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Apple Thinning Guidance for Blocks That Still Have a Crop

Depending on your location and the level of damage from the freeze events on April 8 and April 21, you should now have a reasonably informed idea of your apple crop potential. At this point, most growers likely know whether they have a light crop, a decent crop that still needs some thinning, or little to no fruit at all. In my case, at the Winchester AREC research farm, most blocks appear to have no meaningful crop. However, even here, a few sections of the orchard at relatively higher elevation may still need some thinning.

The purpose of this post is to provide general guidance for thinning decisions in the Winchester/Frederick County area and in Central Virginia, where some orchards still have enough crop to justify thinning.

In both areas, the carbohydrate model showed a very severe carbohydrate deficit between April 11 and April 16. This level of stress was enough to promote natural fruit abscission, which is why I was reluctant to recommend chemical thinning during that period. Growers who applied thinning sprays during that window have likely already seen a very good – strong thinning response, including fruitlets on the ground.

For those who applied thinning treatments last week, between April 22 and April 24, the thinning response may take a little longer to become visible. With the cool weather we are currently experiencing, fruitlet growth and size separation within clusters will likely move slowly. In other words, the difference between fruitlets that will be retained and those that will eventually abscise may not be obvious right away.

For growers who have not applied any thinning treatment yet but still have a crop that requires thinning, there are two possible scenarios:

  1. If your local forecast shows at least one day this week with temperatures above 70°F, no major rain events, and your average fruit size is still below 18 mm, you can still consider using 6-BA and/or NAA. Under these conditions, I would increase the rate by about 30% and include Regulaid or oil to improve thinning activity. These materials are generally less effective when temperatures remain below 70°F on the day of application.
  2. if the weather remains cool and rainy for the rest of this week, it may be better to wait until early next week, around May 4–6. By that time, many blocks will probably have fruitlets larger than 18 mm, and Accede or ethephon-based thinning programs may be more appropriate. The only exception would be blocks where the average fruit size is still below 18 mm; in that case, 6-BA and NAA may still be useful.

As always, please base your thinning decisions on your own block conditions, including actual crop load, fruit size, cultivar, tree vigor, and the amount of freeze damage. This season is highly variable, and the best thinning decision will depend on what you are seeing in each individual block.

Thinning Update: Timing Looks Good This Week

I understand that another wave of freeze hit Winchester and other parts of the state this morning (April 21), and in some areas it caused additional damage—on top of what we already saw on April 8. It’s been a tough stretch, no question. That said, the situation is not uniform across the state. I’ve been hearing from several locations, especially in and around central Virginia, where growers are still expecting a near full crop. For those blocks, thinning is still very much on the table, and that’s really who this update is for.

I ran the carbohydrate thinning model last week using Gala green tip on March 12 and full bloom on April 7, and at that time we were sitting in a pretty severe carbohydrate deficit. That kind of deficit typically drives natural fruit drop, which is why I recommended holding off on any thinning applications last week. I ran the model again today, and the picture has shifted quite a bit. Conditions from Wednesday (April 22) through Saturday (April 25) look like a good window for your main thinning sprays. We’re right in that 6–15 mm fruit size range, the carbohydrate balance has moved into a slight surplus, and temperatures in the 80–85°F range are about as good as it gets for activity from materials like 6-BA and NAA. The model is suggesting an increase in rates by about 30%, but I wouldn’t push it. In my opinion, sticking with your normal rates should be sufficient given how conditions are lining up.

If you already applied thinners last week, I would expect you’ll start seeing a response soon, likely combined with some natural drop—especially in clusters with smaller fruitlets. In that case, I’d hold off and give it some time before making another pass. Waiting until around the middle of next week, say Wednesday or Thursday, should give you a better sense of how things are settling before deciding on any follow-up. Looking ahead, the forecast is calling for some cloudy days next week, which could bring us back into a mild carbohydrate deficit. That may actually help with thinning if additional adjustments are needed.

