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Chemical Thinning of Apples
Chemical thinning applications are probably the most important single spray in a season. Small fruit do not have a strong market and the effects of less 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 24 hours before and 72 hours after the application of the growth regulator thinners (NAA, NAD, 6-BA) may be more important than actual fruit size at time of application. The significance for applicators is that if they are faced with applying a thinner when weather conditions are cold because the fruit is at the ideal size, we recommend they delay the application until more favorable weather is expected. This suggestion applies to the range of fruit size between 5 mm and 17 mm.
Along with the effects of weather we also think that certain materials can be more effective at different fruit sizes. NAA materials, Sevin, and Vydate can be effective from petal fall to fruit sizes of 20 mm. NAA materials, however, should never be applied to Spur Delicious fruit when the size is above 9 mm in diameter. Applications above 9 mm in diameter result in excessive formation of pygmy fruit.
Certain materials should not be mixed together in any one year. Do not make applications of Accel and NAA materials to the same trees. The combination of these two can result in excessive formation of pygmy fruit.
6BA
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, Rite Size, Exillis 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. (See Table 1-16.) 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. 6BA is best used in combination with Sevin or Vydate. Never mix 6BA and NAA products in the same season on Delicious or Fuji, as severe pygmy fruit may result.
Amid-Thin W
Napthalene acetamide (NAD or NAAm) is the amide salt of NAA and is considered a relatively weak thinner. It has less hormonal activity than NAA but remains active over a longer period. It is recommended for use on early cultivars and not recommended for cultivars that mature after McIntosh. Because NAD is a mild thinner, it is often tank-mixed with Sevin or Vydate to increase the thinning response. Application in less than 100 to 150 gallons per acre of NAD has not given satisfactory thinning. Also see Amid-Thin in Part III.
Ethephon
Ethephon (Ethrel, Ethephon II) 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 20 mm in size. As with other PGR-based thinners, ethephon thins more when temperatures are in the 70s. 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 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. Also see ethephon in Part III.
NAA
Napthalene acetic acid (NAA) has been used as a thinner for many years. The material was once available in several similar formulations, but only the Fruitone N formulation is manufactured at present. Once existing stocks of other formulations are used up, these will no longer be available. NAA stimulates ethylene production by fruit tissues, which in turn slows the development of the youngest and weakest fruits more than the older fruit in a cluster. The result is that the weaker fruit cannot compete for resources and they abscise. 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. Timing of application depends in part on cultivar being treated. Delicious and Gala should be treated earlier than other cultivars. Late applications can cause small fruit called pygmies to remain on the tree until harvest. Also see NAA in Part III.
Carbaryl
There are several formulations of carbaryl (carbaryl, Sevin) that are labeled for fruit thinning. The new Sevin XLR 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. Also see Sevin in Part III.
Vydate
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 also be tank-mixed with Ethrel, Accel, 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. Also see Vydate in Part III
General Comments on Thinning
Chemical thinning increases fruit size and enhances return bloom. Many factors influence fruit thinning (examples are given in Table 1-14), and the farm manager will need to consider all these factors when deciding how to chemical thin. Table 1-15 provides general apple thinning guidelines to provide a starting point for these deliberations.
• 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 (MaxCel, Excellis Plus) is not very effective when the temp is below 68°F.
• Carbamates (Sevin and Vydate) 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.
• 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 15 ppm NAA, 2 pints of Sevin XLR, and 1 pint of spray oil per 100 gallons of water applied at petal fall is 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. For a discussion of specific materials used in thinning, please turn to Part III, Chemical Management. Materials are listed alphabetically in this section.