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.

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: https://newa.cornell.edu/apple-carbohydrate-thinning

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: https://newa.cornell.edu/apple-carbohydrate-thinning.

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 https://www.pubs.ext.vt.edu/SPES/SPES-134P/SPES-134P.html.

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.

As of 5:00pm on Sunday, March 19, the weather forecast predicts low temperatures ranging from 18-23°F tomorrow morning. These low temperatures could potentially damage the flowers and flower buds of various fruit trees, particularly stone fruits. However, in the Winchester/Frederick County area, the cold weather over the past two weeks has helped delay the progress of flower buds, minimizing the risk of critical damage. Most apple cultivars in the area are currently between green-tip and half-inch green stages, while peaches are between calyx red and first bloom (for early-blooming cultivars). Sweet cherries are mostly between the tight cluster and open cluster stages. For all these species, the critical temperature that would kill 90% of the buds is higher than what is forecasted for tomorrow. As a result, tomorrow’s frost should not be a significant concern for most orchards in the Winchester area, unless your orchard is located at a low elevation or has early-blooming peach cultivars. Additionally, apricot or plum trees in bloom might experience some damage.

Orchards in central and southwest Virginia face a greater risk from tomorrow’s frost. Current forecasts predict temperatures of 22-23°F in Nelson County, 24°F in Roanoke, and as low as 18°F in Carroll County. Orchards in these locations are generally 10 days to two weeks ahead of those in Winchester, with many peach cultivars possibly in full bloom and post-bloom stages, and apple cultivars between half-inch green and tight cluster stages. If this is the case, tomorrow’s temperatures could cause a 90% kill rate for peach flowers in most of these locations if the frost lasts for 30-60 minutes. For apples, the critical temperatures are 15°F for half-inch green and 21°F for tight cluster stages, so some damage may occur if temperatures drop below 20°F. Please refer to this link for critical temperatures at each developmental stage. http://royaloakfarmorchard.com/pdf/Critical_Temperatures_Frost_Damage_Fruit_Trees_Utah.pdf

What can you do now? Given the weather parameters for tomorrow’s frost, which is characteristic of radiation frost typically accompanied by a clear sky, low wind, and gradual temperature decline starting at midnight and reaching its lowest point just before sunrise, some tools may be effective in raising the temperature around tree canopies by 2-3°F. Wind machines, under-tree and overhead sprinklers, propane heaters, and fire dragons may be useful if the difference between the lowest temperature and the critical temperature is only 3°F. Unfortunately, no measures will be effective if temperatures drop to 18°F tomorrow. We can only hope that the weather forecast is inaccurate and conditions turn out to be more favorable.

What can you do after the freeze? Generally, six hours after a frost event is sufficient to begin observing damage to blossoms. You can collect branches from various locations within the orchard and use a razor blade to cut through peach flowers and apple clusters, looking for signs of damage to the ovary and style (female parts) in the center of each flower. Consult the following link for examples from our previous posts: