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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 (

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)

Ideal Thinning Conditions for Apples in Winchester-Frederick County Area (April 29)

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 (, 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 (( 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

Apple Cultivar Stages and Thinning Practices in Winchester/Frederick County (April-23)

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 (, 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)

For detailed guidelines on thinning practices during petal fall and the 6-18 mm window for Gala, Pink Lady, and other cultivars, please refer to my previous post (

Apple Fruit Thinning: General Notes and Specific Recommendations for Central Virginia

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.

• 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)

• 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)

Optimizing Fruit Size and Preventing Bitter Pit in Apple Crops with Early Season Interventions

Calcium applications:

  • Apply 4-14 pounds of actual calcium per acre each season to help prevent bitter pit and cork spots. This amount is equivalent to applying 15-50 pounds of calcium chloride (CaCl2) or distributing 2-8 pounds of CaCl2 per cover spray.
  • For apple varieties that are particularly susceptible to bitter pit, such as Honeycrisp, Golden Delicious, and York Imperial, it is advisable to use the maximum recommended concentration of calcium.
  • Initiating calcium applications early in the season, starting at the Pink stage of bud development, is more beneficial than applications later in the season.
  • Foliar applications of calcium nitrate are not advised for Delicious and York apple varieties, as they can induce symptoms similar to cork spot.
  • In orchards with significantly low calcium levels, applying calcium sulfate (gypsum) in a band beneath the trees at a rate of 3 tons per acre can improve calcium content in the leaves and fruit. However, it may take two years or more to observe positive outcomes.
  • Avoid applying calcium chloride under conditions that slow drying, such as in the early morning, as it can harm the foliage. This is especially important for apple varieties sensitive to calcium chloride, including Idared and Golden Delicious.
  • A deficiency in boron can hinder the movement of calcium within the tree, potentially affecting fruit quality.

Boron applications:

  • Boron plays a crucial role in the development of flowers and the setting of fruit. A lack of boron can adversely affect both yield and the size of the fruit.
  • Furthermore, boron is necessary for the transportation of calcium within the plant. A deficiency in boron can lead to disorders related to calcium deficiency, such as bitter pit and cork spot in apples.
  • To avoid these problems, it is recommended to apply 0.5-1 pound of boron per acre, which is equivalent to applying 2.5-5 pounds of Solubor per acre.
  • Boron should be applied either at the pink or bloom stage of flower development, mixed in the tank with calcium chloride, or 7-10 days following the fall of the petals.

Prohexadion calcium:

  • Applying prohexadione calcium (PC), found in products like Kudos and Apogee, early in the season at the Pink stage can significantly reduce the incidence of bitter pit and blossom blight, as well as decrease the risk of shoot blight.
  • For optimal results from PC treatments, consider the following guidelines:
    • Apply PC at a rate of 6 ounces per acre.
    • PC’s effectiveness decreases in environments with high pH levels or in water with a high concentration of calcium carbonate (hard water). To counteract this, add ammonium sulfate (AMS) to the spray mixture.
    • Avoid combining PC with calcium or boron in the spray mix, as interactions may diminish efficacy.
    • Enhance the performance of PC applications by including a surfactant in the tank mix. This helps in better distribution and adherence of the product on plant surfaces.
    • Should the surfactant cause foaming, incorporate an anti-foaming agent to mitigate this issue.
    • Both Kudos and Apogee are approved for application at the Pink stage for apples, indicating a targeted timing for use.
    • Research by Sherif in 2019 (unpublished) suggests that applying PC concurrently with or before thinning agents like 6-BA or NAA does not compromise the effectiveness of thinning treatments.

Urea applications

  • Applying urea to the foliage during the bloom period (at a rate of 3 pounds per 100 gallons of water) and subsequently at petal fall and the first cover spray (5-6 pounds per 100 gallons of water) can significantly support cell division. This practice is especially beneficial for Gala apples. Such applications become critical when the primary, or king blooms, which typically produce the largest fruits, are lost to frost damage.
  • The foliar application of urea at the time of bloom can also improve fruit set by prolonging the period of effective pollination, enhancing the chances of fertilization and subsequent fruit development.
  • It’s important to note that foliar-applied nitrogen, while beneficial for fruit set and sizing, does not substitute for soil-applied nitrogen fertilizers. Unlike ground applications, foliar-applied nitrogen does not move into the tree’s woody structure but is instead directly utilized for the development of the fruit.

