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)
With the high daily temperatures we’re experiencing this week, the degree days for Gala and most other apple cultivars in the Winchester/Frederick County area will likely reach the 200DD mark today. This signals the optimal timing for chemical fruit thinning using 6-BA-carbaryl and NAA-carbaryl mixes. I utilized the carbohydrate thinning model on NEWA (https://newa.cornell.edu/apple-carbohydrate-thinning), based on a green tip date of March 7 and full bloom date of April 14 for Gala in my location. The model indicates a severe carbohydrate deficiency, which is very conducive to thinning conditions. It also recommends reducing the chemical thinning materials by 15% compared to standard rates. Please refer to my previous posts ((https://blogs.ext.vt.edu/tree-fruit-horticulture/2024/04/18/apple-fruit-thinning-general-notes-and-specific-recommendations-for-central-virginia/) for the standard rates I’ve recommended and adjust your calculations accordingly. You have a four-day window (April 30 to May 3) to complete your thinning applications. The temperatures over the next few days are expected to be above 80 degrees Fahrenheit, which is ideal for both the uptake and activity of 6-BA and NAA products.
Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 07), Full Bloom (April 14
I believe this is the ideal time to schedule your thinning applications for Gala and other cultivars with a green tip date around March 12 and a full bloom date of April 12. In some areas, Honeycrisp and Fuji reached full bloom a week after Gala. Therefore, make sure to check the Carbohydrate Thinning Model on the NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning) and input the exact dates for your cultivars. Using the weather station data from Roseland and Gala’s phenology data, I ran the model and here are the outputs: three positive indicators for good thinning conditions. First, the accumulated degree days are expected to reach the optimal range of 200-250 DD by this Sunday, April 28. Second, the 6-D weighted average of tree carbohydrate balance is showing a promising decline as we approach April 28. Third, the current weather forecast predicts next week’s temperatures will exceed 80°F, which is ideal for the effectiveness of 6-BA and NAA treatments, with partially cloudy days contributing to the carbohydrate deficiency needed for effective thinning. If this is your first thinning treatment for Gala this year, aim for Sunday, April 28 through Tuesday, April 30. However, if you have already applied a thinning treatment at petal fall, delay your next treatment until the end of next week, around May 2 and May 3, to observe the effects of the initial application.
Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)
In the scenic Winchester/Frederick County area, apple growers are witnessing varied stages of growth across different apple cultivars. Currently, cultivars such as Fuji and Honeycrisp are at the partial petal fall stage, while Gala and Red Delicious have reached complete petal fall. Most notably, the Pink Lady cultivar has reached the critical >6 mm diameter stage, making it the ideal time for fruit thinning.
It’s important to time the application of chemical thinning agents, such as 6-BA and NAA, accurately. According to the Apple Carbohydrate Thinning Model on Cornell’s NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning), the current predictions for accumulated degree days (DD) for Pink Lady apples in Winchester suggest reaching the 200-250 DD window by Thursday, April 25, and continuing through most of the next week. While the model indicates a daily carbohydrate surplus, which isn’t ideal for thinning, a forecasted drop in carbohydrate levels by April 27 should be closely monitored. This dip, though minor, will likely influence the 6-D weighted average, which is a crucial metric for thinning effectiveness.
If you’re targeting a thinning application for cultivars like Pink Lady, which had a green tip date of March 2nd and a full bloom date of April 8, post-April 25th is advisable for starting treatments. The weather forecast suggests that April 28, April 29, and May 1 will offer optimal conditions—both temperature-wise and in terms of carbohydrate levels—for the effectiveness of 6-BA and NAA treatments.
Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Pink Lady- Green tip (March 2), Full Bloom (April 8)
Turning our attention to the Gala cultivar, it hasn’t yet reached the 200-250DD window. However, for those keen on applying a thinning treatment at petal fall, the upcoming Sunday or Monday is recommended. Following this initial treatment, it’s advisable to wait at least 10 days before the subsequent application, ideally timed for when the cultivar enters the 200-250DD window.