Carbohydrate thinning model outputs based on Gala with green tip on March 12, full bloom on April 7, and using the Roseland weather station.

Also, if your green tip and full bloom dates are within about ±3 days of the ones I used, you can still rely on these outputs with reasonable confidence. If your dates differ more than that, it’s best to run the model yourself using the nearest weather station and your exact phenology dates. You can access the model here: https://newa.cornell.edu/apple-carbohydrate-thinning

Overall, this is one of those seasons where variability is driving everything. Some blocks are done, others still need full management. Take a close look at your own situation before making any decisions, and as always, I’ll keep sharing updates as we move forward.

Freeze Damage – What to Do Next

The impact of the recent freeze has been all over the place across the state. Differences in temperature, elevation, dew point, and bud stage have made a big difference, so it’s not surprising that orchards—even blocks within the same farm—are showing very different outcomes. Take some time to look carefully before making any decisions.

Give it a few days before making a final call: If it looks like the crop is gone, I’d encourage you not to rush that conclusion just yet. Some cultivars may still push delayed or “rat-tail” blooms over the next several days. These may have escaped the freeze and could still set a small crop. It’s worth scouting your trees over the next 7–10 days before making a final decision.

If crop loss is severe: If you’re confident that the crop is largely gone, try to avoid additional nitrogen applications unless trees clearly look weak or deficient. With little to no crop, trees will naturally push vegetative growth, and you’ll likely see shoots taking off quickly and the canopy getting dense in a hurry. Extra nitrogen will only make that worse and add more work later, especially during pruning.

If you still have a light crop: If there is still some fruit out there and you want to maintain quality, be prepared for some canopy management. With fewer fruit acting as a sink, trees will still push vegetative growth. Summer pruning may be needed to open things up and allow better light into the canopy.

Managing excessive growth and fire blight risk: You may want to consider prohexadione-calcium products (Apogee, Kudos) to help slow down shoot growth. This can also help reduce the amount of tender tissue that’s highly susceptible to shoot blight.
Also, don’t forget that fire blight risk hasn’t gone away. Any blossoms that were open before the freeze—or delayed blooms that show up afterward—can still be infected. So it’s worth staying on top of that as the season progresses.

Thinning decisions (where crop remains): If you still have a crop and are thinking about thinning, I would hold off for about a week. With the recent warm weather and what’s coming, trees are likely running in a carbohydrate deficit, which can naturally push fruit drop. Applying thinners right now—especially around petal fall—could easily lead to over-thinning.
It’s a good time to check the NEWA carbohydrate thinning model (https://newa.cornell.edu/apple-carbohydrate-thinning). Use your nearest weather station and plug in your green tip and bloom dates to get a better sense of what’s happening in your specific location.

Don’t abandon orchard management: Even in blocks where the crop is minimal or gone, it’s still important to take care of the trees. Keeping foliage healthy and maintaining a functional canopy will pay off in return bloom and next year’s crop. You may adjust your program, but I wouldn’t walk away from it entirely.

I’ll keep sharing updates on thinning and the carbohydrate model for areas where a crop is still present. I know this season started off with a severe frost and has been a tough one for many of you, but we’ll keep working through it together and adjust our approach as we see how things develop over the coming days and weeks.

Devastating Freeze Damage in Winchester and Parts of Virginia

This freeze event turned out to be far more severe than forecasted. The predicted lows for the morning of April 8 were around 27–28°F, but actual temperatures dropped much lower. In our Winchester area, I recorded temperatures ranging from 19.5 to 24°F depending on elevation. The level of damage we are seeing is unlike anything I’ve observed in apples over the past eight years at Virginia Tech. I held off reporting yesterday because, quite honestly, I didn’t believe what I was seeing and wanted to confirm the extent of the injury.

Unfortunately, we are looking at a near-complete crop loss in apples in this area. Across multiple cultivars—‘Pink Lady’, ‘Gala’, and ‘Honeycrisp’—and across all developmental stages, from first pink to full bloom, flowers were completely killed. When dissected, both king and side blooms were brown and water-soaked, and entire clusters appeared “roasted.”

