MSU Extension

Chemical thinning is the most difficult annual grower practice, yet the most important. Too little thinning will produce a crop of small fruit and poor return bloom. Over-thinning will produce a crop of large apples with quality issues. Michigan State University Extension recommends Precision Cropload Management as a strategy to use all available techniques and information to thin to a precise target crop load. Thinning early and using crop load tools will make thinning easier, more successful, and precise, and also get best fruit quality with good return bloom.

The first step is to evaluate your block and set a target crop load. We use the target crop load to dormant prune to a 2x target bud load. If your target crop load is 100 apples on a tall spindle tree, then the dormant fruit bud load would be 200 per tree. Trees often have five times this target bud load before pruning. If still in the dormant season, prune each tree to 2x of the bud load. Then plan on using the Predicting Fruitset Model and the Carbohydrate Model to help fine tune thinning activities and rates. Plan on thinning early, starting at full bloom or at least at petal fall. Early applications of thinners will help get the thinning started. Usually thinning at full bloom or petal fall will thin at mild levels (0 to 15% thinning). In 2013 and 2014, aggressive thinning occurred at these timings because there was significant tree stress as predicted by the carbohydrate model and the predictions were correct.

At pink, set up the Predicting Fruitset model by marking 15 blossom clusters on 5 trees per block. These marked clusters will be used at the 6 mm stage to make set measurements and ultimately predict fruitlet abscission. The next step will be to spray a thinner at full bloom. I believe your three choices are ATS, NAA or MaxCel. One of these thinners should be used to get some thinning started. At full bloom, the set is unknown but apple trees usually will set 200 percent of a crop and perhaps more, if bud load was not reduced during the dormant season. At full bloom you will have some of the information to help predict fruitset, which is spring weather conditions and forecasts as well as flower health, potential bee activity, potential pollination and potential fertilization. If conditions appear to be good and the forecast is good, then fruitset should be good. Thinning at full bloom is a positive action. If conditions are poor, then wait until petal fall to thin. Before applying a full bloom thinner, use the predictions of the Carbohydrate Model. Throughout the chemical thinning window, the Carbohydrate Model can be used to adjust thinning rates up or down depending on the amount of stress predicted. If predictions are for no stress, then apply normal thinning rate or higher rates. If high stress is predicted, reduce rates or do no thinning. The same is true at other thinning times.

Next is the petal fall to 6-mm spray. Once again fruitset is unknown, but more actual bloom and bee information has occurred. Consult the Carbohydrate Model for stress prediction and adjust rates accordingly. Apply thinners when weather is favorable. Now at 6-mm stage, measure the blossom clusters marked at pink. This will give a baseline measurement to base fruitlet growth in the next measurement (about 5 to 7 days later). The difference between these two measurements will be used by the Predicting Fruitset Model to predict percent set and thus, determine if the target crop load is close. If not, thin again and measure again, but use the Carbohydrate Model to adjust this next thinning treatment.

Repeating fruit measurements; add this data to the Predicting Fruitset model; check the Carbohydrate Model; and re-thin. This routine will be continued until the thinning is correct or the fruits are too big to chemically thin. This leaves only hand-thinning to correct the crop load.

Using these tools and information will first get some thinning done early and the models will help adjust rates and repeat applications. By performing these Precision Cropload Management activities, early, successful, target thinning can be accomplished and growers will gain great experience and confidence in their thinning program while reducing the personal stress that so often makes thinning a difficult activity. For more information and to get a copy of the Predicting Fruitset Model, visit www.apples.msu.edu.