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.

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).

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.

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.

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.

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 (https://blogs.ext.vt.edu/tree-fruit-horticulture/2024/04/18/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.

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.

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.