
6BA is a cytokinin--an active fruit thinner that can also enhance cell division. Therefore, a purported advantage of 6BA is an increase in fruit size above that achieved by thinning alone. There are several formulations of 6BA labeled for thinning apple (e.g., MaxCel, RiteWay or Excilis 9.5SC, Exilis Plus), each with slightly different concentrations and different label restrictions, requiring the applicator to carefully read the label before use. 6BA is an effective thinner at a concentration of 75 to 150 ppm, and many varieties are thinned satisfactorily at about 100 ppm. Varieties, such as Fuji and spur-type Delicious, that are considered difficult to thin with NAA may thin more easily with 6BA. Regardless of the formulation used, 6BA thins best when daytime high temperatures reach 70°F for several days during and following the application. The efficacy of 6BA is enhanced when used in combination with carbaryl or Vydate. Never mix 6BA and NAA products in the same season on Delicious or Fuji, as severe pygmy fruit may result.
Napthalene acetamide (NAD) is the amide salt of NAA and is a relatively mild thinner. It has less hormonal activity than NAA but remains active over a longer period. Because NAD has fewer side effects on vegetative growth, it is recommended for use on sensitive early cultivars, and as an early thinner at the late bloom to petal fall timing. Because NAD is a mild thinner, it is usually used either as a first step in a multiple spray thinning strategy or tank-mixed with Sevin or Vydate at petal fall to increase the thinning response. Application in less than 100 gallons per acre of NAD has not given satisfactory thinning. Amid-Thin should not be applied to Delicious or Fuji, as pygmy fruit may result.
Ethephon (Ethrel, Ethephon 2, Motivate,Verve) is most effective as a thinner when fruits are greater than 17 mm in diameter, and it is especially valuable when other thinners have been used and insufficient thinning has occurred. Ethephon offers the opportunity for "rescue thinning" as it has been shown to effectively remove apples up to 22-27 mm in size. As with other PGR-based thinners, ethephon thins more when temperatures are warm (in the 70s to low 80s). Ethephon may be mixed with carbaryl and with horticultural spray oil to increase the thinning response, if needed. Golden Delicious and Rome are easily overthinned with ethephon, and the thinning response to ethephon is especially great when daytime highs reach the upper 80s. Lower rates and caution are called for in these circumstances.
Addition of a nonionic surfactant can enhance treatment effectiveness. Buffering spray solution to a pH of 3 to 5 can improve performance where water is alkaline. Use a spray volume sufficient to cover trees thoroughly and uniformly.
Napthaleneacetic acid (NAA) has been used as a thinner for many years. Fruitone L, PoMaxa 3.5%, Refine 6.25L and Refine 2L liquid formulations and Fruitone N and Refine 3.5 WSG dry formulations are currently registered for use as chemical thinners for apple and pear. NAA may be applied between bloom and 17-mm fruit diameter, although the traditional target window for optimal response to NAA is 10- to 12-mm fruit diameter. When possible, apply chemical thinners such as NAA when daytime high temperatures are forecasted to be in the 70s for several days. Timing of application depends in part on the cultivar being treated. Delicious, Fuji, and Gala should be treated earlier than other cultivars. Late applications to these varieties can cause small fruit, called pygmies, to remain on the tree until harvest.
Several formulations of carbaryl (carbaryl 4L, Sevin SC and Sevin XRL Plus) that are labeled for fruit thinning. The Sevin XLR Plus label indicates that it can be used for thinning fruit between 80 percent petal fall and a fruit size of 16 mm. Sevin is a mild thinner and is used in combination with other thinners. Some problems have been encountered with poor fruit finish under extremely humid, warm conditions, especially when oil is used as an adjuvant.
In light of the toxicity of carbaryl to honey bees, we strongly urge you to follow the cautions listed on the label.
Vydate L may be used as a thinner in Pennsylvania, New Jersey, Virginia, and West Virginia. Vydate is a carbamate like Sevin and has similar thinning properties. Work in Virginia suggests that the thinning response may be dose dependent. Since Vydate is a little less toxic to certain predators than Sevin, it may be a better choice than Sevin.