Apple Carbohydrate Thinning Model-Weather Station (Winchester, VA), Gala- Green tip (March 7), Full Bloom (April 14)
In various parts of Virginia, apple orchards are currently transitioning through stages ranging from full bloom and petal fall to having fruits about 6-18 mm in size. Given this period of active growth, it’s an ideal time to discuss the crucial practice of apple fruit thinning—especially vital this year due to the heavy bloom and minimal spring frost damage. Proper thinning is essential; without it, the excess blossoms and small fruitlets from this year could negatively impact both next year’s yield and the quality of this year’s harvest. The development of next year’s flowering buds in apples is largely determined during the first four to five weeks post-full bloom. In years with heavy bloom and fruit set, like this one, failing to adequately thin can lead to too many fruits drawing on the tree’s carbohydrates. This overburden can drastically reduce the number of viable buds for the following year, potentially resulting in no crop for varieties prone to alternate bearing, such as Honeycrisp, Fuji, and Winesap.
Moreover, maintaining too many fruits on the tree can also degrade fruit size and overall quality at harvest—particularly for the Gala variety. It’s important to note that fruit size at harvest is primarily influenced by the rate of cell division and the total number of cells per fruit, rather than merely by cell expansion. This cell division occurs mainly in the initial weeks following full bloom, making it critical to reduce competition among the fruitlets so that more resources are directed to fewer fruits, thus enhancing their growth and quality.
With these points in mind, I’d like to offer some specific recommendations for thinning applications at petal fall and the 6-18 mm fruit size window (the main thinning window). At the end of this post, I will also share outputs from the Carbohydrate Thinning Model and my personal advice for apple growers in Central Virginia and other regions currently within the 6-18 mm fruit size stage. For those in the Winchester and Frederick County areas, where orchards are still transitioning between full bloom and petal fall, other posts will follow as needed.
Effective thinning sprays at petal fall to 5 mm fruit diameter.
Notes: • To prevent the formation of pygmy fruit, avoid applying NAD and NAA to Red Delicious and Fuji apples after petal fall. • When using NAD as a thinning spray, ensure that the spray volume is at least 100 gallons per acre, as lower spray volumes may decrease efficacy. • Carbaryl can be used alone as a thinning spray between petal fall and when the fruit diameter reaches 15 mm. However, it is more commonly used in combination with either NAA or 6-BA when the fruit diameter is between 7 and 15 mm. • When using carbaryl alone, increasing the rate of application will not enhance thinning activity, as carbaryl’s effectiveness is not highly responsive to changes in application rate.
Recommended chemical materials and rates for thinning fruits (6-15 mm)
Notes: • Exilis 9.5 SC contains a high concentration of 6-BA (9.51%) compared to Maxcel (1.9%) and Exilis Plus (2%). Refine is available in two concentrations: Refine 3.5 and Refine 6.25, with the latter having a higher concentration of NAA. All dosages in the table are based on the concentrations found in Maxcel (6-BA) and PoMaxa (NAA). • The recommended application rates in the table are for mature trees (6 years or older). For younger trees (4th and 5th leaf), reduce the rate by 25% to 50%. • Do not use less than 100 gallons per acre for thinning treatments. • Thinning is more difficult in the upper parts of the canopy compared to the lower parts. • Thinning is easier when the crop is heavy, as opposed to when the crop is light. • Use the carbohydrate thinning model to time your thinning applications. Alternatively, as a general rule of thumb: Thinning is more challenging when it is sunny; overcast conditions for 2 to 3 days facilitate easier thinning; thinning materials are less effective in cool weather (below 65°F) and more effective in warm weather (above 70°F).
Apple Carbohydrate Thinning Model Outputs-Pink Lady- Roseland-VA
The Carbohydrate Thinning Model uses critical environmental factors such as temperature and solar radiation to assess the carbohydrate status of trees. This status helps determine how trees will respond to chemical thinning agents. When trees are in a carbohydrate-deficient state, they are more likely to shed fruit, making them more receptive to thinning applications. Conversely, when conditions are favorable for photosynthesis—sunny and warm—trees are less likely to respond to thinning as they prefer to retain their fruits. Remember, fruits carry seeds, and seeds represent the future generation of trees—an easy concept to grasp. The model doesn’t rely on a single day’s data; instead, it averages the carbohydrate balance over six days to decide if a tree has a deficiency or surplus. This helps determine whether to apply treatments, how much response to expect from the trees, and whether to adjust the quantity of thinning materials to avoid over- or under-thinning. The model also tracks accumulated degree hours from the time of bloom, which is why knowing the exact day of full bloom is crucial. It predicts the optimal thinning period, which usually occurs between 200 to 250 degree days (DD)—coinciding nearly perfectly with the 6-18 mm fruit size window.
