Chemical thinning – Facts for Fancy Fruit

Chemical thinning

We’re getting close to the time when growers need to make chemical thinning decisions – for many the most perplexing and risky decision they will make all year.  This is usually a tricky call to make. Remember that only about 5-10% fruit set is usually enough for a full crop. In other words, 90 out of 100 flowers can drop from the tree, and we would still have too many developing fruitlets and would need to thin.

As apple crops approach petal fall, it’s time to start chemical thinning.

The effectiveness of a chemical thinner application depends on many factors, and to hit it just right takes as much art as science. That’s a fancy way of saying that we don’t really understand why different orchards respond differently to a given thinner application. But we know they do. That’s why it’s impossible to develop a recipe approach to thinning. So let me explain a little about how thinners work, then discuss some specific strategies.

From the time of bloom and for the next month or so, there are thousands of flowers and developing fruitlets on the tree, struggling to get enough resources to grow. On a semi-dwarf tree, there can easily be 5000 flowers (yes, we’ve counted!). By resources I mean food in the form of carbohydrates. These carbohydrates come from stored sources in the tree but especially from leaves taking light energy and converting it to carbohydrates through the photosynthetic process. At this time of year, leaf area for photosynthesis is limited, so there is a shortage in the supply of carbohydrates. Because the demand exceeds the supply, fruitlets compete for carbohydrates and the strong survive. The weak flowers or fruitlets lose out and drop off, which we call fruit drop or June drop. The thinners we commonly use in Indiana exacerbate this shortage, so that even more fruitlets drop off. Some, like NAA, reduce photosynthesis so there is less carbohydrate supply. Others (such as Sevin) decrease the flow of carbohydrates from leaves to fruitlets, thereby also decreasing the supply. The Maxcell-type thinners increase respiration, burning up more carbohydrates so less is left over for developing fruitlets. So in these 3 different ways, thinners increase the shortfall of carbohydrates resulting in increased fruit drop. Keeping this in mind allows growers to predict the response to thinners from year to year. For example, a lot of cloudy weather soon after bloom means less light for photosynthesis, less carbohydrate and increased fruit drop. In that situation growers may want to back off a little with their thinner rates. Thinners work best when the weather is warmer. The optimal temperature is around 70˚F and below 60 you may as well not bother – most thinners are not going to have much effect when it’s that cool. When the temperature is 80˚F or above, be careful – thinners can have very strong effects at those temperatures. Dr. Jim Schupp at Penn. State University knows more about thinning than anyone I know. Jim says “There is no “safe” thinner at high temperatures”. Tread carefully.

It turns out that some of our most biennial varieties (Fuji, Golden Delicious) are also some of the more difficult to thin. So not only is thinning more difficult, the consequences of inadequate thinning are greater. Keep in mind your own experience on your orchard, but with Fuji you might want to start with a full rate of Maxcell soon after petal fall. Wait a full 2 weeks to see the response to the thinner application before applying more thinners. If another application is needed, I’d suggest ONE of the following, depending on how aggressive you want to be. In order from conservative to most aggressive, I’d suggest:

Maxcell again


Maxcell + sevin

Maxcell + ethrel

Maxcell + oil

Keep in mind these are general thoughts based on my experience and published research, but things work a little differently on different farms, so mix these thoughts with your own experience to come up with a plan. Most products do not thin Fuji enough. I’d put NAA/NAD, carbaryl and ethephon in this category. I’d stay away from NAA and NAD because of the tendency to form pygmies. Starting at petal fall gives you some time for a follow up application 2 weeks later if necessary and spreads the risk.  This is often referred to as “The Nibble Approach”. The single application approach is putting all your eggs in one basket and too risky for many growers.

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