Estimating Mean Fruit Weight and Mean Fruit Value for Apple Trees: Comparison of Two Sampling Methods with the True Mean

Abstract

Data obtained over two years from chemical thinning experiments with `Redchief Delicious' apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Manst.] on Malling 26 (M.26) rootstock were used to estimate mean fruit weight (MFW) and mean fruit value (MFV) using two sampling methods. The estimated values were compared with the true MFW and the true MFV calculated from the entire crop from a tree. Statistical techniques were used to assess agreement between the values obtained with estimation methods and the true values. Estimates of MFW obtained from a 20-fruit sample per tree may differ from the true value by ≈13% and estimates obtained from weighing all fruit on three limbs per tree may range from 11% to 19% of the true mean. Estimates of MFV obtained from packouts of a 20-fruit sample may differ from the true value by about $0.04 (U.S. dollars)/fruit and estimates from packing out all fruit on three limbs per tree may differ from the true mean by about $0.07/fruit. Analysis of variance was performed on each data set. The resulting P values differed for the three methods of calculating MFW and MFV. Therefore, erroneous conclusions may result from experiments where MFW and MFV are estimated from subsamples. Error associated with estimating fruit weight and fruit value from the sampling methods employed in this study may be larger than many pomologists can accept. Until protocols for sampling apple trees, which account for the important sources of within-tree variation, are developed, researchers should consider harvesting the entire crop to calculate MFW and MFV.