Category Archives: MaluSim

Thinning Update: Timing Looks Good This Week

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

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

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

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

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

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

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

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

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

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

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

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

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

Ideal Conditions for Apple Fruit Thinning This Week: Model Results for Winchester and Central Virginia

As you know, despite the freeze damage we experienced on April 9, several apple cultivars across multiple locations are still carrying at least a few times more fruit than desired. Chemical fruit thinning remains our best option before it’s too late and we are left with the laborious task of hand thinning. The best timing for applying 6-BA and NAA for fruit thinning is during the 6–18 mm fruit size window, and we are currently within that window for ‘Gala’ and most other apple cultivars in the Winchester/Frederick County area. To help guide thinning applications and ensure trees are responsive to treatments, I ran the Cornell Apple Carbohydrate Thinning Model (https://newa.cornell.edu/apple-carbohydrate-thinning). Based on key parameters, the model helps determine both the timing and rate of thinning applications. Using data from the weather station located at our research center in Winchester, and with a green tip date of March 16 and a full bloom date of April 12 for ‘Gala’, the model predicts that starting tomorrow—Tuesday, April 29—and continuing through the end of the week, conditions will be ideal for thinning treatments. The trees’ accumulated carbohydrate balance is generally negative, which is exactly what we want for effective thinning, and temperatures—especially on Wednesday and Friday—will be perfect for the performance of 6-BA and NAA. The model also recommends using the standard rate for thinning materials.

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

For growers in Central Virginia, I also ran the model using the Roseland, VA weather station, with ‘Gala’ green tip and full bloom dates set as March 14 and April 10, respectively. As shown below, if you haven’t already applied your thinning sprays, we are still within the optimal thinning window. The model suggests that if you apply tomorrow (April 29), you should increase the thinning rate by 30%. However, based on the daily carbohydrate balance, I believe that applications made this Wednesday and Thursday will likely require only the standard rate.

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

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

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

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

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

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

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

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

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

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

Thinning Recommendations and Timing for Central Virginia and Winchester/Frederick County Apple Orchards

Clusters of apples on the same branch exhibit noticeable variations in fruit sizes, posing a challenge for thinning decisions

We base our thinning recommendations on the carbohydrate thinning model results from the NEWA website, which I previously discussed in earlier posts. Last week, I ran the model for Central Virginia using Gala’s green tip and bloom dates of March 6 and 29, respectively, along with data from the Crozet weather station (Chiles peach orchard). Based on the model, I recommended that growers apply their primary thinning treatment between April 17 and 20, ideally on Wednesday or Thursday, when both temperature and tree carbohydrate balance were optimal for thinning with 6-BA and NAA products. Those who have already followed these recommendations and applied treatments last week should now see that it was the right decision, given the cool weather and carbohydrate surplus this week.

For growers who did not apply a thinning treatment last week but plan to start this week, I suggest one of two options: A) If the majority of your crop has an average fruit size more than 12mm, apply the treatment tomorrow (April 26), or Thursday or Friday (weather permitting). B) If the majority of your crop has a fruit size under 12mm, wait another week for a warmer temperature window and/or lower carbohydrate levels. This year, it is not uncommon to see two distinct crops with different sizes on the same tree, particularly for Gala, Pink Lady, Fuji, and Reds. If the majority of your crop has an average size of >12mm, follow option A; otherwise, go with option B.

Green tip (Mar 6), full bloom (Mar 29), weather stations (Crozet, Chiles peach orchard), percent flowering spurs (51-75%). Note: If thinning treatments are scheduled for tomorrow, the model suggests increasing the thinning materials by 30% to account for carbohydrate surplus.

Regarding thinning recommendations for apple blocks in the Winchester/Frederick County area, the model results, using Gala’s green tip and bloom dates of March 7 and April 12, respectively, along with data from the Winchester weather station (VT AHS AREC), show that we have not yet reached the 200-250 accumulated degree days (DD). These values are expected to be reached this weekend (April 29 and 30), coinciding with low carbohydrate levels and relatively acceptable temperatures. However, rain is anticipated over the weekend. With this in mind, I suggest the same two options as I did for Central Virginia. Based on apple blocks at our research center, I would choose option B for the majority of our varieties and wait another week, hoping for better thinning conditions in terms of temperature and tree carbohydrate status. I typically recommend applying 6-BA and NAA only when the temperature is above 65°F on the application day, with the optimal range being 80-85°F. I hope to see warmer temperatures by next Thursday or Friday, allowing for our primary thinning treatment.

Green tip (Mar 7), full bloom (April 12), weather stations (Winchester, VT AHS AREC), percent flowering spurs (51-75%). Note: If thinning treatments are scheduled for tomorrow, the model suggests increasing the thinning materials by 30% to account for carbohydrate surplus.

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%

Fruit Thinning for Apple Orchards in Winchester/Frederick County- May 9, 2022

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.

The Carbohydrate Model & Thinning Recommendations-Central Virginia-May 1, 2022

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.

Apple Fruit Thinning: An Overview & Recommendations

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://treefruithorticulturevaes.wp.prod.es.cloud.vt.edu/?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.

MaluSim models for 11 May 2015

For both Winchester and Central Virginia, the Malusim model is predicting a mild to no carbohydrate stress over the next week. Although temperatures at the time of application, are less important then the carbohydrate running average (2+4 Running Average), expect chemical thinners applied Sunday (10 May) or Monday (11 May) to cause more thinning than applications made late last week or for the rest of this week. Expect a “typical” response from chemical thinners applied Tuesday (12 May) through the rest of this coming week.

Peck Winchester MaluSim 5_11_15Peck Central VA MaluSim 5_11_15

The National Weather Service is predicting sunny weather later this week, as well as daytime temperatures in the 70s. This should result in ideal conditions for plant growth, and thus less carbohydrate stress in the apple trees.

One of the weaknesses of weather-based models, is the need to use forecast data to make decisions about future events. When I ran the MaluSim models for both Winchester and Central Virginia last Thursday, the forecast was for hot, cloudy weather. Although it was in the mid-80s in most of Virginia over the weekend, there was also a lot more sunlight than predicted. Temperatures are generally a lot more reliable than cloud cover predictions, but both seemed to be less severe than originally forecasted.

Cloudy weather predicted for later in the week is causing wild swings in the MaluSim model output. My expectation is that actual carbohydrate levels will be more moderate and that running average values will remain in the 20 to -20 g CHO/day range.

Full model output:

Peck Winchester MaluSim 5_11_15.pptx

Peck Central VA MaluSim 5_11_15.pptx