Tag Archives: Thinning

MaluSim Carbohydrate Model for May 12, 2014

Fruitlets in the Central Virginia area are quickly sizing up (reports are in the 12-18 mm range), and growers have only a couple-few more days to use NAA and 6-BA products for thinning. In the Winchester area fruitlets are just entering the 10 mm thinning window, but with the hot weather over the past few days it is likely that fruit will be growing at a rapid pace (greater than 0.5 mm per day).


















Table 1. Fruitlet sizes at the Alson H. Smith, Jr.AREC in Winchester.

Fruit diameter (mm)
Cultivar 12-May
Empire 10.4
Fuji 7.0
Gala 8.7
Goldens 8.0
Idared 8.1
Pink Lady 9.2
Red Delicious 8.0
York 7.9

In Central Virginia, the MaluSim model is showing a moderate carbohydrate deficit for the next three days, and then the carbohydrates balance shows positive values through the weekend.  A large deficit that was predicted to occur over the past weekend never materialized, largely because there was more sunlight than forecasted. Additionally, as the expanding leaf surface transitions from being a carbohydrate sink to a carbohydrate source the model will continue to show a positive carbohydrate balance. Growers should target blocks that require more aggressive thinning on Tuesday and Wednesday at normal or slightly reduced rates (unless trying to thin larger fruit) and then target blocks with lighter starting crop loads towards the end of the week.

In Winchester, bud break was 6 days later than in Central Virginia, meaning that many leaves are still sinks for carbohydrates and more severe deficits occured over the past few days.  However, as the Winchester area enters into the thinning window, less severe carbohydrate deficits are predicted to occur. Growers should start thinning their earliest blooming cultivars on Tuesday evening or Wednesday and then move to more lightly cropped blocks towards the end of the week and over the weekend.

Click below to download the full pdf reports:

Central VA MaluSim 5_12_14

Winchester VA MaluSim 5_12_14

ALERT! MaluSim Carbohydrate Model for May 9, 2014

Central Virginia growers, please take note. For the MaluSim model, I have been using weather forecast data for Tyro, VA from Intellicast.com. It was pointed out to me today that the data from that location is significantly cooler than other locations in the region. I have rerun the model using Intellicast.com data from Piney River, VA. While only 5 miles apart geographically, the two forecasts are very different, with Piney River being 5-8F warmer than Tyro. Why that is the case, I do not know. I make no claim to be a weather forecaster!


Bottom line is that you need to look at the details of the weather data that I am using in the model by downloading the pdf file. <<Central VA MaluSim 5_9_14>>. Compare these data to your own location to make sure that they represent your conditions. As I have repeatedly said every time I discuss the MaluSim model, the output data is only as good as the weather forecasts that are used as inputs.

Using the Piney River forecasts, the MaluSim model is predicting a much greater CHO deficit than Tyro. If temperatures are going to be in the 80sF at your location for the next couple-few days, then consider reducing rates or waiting until cooler temperatures return next week. Also, remember that the greatest thinning activity is obtained when fruitlets are between 8-12 mm. Waiting until fruitlets are 12-16 mm will result in less severe thinning. But once fruit gets closer to 20 mm, they become much less likely to be thinned by NAA or 6-BA applications.

MaluSim Carbohydrate Model for May 8, 2014

Slide3Due the rapidly changing weather forecasts, I ran the MaluSim model again on Thursday. For Central Virginia, the model is showing values in the -20 to 20 range for the next seven days. This means Slide3that growers should expect an “average” response from their chemical thinners.  Growers in the Central Virginia region should target the next couple-few days to make their thinning sprays as fruitlets are at the ideal size for applications of NAA and 6-BA. Fruitlets grow at ~0.5 mm/day in cool temperatures, and as you can see from the table below, up to 1 mm/day with warmer temperatures.


Table 1. Fruitlet sizes at Silver Creek Orchards.