Fruit Russeting in Golden Delicious and Scarf Skin in Gala:

  • Fruit russeting and scarf skin in apples is often associated with high humidity during the first 30-40 days of fruit development.
  • To address this issue, apply 2-4 applications of GA4+7 (such as ProVide 10SG or Novagib 10L), starting from petal fall and continuing at 7-10 day intervals.

Carbohydrate Thinning Model Updates for Central Virginia- April 18

In my latest blog entry, I highlighted that this week, particularly on Wednesday and Thursday, presents an optimal window for fruit thinning treatments, especially for Gala, Pink Lady, and possibly Fuji. Taking into account the green tip and full bloom dates of March 6th and March 29th respectively for Gala apples in Central Virginia and data from the Crozet weather station, the model forecasts a moderate to severe shortfall in daily carbohydrate levels over the next three days, impacting the 6-day weighted average. As such, it is crucial to apply thinning treatments judiciously, with a focus on 6-BA and NAA. However, over-thinning may occur if high rates are used. Therefore, the model suggests reducing thinning material rates by 15% for applications scheduled on Wednesday, April 19th. This recommendation may extend to Thursday and Friday, based on current forecasts. In practical terms, if your standard application rate for Gala consists of 64 fl oz of Maxcel, 1 qt of Sevin, and 1 pt of Regulaid per 100 gal/acre, a 15% reduction corresponds to 54 fl oz of Maxcel and 27 fl oz of Sevin. No adjustments to the surfactant are needed.

I encourage you to utilize the model with the weather station nearest to your location, inputting accurate green tip and bloom dates for your orchard. If the degree days (DD) column displays values under 200, but your king fruits measure over 7mm in size (averaging around 15-20 fruitlets), I still advise proceeding with thinning treatments this week, rather than postponing to the following week and potentially encountering difficulty in finding an appropriate temperature window for application. To access the model, use the following link:

Apple Orchard Thinning Recommendations for Central Virginia and Winchester

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%

Chemical Thinning Decisions: A Refresher on Key Factors and Tools

Chemical fruit thinning is a critical management practice in commercial apple orchards, aiming to optimize crop production by regulating the number of fruit per tree. This technique involves the application of chemical thinners during the spring to trigger intentional fruit abscission or shedding, known as “thinning.” This blog post delves into the purpose of chemical fruit thinning, its effectiveness, factors influencing its efficacy, and tips for growers to better predict and achieve desired thinning outcomes.

Chemical fruit thinning serves to:

  • Increase fruit size: By directing carbohydrates to fewer fruits, thinning promotes the growth of larger fruits, preventing the production of small, undersized fruit.
  • Improve fruit quality: Thinning enhances fruit color by increasing light and nutrient availability.
  • Prevent biennial bearing: Thinning helps maintain an annual bearing cycle by encouraging the development of sufficient fruiting spurs for the next year.

Various factors influence the effectiveness of thinning sprays, including:

  • Chemical selection and rates: Choose chemical thinners based on the fruit growth stage and desired level of thinning, and adhere to label guidelines for proper rates.
  • Fruit growth stage and application timing: Apply chemical thinners according to the fruit growth stage, as this can impact efficacy. Most thinning sprays should be completed by 15 mm fruit diameter.
  • Use of oils and surfactants: Adding spray oils and surfactants to chemical thinners can improve efficacy but should be used cautiously unless heavy thinning is required.
  • Weather: Light and temperature conditions before, during, and after application affect thinning spray efficacy. Low light and high temperatures can increase thinning response.
  • Fruit variety: Different apple varieties respond differently to chemical thinners. Consult the following table for a categorization of easy, moderate, and hard-to-thin apple varieties.
  • Block health and age: Healthy, vigorous, and mature orchard blocks tend to be more challenging to thin than stressed or younger blocks.
  • Pruning: Previous dormant pruning can impact thinning spray efficacy, with heavily pruned blocks potentially requiring little to no thinning.

Rates of Chemical Thinners

The rate of chemical thinners is another important aspect that growers need to carefully consider. Most chemical thinners except for carbaryl, oxamyl, and ethephon have rates listed in parts per million (ppm) and fluid ounces (fl. oz.) according to the spray volume. The labels for each chemical thinner commonly supply a conversion chart from ppm to fluid ounces based on the spray volume (gallons per acre). The labels for most chemical thinners typically list a broad range of rates. The level of thinning action is dependent on the rates. When lower rates are used, a lower thinning action can be expected, and when higher rates are used a higher thinning action occurs. When deciding what rates to apply, carefully consider the number of set fruit (crop load), factors which can affect the efficacy (i.e. variety), and always follow the label.

