As of 2019, NovoSource™ Lime-Sulfur Solution. can be used for apple crop thinning in Pennsylvania and several other eastern states. This registration extends the range of effective chemical thinning options to include bloom. Additionally, seven variety-specific pollen tube growth models are available to commercial apple growers in 2019 through the Network for Environment and Weather Applications. These models are used to help with application timing of lime sulfur thinning programs.
Lime sulfur thins flowers by inhibiting pollen germination and pollen tube growth, preventing fertilization of the flower. Unlike other pollinicides, LS has about 24 hours of kick-back after the pollen grain germinates. This post-germination activity extends the time for making an effective thinning spray. LS is also a photosynthetic inhibitor. A temporary reduction of apple leaf photosynthesis can also contribute to thinning by causing a brief period of carbon stress. Spray oil can be added to LS to enhance its penetration and boost its efficacy.
Application of LS as a blossom thinner targets the pistil of unfertilized flowers. Thorough coverage is essential. For well-pruned dwarf and semi-dwarf trees, a spray volume 80-100 gallons per acre is typically used. For small trees with narrow open canopies, 50 gallons per acre may be enough, while older larger trees may require 200 gallons per acre to obtain thorough coverage. Avoid excessive spray volume to minimize leaf damage and fruit russeting. The action of LS is based on its concentration, not on the rate per acre. Do not concentrate the chemicals when spraying at lower volume.
LS is applied at 6-10% (v/v) when used alone. When mixed with oil, use LS at 1.5 to 2% (v/v). Oil options include fish oil at 2% (v/v), dormant petroleum oil at 1%, or summer oil at 1 to 1.5% v/v. Trials in the mid-Atlantic indicate that an oil + LS combination is more effective than LS alone.
Two LS applications during bloom are suggested if possible. If weather conditions are not conducive to a 2nd application or if less aggressive blossom thinning is desired, a solitary application would still be of benefit in multi-step thinning programs (nibble approach). In years of a protracted bloom period, a 3rd application is possible. Do not make more than 3 applications of LS for blossom thinning per season.
NovaSource™ LS has an REI of 48 hours. Refer to the label for a list of PPE for pesticide handlers, applicators and for early re-entry. Note that LS is hazardous to eyes, so spray applicators and other handlers must wear goggles or a face shield.
Postbloom Chemical Thinning of Apples
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 two days before and the four days after the application of the growth regulator thinners (NAA, NAD, 6BA) can have a major impact on the efficacy of a thinner 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 and 17 millimeters. Likewise, thinners applied when daytime high temperatures exceed 85°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 the carbamates (carbaryl and Vydate) can be effective from petal fall to fruit sizes of 17 millimeters. NAA materials, however, should not be applied to Spur Delicious fruit when the size is above 9 millimeters in diameter. Applications above 9 millimeters 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.
Materials for postbloom thinning of apples
6BA
6BA is a cytokinin-an active fruit thinner that can also enhance cell division. Therefore, an 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., 6-BA, MaxCel, RiteWay, Exilis 9.5SC), 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 spurtype 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 to 75°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.
Amid-Thin W
Napthalene acetamide (NAD) is the amide salt of NAA and 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, as pygmy fruit may result.
Ethephon
Ethephon (Ethephon 2, Motivate, Verve) is most effective as a thinner when fruits are larger than 17 millimeters 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 24 to 27 millimeters 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.
NAA
Napthaleneacetic acid (NAA) has been used as a thinner for many years. Fruitone L and PoMaxa and Refine 3.5L liquid formulations and Fruitone N and Refine 3.5 WSG dry formulation are registered for use as chemical thinners for apple and pear. NAA may be applied between bloom and 17-millimeter fruit diameter, although the traditional target window for optimal response to NAA is 10- to 12-millimeter 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.
Carbaryl
Several formulations of carbaryl (carbaryl 4L, Sevin 4F, Sevin SC, and Sevin XLR Plus) 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 millimeters. 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 and preferentially use the XLR formulation.
Vydate (oxamyl)
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 millimeters in diameter, to when it is approximately 20 millimeters 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 russetprone 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.