Based on data from the weather station in Roseland, VA, and the growth stages of Pink Lady and Gala apples, the model predicts that by Saturday, April 20, Pink Lady trees will reach about 200DD. This is the ideal time to start applying thinning materials, continuing through the 250 DD mark. However, the model also indicates a carbohydrate surplus on April 19 and the following three days. Therefore, it recommends increasing the thinning material rate by 30% above standard. Since the forecast predicts temperatures above 70°F this Saturday, which enhances the uptake and efficiency of agents like NAA and 6-BA, thinning applications are best done on that day.
Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Pink Lady- Green tip (March 4), Full Bloom (April 6)
For Gala apples, which reached green tip on March 12 and full bloom on April 12, the 200-250DD window has not yet been reached. Nonetheless, if you wish to apply chemical thinning now using 6-BA and carbaryl to promote cell division, you can use 48 fl oz of 6-BA and 1 pt/100 gal of carbaryl. However, it’s important to wait at least 10 days to observe the effects of this treatment before proceeding with another. Based on the current weather forecast and degree days, the 6-18 mm fruit size window should still be open in 10 days if you decide on a second thinning application.
Apple Carbohydrate Thinning Model-Weather Station (Roseland, VA), Gala- Green tip (March 12), Full Bloom (April 12)
As we enter the optimal window for thinning treatments in the Winchester/Frederick County area, it’s important to consider the best options for your apple trees. In a follow-up to our previous post, we have some important updates for those who chose option A or B.
If you went with option A, and applied your thinning treatments last week (April 25-27) because you had larger fruit, you won’t need to do any further treatment for at least two weeks after this treatment. Wait until you can see signs of fruit abscission and/or fruits showing distinct segregation in size before deciding if another thinning treatment is necessary. For those who went with option B due to the low temperatures, this Friday and over the weekend may provide better thinning conditions. Temperatures are expected to rise above 65F on Friday and reach 72F on Sunday, with even higher temperatures on Monday. If your average fruit size is 15mm or above, start thinning on Friday and Saturday. If your fruits are less than 15mm, wait until Monday when temperatures are expected to reach around 78F and remain warm and cloudy for the rest of the week. Trees are currently in a carbohydrate surplus state, which could sustain until Friday, so you may need to pump up the rate to 30% more than the standard rate. Check the model the day before the application to see the recommended rate in the last column.
Green tip (Mar 7), full bloom (April 12), weather stations (Winchester, VT AHS AREC), percent flowering spurs (51-75%). The carbohydrate thinning model can be accessed at: https://newa.cornell.edu/apple-carbohydrate-thinning
For apple growers in Central Virginia, the same advice applies, except that you can start thinning treatments on Thursday if the forecasted temperature stays at 65F or higher. If your current fruit size is less than 15mm, consider Monday (5/8) as the main thinning day. Use the model to determine whether to apply the standard rate or increase it. If the model suggests adding oil to the tank to increase efficiency, it’s worth considering, especially if your fruits have exceeded 15mm and this is your final chance to get the top part of the canopy in shape. However, keep in mind that Captan should not be used 4-5 days before and after applications containing oil, as this can cause significant damage to fruit quality. In summary, timing and temperature are critical when it comes to thinning treatments for apple trees. Follow these guidelines to ensure optimal results for your fruit harvest.
Green tip (Mar 6), full bloom (Mar 29), weather stations (Crozet, Chiles peach orchard), percent flowering spurs (51-75%). The carbohydrate thinning model can be accessed at: https://newa.cornell.edu/apple-carbohydrate-thinning
When it comes to thinning apple trees, it’s important to consider the optimal thinning conditions that increase return bloom, especially for cultivars with a tendency for biennial bearing, such as Honeycrisp, Golden Delicious, and Fuji. If you’re looking to improve return bloom for Honeycrisp and Golden Delicious, we have a recipe that has been endorsed by many and is based on our own research over the past few years.
First, use NAA applications for fruit thinning at a fruit size of 6-15 mm, or within 30 days of full bloom. The recommended mixture is NAA @ 3 oz + Sevin @ 1 qt + Regulaid @ 1pt/100 gal/acre. In addition to NAA applications for thinning, NAA applications (@ 2-4 oz/100 gal without carbaryl or oil) should start at a fruit size of 30-35 mm and repeated 2 times at 7-10 day intervals. The rates of NAA are based on Refine 3.5 and PoMaxa. Make rate adjustments if using Refine 6.25.
