May 31st Apple Thinning Update Based on the Carbon Balance Model

It is predicted to cool off and remain sunny by Friday, and it may be prudent to wait until then to apply any final chemical thinners. It should be noted that the model outcomes are based upon FREC temperatures, FREC sunlight and FREC bud break, so these outcomes are not totally accurate for other locations. Also, the projection is only as good as the weather forecast that it is based upon. Nevertheless, the carbon balance model helps us to gain a better understanding of how apple trees will react to chemical thinners based on the weather. 

Dr Alan Lakso at Cornell University developed this carbohydrate balance model for apple, and we are testing it for predicting fruit set. The model estimates carbohydrate supply and demand, and then calculates the carbon balance of the trees. During times of high supply and low demand (sunny and cool), the balance is positive and it is difficult to thin chemically. When the balance is slightly negative (0 to -20°F), chemical thinning becomes easier. When the balance drops to the range of -40°, the trees are under significant carbohydrate stress, and chemical thinning will be strong. Below -40°, the natural stress may be so severe that some fruits will be shed even when thinners aren’t used. Below -40° the response to chemical thinners is likely to be very strong.