1-Apr 28-Apr 30-Apr 2-May 6-May 8-May
Empire Silvertip Petal Fall 5.6 7.3 10.9 12.1
Red Delicious 5.7 6.5 9.3 10.6
Pink Lady 5.4 7.5 8.3 10.4
Gala 5.4 6.4 8.8 10.6
Fuji 4.4 5.8 7.8 9.9

Much warmer day and night temperatures are forecast for the Winchester area, so the MaluSim model is showing a persistent carbohydrate deficit for the next five days. However, we are still at petal fall to ~5 mm for most cultivars, so hopefully by the time fruit are at 10 mm more moderate weather conditions will reduce the chance of over thinning. Chemical thinning applications made when either the day-by-day or running average value is below -60 g CHO/day will likely result in aggressive thinning.

I will run the models again early next week. Please download the below pdf files for the full report.

Central VA MaluSim 5_8_14

Winchester VA MaluSim 5_8_14


MaluSim Carbohydrate Model for May 6, 2014

Today, I ran the MaluSim model for both Central Virginia <pdf> and  Winchester <pdf>.

Central VA MaluSim 5_6_14 Winchester MaluSim 5_6_14

For Central Virginia, fruitlets are just starting to approach the 10 mm stage. It looks like the best chance to get a strong thinning response will be Wednesday and Thursday. After that, the model is showing that chemical thinners will have more of an “average” response. Target blocks that have a heavy crop load on Wednesday and Thursday, and leave blocks with more frost damage and lighter crop loads to thin on Friday and over the weekend.






Table 1. Fruitlet sizes at Silver Creek Orchards.

1-Apr 28-Apr 30-Apr 2-May 6-May
Empire Silvertip Petal Fall 5.6 7.3 10.9
Red Delicious 5.7 6.5 9.3
Pink Lady 5.4 7.5 8.3
Gala 5.4 6.4 8.8
Fuji 4.4 5.8 7.8

For the Winchester area, most blocks are at petal fall or just beyond. We’re forecasted to have some summer-like weather over the next several days, which will cause a fairly strong carbohydrate deficit in the trees. Petal fall thinning sprays rarely over thin, even in hot weather, however blocks with fruitlets approaching 10 mm may see a strong thinning response from applications made over the next three to four days.

I will run another set of models on Friday. I will also start posting fruitlet sizes from fruit at the AREC. Download the above linked pdf files to view the full reports.

MaluSim Carbohydrate Model for May 2, 2014

Fruitlets in Central Virginia are growing fast, and I suspect that most growers will be putting on their 10 mm thinning sprays over the next 7 days. In the Winchester area, we’re just at petal fall, so I will start running the MaluSim model next week.

For Central Virginia, using Intellicast forecasts, the MaluSim model is predicting a slight carbohydrate deficit through the early part of next week. Growers should expect a normal to slightly aggressive response from their chemical thinners that are applied today through Monday. I’ll run another model on Tuesday to assess the impact of thinner applied in the middle to end of next week.

Table 1. Fruit phenology at Silver Creek Orchard.

1-Apr 28-Apr 30-Apr 2-May
Empire Silvertip Petal Fall 5.6 mm 7.3 mm
Red Delicious 5.7 mm 6.5 mm
Pink Lady 5.4 mm 7.5 mm
Gala 5.4 mm 6.4 mm
Fuji 4.4 mm 5.8 mm

Download the full report <Peck Central VA MaluSim 5_2_14>.

MaluSim Carbohydrate Model for April 30, 2014

Growers in the Central Virginia growing region are reporting that their bloom is stretched out quite a bit with some side bloom still in flower and some king bloom fruit approaching 5 to 6 mm in fruitlet size. In general, most chemical thinners are more active on the smaller, slower growing fruit, so this spread may prove to be advantageous.

Peck Central VA MaluSim 4_30_14

For those Central Virginia growers who are starting to map out their thinning schedule, I’ve started running the MaluSim carbohydrate model. I will discuss the details of how the model works, as well as my research results on validating the model at the meeting on Thursday, May 1.

