First Carbohydrate Model Run for 2021

Full bloom triggers the beginning of the MaluSim carbohydrate growing degree days and the recommendations on efficiency of the various thinning products.

The MaluSim Carbohydrate Model

As mentioned in the MaluSim article several weeks ago, there are several changes that have been added to the carbohydrate model. These changes were implemented based upon several years of experiments by horticulturists at Cornell University the developers of the model. In the last two weeks flower development continued and several orchards have reported full bloom of Gala. Unfortunately we had a cold spell with growers reporting variable frost damage to fruit trees. The table below shows some of the low temperatures from NEWA weather stations from around the state as well as the duration in hours temperatures were below freezing.

Table: Low Temperatures the night of April 21, 2021 into April 22, 2021

Bold font temperatures are sites that did not drop below freezing.

The image below shows the printout from the Apple CHO Thinning v2021, that can be accessed at the NEWA web site under the Crop Management tab. To operate the model, you will need to choose a weather station closest to your orchard location; input the date of Green Tip and Full Bloom date for each cultivar of apple that you will be thinning. In our output we chose Gala as the subject cultivar; primarily since most growers in the state have Gala trees. Note that the model can generate generic dates of green tip and full bloom for your area. However, due to variations in elevation, exposure, cultivar and tree age it is better to use your actual dates you observe your growth stages.

You will also need to enter the percent flowering spurs. This can be done by choosing two branches on opposite sides of representative trees and counting the number of potential flower buds and the total number of buds. The percent of buds that actually are flowering is determined by dividing flower clusters by the total number of buds and multiplying by 100. I indicated at the virtual Spring Orchard Meeting on April 22 that I am running the model assuming 76–100% flowering spurs. This was due to a lower crop in 2020 from frost damage events. Observations this year prior to April 21st showed that nearly every bud was expected to produce flowers.

The events of April 21–22 may have changed that selection and you may need to adjust the flowering percent down. The percent flowering spurs was added based on research that has shown one of the greatest variables to impact final crop load is the number of flowers on a tree. The general concept is that if there are more flowers there will be more fruit and vice versa.

The final ‘new’ parameter added to the model is targeting the period when apples appear to be the most susceptible to the action of chemical thinners. Going back to research work at Virginia Tech by Ross Byers and also more recent work apples seemed to be most responsive to thinners when they were 10 to 12 millimeters in diameter. The growing degree day model that best matched this diameter timing was 200 to 250° GDD Base 4°C. While it is a good biological set point you still need to have the proper weather conditions for the thinner to work. You could be in the 200 to 250° range but if temperatures are off or carbohydrate deficits are not high your thinning results may be less than what you expect.

Finally, click on the green “Calculate” button and the model will show the maximum and minimum temperatures for each day. Daily solar radiation, daily carbohydrate balance, a 7-day average balance and number of growing degree days with a base of 4°C (39°F). The results will automatically be calculated and displayed. As indicated at the bottom of the table the color of the text in the 2nd through 7th column will indicate the efficacy of applying a thinner at that date. The column on the far right will suggest the concentration of a thinner chemical. that will it need to be increased, held constant or decreased. This assumes that you have a standard chemical thinner material and a rate that you have used in the past that worked well.

Please remember that the recommendation is very dependent on the weather forecast. If the actual weather differs greatly from the forecast weather, thinning recommendations will change. The best use of the model is to run it the day you intend to apply a thinning spray. The next best is to run the model late in the afternoon the day before you will be applying your thinner.

Finally, you will notice the format of the tables is different from previous years. Instead of standard Excel tables we are sending actual screen shots from the NEWA Carbohydrate Thinning Model for each date. In the first column, the date that is shaded in green and in bold is the date the model was run. Usually it is run first thing in the morning.

In summary, chemical thinning is still not completely understood. Thinning with PGRs is still subject to the variances of weather and weather forecasts. However, we are gaining more knowledge of how these factors can impact the outcome of thinners.

If you have any questions please drop me an email at rmc7@psu.edu

Apple Carbohydrate Thinning Models for Pennsylvania Counties

• Adams County
• Bedford County
• Butler County
• Erie County