Tag Archives: Malusim

Optimal Timing for Thinning Apple Trees and Increasing Return Bloom

As we enter the optimal window for thinning treatments in the Winchester/Frederick County area, it’s important to consider the best options for your apple trees. In a follow-up to our previous post, we have some important updates for those who chose option A or B.

If you went with option A, and applied your thinning treatments last week (April 25-27) because you had larger fruit, you won’t need to do any further treatment for at least two weeks after this treatment. Wait until you can see signs of fruit abscission and/or fruits showing distinct segregation in size before deciding if another thinning treatment is necessary. For those who went with option B due to the low temperatures, this Friday and over the weekend may provide better thinning conditions. Temperatures are expected to rise above 65F on Friday and reach 72F on Sunday, with even higher temperatures on Monday. If your average fruit size is 15mm or above, start thinning on Friday and Saturday. If your fruits are less than 15mm, wait until Monday when temperatures are expected to reach around 78F and remain warm and cloudy for the rest of the week. Trees are currently in a carbohydrate surplus state, which could sustain until Friday, so you may need to pump up the rate to 30% more than the standard rate. Check the model the day before the application to see the recommended rate in the last column.

Green tip (Mar 7), full bloom (April 12), weather stations (Winchester, VT AHS AREC), percent flowering spurs (51-75%). The carbohydrate thinning model can be accessed at: https://newa.cornell.edu/apple-carbohydrate-thinning

For apple growers in Central Virginia, the same advice applies, except that you can start thinning treatments on Thursday if the forecasted temperature stays at 65F or higher. If your current fruit size is less than 15mm, consider Monday (5/8) as the main thinning day. Use the model to determine whether to apply the standard rate or increase it. If the model suggests adding oil to the tank to increase efficiency, it’s worth considering, especially if your fruits have exceeded 15mm and this is your final chance to get the top part of the canopy in shape. However, keep in mind that Captan should not be used 4-5 days before and after applications containing oil, as this can cause significant damage to fruit quality. In summary, timing and temperature are critical when it comes to thinning treatments for apple trees. Follow these guidelines to ensure optimal results for your fruit harvest.

Green tip (Mar 6), full bloom (Mar 29), weather stations (Crozet, Chiles peach orchard), percent flowering spurs (51-75%). The carbohydrate thinning model can be accessed at: https://newa.cornell.edu/apple-carbohydrate-thinning

When it comes to thinning apple trees, it’s important to consider the optimal thinning conditions that increase return bloom, especially for cultivars with a tendency for biennial bearing, such as Honeycrisp, Golden Delicious, and Fuji. If you’re looking to improve return bloom for Honeycrisp and Golden Delicious, we have a recipe that has been endorsed by many and is based on our own research over the past few years.

First, use NAA applications for fruit thinning at a fruit size of 6-15 mm, or within 30 days of full bloom. The recommended mixture is NAA @ 3 oz + Sevin @ 1 qt + Regulaid @ 1pt/100 gal/acre. In addition to NAA applications for thinning, NAA applications (@ 2-4 oz/100 gal without carbaryl or oil) should start at a fruit size of 30-35 mm and repeated 2 times at 7-10 day intervals. The rates of NAA are based on Refine 3.5 and PoMaxa. Make rate adjustments if using Refine 6.25.

For improving return bloom in Fuji and Red Delicious, we recommend using ethephon. Don’t use NAA with Fuji and Red Delicious. Apply 1-4 applications of ethephon at 0.5 pt/100 gal (without carbaryl or oil) starting at a fruit size of 30-35 mm in diameter and repeated at 7-10 day intervals. A surfactant is not necessary if ethephon is applied with cover sprays. Ethephon at this low concentration and stage of fruit development will not cause any thinning, but it should improve return bloom.

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.

Fruit Thinning Decisions for Apple Orchards in Winchester/Frederick County- May 3, 2022.