The MaluSim model is most effective at predicting thinning efficacy at the 10 mm timing, and has a much lower predictive value for flower, petal fall, or 20-25 mm thinning applications.

As of today, the long-term forecast shows a long string of days with mild temperatures and lots of sun. From these forecasted data, the MaluSim model <<pdf>> is showing no extremes in carbohydrate supply, meaning that chemical thinners will have a “average” response. For a detailed look at the model, as well as interpretation table, download the above linked pdf.

For the Winchester area, I will assess fruit development on Friday to decide when the first model run is warranted.

A big thank you to John Saunders for helping with the MaluSim modeling work this year. Last year, Dr. Mizuho Nita (grape plant pathologist at our AREC) installed weather stations at Silver Creek and set up a network relay system that allows me to download meteorological data from Silver Creek right to my computer. If you work with Mizuho on grape disease related issues, please thank him as well!

Notice of Date Change for Central Virginia Orchard Meeting

April 29, 2014

Dear Central Virginia Fruit Producer:

Due to the quick progress of our trees this spring, our third orchard production meeting of this year HAS BEEN MOVED FORWARD TO THIS THURSDAY, May 1st. We will be meeting at 11:00 a.m. at the office of Saunders Bros. Nursery, located at 2717 Tye Brook Hwy, Piney River, VA 22964; Tel.: (434) 277-5455; http://www.saundersbrothers.com/. Bennett Saunders will be our host.

A special focus of this meeting with be crop thinning considerations provided by Extension specialist Gregory Peck. In addition, Drs. Yoder and Bergh will be on hand to discuss disease and insect pest issues, respectively. Please come to discuss fruit production issues and concerns with other fruit growers and Extension fruit specialists. You are encouraged to bring plant and insect samples for identification, but please have these contained in a plastic bag.

Please bring your own bagged lunch to this meeting. Cold drinks will be provided by the host orchard.

Orchard Events Website:


Remaining 2014 Central Virginia Orchard Meeting Schedule

· May 20th Fitzgerald Orchard Lowesville, VA

· June 3rd 7K Farm Rustburg, VA


Michael W. Lachance

Extension Agent

If you are a person with a disability and desire any assistive devices, services or other accommodations to participate in this activity, please contact the Nelson County Extension Office at (434) 263-4035 to discuss accommodations five days prior to the date of the meeting.

MaluSim Carbohydrate Models: 2013 Season Recap for Winchester and Central Virginia

Slide3 Slide3Attached are the final MaluSim carbohydrate models that I will run for 2013.

Please print out a copy of the below pdf file to add to your records. Later in the season, I will be asking for feedback from you about how you used the model, how well the model predicted thinner response in your orchard, and if I should continue running the model in future years. Please feel free to send me additional feedback at anytime.

Peck Central VA MaluSim 5_29_13

Peck Winchester VA MaluSim 5_28_13

Suggestions for late thinning and return bloom applications

By this time, apples in most of Virginia are 13-20 mm in fruit size. Once apples reach this size, the trees tend to be less responsive to the typical 10 mm (carbaryl plus NAA or 6-BA) applications. However, if additional thinning is needed, growers can still use ethephon (sold under the trade names Ethrel and Ethephon 2). Ethephon is most effective when fruit size is 14 to 28 mm in diameter. In most years, it is difficult to chemically thin apples larger than 24 mm.

Ethephon has variable responses due to air temperature and humidity, as well as spray water volumes. Overthinning is more likely with this material than with other thinners. For this reason, ethephon has been primarily used when earlier thinning applications were not successful, on very difficult to thin cultivars, and/or when return bloom has been a severe problem. Thinning results with ethephon varies amongst cultivars. Ethephon is not very effective on Gala, but it is very effective on Golden Delicious and Rome. Where water is alkaline, buffering the spray solution to a pH of 3 to 5 will increase chemical stability and effectiveness. Do not use before a light rain or dew or when post-application temperatures are predicted to be greater than 90˚F because excessive thinning may occur.

