Accurate early-season estimates of yield and fruit size distribution would aid apple growers and packers to estimate harvest labor, the number of bins, and the amount of storage space needed for the upcoming harvest. Accurate estimates of fruit size would also allow packers to better negotiate with potential buyers and retailers to develop marketing plans for the upcoming crop. There is currently no method to estimate packout at harvest from early-season fruit size estimates. The first step in estimating fruit size distribution requires models to estimate fruit size at harvest from early-season measurements. Apple fruit diameter at harvest was reasonably well predicted from early-season fruit diameter measurements for orchards within fairly close proximity (Batjer et al., 1957; Forshey, 1971). Marini et al. (2019a) recently reported that fruit diameter and fruit weight at harvest could be estimated from fruit diameter measurements at 60 d after full bloom (DAFB) for three cultivars growing in three fruit-growing regions of Pennsylvania, and coefficients of determination associated with the linear models ranged from 0.54 to 0.96. Adding the ratio of (fruit weight/fruit diameter) at 60 DAFB to the model with fruit diameter at 60 DAFB, produced less biased models with improved coefficients of determination, and predicted values were more similar to the observed values. The fruit weight/diameter ratio was positively related to cumulative growing degree days for the 60 d before the fruit were measured and tended to be lower in years when fruits were exposed to frosts.
The second step in estimating fruit size distribution for an individual tree involves sampling a tree adequately to obtain an unbiased estimate of early-season fruit size that represents the fruit size distribution for the whole tree. To assess the effect of various treatments on average fruit size on trees, researchers typically measured 5 to 35 fruit per tree (Byers and Carbaugh, 1991; Dozier et al., 1980; Greene, 1986; Miller, 1982; Schupp et al., 2002). Marini (2001) reported that estimates of mean fruit weight from a 20-fruit sample per tree differed from the true mean value by ≈13%. Estimates obtained by weighing all fruit on three limbs per tree differed from the true value by 11% to 19%. He suggested that sampling ≈25% of the fruit on a tree may be necessary to estimate mean fruit weight accurately. Over the past 15 years, many semidwarf orchards with relatively large trees have been replaced with more intensive orchards with smaller tree canopies. Smaller canopies have less variable light distribution, and fruit size may also be less variable. To determine the effect of canopy side on fruit size distribution, Marini et al. (2019b) divided vertical axis trees into eight vertical wedges or slices, and at harvest all fruit within each section were weighed. Fruit from most canopy sections were normally distributed, and distributions were similar for most sections. Therefore, we should be able to estimate fruit size distribution for a tree at harvest by measuring all fruit from a vertical section of tree representing at least 12.5% of the canopy.
Although Batjer et al. (1957) used early-season fruit diameter measurements to develop tables of estimated fruit diameter at harvest, they did not verify the models by comparing predicted fruit diameter to actual fruit diameter. They also did not consider how a tree should be sampled to obtain estimates that are representative of the whole tree. To our knowledge, no one has attempted to estimate the size distribution of a small population of trees at harvest from a sample of fruit measured early in the season. The final step in developing a protocol for estimating fruit size distribution for a group of trees at harvest from early-season fruit diameter measurements is to compare predicted fruit size distributions obtained from a sample of fruit with the actual distribution obtained from the whole tree at harvest. Therefore, the objective of this study was to determine whether previously published sampling protocols and regression equations developed with data from several orchards and several years (Marini et al., 2019a, 2019b) can be used to predict distributions of fruit weight and box sizes for a small population of trees. To test this hypothesis, distributions of fruit weight estimated by measuring diameters of all the fruit developing in about 12.5% or 25% of the volume of the canopy at 60 DAFB were compared with the actual distributions obtained by weighing all fruit on the tree at harvest.
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