Based on our field assessments, apple mortality reached approximately 100% in ‘Pink Lady’ and ‘Honeycrisp’ and about 98% in ‘Gala’. In peaches, mortality was 100% in ‘SunHigh’ and around 98% in ‘Redhaven’, with slightly better survival at higher elevations. In cherries, damage was estimated at around 56%. In practical terms, this represents a complete wipeout for apples and peaches in many Winchester-area blocks. However, conditions may vary across Virginia. Some regions did not experience temperatures below critical thresholds, and those areas may still have a reasonable crop if no additional stress events occur.

At this stage, it is important to continue monitoring orchards carefully. Some cultivars may produce secondary or late flowers that could have escaped injury, although any surviving bloom is likely to contribute only a small fraction of a normal crop. Management plans will need to be adjusted accordingly, including decisions related to thinning and pest management, all of which should now be aligned with the expected reduced crop load.

I wish I had better news to share, but it is important to provide a clear and accurate picture so informed decisions can be made early.

Apple flowers at full bloom, completely killed by the April 8 freeze event.

Apple flowers at the pink stage were also completely killed by the freeze. The image on the right shows the same cluster as the left after removing the petals.

Key Early-Season Spray Programs in Apples

As we move through pink and bloom in most parts of the state, many of the decisions we make in apple orchards can affect fruit quality at harvest and even influence next year’s crop. Early-season sprays are not only about protecting foliage and fruitlets; they also play an important role in managing fruit size, bitter pit, vegetative growth, fruit finish, and return bloom.

Calcium starts early, not late
When growers think about bitter pit management, calcium often comes to mind as a late-season program. In reality, early-season calcium sprays are highly important, especially for cultivars such as Honeycrisp, Golden Delicious, and York Imperial that are prone to bitter pit and cork spot. A seasonal target of 4 to 14 lb of actual calcium per acre is recommended, which is roughly equivalent to 15 to 50 lb of calcium chloride per acre, or about 2 to 8 lb per cover spray depending on the program. Calcium sprays that begin around pink are usually more effective than waiting until later in the season.

Boron supports both fruit set and calcium movement
In most programs, 0.5 to 1 lb of actual boron per acre is sufficient, which is about 2.5 to 5 lb of Solubor per acre. This can be applied at pink, at bloom, or about 7 to 10 days after petal fall. Boron can be included with calcium chloride in the spray tank, but one important point is to avoid pre-mixing Solubor and calcium chloride together in a small volume of water before adding them to the main tank. They should be added separately.

Prohexadione-calcium is more than a growth regulator
Prohexadione-calcium products such as Kudos and Apogee can be very useful when applied early, particularly at pink. Most growers think of these products mainly for shoot growth control, but early applications can also help reduce bitter pit and suppress blossom blight and later shoot blight development. In vigorous blocks, that combination of benefits can make prohexadione-calcium one of the most valuable early-season materials.

The standard rate highlighted in your handouts is 6 oz per acre. Performance can drop when spray water is alkaline or contains high levels of calcium carbonate, so hard water should be conditioned with ammonium sulfate. A surfactant can improve uptake, although some combinations may foam and require an anti-foam material. One important limitation is compatibility: prohexadione-calcium should not be tank-mixed with calcium or boron. It should also be kept in mind that prohexadione-calcium is not a substitute for calcium sprays.

Urea can help when fruit set or fruit size is at risk
Foliar urea remains a useful tool early in the season, particularly when growers are trying to promote cell division and improve fruit sizing. It is especially valuable in Gala and in situations where king blooms have been damaged by frost, since the loss of king fruit can shift the crop toward smaller fruit. Applications at bloom at 3 lb per 100 gal, followed by petal fall and first cover applications at 5 to 6 lb per 100 gal, can help support fruit set and early fruit growth.