For improving return bloom in Fuji and Red Delicious, we recommend using ethephon. Don’t use NAA with Fuji and Red Delicious. Apply 1-4 applications of ethephon at 0.5 pt/100 gal (without carbaryl or oil) starting at a fruit size of 30-35 mm in diameter and repeated at 7-10 day intervals. A surfactant is not necessary if ethephon is applied with cover sprays. Ethephon at this low concentration and stage of fruit development will not cause any thinning, but it should improve return bloom.
Despite the frost damage we witnessed on March 20 statewide and in some locations on April 8-9, you might be astonished by the quantity of fruits that still need chemical or hand thinning to attain the targeted size and quality. That being said, it is crucial to assess the extent of frost damage in your various blocks and varieties before deciding whether thinning treatments are necessary this season. To learn about the materials used for apple fruit thinning and the stages at which these chemicals are applied, please refer to my previous blog article. The present blog post aims to share the results of the apple carbohydrate thinning model and provide recommendations on timing, rates, and other considerations for orchards in Central Virginia and the Winchester/Frederick County area.
For Central Virginia, I ran the model today (April 16 at 12:00 pm) using the green tip and full bloom dates for Gala as March 6 and March 29, respectively. You should take into account your own dates and varieties when running the model. However, I believe these two dates are suitable for most apple varieties and orchards in Central Virginia. According to this data, we have already reached an accumulated growing degree day (base temperature = 4C) of 209, which falls within the optimal range (200-250) for thinning applications. This range often coincides with a fruit size of 6-15 mm, traditionally considered the most favorable period for fruit thinning. In terms of carbohydrate status, the daily carbohydrate level is predicted to experience a surplus for the next two days (April 17-18), but the 6-day weighted average will reach -15 g/day tomorrow. Trees generally respond more effectively to thinning materials when the 6-day weighted average values range from -10 to -40 g/day. Furthermore, the current forecast indicates temperatures between 65 to 85F this week (Monday to Friday), which is ideal for NAA and 6-BA uptake and effectiveness.
Considering all these factors, I strongly recommend applying your main fruit thinning application within the next four days. If you can complete all thinning applications within two days, opt for Wednesday and Thursday, as temperatures are expected to be around 80F. For tomorrow’s treatment, a standard rate of thinning materials is advised. For example, if you typically apply 64 fl oz of 6-BA (e.g., Maxcel or Exilis plus), 1 qt of Sevin or Carbaryl, and 1 pt of Regulaid per 100 gal/acre as the standard rate, use the same rate tomorrow. For applications on Wednesday and Thursday, more carbohydrate deficiency is expected which may lead to a recommended 15% reduction in your standard rate. However, this cannot be confirmed yet, as it is based on the 6-day average (2 days before thinning and the following four days). Nevertheless, with a projected carbohydrate surplus on Monday and Tuesday and a deficit in the subsequent three days, I believe a standard rate should be used for Wed and Thu applications.
The apple carbohydrate thinning model outputs for Gala apples, incorporating data from the Crozet (Chiles Peach Orchard) weather station, a green tip date of March 6, a bloom date of March 29, and a flowering spur percentage range of 51-75%
For orchards in the Winchester/Frederick County area, I ran the model for Gala with green tip and bloom dates of March 7 and April 12, respectively. As the model outputs below indicate, the optimal thinning window has not yet arrived. The accumulated DD is below the recommended 200 value, and our average fruit size is still under 5 mm. Therefore, I anticipate that our main thinning window will be around Monday-Tuesday next week (April 24-25), but I will keep you updated if conditions change.
