Sampling Apple Trees to Accurately Estimate Mean Fruit Weight and Fruit Size Distribution in: HortScience Volume 54 Issue 6 (2019)

Discussion

Understanding within-tree variation is important to reliably estimate the average fruit size or fruit size distribution for a tree. De Silva et al. (2000) identified systematic trends in FW within the canopy of relatively large 4- to 5m-tall slender-pyramid ‘Gala’ trees. On lower limbs FW increased from the base outwards, but the opposite was true for the upper limbs, and at the base of the limbs FW increased from the lower limbs to the upper limbs. They suggested that sampling along a limb will result in more efficient estimates of mean FW compared with random or systematic sampling. Previous attempts to estimate fruit size or fruit size distribution were often based on samples of fruit rather than for the entire population of fruit on the tree. Zhang et al. (1995) sampled 20 ‘Gala’ fruit from three canopy positions of five trees in 15 orchards in two seasons, and average FW was positively correlated with the sd. Like our study, they reported that FW for the 30 combinations of years and orchards was normally distributed, but their sd (20-30 g) were nearly 6 times greater than for ‘Fuji’ in our study (3.6-5.0 g). Lower variation in our study was expected because our trees were small, and effects of within-canopy shading and other factors were probably minimal. Previous reports did not provide details concerning fruit thinning, and the follow-up hand thinning in this study likely enhanced fruit size uniformity. Using the mean and cv, Zhang et al. (1995) estimated the percentage of fruit that would fall into a given fruit count size. De Silva et al. (1997) recorded the diameters of a sample of apples at 14-day intervals throughout the season and attempted to predict fruit size distribution at harvest. Fruit size distributions of their sampled fruit changed during the season and the predictions were not very accurate.

In recent years, the apple industry has generally transitioned to relatively small trees less than 4 m tall and with narrow canopies less than 2.0 m in diameter at the widest point. Light distribution throughout the canopy of these modern trees is likely more uniform, leading to less systematic variation in fruit size and quality. Harvesting a relatively large proportion of the fruit on small trees is much easier than with large trees. Dorsey and McMunn (1938) were probably the first to evaluate sampling methods for estimating apple fruit size. They found that sampling 100 fruit per tree, harvesting all fruit on one limb per tree, or harvesting all the fruit from a vertical section of the tree did not provide accurate estimates of average FW. These were large trees, and they found that harvesting about 25% of the crop (91.0 kg/tree) provided estimated mean FWs within about 10% of the true mean. Marini (2001) used relatively small central leader ‘Redchief Delicious’/M.26 trees with about 400 fruit per tree and found that a random sample of 20 fruit per tree (5% of the crop) was nearly as accurate as a 60-fruit (17% of the crop) sample obtained by harvesting all fruit on three limbs. Trees used for the present study were much smaller, with an average of only 88 and 147 fruit per tree for ‘Gala’ and ‘Fuji’, respectively. As previously reported, the estimated FW more closely approximated the true value as the proportion of fruit sampled from trees increased.

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To our knowledge, this is the first report in which fruit size distributions from different canopy sections were compared. The fact that the fruit size distributions were similar for different canopy sections supports the concept that canopy position affects fruit size to a much less extent in small trees than in larger trees, where light distribution is less uniform. Our results indicate that sampling all fruit from about 25% of the canopy will provide estimates of average FW within 7% of the true mean and the distribution of fruit size will be similar to the distribution for the whole tree. If very accurate estimates are needed, then harvesting all fruit from the ordinate or coordinate sections (50%) of the canopy is suggested.

Methods have been published to predict apple fruit size at harvest from early-season measurements (Batjer et al., 1957; Forshey, 1971; Marini et al., 2019). The next step in predicting fruit size distribution at harvest from early-season fruit measurements is to measure FD or FW of all the fruit from at least 25% of the canopy 60 d after bloom. Using previously published models to predict fruit size at harvest from early-season measurements (Marini et al., 2019), the predicted distributions can be compared with the true distributions using all the fruit on the whole tree. Agricultural engineers are developing technologies to nondestructively count and measure fruit in orchards and our data indicate that recording data all fruit on one side of a small tree can provide accurate estimates of fruit size and fruit size distribution.

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