The label recommends applying one to two dilute sprays between 5 and 30 days after full bloom. This period coincides with petal fall, when fruit is approximately 5 mm in diameter, to when it is approximately 20 mm in diameter. The application rate should be 2 to 4 pints per acre, and not more than 8 pints in any one year. Vydate can be tank-mixed with ethephon, 6BA, or NAA. There is a warning about the possibility for increased russeting on russet-prone cultivars such as Golden Delicious or Stayman. A surfactant such as Regulaid, LI 700 or Tween 20 can be used to increase the effectiveness of Vydate. Do not apply oil with Vydate, as russeting can be increased.
General Comments on Thinning
Chemical thinning increases fruit size and enhances return bloom. Many factors influence fruit thinning, and the grower will need to consider all these factors when deciding how to chemical thin.
- Use chemical thinners only in blocks where bloom density and pollination were adequate to set an excessive crop.
- First key to making the right call on thinning: accurate assessment of initial set.
- Second key: understanding the role of light and temperature and getting an accurate weather forecast.
- Sunny weather: harder to thin. Heavy clouds for 2 to 3 days: easy to thin.
- Cool weather (below 65°F): less thinning. Hot weather (above 80°F): more thinning.
- "2X4"--The temperatures and sunlight on the 2 days before, and the 4 days following thinner spray are the most crucial.
- Use fruit diameter as a centering date, but the best timing is a blend of temperature, light, and fruit diameter.
- 6BA is not very effective when the temp is below 68°F.
- Carbamates (Sevin and Vydate) are still somewhat effective in suboptimal temperatures.
- NAA also has some thinning activity when temperatures are suboptimal, but this increases the chances of mummies and pygmies.
- Mummies and pygmy fruits can result from postbloom sprays of certain thinners (NAA and 6BA), and this risk rises with increasing chemical rate and with later thinning timing.
- All thinners work best when temps in the 70s, and all chemistries have the potential to overthin when temperatures are in the mid- to high 80s. There is no "safe" thinner at high temperatures.
- Adding carbaryl in a tank mix with NAA or 6BA increases thinning response. Adding oil at 1 quart per 100 gallons of finished spray mix boosts efficacy of all thinners.
- Varieties once considered chemically hard to thin (Golden Delicious, Gala, Fuji) may not be hard to thin with 6BA/ carbaryl tank mix.
- For most effective thinning, no less than 100 gallons of water per acre should be used for adequate coverage, and sufficient wetting time must be allowed to permit foliage to absorb the thinner.
- Use extra caution if freezing temperatures occurred during bloom. Delay thinning until you are certain that fruits are growing. Consider reduced rates of chemical thinner in this circumstance.
- Trees less than 5 years old are more apt to be overthinned so lower rates of thinners and fewer applications are warranted.
- Be sure to leave several nonsprayed trees so that you can check the results of using thinner.
- In some instances, it may be desirable to remove all fruit from the trees. In such cases, a mixture of 3 quarts of 6BA, 1 quart of carbaryl, and 1 quart of spray oil per 100 gallons of water applied at petal fall is largely effective.
While chemical fruit thinning is not an exact science because of differences between orchard blocks, cultivars, sites, and years, nevertheless the materials are standard.
When fruit diameter reaches 18 mm, apples become difficult to thin with NAA and 6BA. Once the fruit reach about 22 mm, they begin to become unresponsive to chemical thinners. Apple fruits grow about 1 mm per day in warm weather, so when fruit are 18 mm in diameter, you have only 4-6 days to apply chemical thinners.
The two chemistries that still have thinning activity at this advanced stage of fruit growth are carbaryl and ethephon. Where mild thinning is all that is required, carbaryl at 1 pint to 1 quart per 100 gallons may suffice. To create a moderately strong late thinning spray, add 1 quart of spray oil per 100 gallons of finished spray mix to the 1 quart carbaryl rate. Oil and captan cause phytotoxicity, so if you are using oil in this spray, keep captan out of the orchard for the next two cover sprays.
If a strong thinning combination is called for, then combine ethephon, at 1.5 pints per 100 gallons, with 1 quart carbaryl and oil. One quart horticultural spray oil can be added to this tank to boost the thinning response. Golden Delicious and Rome are very sensitive to ethephon. Reduce the ethephon rate to 12 fluid ounces per 100 gallons for Rome and to 1 pint per 100 gallons for Golden Delicious.
Source: Penn State Tree Fruit Production Guide, January 2018