As you know, despite the freeze damage we experienced on April 9, several apple cultivars across multiple locations are still carrying at least a few times more fruit than desired. Chemical fruit thinning remains our best option before it’s too late and we are left with the laborious task of hand thinning. The best timing for applying 6-BA and NAA for fruit thinning is during the 6–18 mm fruit size window, and we are currently within that window for ‘Gala’ and most other apple cultivars in the Winchester/Frederick County area. To help guide thinning applications and ensure trees are responsive to treatments, I ran the Cornell Apple Carbohydrate Thinning Model (https://newa.cornell.edu/apple-carbohydrate-thinning). Based on key parameters, the model helps determine both the timing and rate of thinning applications. Using data from the weather station located at our research center in Winchester, and with a green tip date of March 16 and a full bloom date of April 12 for ‘Gala’, the model predicts that starting tomorrow—Tuesday, April 29—and continuing through the end of the week, conditions will be ideal for thinning treatments. The trees’ accumulated carbohydrate balance is generally negative, which is exactly what we want for effective thinning, and temperatures—especially on Wednesday and Friday—will be perfect for the performance of 6-BA and NAA. The model also recommends using the standard rate for thinning materials.

For growers in Central Virginia, I also ran the model using the Roseland, VA weather station, with ‘Gala’ green tip and full bloom dates set as March 14 and April 10, respectively. As shown below, if you haven’t already applied your thinning sprays, we are still within the optimal thinning window. The model suggests that if you apply tomorrow (April 29), you should increase the thinning rate by 30%. However, based on the daily carbohydrate balance, I believe that applications made this Wednesday and Thursday will likely require only the standard rate.