Thinning Spray Applications

The factors of application timing, chemical thinners, and rates should be carefully considered, evaluated, and selected to develop a thinning spray application which will achieve the desired thinning response. The following tables list several common thinning sprays according to the fruit growth stage, materials, rate ranges, and perceived thinning actions.

Effective thinning sprays (petal fall – 5 mm fruit diameter)

Effective thinning sprays (6 mm – 15 mm fruit diameter)

Effective thinning sprays (16 mm – 25 mm fruit diameter)

Carbohydrate Thinning Model

The Cornell Apple Carbohydrate Thinning Model is a valuable tool that assists growers in making informed chemical thinning decisions. Developed by scientists at Cornell University, this model enhances the effectiveness of thinning sprays by calculating the carbohydrate balance in trees, accounting for weather factors such as solar radiation and temperature. Growers can easily access and operate the model through the Network for Environment and Weather Applications (NEWA) webpage, hosted by Cornell University, at the following web address:

To utilize the model, begin by selecting the weather station nearest to your orchard blocks. If you prefer to use your own weather station, detailed instructions for uploading your weather station data are available on the NEWA webpage. Once the closest weather station is selected, input your current date, green tip date, and bloom date for the orchard block you want to analyze. The model will then generate a table displaying daily maximum and minimum temperatures, solar radiation, total carbohydrate status, and accumulated degree days since bloom. In the far-right column titled “thinning recommendations,” you will find chemical thinner rate suggestions based on the weather forecast.

Screenshot: Navigating the Cornell Apple Carbohydrate Thinning Model

Disclaimer: The application rates mentioned in this article are based on 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 you are using. The degrees of thinning action are listed according to the author’s personal field experiences. Thinning action may vary from orchard to orchard and block to block.

For additional information on apple fruit thinning, please refer to our extension publication, “Crop Load Management in Commercial Apple Orchards: Chemical Fruit Thinning,” available at

Assessing the Extent of Frost Damage: Unexpectedly Devastating Effects on Apples and Severe Damage in Peaches and Cherries

Temperature data loggers, situated at various locations within our research farm in Winchester, VA, have documented temperatures as low as 18F this morning (March 20, 2023). In several fruit-growing regions across the state, temperatures in the range of 14-18 F have also been recorded, resulting in a severely detrimental hard freeze, causing considerable damage to most fruit trees in Virginia, including apples, peaches, cherries, apricots, and plums.

Upon assessing the damage to our blocks, we were surprised to find that our apple cultivars, which were transitioning from half-inch green to tight clusters, exhibited severe damage to both king and side blooms. In both Pink Lady and Gala, we recorded a staggering 87% flower damage in king blooms and 60% damage in side blooms. Peach flowers (mostly at the pink stage) sustained damage ranging between 20 to 70%, while sweet cherry flowers (mostly at the tight cluster stage) sustained damage around 80%. Given that our location is approximately 10-14 days behind most of the other orchards in central and southwest Virginia with regards to flower development, the presented figures offer only a glimpse into the overall extent of the damage caused across the state.

While we maintain hope that our other apple cultivars, currently in the green-tip and ¼ inch green stages, have survived the freeze, a comprehensive assessment of the buds’ survival can only be performed later, once they have grown larger. I have included some images below that you can use as a guide to determine the extent of damage to your orchards.

Assessment of blossom damage was conducted on sweet cherry flowers at the ‘tight cluster’ stage, using longitudinal sections. This evaluation followed a freeze event that took place on Monday, March 20, 2023, in Winchester, VA. Blue and red arrows refer to viable and damaged flower styles and ovaries.
Assessment of blossom damage was conducted on ‘Pink Lady’ appleflowers at the ‘tight cluster’ stage.This evaluation followed a freeze event that took place on Monday, March 20, 2023, in Winchester, VA. Blue and red arrows refer to viable and damaged flower styles and ovaries.
After a freeze event that occurred on March 20, 2023, longitudinal sections of ‘Sunhigh’ peach flowers at the ‘Pink’ stage reveal viable (blue arrow) and damaged (red arrow) flower styles and ovaries.