Prediction of fruit set in apple to facilitate the chemical fruit thinning

The decision, whether a chemical fruit thinning in an apple orchard is necessary and to what extent, is often difficult. Currently growers take a decision without knowing the extend of natural fruit drop (June drop). In recent years two methods for prediction of fruit set respectively the intensity of June fruit drop in apples have been examined. The first method is based on calculating fruit growth of marked fruits shortly after bloom, by measuring the fruit diameter on two dates. The presumption of the model is, that fruits with growth below a critical value, will abscise at the June drop. The survey and evaluation of the first experimental year revealed, that the parameter values given in the literature, cannot be transferred in our growing conditions. By adjusting the model parameters, based on field experiments in subsequent years, the reliability of fruit set prediction was significantly improved. In a second experiment, young fruits were measured with a near-infrared spectrometer. The intention was to prove if the physiologic changes in abscissing fruits may be detected, and thereby if a prediction of the fruit set is possible. It was observed that, at certain wavelengths, the spectra of falling and remaining fruits were clearly different, which could be confirmed by a regression model. To what extent, differences between varieties and years are influencing the mo