There is another benefit at bloom: foliar urea can help extend the effective pollination period. That said, foliar nitrogen should not be viewed as a substitute for a sound ground nitrogen program, because it does not move down into woody tissues the same way soil-applied nitrogen contributes to whole-tree reserves. Its role is more immediate and is mainly tied to fruit set and sizing.

Fruit finish problems begin soon after bloom
For Golden Delicious and Gala, fruit finish issues such as russeting and scarf skin are often tied to humid conditions during the first 30 to 40 days of fruit development. Where these problems are common, GA₄₊₇ products such as ProVide 10SG or Novagib 10L can be useful. The handouts recommend 2 to 4 sprays beginning at petal fall and repeating at 7- to 10-day intervals. This is one of those programs where timing is much more important than waiting until symptoms are visible, because by then the damage to fruit finish has already started.

Young trees need a different mindset
For second- and third-leaf trees, the goal is often not crop retention but defruiting, so the tree can put more energy into canopy and framework development. There are several approaches, including blossom thinning with lime sulfur plus oil during bloom and post-bloom thinning with 6-BA or NAA plus carbaryl at petal fall and again at 6 to 12 mm fruit size. Ethephon plus carbaryl plus oil can also work at 10 to 15 mm, but this mix should be avoided on weaker or medium-vigor cultivars on dwarfing rootstocks because of the risk of excessive growth suppression.

Defruiting young apple trees using blossom thinning with lime sulfur and oil (left) and post-bloom chemical thinners (right).

Thinning Update: Week of May 6 – Winchester and Central Virginia

If you haven’t applied any thinning treatments yet and are planning to start this week, I’ve run the carbohydrate thinning model for both Winchester and Central Virginia to guide your decisions.

For Winchester/Frederick County:
Last week offered ideal thinning conditions—fruit size ranged from 6–15 mm, the carbohydrate model indicated a deficit (favorable for thinning), and the temperatures were warm enough for effective 6-BA and NAA activity. If you applied thinning sprays last week, you’re in great shape—no further action is needed this week or until at least next Monday. Keep in mind that it usually takes 10–14 days under current weather conditions to evaluate thinning response. If you didn’t thin last week and want to start now, here are your options:

Before applying anything, I recommend closely inspecting your trees and gently shaking some branches to check for natural fruit drop. If you still believe thinning is needed, don’t delay—waiting until next week may be too late.

Cornell Apple Carbohydrate Thinning Model for Winchester, VA. Green tip date is March 16, and full-bloom date is April 12.

For Central Virginia:
The same guidance applies. If you applied thinning sprays last week, no action is needed this week. If not, refer to the thinning recommendation table below for this week’s guidance.

Cornell Apple Carbohydrate Thinning Model for Roseland, VA. Green tip date is March 14, and full-bloom date is April 10.

Apple Carbohydrate Thinning Model Outputs – Central Virginia – Roseland, VA

The Apple Carbohydrate Thinning Model uses key environmental variables—primarily temperature and solar radiation—to estimate the carbohydrate status of apple trees. This status helps predict how responsive trees will be to chemical thinning agents. When trees are carbohydrate-deficient, they are more likely to shed fruit, making them more responsive to thinning. Conversely, when conditions are favorable for photosynthesis—sunny and warm—trees tend to retain their fruit and respond less to thinning applications.

Importantly, the model does not rely on a single day’s data. Instead, it calculates a six-day weighted average of carbohydrate balance to determine whether the tree is in a state of surplus or deficit. This running average guides decisions on whether thinning materials should be applied, what kind of response to expect, and how much product to use to avoid over- or under-thinning. You can refer to Column 6 in the table below to view the six-day average.

In addition, the model tracks accumulated degree days (DD) from full bloom, which is why recording the exact date of full bloom is essential. The optimal thinning window usually occurs between 200 and 250 DD—coinciding closely with the 6–18 mm fruit size window. See Column 7 in the table below for the current degree-day accumulation.

