Getting to the bottom of fruit drop

This has been a banner year for fungus and bugs in SW Idaho. I had a pretty good fruit set on my peach, and on 2 apples. By now, 100% of the apples have dropped and there’s only one peach standing. The trees are all young - none older than 4 years but the apples carried fruit to completion last year. I have been using Regalia for the first time this year, and it’s been effective at controlling scab on my apples - but I did read some articles on it having a fruit thinning effect.

Most of the fruit dropped when smaller than a nickel. This was the last hanging apple which dropped the other day. Looks like some bug bites and some rot. What do people do to prevent fruit drop?

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Drop? Massive thinning is all I’ve seen/needed. Having all apples drop is very unusual. Mine won’t drop when sprayed with chemical thinners like Sevin.

Maybe your spray was the reason. But in sunny climates having all apples drop is very unusual.


Young trees can set one year and not the next. I would not draw too many conclusions from this one year with young trees.


To elaborate on what’s been said already this article explains a lot Apple Crop Load Management: Chemical Thinning.
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Apple Crop Load Management: Chemical Thinning

Chemical thinning applications are probably the most important single spray in a season.



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.

Blossom Thinning of Apples

As of 2019, NovaSource™ 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 apples6BA

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 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.


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.


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.

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.

Late thinning

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 2020




Professor of Pomology


Tree fruit production Orchard management systems Crop load management of tree fruit Fruit tree pruning and training



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Nitrogen Fertilization of Peach Trees ARTICLES

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Tree Fruit Production Guide GUIDES AND PUBLICATIONS




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The Pennsylvania State University,323 Agricultural Administration Building,University Park, PA 16802



Interesting, thanks. A lot of that is over my head - but I gathered that low carbohydrate reserve cause thinning and that covering the leaf surface with a spray can cause temporary carbon stress. Either of those could explain my situation. My trees are growing a lot this year, and they may just not have the carbs to make fruit too. And maybe I need to try a lower concentration of Regalia, or spray lower volume as well.

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Poor pollination makes fruit drop when it’s very small. Your fruit is beyond that stage. Insects burrowing in the fruit can cause early fruit drop which is likely. Certain fruit sprays such as those mentioned can cause fruit drop. If a tree is defoliated by insects such as Japanese beetles it may cause the tree to not have enough stored carbohydrates to set much fruit the next year. An excessive large fruit crop the previous year can cause reduced fruit set. Trees to close or shading your fruit tree can cause it to not get enough sun or nutrients for excess carbohydrate storage. Excessive pruning causes a tree to put its nutrients towards growing branches instead of fruit. Hopefully that explains part of the problems.

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I wonder if Surround sprayed on the leaves is a factor in fruit drop?

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Thanks for laying out those possible reasons. I do suspect bugs being the most likely this season. Next year I’ll have to look into some bug solutions. I started using Spinosad this year but I think it was too little too late.

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@scottfsmith explained a lot to me about surround years ago which can work for you @kunsangsean .
Asked this question Is Surround just Kaolin Clay?

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Yeah I’ve been reading about how to use Surround on here, it seems the next logical choice. I just have to wrap my head around the aesthetics of it for my urban front yard orchard. I’ve seen some photos where the whole tree is white/grey and others where it’s really not that noticeable or just the fruit.

Cannot think why anyone would wish to spray an unnecessary chemicals on home produce.

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For informed backyard growers, we spray chemicals because it is necessary.