651 Methods for Early Forecasting Apple Size at Harvest Guglielmo Costa, Massimo Noferini, Fabrizio Bucchi, and Luca Corelli-Grappadelli Dipartimento di Colture Arboree, University of Bologna, Via Fanin 46, 40127 Bologna, Italy Keywords: Malus domestica, fruit growth, expolinear model, heat unit accumulation, growing degree days Abstract 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 R 2 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. INTRODUCTION For many practical reasons, it is desirable to know in real time the growth progress of apple fruit under orchard conditions for many practical reasons. On the one hand, growers may better fine-tune management practices; on the other, the ability to forecast average fruit weight at harvest is of great value for marketers. A number of papers have been published over the years, indicating the importance of the early season growth for determining final fruit weight (Bergh, 1985; Lakso et al., 1989; Warrington et al., 1999; Stanley et al., 2000). A general agreement exists on the assumption that the process of cell division, occurring in the early stages of fruit growth, sets the potential final fruit weight. This can be attained, provided unlimited resources are available to the fruit for growth. Temperature may often be limiting during the post-bloom, initial phase of fruit growth, resulting in reduced cell division, or expanding the length of the cell division period (Berg, 1990). Warrington et al. (1999) demonstrated this effect in a growth chamber study, where fruits growing on trees exposed to low temperature regimes were smaller at harvest then fruits from warmer treatments. Because of this consideration, and based on the observation that fruit relative growth rate (RGR) at about 1–5 weeks after bloom is highly correlated to final fruit size (Lakso et al., 1989), Stanley et al. (2000) proposed to use the degree day accumulation at 50 DAFB as a predictor of final fruit size. They reported good correlations between these parameters for three fruit districts and seasons in New Zealand. According to the expolinear model of fruit growth (Lakso et al., 1995), a prediction of final fruit weight might be made at about 50-80 DAFB (Costa et al., 2001b; Bucchi and Corelli Grappadelli, 2002). This approach is based on the assumption that fruit growth occurs in a linear fashion, once cell division is over. Therefore, by estimating the slope of this linear phase at its inception it should be possible to forecast final fruit weight, in the absence of growth limitations (e.g. water stress, nutrition imbalances). The present paper reports a comparison between these two methods of early apple size prediction for several cultivars, (‘Gala’, ‘Golden Delicious’, ‘Fuji’), over several years and from different trials carried out in Bologna. Proc. XXVI IHC – Deciduous Fruit and Nut Trees Ed. A.D. Webster Acta Hort. 636, ISHS 2004 Publication supported by Can. Int. Dev. Agency (CIDA)