Based on weather data from the Roseland, VA station and the growth stage of ‘Gala’ apples, the model projects that trees will reach approximately 200 DD by Saturday, April 26. This signals the start of the ideal thinning window, which will extend through the 250 DD threshold. With forecasted temperatures staying above 75°F from Saturday through Wednesday, conditions are favorable for effective uptake of thinning agents such as NAA and 6-BA.

However, the model also indicates a carbohydrate surplus from April 26 through at least the next three days. As a result, it recommends increasing the thinning material rate by 30% above standard. You can find these suggested rates in the last column of the table below. Under surplus conditions, the risk of over-thinning or excessive natural fruit drop is reduced. Still, conditions may shift by Tuesday or Wednesday, so rate adjustments might be necessary.

I’ll re-run the model early next week and share updated guidance. In the meantime, you can run the model yourself at: https://newa.cornell.edu/apple-carbohydrate-thinning
This will allow you to select the closest weather station and enter your specific green tip and full bloom dates for a more localized prediction.

Bottom line: The thinning window begins this Saturday and will continue through the coming week. Plan your applications accordingly.

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 14), Full Bloom (April 10)

Rescuing Apple Crops After a Frost or Hailstorm: Can 6-BA + GA Treatments Help?

One way growers have managed to salvage part of their crop in years when a hard freeze or hailstorm hits during apple bloom is by applying a combination of plant growth regulators—specifically 6-benzyladenine (6-BA) and gibberellins GA₄+₇—found in products like Promalin® and Perlan®. These products essentially mimic the natural hormones produced by developing seeds after successful fertilization. By applying them during bloom or shortly after a frost or hail event, it’s possible to induce parthenocarpic fruit set—a process in which fruit develops without fertilization, resulting in seedless apples.

How It Works: When fertilization doesn’t occur—due to frost injury, hail damage, or disrupted pollination—flowers typically abscise. But applying GA₄+₇ and 6-BA can supplement those missing signals. Gibberellins stimulate fruit cell enlargement, and cytokinins like 6-BA promote cell division. Together, they initiate fruit development in flowers that would otherwise abort. This treatment has shown success across many trials and real-world orchard scenarios, with growers reporting the ability to recover 20–30% of a normal crop. While that might not seem like much in a bumper crop year, it can make a huge difference in disaster years—sometimes being the difference between breaking even and facing a total loss.

For best results, the sprays should be applied as soon as possible after the damage, ideally within 24 to 48 hours. There’s still a chance of success if applied up to 4–6 days later, but the earlier the application, the better the outcome. It’s important to remember that completely dead flowers cannot be revived, but if the blooms are partially damaged and still viable, there’s a good chance they will respond. The optimal timing is from bloom through petal fall. Later applications, once fruitlets are already forming, are far less effective. Conditions at and following application also matter—moderate temperatures and no immediate refreeze improve absorption and success.

Managing Expectations: While the benefits are real, it’s important to manage expectations:

  • Results vary by cultivar, timing, extent of damage, and orchard conditions.
  • The treatment won’t help if all flowers are completely destroyed.
  • Parthenocarpic fruits are usually of good eating quality, but they may be slightly smaller, more elongated, and are sometimes more prone to dropping if they’re growing alongside seeded fruit.

Nonetheless, when applied under the right conditions, GA₄+₇ + 6-BA sprays have repeatedly increased fruit set and overall yield, even in the absence of pollination. The resulting apples typically retain their flavor, sweetness, and firmness. In most cases, there’s no downside in terms of market quality, especially when crop load is light.

In addition to PGRs, applying low-biuret urea at a rate of 3 lb/acre during bloom can also help improve fruit set, particularly under stressful conditions. Urea serves as a readily available nitrogen source, supporting early cell division and metabolic activity in developing fruitlets. When used in conjunction with GA₄+₇ and 6-BA, it can enhance the fruit set response and overall effectiveness of frost recovery treatments.

