This week of April 24-30 is the perfect timing for chemical thinning applications for both Winchester and Central Virginia. It’s as the books says about the optimal thinning conditions. Apples are in the size of 6-9 mm, compete aggressively with each other and consume a lot of the tree carbohydrate reserve; temperatures are above 80’s in most days and subsequently the transpiration rate will be high; some days are partially cloudy and will lead to fewer photoassimilates; and more importantly, the carbohydrate model predicts carbohydrate deficiency up to -60 g/day. The only thing that works against us is the high chance (90%) of rain tomorrow (Monday) and after tomorrow (Tuesday). However, we still have this Wednesday and Thursday as two ideal days to chemically thin our fruitlets.
If you were wondering about the best thinning applications to use at this stage, I would recommend using 6-BA for most varieties. Best results will be obtained when you mix 6-BA with carbaryl. NAA is another thinner you can use at this stage and also will give better results when mixed with carbaryl (i.e. Sevin). The mix of 6-BA and NAA might work well with most varieties and there is a good logic of having these two thinners mixed together at this time. However, this mix (6-BA + NAA) SHOULD NOT BE USED with Delicious and Fuji as it will likely result in the development of pygmies. With the coming hot weather this week, adding oil to the tank may result in overthinning. There are many variations among apple varieties and some are more sensitive to chemical thinners than others. The following table (Table 1) gives some indications about the sensitivity of different apple varieties to chemical thinners.

One final note, I run the model for Winchester and Central Virginia based on the information I retrieved today (April 23) from the weather stations in AHS Jr. AREC, Winchester and Batesville, respectively, and there is no guarantee that weather forecasting will remain unchanged the whole week. However, if I have detected any major change in the temperature and solar radiation values over the week, I will run the carbohydrate model again and keep you posted.