Methods for early forecasting apple size at harvest

Two methods for of forecasting fruit size at harvest between 50 and 80 days after full bloom (DAFB) were applied to several apple fruit growth datasets derived from studies conducted over many years with the cultivars Gala, Golden Delicious and Fuji. The degree day accumulation at 50 DAFB, and fruit size at 50 DAFB were correlated with the fruit size at harvest, with R-2 values of up to 0.78. The second method for forecasting involved the extrapolation to the date of actual harvest of an estimate of the slope of the linear phase of apple growth, as depicted by the expo-linear model, obtained between 50 and 80 DAFB. The R2 for this estimate was 0.87. Despite the good performance, the accuracy of the second method, as tested in any single year/cultivar combination, was variable, e.g. from 7% to 22% for two 'Golden Delicious' datasets. Combining the fruit growth data with a study of the absolute growth rate (AGR) might in some cases improve the forecast, since AGR often shows trends that may become apparent in terms of fruit weight only later. It also appears that a two-pronged approach, based on a combination of both methods, might be the most desirable, particularly in view of the effort required of the grower.