More About Apple Chemical Thinning-May 2024

From the thinning treatments conducted between April 28-30 in the Winchester/Frederick County area, we can now observe a noticeable difference in fruit size within the same cluster. The smaller fruits, which are likely to fall off, can now be easily detached with a gentle touch or slight pull. As for the chemical thinning applied last week (May 2-3), the results are starting to become visible for some, but not all, varieties. It’s important to note that due to several days of low carbohydrate levels, as expected, some fruits have started to drop naturally—without any chemical intervention. These fruitlets measure between 8-11 mm in diameter. The fruits that are responding to our thinning from 10-12 days ago are about 13-15 mm in diameter. Therefore, hold off on deciding whether to apply another thinning treatment based on the smaller fruits under 11 mm. Wait a bit longer to see how the last treatment worked, checking for clear size differences in the same cluster or if the fruits can now be easily pulled off.

Apple fruitlets from a Pink Lady apple tree, displayed in two groups. The upper section shows fruitlets with diameters of 8-11 mm that have either fallen or are about to fall due to natural fruit abscission. The lower section presents fruitlets measuring 13-16 mm, which detach easily upon touch or gentle pull, indicating a response to the chemical thinning applied on April 29th. Some king fruits now measure 24 mm.

If you find that your fruit size is still under 18 mm and you need another thinning pass, you can still use 6-BA and NAA products along with carbaryl and a surfactant—or better yet, oil. For fruits averaging 15-20 mm, Accede can be used. It contains 10% of ACC, a precursor to the ethylene hormone, crucial for this process. You should apply 23-46 fluid ounces per acre, ideally when the king fruit is between 15-20 mm. If the temperature is above 90°F on the day you plan to apply, consider reducing the rate. For best results, apply Accede in slow drying conditions, like early morning or at night.

On the other hand, Ethephon is best for ‘rescue thinning’ when your fruits are between 18-25 mm. Ethephon works best when temperatures are between 70-80°F on the day of application and the following two days. Avoid using Ethephon when it’s cooler than 70°F or hotter than 80°F, as it may be ineffective or cause overthinning. Also, don’t use it if the forecast predicts temperatures over 85°F for the application day or the next two days. When applying Ethephon, use at least 100 gallons per acre to ensure effective distribution. Please refer to the attached table for the rates of ethephon and carbaryl for different cultivars, based on the research by Autio and Cowgill (https://ag.umass.edu/fruit/fact-sheets/f-129r-late-season-rescue-thinning-with-ethephon).

Apple Thinning Advisory: May 6-10 Insights for Winchester/Frederick County and Central Virginia Growers

In the Winchester/Frederick County area, it appears that apple trees may be overly responsive to thinning treatments applied this week (May 6-10). Therefore, I generally advise against implementing any thinning treatments during this period, especially if you have already done so at petal fall (April 25-26) or the previous week (April 29-May 3). Examining the carbohydrate balance chart below, it’s evident that trees have been consistently deficient since April 24. While this deficiency is typically conducive to chemical thinning treatments, it also poses a risk of natural fruit abscission. Given the forecasted temperatures exceeding 80°F with mostly cloudy days this week, I anticipate further carbohydrate deficiency. If you administered the first thinning treatment on April 25-27, you should observe its effects this week (May 7-8). However, if your initial treatment was conducted last week (April 29-May 2), simply wait until May 12-13. By then, you should notice some fruits dropping off easily upon touch, with a noticeable difference in size between retained and dropping fruits.

Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 7), Full Bloom (April 14)

The situation in Central Virginia presents less risk, with more days of carbohydrate surplus following full bloom. Nevertheless, there has been a significant decline in carbohydrate balance this past week, likely to persist due to high temperatures and reduced solar radiation on certain days this week. Hence, I also advise against thinning treatments this week, recommending waiting until May 10th to ascertain if carbohydrate levels stabilize. Growers who have already applied two thinning treatments (at petal fall and last week) likely have completed thinning for the season, achieving an optimal crop load. For those who applied their first treatment on April 28-30, trees are likely to respond favorably to thinning materials, with a noticeable size separation expected this week (May 8-10).

Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)