Data from today’s Winchester MaluSim simulation (pdf) shows that apple trees have been in a mild carbohydrate deficit for the last couple of days and will likely continue to be in a mild deficit for three to four days. This means that you should expect average to slightly aggressive thinning response from materials apples during this past week. A more severe carbohydrate deficit is predicted for next week, but this will likely be beyond the 10 mm thinning application for most orchards and cultivars. At the AREC, many cultivars are already at or beyond 15 mm. The three-day rolling average shows less of a deficit over the past few days, but also shows that we are heading into a period of severe carbohydrate deficit next week.

We are unsure how well the MaluSim model predicts responses to ethephon-based materials used for late- or re-thinning at ~20 mm. However, it is something that I am testing this year.

As a reminder, MaluSim is based on an idealized Empire tree and uses weather data from the Winchester AREC. Therefore it may not represent conditions in your orchard or with different cultivars. However, please let me know if you are using this data for making management decisions and how well it is or isn’t working so that we can improve the model for future years.

As I mentioned in the comment to my last post, I am out of town and unable to run a MaluSim simulation until I have access to my lab computer on Friday. However, I just compared the predicted temperatures and sunlight levels that I used as inputs for the model I ran on Monday (April 30), and the current forecast. For the next few days, the Winchester area is predicted to have slightly warmer day time temps and slightly cooler night temps than previously forecast, but overall the temperature forecasts match up reasonably well (see table below). There will be thundershowers over the next few days, which makes predicting total daily light levels difficult. However, the warmer day temps will like drive the carbohydrate status in the tree to be more negative (that is, in a greater deficit). My interpretation of Monday’s model was to expect average to slightly greater than average responses from chemical thinners. With today’s forecasts, I suspect that apple trees will be even more responsive to chemical thinners for the next three days (Thurs-Sat). I’ll verify this when I run the model again on Friday.

Click on this link to see the MaluSim simulation for April 30, 2012 (pdf). As I’ve mentioned previously, it is recommended to look for three-day trends in carbohydrate balance. My colleagues in New York who have been working with the MaluSim for several years now have suggested using a multi-day running average carbohydrate balance, as opposed to the day-by-day data that I have been presenting. Within the file, the first simulation chart is for the day-by-day data, and the second simulation chart is for the three-day running average. The running three-day average is meant to smooth out the data to make it easier to see the trends. Since I haven’t compared previous years’ thinning efficacy using a three-day average, I am less familiar with how to interpret the numbers. Nonetheless, if you look at either the day-by-day data or the three-day running average data, you’ll see that we’re heading into a period of carbohydrate deficit. This means that your chemical thinners will have average to slightly aggressive activity if applied tomorrow (Tuesday) through Thursday.

As a reminder, the model is based on an idealized Empire apple tree that has an average crop load, average number of extension shoots, and has not had any other stress related damage (e.g. frost). Therefore, the model is only meant as a guide for understanding the effects of weather on the activity of your chemical thinners. You should base your thinning applications on the crop situation in your blocks, as well as your experience.

Most cultivars at the AREC are at or beyond the 10 mm fruitlet size. Although  differential separation between fruitlets is not extreme this year (especially in clusters with no king bloom due to the frost), it is still assumed that weaker fruit will be thinned more easily than stronger (i.e. larger) fruit. Since there is a great amount of variability in fruitlet sizes within trees and within orchards, make sure to properly assess blocks before applying chemical thinners.

Please let me know if you have any questions. You can use the comments feature on this blog site, or send me an email at greg.peck@vt.edu.

As a final note to today’s post, I want to thank Drs. Alan Lakso and Terence Robinson from Cornell University who have spent many years working on developing the MaluSim model. They have been very generous in sharing the model and their experiences interpreting the model for use in predicting thinning efficacy.

Here is the MaluSim simulation for April 26 (pdf) that I presented at the In-depth Fruit School in Winchester last night. As you can see from both the weather data and the simulation, it looks like the next 3-day period where there is a carbohydrate deficit will be next week. Chemical thinners applied at the earlier end of the deficit will likely have more activity than those applied over the weekend. Since the model predictions are only as good at the forecast data that are used as inputs, it is be important to keep an eye on the temperatures towards the end of next week. I’ll run another simulation on Monday. In previous years, the model has shown that carbohydrate deficits of less than -60 g CHO/day can cause over thinning.

As I discussed last night, the MaluSim model should be used as another tool in the toolbox for understanding the interaction between environmental conditions (primarily temperature and sunlight) and thinning. At this point in time, the model is not able to predict the actual amount of thinning you can expect for any given application. But it does provide some indication as to whether to expect greater or lesser activity from your thinning chemicals.

Please let me know how you are using this information for making management decisions. In future years, we hope to be able to further refine the model to account for more specific inputs (for example, cultivar, crop load, frosts and other physiological stresses). Your input will help us understand how the model is being used and how we can increase its utility.

In the linked pdf file, I also include data on the current fruitlet size for several varieties that we grow at the AREC (fruitlet sizes listed on the simulations are for Empire, which is the standard tree used for the model).

Also included is a slide on year to date data on precipitation. Despite some rains last week and over the weekend, soil conditions are still abnormally dry.

I ran the MaluSim model again today, April 23 (pdf). As you can see from the simulation within the pdf file, we are predicted to have a fairly significant carbohydrate surplus over the next week. This is the result of the forecasted cold temperatures for next ten days.

This data suggests that you can expect an average to below average response from thinning chemicals applied over the next week. In orchards with little to no frost damage, growers may want to increase their rates or add adjuvants, such as oil, to the tank. With the cool weather, fruitlets growth rate has significantly slowed, so there might be another opportunity to thin early next week before fruitlets become too large. Otherwise, orchards that need additional thinning beyond the 12 mm fruitlet size may be treated with ethephon (or ethephon +/- carbaryl +/- oil) up until about 25 mm.

Fruitlets measured at the AREC today were within the size range (8-12 mm) that is typically recommended for fruit thinning:

Empire = 11.7 mm  |  Red Delicious = 10.0 mm  |  Golden Delicious = 8.8 mm

I will continue to run the MaluSim model until chemical thinning is no longer a viable option in the Winchester area.

I ran the MaluSim model today (4/19 pdf). Despite the low-light conditions that are expected with this weekend’s cloudy weather and rain, the predicted cooler temperatures will allow plants to maintain a relatively high carbohydrate status. With the MaluSim model, this is depicted as a numerically positive carbohydrate balance. Based on the current weather forecasts (I checked several and also ran a MaluSim simulation using National Weather Service data) we can expect an average to below average response from chemical thinner applications that are put on tomorrow (Friday) through the early part of next week. In blocks with no or minimal frost damage, this would suggest that you can increase your rates, or add an adjuvant, such as oil to increase thinning activity. In blocks with frost damage some additional thinning may result and you may want to reduce rates.

Many varieties are approaching 10 mm size; however, the model is not showing a good “thinning window” over the next 10 days. Fruitlet growth may slow down over the next few days with the cooler temperatures, but it is unclear if we will have warmer temperatures and a better window towards the end of next week. I’ll run another MaluSim simulation again early next week.