Apple Thinning Update Based on the Carbon Balance Model

Posted: May 24, 2011

Dr Alan Lakso at Cornell University has developed a carbohydrate balance model for apple, which we are testing for predicting fruit set. This 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 predicted to be very strong.

We ran the model based on weather data for upper Adams County and it shows several windows of carbohydrate stress in May 2011 when thinning may have been very strong (see figure). On Tuesday morning, May 24, 2011, we entered weather predictions for the coming week. The model predicts that there will be a large block of time over the next several days when fruits will be very sensitive to chemical thinning.

The model is still experimental, but I believe it reflects stresses that affect fruit set. While some blocks will still need to be thinned, growers are advised to monitor the weather forecast, to carefully re-assess the need to thin, and to consider reducing the strength of the thinner tank mix. This is done primarily by using lower rates of the thinners themselves, but also by reducing or dropping additives that promote thinning.

Contact Information

James Schupp
  • Professor of Pomology
Phone: 717-677-6116