Tag Archives: Thinning

MaluSim Carbohydrate Models for May 16: Winchester and Central Virginia

MaluSim models were run on May 16 for Winchester and Central Virginia.


As I spoke about at the Winchester breakfast meeting on Thursday morning, the Winchester forecast is calling for warm and cloudy weather starting today and running through most of next week. From many year’s of research, we know that these are the conditions under which there is a fair amount of natural thinning and that chemical thinners can be particularly active. When the weather data is run through the MaluSim model, we see that there is a carbohydrate deficit between -40 to -60 g CHO/day predicted for the next 10 days. This means that chemical thinners that are applied over the next several days will likely be fairly active. With this in mind, growers should consider reducing rates and/or not using a surfactant. Growers with blocks that sustained freeze damage on Tuesday morning, should consider further reducing rates. As always, you should be checking your trees and making decisions based upon the conditions in your own orchard.

For Central Virginia (Piney River), the carbohydrate deficits over the next week will remain in the negative, but not be as low as the Winchester area. This means that chemical thinners applied from today through the weekend will likely be slightly aggressive when used at standard rates. Growers should be regularly checking their blocks for current conditions. By now, you should be able to see some effect of the cloudy weather that the region experienced early last week.

Here are the pdf files that contain the weather data and the MaluSim models:

Peck Central VA MaluSim 5_16_13

Peck Winchester VA MaluSim 5_16_13


MaluSim Carbohydrate Models for May 13: Winchester and Central Virginia

Slide3Slide3The MaluSim carbohydrate model was run on May 13 for both Winchester and Central Virginia.

In Central Virginia, many later blooming cultivars are around 10 mm fruitlet size and can still be chemically thinned. From applications made over the past weekend, growers can expect average responses to chemical thinners. As we move through the week, greater and greater carbohydrate deficits are predicted, which will increase the potency of chemical thinners.

For Winchester, the model is predicting mild carbohydrate deficits early in the week, and much more severe deficits towards the end of the week and over the weekend. Applications made this past weekend through the middle of the week will likely have an average to slightly aggressive response. However, applications make towards the end of the week may result in very aggressive thinning activity. If the weather forecasts hold up, growers should consider reducing rates for applications made towards the end of the week.

I will run the model again on Thursday, May 16.

Fruitlet sizes from the Winchester AREC:

  • Honeycrisp: 8.8 mm
  • York: 8.0 mm
  • Fuji: 8.6 mm
  • Suncrisp: 8.4 mm
  • Pink Lady: 9.2 mm
  • Empire: 11.8 mm

One more note, there are widespread frost/freeze predictions for most of Virginia for Tuesday morning. If a significant frost/freeze event occurs, growers should be cautious with their thinning applications until they can assess if damage has occurred. However, the forecast for Winchester predicts the temperature will only go to 32F, which will not be cold enough to cause significant damage. The forecast for fruit growing regions to the south of Winchester appears to be in the mid-30s to low 40s.

To see more information, click on the linked pdf files below:

Peck Winchester VA MaluSim 5_13_13

Peck Central VA MaluSim 5_13_13

MaluSim Carbohydrate Models for May 9: Winchester and Central Virginia


MaluSim carbohydrate models for Winchester and Central Virginia were run on Thursday morning.


Over the next day or so, for Central Virginia, the carbohydrate balance will remain in the negative, but quickly move to the positive over the weekend. From thinning applications made Thursday (5/9) or Friday (5/10), growers should expect a fairly “normal” response. However, forecast data from Intellicast.com, suggests that sunnier, but cooler weather will move into Virginia early next week, followed by a warming trend starting on Wednesday. Based on the MaluSim model, this weather pattern will cause apple trees to have positive carbohydrate values through the early part of next week, and thus standard rates of thinners used on Saturday or Sunday will be less effective than expected. In blocks where heavier thinning is needed, growers should consider holding off on their 10 mm thinning application until Tuesday or Wednesday when warmer temperatures return. However, once fruit gets to be 15 mm or larger, NAA and 6-BA are less effective thinning agents. The other approach is to increase the rates used in thinning applications made over the weekend through Monday.

The above scenario is similar for Winchester but since fruit size is smaller (generally 6-8 mm), growers can wait for next week’s warming trend to make the 10 mm thinning applications. Additionally, with the smaller fruit size in Winchester, the cloudy weather that we’ve experienced over the last few days should cause less natural thinning than what might occur in Central Virginia.

I’ll run the model again on Monday.

Current fruitlet sizes at the Winchester AREC:

  • Empire: 8.0 mm
  • Golden Supreme: 6.3 mm
  • Fuji: 6.5 mm
  • Golden Delicious: 5.7 mm
  • Suncrisp: 6.0 mm
  • Pink Lady: 7.2 mm
  • Idared: 7.1 mm
  • York: 6.0 mm

For more information, download pdfs of the models:

Peck Central VA MaluSim 5_9_13

Peck Winchester VA MaluSim 5_9_13

Impromptu meeting for Central Virginia on May 8

On Wednesday May 8, we will have a commercial fruit growers’ meeting at Saunders Brothers Orchards starting at 11AM. The meeting was requested by growers in the Central Virginia area to discuss fruit thinning, as well as current disease and insect management issues.

For more information, please contact Michael Lachance at Lachance@vt.edu or 434 263 4035.

