Moderate temperatures and adequate sunlight are resulting in slight carbohydrate deficits according to the MaluSim model.
This means that chemical thinners applied today through the first part of next week should cause an “average” response. Warmer temperatures are forecast for the middle of next week, which should result in greater carbohydrate deficits. I will run another simulation for Central Virginia, as well as the first Winchester run on Monday.
Central Virginia MaluSim for 1 May 2015:
Today, I ran the first Central Virginia MaluSim model for 2015. I will start the Winchester models later this week.
For Central Virginia, I use data from a weather station set up at Silver Creek Orchards and managed by my colleague, Dr. Mizuho Nita. A big thank you to Dr. Nita for allowing me to install the necessary instrumentation and for maintaining the station and associated software.
This year, I will be using forecast data from the National Weather Service. In past years, I used Intellicast.com data because they provided 10-day forecasts with cloud cover predictions. Intellicast no longer provides the cloud cover data in an easy to use format, so I am switching to the National Weather Service’s forecasts. Note that these forecasts only project five days into the future.
We will discuss the details of the model at Tuesday’s meeting at Saunders Brother. 
However, this first model run suggests that the relatively cool weather, and mostly sunny weather that has occurred since bud break has resulted in a moderate carbohydrate deficit in apple trees. Growers can expect average results from chemical thinners applied the last couple-few days through to the weekend. In blocks where significant thinning is needed, growers should apply carbaryl at petal fall and then look to the warmer weather that is predicted for the weekend and early next week for their 10 mm application.
Central Virginia weather data, MaluSim data, and an interpretation chart are in the pdf linked to below:
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.
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:
LATE (“RESCUE”) THINNING
Where late (14-25 mm) thinning is still possible growers can use the following materials and combinations of materials in order of greater thinning potency:
Ethephon applications can cause over thinning, so use the lowest rates and tank mix with the least number of other materials unless significant fruit removal is desired.
PROMOTING RETURN BLOOM IN APPLE
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
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.
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:
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:
Fruit in the Winchester area is at the ideal size for chemical thinning.
Thinning 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:
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).
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 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.
Due 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 
that 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.
