Remember that many chemical thinners don’t work very well when daily temperatures are below about 65 F. So here we are with warm days ahead, which means rapid fruit growth, perhaps up to 1 mm per day. Apple fruit here in Lafayette are about 12mm which leaves us some good options for chemical thinning. However in more southern areas, fruit is considerably larger, maybe 15 mm or more. Even if chemical thinners have been applied, in many cases these applications didn’t seem to have worked very well so we still have too many fruit on the tree.
So what to do at this point? There are really two main options for chemically thinning in this situation: ethephon and sevin. While ethephon can be a very effective thinner, it tends to be pretty unpredictable and so the risk associated with this product is higher than many growers are comfortably with. Sevin can thin fruit up to 20 mm and is more predictable so is probably the best option for growers who have larger fruit that need additional thinning. But with warm temperatures ahead and fruit sizing quickly, growers will need to act fast.