MaluSim Carbohydrate Models for May 6: Winchester and Central Virginia



MaluSim models were run on Monday May 6 for Winchester and Central Virginia (Piney River). The outputs from both locations were fairly similar, with carbohydrate deficits predicted over the next six or so days. This is mostly due to the cloud cover from a series of storms that will pass through the region. With sunnier conditions and warmer temperatures, the model predicts a carbohydrate surplus by Saturday.

Apple trees are still at late bloom to 4 mm fruitlet size in the Winchester area, and growers will probably not need to start thinning until later in the week or early next week.

In Central Virginia, fruitlets are ranging from 8 to 15 mm. Growers should consider reducing rates over the next few days as the model is predicting an aggressive response to standard rates of chemical thinners. However, as we move towards the end of the week, growers should use full rates to ensure an effective response.

It is unclear how much “natural” thinning will occur from this prolonged stretch of cloudy weather. However, work by Dr. Byers suggests that 3 days of cloudy weather with temperatures at 70F (this was a controlled study in growth chambers) can cause 50% of the fruitlets to abscise. With the prolonged stretch of cloudy weather that we are experiencing, growers should consider being less aggressive with rates until the end of the week.

Peck Winchester VA MaluSim 5_6_13

Peck Central VA MaluSim 5_6_13

MaluSim Carbohydrate Models for May 2: Winchester and Central Virginia

MaluSim Central VA 5_2_13


MaluSim Wincheter 5_2_2013



Today, I ran the first MaluSim Carbohydrate models for 2013. This includes a simulation for Winchester, and a simulation for Central Virginia (Piney River, VA). Thanks to Bennett Saunders for sending us the data to use for Central Virginia.

The Malusim model was developed by Drs. Alan Lakso and Terence Robinson at Cornell University to estimate the carbohydrate balance in an ‘Empire’ apple tree based upon many years of detailed research and information found in the scientific literature. In recent years, the model has been used by researchers and extension specialists in New York as a tool to help assess the application timing for applying thinning materials at the 10 mm fruitlet size. Carbohydrate status, and therefore the MaluSim model, is not as important for other times when chemical thinners might be applied (i.e., bloom, petal fall, 20 mm), so it is not as clear how well the model does to predict the response of chemical thinners used at timings other than 10 mm.

For the past several years, I have been running trials at the Winchester AREC to see how well the MaluSim model performs under conditions found in Virginia. Many researchers, including myself, are still working towards understanding the best way to use the model. From what we have found, when weather forecasts are used to predict the future carbohydrate status of an apple tree, the MaluSim model can provide a warning as to when there might be a chance for severe under- or over-thinning. This is particularly true when there are three- or four-day trends with large carbohydrate surpluses or deficits predicted. For this reason, in addition to the data points for each day, I show a four-day running average (the black line on the charts). To be consistent with other researchers and extension specialists, the four-day running average is calculated from the date the data is show to three days into the future.

Please keep these caveats in mind if you are using the MaluSim model to guide your thinning application timing:

  • We’re still learning how to best use the model. At this point, it is just one more tool in the toolbox. There is no tool that is better than your own experience with chemical thinning on your own farm.
  • Future predictions of carbohydrate status are based upon weather forecasts from Intellicast.com for Winchester and Piney River. Temperature forecasts are probably pretty good for 3-5 days, but cloud cover (which is how I calculate the future solar radiation) is much less reliable. Although I show predictions out 10 days, my confidence that they are correct decreases significantly after 5 days. Nonetheless, it is useful information–just remember to keep looking for updated models so that you have the most up-to-date information. I will do my best to post a model for Central Virginia and Winchester twice a week.
  • Look at the day that you want to spray, and the 2-3 days after that. If there are periods of severe surplus or deficit, consider adjusting the rates or timing of your thinning applications.
  • The model is based upon the phenology of ‘Empire’. This is because the researchers in NY had a large body of data on Empire to use when they created the model. Of particular note, you should be aware that the model “clock” starts at silver tip for Empire. Cultivars with much earlier or later bud break may not correlate as well with the model values.
  • The models that I am running use data from the Winchester AREC, and Saunders Brothers Orchards in Piney River, VA. You need to consider whether or not those locations represent the weather in your orchard.
  • The model only accounts for temperatures and sunlight. Other weather events, such as rainfall or frosts may cause other impacts on the efficacy of your thinning materials that are not going to be represented in the model.

In the below pdf files, I provide a chart of the weather data that are used to run each model (daily maximum temperature, daily minimum temperature, and daily solar radiation), a chart of the MaluSim model with day-by-day data and a four-day running average, and a chart with the potential response a chemical thinner might have for different levels of carbohydrate balance in the tree.

So, what do today’s models show? Some very advanced blocks of early blooming apple cultivars in Central Virginia are getting close to the 10 mm thinning stage. From applications that occur today and tomorrow, you should expect “normal” to “slightly aggressive” response from your standard thinning rates, but nothing that suggests sever overthinning. However, there is a period of unsettled weather predicted for Mon-Wed of next week that could cause the carbohydrate levels to drop in the trees. It all depends on the amount of cloud cover we get over those days. Temperatures are going to remain moderate over that period, so even with the potential for some cloud cover, it doesn’t look like a severe carbohydrate deficit. Cloud cover is the hardest thing to predict, and predictions are only good for 3 days at best.

In Winchester, we are still at late bloom to petal fall, so there’s probably another week before the carbohydrate model data needs careful interpretation.

I will talk more about the model at tonight’s In-depth fruit school in Winchester.

Peck Central VA MaluSim 5_2_13

Peck Winchester VA MaluSim 5_2_13