Ethephon is usually combined with carbaryl or oxamyl (Vydate L). For greater thinning activity, ethephon can also be tank mixed with NAA and/or spray oil.

I have several research trials underway to look at alternative and hopefully more consistent late thinning materials. However, at this time, ethephon is still the standard material when fruit size is greater than 14 mm.

Once fruit is larger than 28 mm, hand thinning will need to be used to remove additional fruit. Hand thinning will have a positive impact on final fruit size and return bloom for up to about 45 days after full bloom. In 2013, this is around the first week of June for central Virginia and the second week of June for the Winchester area.


Ethephon (sold under the trade names Ethrel and Ethephon 2) can promote flower bud formation when applied from petal fall to about 6 to 8 weeks after full bloom. The greatest effect is from applications made 0 to 4 weeks after bloom. However, since ethephon can cause substantial fruit thinning, multiple weekly applications at rates 1/2 that of the thinning rate are recommended starting when fruitlets are greater than 30 mm. When possible, it is best to wait until after “June” drop has occurred. At a minimum, wait 7-10 days after the last thinning application before starting ethephon return bloom sprays. Additionally, do not apply ethephon to trees that are stressed or trees that are low in vigor.

A single ethephon application can be used at a high rate (up to 900 ppm). However, more consistent results are often obtained from multiple (3-4) applications made at 10-14 day intervals using lower rates (150-300 ppm). One common, and often effective strategy, is to make two applications in June and two applications in July.

Another strategy is to make 2-4 applications of ethephon at 150 ppm tank mixed with NAA at 5-10 ppm (or 2.5-5 ppm when tank mixed with spray oil). This approach has been beneficial for strongly biennial cultivars.

Sensitivity to ethephon is very different amongst cultivars, thus it is important to choose a rate specific to each variety. Do not exceed 8 pints per acre per year. If trees are over-cropped ethephon may not effectively give adequate return bloom the following season. Higher soluble solids and lower starch levels at harvest may be expected with some cultivars, particularly with high rates and/or late season applications. No loss of firmness has been detected with ‘Red Delicious’ at the optimum harvest date.

Ethephon sprays can reduce tree growth (dependent on timing and amounts used) and thus may not be desirable for young non-bearing trees if maximum tree growth is desirable.


Hand thinning peaches to 6-8 inches apart on the branch will result in increased final fruit size and help prevent limb breakage. This activity will be most effective when completed over the next couple-few weeks. Hand thinning peaches later in the season will not have as much of an impact on final fruit size.

MaluSim Carbohydrate Models for May 20: Winchester and Central Virginia

On May 20, I ran the MaluSim carbohydrate model for Winchester and Central Virginia. This will be the last MaluSim posting using forecasted data for the season. Sometime in the week or two I will post a season recap using only recorded data.

Slide3Slide3In Winchester, as predicted, we have been experiencing hot and cloudy weather over the last few days. Thinning applications made from late last week through Monday will likely be fairly active, with some easy to thin cultivars (e.g. Red Delicious) potentially having significant fruit drop. However, from Tuesday through the end of the coming week, growers should expect an average response from their thinning applications.

In Central Virginia, there was a similar trend but with somewhat cooler daytime temperatures, the carbohydrate deficits were less severe. Similar to Winchester, thinning applications made over the next several days should have a fairly typical response.

Peck Winchester VA MaluSim 5_20_13

Peck Central VA MaluSim 5_20_13

Fruitlet sizes and growth rates at the Winchester AREC:

  • Empire: 19.1 mm; 1.5 mm/day
  • Fuji: 13.6 mm; 1 mm/day
  • Gala: 13.8 mm; 0.8 mm/day
  • Pink Lady: 14.1 mm; 0.9 mm/day
  • Red Delicious: 13.8; 0.9 mm/day