Category Archives: MaluSim

MaluSim Carbohydrate Model for May 27, 2014

I ran a MaluSim model using data from the weather station located at the Alson H. Smith, Jr. AREC in Winchester. This will be the last model run for the year.

Slide3 No forecast data was used in this run. Based on the output, I would expect chemical thinners that were applied May 14-16 to have an average response, while thinners that were applied on and after May 17 to have minimal effect. The effectiveness of thinners is greatly influenced by fruit size, so if the model showed a carbohydrate status that was greater than 20 g per day, and fruitlets were greater than 15 mm at the time of application then chemical thinners may not have been particularly effective.

Growers should compare the results of their chemical thinning program with the modeled carbohydrate status to determine if the model accurately predicted the level of thinning you expected from your applications.

I would appreciate hearing from growers about how they used the model to make chemical thinning decisions. Both positive and negative feedback is appreciated.  You can use the comment feature in the blog site, or just send me an email.

Download the full report:

Winchester VA MaluSim 5_27_14

MaluSim Carbohydrate Model for May 22, 2014

On Thursday, I ran the MaluSim model for Winchester.


As I suggested in my last post, the strongly negative carbohydrate balance forecasted for May 20 and 21 were a product of poor cloud cover forecasting. In fact, since May 16 the carbohydrate status has been in the positive range. Chemical thinning applications made from May 17 through the next several days will likely have an moderate to poor response. Now that fruitlets are 15 to 20 mm in diameter, NAA and 6-BA will have minimal thinning efficacy. Growers who need to thin additional fruit from their trees should consider ethephon applications.

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

Variety 12-May 15-May 20-May 22-May
Empire 10.4 12.2 15.1 17.9
Fuji 7.0 9.0 12.1 14.8
Gala 8.7 11.3 13.9
Goldens 8.0 9.5 12.7 14.9
Idared 8.1 9.5
Pink Lady 9.2 11.0 13.6 16.6
Red Delicious 8.0 10.1 11.4
York 7.9 10.7 13.8 16.1

As I did for the Central Virginia models, I will run one more season recap model early next week.

Download the full report:

Winchester VA MaluSim 5_22_14

MaluSim Carbohydrate Model for May 19, 2014

On Monday, I ran the MaluSim model for Winchester and a season recap for the Central Virginia region.


For the Winchester area, the carbohydrate status has been in the -20 to 20 g CHO/day range since May 15. This means that thinning sprays made between last Thursday and today are predicted to have an average response. For the next couple of days, the model is showing a significant carbohydrate deficit. This is being caused by the cloudy weather that is predicted, as neither day nor night temperatures are forecast to be very high. Cloud cover remains the hardest variable to forecast. If the next two days are sunnier than forecast, or if the rain showers occur at night, then I would not expect the actual carbohydrate balance to go into such a low deficit. However, if the weather forecasts that I used in the model are accurate, then growers should avoid thinning on Wednesday. Either way, the best predictor of the potency of chemical thinners is the 2 days before, day of, and 3 days after (2+4-day) running average (the black line). As you can see, the running average remains between 0 and 20 g CHO/day, meaning that chemical thinners will have an average response.

For the Central Virginia region, I’ve run a recap of the season using weather station data through Monday, May 19. Growers should compare their thinning results to the carbohydrate status at the time they made their application to see how well the model predicted the potency of thinners this year. From my experience, using the MaluSim model as a forensic tool can really help explain why chemical thinners work so differently from one year to the next. As I repeatedly have said, the most difficult part of using the MaluSim model to make real time decisions is having accurate weather data to use as inputs. As I mention above, cloud cover can really impact the carbohydrate status in the tree, and thus the responsiveness of thinners, but it is the hardest variable to forecast. Additionally, weather can be very local. We experienced this issue with the major differences between Tyro and Piney River weather forecasts. We also saw a lot of variable temperature data, with several days between May 8 and 14 being 5 or more degrees Fahrenheit warmer than forecast.

Even with these issues, I hope that the model is providing useful information that helps you make sound management decisions. If you have any feedback, please don’t hesitate to contact me.

I will continue to run the Winchester model until fruit size is too big to thin with NAA and 6-BA.

Download the full reports:

Central VA MaluSim 5_19_14

Winchester VA MaluSim 5_19_14

MaluSim Carbohydrate Model for May 15, 2014

Fruit in the Winchester area is at the ideal size for chemical thinning.

Slide3Thinning applications made on Monday through Wednesday of this week will likely see an excessive amount of thinning, especially if  fruit was 8-12 mm in diameter. Thinners that were applied on Thursday will likely have a moderate to strong response. Thinners applied Friday (if it stops raining!) through the weekend will have a normal to weak response. If the weather forecast holds, growers should consider slightly increasing rates and using a surfactant for thinning applications made over the weekend. I’ll run another MaluSim model on Monday.

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

Variety  12-May 15-May
Empire 10.4 12.2
Fuji 7.0 9.0
Gala 8.7 11.3
Goldens 8.0 9.5
Idared 8.1 9.5
Pink Lady 9.2 11.0
Red Delicious 8.0 10.1
York 7.9 10.7

Download the full Winchester report:

Winchester VA MaluSim 5_15_14

Fruit in the Central Virginia region is now beyond the size where chemical thinning with 6-BA or NAA is effective. If additional thinning is needed, growers should use ethephon. While these late or “rescue” thinning applications are not very responsive to the carbohydrate status modeled with MaluSim, ethephon does tend to be more active when applications are made on hot days. I will run a season recap for the Central Virginia region in a few days (I was having technical issues accessing the weather station data).

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 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 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!