Crop Load Adjustment Based on Branch Vigor for Producing Uniform Fruit in Young Apple Trees

A series of investigations were conducted to identify an efficient cultivation method for producing uniform fruit in young ‘Hongro’ and ‘Fuji’ apple trees through vigor balance control and crop load adjustment at commercial orchards in Pocheon, South Korea, for two consecutive growth seasons (the third and fourth leafy years), namely, the years 2012 and 2013. There was a highly significant correlation between the cross-sectional area of the main stem (the trunk) and those of all the branches emerging from the main stem, with coefficients of 0.857** (P < 0.001) in ‘Hongro’ and 0.699** (P < 0.001) in ‘Fuji’. The vigor balance, defined as the ratio of branches to the main stem, was the most densely distributed in the range of 3.0 to 3.5 for diameter and around 1.2 for cross-sectional area in both varieties. The present study evaluated the level of crop load by measuring flower cluster density, crop density, and yield efficiency, and found the most densely distributed range was lower than the mean value. The apple trees showed higher productivity with a vigor balance of 1.1 to 1.3 in both varieties, but showed lower productivity with a vigor balance above 1.3 in ‘Hongro’ and below 1.1 in ‘Fuji’ trees in the third and fourth leafy years. Upon adjusting crop load by yield efficiency, the present study obtained higher productivity with a higher degree of yield efficiency in the two consecutive years. However, fruit size and other characteristics demonstrated little change by vigor balance control and crop load adjustment. The uniformity of fruit size and other characteristics was improved by the individual or combined application of vigor balance (from 1.1 to 1.3 in both varieties) and crop load adjustment (from 0.9 to 1.3 kg per cm2 TCA in ‘Hongro’ and from 0.7 to 1.1 kg per cm2 TCA in ‘Fuji’). Notably, the uniformity of fruit size and total soluble solids was efficiently improved by the application of crop load adjustment and vigor balance control, respectively.