Tomorrow and after (May 4-5) could be a good (but not ideal) time for fruit thinning applications for apple orchards located in Winchester and the surrounding area. In our Winchester research orchard, the average fruit diameter for Gala, Honeycrisp, Pink Lady, Red Delicious, and Fuji is about 6.5 to 8.5mm, which is inside the size range (6-15mm) we usually target for thinning treatments. Also, the carbohydrate thinning model shows low carbohydrate levels in the past two days, today, and the following four days, which should also ensure a good response to thinning treatments applied tomorrow and Thursday (May 5). However, it should be noted that we are not within the “perfect” thinning window yet. We typically target accumulated degree days between 200-250 DD. We got only 143 DD, which is not bad…but also not ideal. Also, based on the weather forecast, it will be rainy tomorrow until 8:00 am and windy (>10 mph) by noon, which gives you only a few hours to apply thinning treatments. The daytime temperature tomorrow and Thursday is fine for 6-BA and NAA treatments, but I would generally wait for extended periods of 75 to 85 oF for a better thinning response.

The second potential thinning window will likely be on May 10-12. As we approach there, fruit size will be around 11-12 mm, degree days will be within the perfect range (200-250), the temperature will be just perfect (based on the current forecast), but we can’t predict if the tree carbohydrate balance would remain on the deficit side by that time. We can, however, increase the rate of thinning materials to compensate for high carbohydrate levels.

So, after considering all these factors, I would focus tomorrow on thinning the hard-to-thin cultivars (e.g. Gala, Fuji, Golden Delicious, and Gold Rush) that generally require more than one thinning treatment. But, for other cultivars, I would wait for the May 10-12 window. It should be worth noting that blocks you decide to spray tomorrow can’t/shouldn’t be resprayed with thinning treatments on May 10-12. For these blocks, I would suggest that you give it 2-3 weeks at least to see the effect of the first thinning treatment and decide if you will need additional applications. For tomorrow’s thinning applications, the model recommends that you apply the standard chemical thinning rate.

The Cornell Apple Carbohydrate Thinning Model (May 3, 2022).

Finally, if you are nearby, you may need to consider attending our in-depth meeting tomorrow, May 4 at 7:00 pm at the Alson H. Smith, Jr., Agricultural Research and Extension Center (AHS AREC): 595 Laurel Grove Road, Winchester, VA 22602. I will cover critical topics regarding crop load management and return bloom, and other specialists will provide seasonal updates on disease and insect management.

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://blogs.ext.vt.edu/tree-fruit-horticulture/?s=Accede

Table 5: Easy, moderate, and hard to thin apple cultivars.


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

MaluSim Model Data for 7 May 2015

Due to warm days (>80F) and intermittent cloud cover, the MaluSim carbohydrate model is showing a strong carbohydrate deficit for both Winchester and Central Virginia over the next four to five days. Expect an aggressive to very aggressive response to chemical thinners applied today through early next week. Reduce rates, and/or do not include oil or other surfactants if overthinning is a concern.Peck Winchester MaluSim 5_7_15

Peck Central VA MaluSim 5_7_15

Pay close attention to the weather forecasts. If actual temperatures are closer to 90F and/or there are prolonged periods of cloud cover, then overthinning will likely occur.

The greatest amount of thinning occurs when fruitlets are on average between 8-12 mm in diameter. Thinning when fruitlets are slightly smaller or larger will result in less thinning, which may be desirable if there is a severe carbohydrate deficit.

Cooler temperatures forecasted for the middle to end of next week should result in less carbohydrate stress and reduced chances of overthinning. However, fruitlets that are greater than 15 mm in diameter are more difficult to thin with NAA or 6-BA.

During these warm days, expect the average fruitlet size to increase by at least 0.5 mm per day.

Full MaluSim model output:

Peck Winchester MaluSim 5_7_15.pptx

Peck Central VA MaluSim 5_7_15.pptx

Central Virginia MaluSim for 4 May 2015

Warmer temperatures and cloud cover from some rainstorms later in the week will cause a moderately stronger carbohydrate deficit for Central Virginia.

Peck Central VA MaluSim 5_4_15

Thinning applications made Tuesday through Thursday will result in the strongest response this week. Long term weather forecasts predict days in the 80’s and nights in the 60’s for the next 10 days. Carbohydrate levels will likely remain in the 0 to -40 range during that period.  To get the strongest response possible, make your applications when average fruit size is 8-12 mm in diameter.

Due to a technical glitch, I was unable to run the MaluSim model for Winchester. I hope to have the problem resolved Tuesday morning. I will post an update for Winchester as soon as possible.

Full MaluSim report:

Peck Central VA MaluSim 5_4_15.pptx