Tomorrow and after (May 4-5) could be a good (but not ideal) time for fruit thinning applications for apple orchards located in Winchester and the surrounding area. In our Winchester research orchard, the average fruit diameter for Gala, Honeycrisp, Pink Lady, Red Delicious, and Fuji is about 6.5 to 8.5mm, which is inside the size range (6-15mm) we usually target for thinning treatments. Also, the carbohydrate thinning model shows low carbohydrate levels in the past two days, today, and the following four days, which should also ensure a good response to thinning treatments applied tomorrow and Thursday (May 5). However, it should be noted that we are not within the “perfect” thinning window yet. We typically target accumulated degree days between 200-250 DD. We got only 143 DD, which is not bad…but also not ideal. Also, based on the weather forecast, it will be rainy tomorrow until 8:00 am and windy (>10 mph) by noon, which gives you only a few hours to apply thinning treatments. The daytime temperature tomorrow and Thursday is fine for 6-BA and NAA treatments, but I would generally wait for extended periods of 75 to 85 oF for a better thinning response.
The second potential thinning window will likely be on May 10-12. As we approach there, fruit size will be around 11-12 mm, degree days will be within the perfect range (200-250), the temperature will be just perfect (based on the current forecast), but we can’t predict if the tree carbohydrate balance would remain on the deficit side by that time. We can, however, increase the rate of thinning materials to compensate for high carbohydrate levels.
So, after considering all these factors, I would focus tomorrow on thinning the hard-to-thin cultivars (e.g. Gala, Fuji, Golden Delicious, and Gold Rush) that generally require more than one thinning treatment. But, for other cultivars, I would wait for the May 10-12 window. It should be worth noting that blocks you decide to spray tomorrow can’t/shouldn’t be resprayed with thinning treatments on May 10-12. For these blocks, I would suggest that you give it 2-3 weeks at least to see the effect of the first thinning treatment and decide if you will need additional applications. For tomorrow’s thinning applications, the model recommends that you apply the standard chemical thinning rate.
The Cornell Apple Carbohydrate Thinning Model (May 3, 2022).
Finally, if you are nearby, you may need to consider attending our in-depth meeting tomorrow, May 4 at 7:00 pm at the Alson H. Smith, Jr., Agricultural Research and Extension Center (AHS AREC): 595 Laurel Grove Road, Winchester, VA 22602. I will cover critical topics regarding crop load management and return bloom, and other specialists will provide seasonal updates on disease and insect management.