The apple carbohydrate thinning model outputs for Gala apples, incorporating data from Winchester (VT AHS AREC) weather station, a green tip date of March 7, a bloom date of April 12, and a flowering spur percentage range of 51-75%
For those who responded humbly to the recommendations of my previous blog post and sprayed their Galas, Fujis and other hand-to-thin cultivars with chemical thinners on May 4 and 5…Lucky You…I believe that was a wise decision. We had several cold and/or rainy days in the past few days, along with carbohydrate surplus conditions that were unsuitable for any thinning treatments. This week, there is another potential window for thinning for those who chose to wait for a larger fruit size or more perfect thinning conditions. According to the carbohydrate model outputs (see below), the accumulated degree days will be within the perfect range (200-250 DD) on May 13 & 14. There is also a potential decline in tree’s carbohydrate level which should increase the response of apple trees to thinning materials applied in these two days. As of today, May 9, the average fruit size for our Fuji, Gala, Goldens and Honeycrisp in Winchester’s research farm is evolving around 10 mm which is also the perfect size for thinning. Having said that, I think it won’t be possible to spray on Friday and Saturday due to the rain; so, you better apply your thinning treatment on Wed and Thu this week (May 11 and 12); or Sunday (May 15) if weather allows. For blocks treated for thinning on May 4&5, no additional thinning treatments are needed at this time. You need to wait at least 2 weeks after the 1st application to decide if additional thinning is required.
The Cornell apple carbohydrate thinning model outputs for Winchester.
It’s worth noting that the outputs and recommendations of the model above are based on our Gala’s green tip date of Mar 15 and full bloom date of April 17. If you have different dates for your Gala or other apple cultivars, you will see different model outputs. To access the model, use this link: https://newa.cornell.edu/apple-carbohydrate-thinning.
Our collaborating apple growers in Central Virginia informed me that fruit sizes for most apple cultivars are now between 6-12 mm; the prime window for fruit thinning applications. I, therefore, ran the apple carbohydrate thinning model this morning to determine the optimal application timing based on the carbohydrate status, growing degree days, and thinning efficacy. As you will see below, we have already approached accumulated degree days of 235 (base temp= 4C), which is within the optimal range (200-250) for thinning applications. This range often coincides with fruit size 6-15 mm, which has always been considered the sweetest spot for fruit thinning. As for the carbohydrate status, the daily carbohydrate level is predicted to be in the deficit status for the next three days (May 2-4), and the 7-days weighted average will be at -2.49 g/day for tomorrow.
As you know, trees respond better to thinning materials when the 7-day weighted average values are in the range of -10 to -40 g/day, which is unlikely to be achieved in the following three days and therefore, the model predicts a mild response to thinning materials, and suggests increasing thinning rates by 30%. So, if you decide to apply thinning sprays tomorrow and if you usually use per acre rates of (64 fl oz of Maxcel and 1 qt of Carbaryl, and 1 pt of Regulaid/100 gal) for thinning your Gala trees, you should use (83 fl oz of Maxcel and 1.3 qt of Carbaryl and 1 pt of Regulaid/100 gal/acre) to compensate for the mild thinning action. As you may have noticed, the rate of the non-ionic surfactant remains the same per 100 gal.
My recommendations: after considering the fruit size, the carbohydrate level, and the predicted thinning action, I would seriously consider applying thinning sprays in the following three days (May 2-4); and with the forested thunderstorms on Tuesday and Wednesday, I would surely consider finishing a major portion of thinning applications tomorrow (Monday, 5/2). The temperatures in the following three days are around 80 F, perfect for NAA and 6-BA uptake and effectiveness. Also, the cloudy days on May 5-7 might reduce the carbohydrate level, at least partially, allowing for better thinning. If the current forecast is accurate, the second potential thinning window will be on Monday and Tuesday next week (May 9-10). I would use this window for cultivars currently at 6-8mm fruit diameter.
The Cornell Apple Carbohydrate Thinning Model (May 1, 2022).
Important remarks regarding the Cornell apple carbohydrate thinning model:
The model interface on NEWA (https://newa.cornell.edu/apple-carbohydrate-thinning) is slightly different this year, but the model inputs and outputs are the same.
The model collects data for temp and solar radiation from the weather station to calculate the tree’s carbohydrate daily balance and 7-day average (2 days before, the day of thinning and the 4 days after ).
For orchards in Central Virginia, I usually use the weather station in Tyro (Silver Creek), but I could not see this station listed on NEWA this morning, probably for a technical reason, and therefore I used the Crozet (Chiles peach orchard) instead. It would help if you used the nearest station to your location.
The model requires inputs for the green tip and full bloom dates. For the model I ran today, I used a green tip date of March 15 and a full bloom date of April 10 for Gala apples. If your dates are different, the outputs and recommendations will be different.
