The average fruit size for our Gala, Pink Lady, Red Delicious, Fuji, and Honeycrisp in the Winchester area is 6.00, 6.11, 6.04, 5.72, and 6.37 mm, respectively. So, you may be wondering whether you should put any thinning application this week and if yes, when to spray and at what rate?!
The short answer is yes. You may consider applying chemical thinners, e.g. 6-BA or NAA, for your apple trees this week, preferably tomorrow, May 2nd, and Sunday (if it’s not raining). Based on the 10-d weather forecast for Winchester, it will be only Saturday and Sunday this week with temp. above 70 oF. Both 6-BA and NAA work better when the temp is greater than 65 oF. As for the rate, the carbohydrate model recommends increasing chemical thinning rate by 30%. Two things to consider here: a) Apple cultivars respond differently to thining materials. Golden Delicious, Fuji, and Cameo are usually hard-to-thin, whereas Pink Lady, Gala, Honeycrisp and Granny Smith are easier to thin. For the rates and materials recommended for Gala, Fuji, Honeycrisp, Red Delicious, Pink Lady, and Golden Delicious, use this link (https://blogs.ext.vt.edu/tree-fruit-horticulture/2020/04/02/fruit-thinning-scenarios-2020/). b) The rates mentioned on 6-BA and NAA product labels are for dilute applications (100 gal/acre). To convert from dilute to concentrate, please refer to my other post at (https://blogs.ext.vt.edu/tree-fruit-horticulture/2020/04/02/how-to-calculate-the-tree-raw-volume-trv-and-chemical-rates-for-concentrate-spraying/).
Sherif M. Sherif, Assistant Professor at Virginia Tech, School of Plant and Environmental Sciences,
AHS Jr. Agricultural Research and Extension Center, 595 Laurel Grove Road, Winchester, VA 22602 , Email: firstname.lastname@example.org, Tel. 540-232-6035