In the scenic Winchester/Frederick County area, apple growers are witnessing varied stages of growth across different apple cultivars. Currently, cultivars such as Fuji and Honeycrisp are at the partial petal fall stage, while Gala and Red Delicious have reached complete petal fall. Most notably, the Pink Lady cultivar has reached the critical >6 mm diameter stage, making it the ideal time for fruit thinning.
It’s important to time the application of chemical thinning agents, such as 6-BA and NAA, accurately. According to the Apple Carbohydrate Thinning Model on Cornell’s NEWA website (https://newa.cornell.edu/apple-carbohydrate-thinning), the current predictions for accumulated degree days (DD) for Pink Lady apples in Winchester suggest reaching the 200-250 DD window by Thursday, April 25, and continuing through most of the next week. While the model indicates a daily carbohydrate surplus, which isn’t ideal for thinning, a forecasted drop in carbohydrate levels by April 27 should be closely monitored. This dip, though minor, will likely influence the 6-D weighted average, which is a crucial metric for thinning effectiveness.
If you’re targeting a thinning application for cultivars like Pink Lady, which had a green tip date of March 2nd and a full bloom date of April 8, post-April 25th is advisable for starting treatments. The weather forecast suggests that April 28, April 29, and May 1 will offer optimal conditions—both temperature-wise and in terms of carbohydrate levels—for the effectiveness of 6-BA and NAA treatments.
Turning our attention to the Gala cultivar, it hasn’t yet reached the 200-250DD window. However, for those keen on applying a thinning treatment at petal fall, the upcoming Sunday or Monday is recommended. Following this initial treatment, it’s advisable to wait at least 10 days before the subsequent application, ideally timed for when the cultivar enters the 200-250DD window.
For detailed guidelines on thinning practices during petal fall and the 6-18 mm window for Gala, Pink Lady, and other cultivars, please refer to my previous post (https://blogs.ext.vt.edu/tree-fruit-horticulture/2024/04/18/apple-fruit-thinning-general-notes-and-specific-recommendations-for-central-virginia/).