After all the frost events and subfreezing temperatures in the past four weeks, we (the state of Virginia) still have a medium-heavy apple crop, and we should start planning for the fruit thinning treatments. However, given the geographical distribution of apple orchards throughout the state, which results in different developmental stages, growing degree days, elevation, etc., thinning recommendations would vary from one location to another. So, this post aims to give a general overview of thinning materials, rates, and application timing as well as a few suggestions to consider when thinning certain cultivars or using specific materials. But I will follow this with other blog posts focusing on particular locations as required.
Table 1: Common chemical thinners, tradenames, and manufacturers.
Notes
Exilis 9.5 SC contains 9.51% of 6-BA, compared to 1.9% in Maxcel and 2% in Exilis Plus.
There are two forms of Refine; Refine 3.5 and Refine 6.25. The later contains a higher concentration of NAA.
Table 2: Effective thinning sprays at petal fall to 5 mm fruit diameter
The application rates are based on PoMaxa for NAA, Amid-Thin for NAD and Carbaryl 4 L for Carbaryl.
Table 3. Thinning materials and rates for 6-18 mm fruit diameter
The application rates are based on PoMaxa for NAA, Maxcel for 6-BA, Amid-Thin for NAD and Carbaryl 4 L for Carbaryl.
Do not use NAD and NAA with Red Delicious or Fuji, as this will result in pygmy fruits.
To achieve better results with NAD, use it in 100 gallons or more per acre.
Carbaryl can be used alone for fruit thinning between petal fall and 15 mm fruit diameter. However, it is better to combine it with either NAA or 6-BA for thinning fruits at 7-15 mm.
Carbaryl is not rate-responsive when used alone, so increasing the rates of carbaryl will not necessarily improve thinning efficiency.
6-BA enhances cell division and fruit size compared to other fruit thinners. Therefore, it is recommended to use it when thinning small-fruited cultivars such as Gala and Ginger Gold.
Temperatures between 75 – 85 oF are ideal for 6-BA uptake and effectiveness. 6-BA is not effective when temperatures are below 68°F.
NAA can reduce the overall fruit size of the harvested crop compared to other chemicals used in fruit thinning.
NAA applications for fruit thinning can also enhance return bloom in biennial bearing cultivars, such as Honeycrisp and Golden Delicious. Research from my lab at Virginia Tech showed that NAA applications within the first 30 days of bloom are critical for flower bud formation and return bloom. NAA applications at 40 and 50 days of bloom have minimal effect on return bloom in Honeycrisp.
Adding a non-ionic surfactant (e.g. Regulaid @ 1pt/100 gal) to 6-BA and NAA spray mixtures improves thinning efficiency.
We use the Cornell Apple carbohydrate thinning model on the NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning) to determine the optimal timing for thinning treatments. The model collects temperature and solar radiation data from the nearest weather station to your location to predict the carbohydrate status of the tree. Under the carbohydrate deficit status, trees become more responsive to thinning treatments; and the opposite is true under the carbohydrate surplus conditions.
Sunny, cool days and cold nights promote carbohydrate accumulation, resulting in low thinning efficiency; whereas cloudy, hot days, and warm nights lead to carbohydrate deficiency, which is good for fruit thinning.
The upper parts of the canopy are harder to thin than the lower parts.
If you have a heavy crop, thinning will be easier than having a light crop.
Vigorous trees are harder to thin compared to compact and dwarf trees.
Table 4: Effective thinning treatments at 16 mm-25 mm fruit diameter.
Ethephon should be applied as a “rescue thinning” treatment if first thinning sprays were insufficient. It is most commonly used when fruit size is between 18 and 26 mm.
When aggressive thinning is needed, mix carbaryl or NAA with ethephon.
Ethephon can result in severe over-thinning, particularly at high temperatures (> 90 oF).
Accede, a new thinning product from Valent USA, can also be used at this stage. For more information about Accede, read our blog post: https://blogs.ext.vt.edu/tree-fruit-horticulture/?s=Accede
Table 5: Easy, moderate, and hard to thin apple cultivars.
Disclaimer
Application rates mentioned in this article are based upon a concentrate spray volume of 100 gallons per acre and product labels at the time of publication. When applying chemical thinners use the rates indicated on the labels of the products that you are using. The degree of thinning action are listed according to the author’s personal field experiences. The degree of thinning action may vary from orchard to orchard and block to block.
For more information regarding apple fruit thinning, you can read our extension pub “Crop Load Management in Commercial Apple Orchards: Chemical Fruit Thinning” at https://www.pubs.ext.vt.edu/SPES/SPES-134P/SPES-134P.html.
