Various measures of tree vigor, yield, and yield efficiency of apple trees in the 1990 NC-140 systems trial as influenced by location, cultivar, and orchard system

In the 1990 NC-140 Apple Systems Trial (4, 5), the treatments included Slender Spindle (SS)/ M.9EMLA, B.9, and Mark; Vertical Axis (VA)/ P.1, M.26EMLA, O.3, M.9EMLA, and Mark; and Central Leader (CL)/ M.26EMLA and Mark. In the Illinois and Ontario trials the cultivars were 'Empire' and 'Jonagold'; in North Carolina and Virginia the cultivars were 'Empire' and 'Early Red One Delicious' (a non-spur strain). At the termination of the trial in 1999, above-ground tree weight data were collected by cooperators at four sites. In combination with data collected previously, these data provided the opportunity to evaluate the relationship between scion weight and trunk cross-sectional area (TCA) as well as to calculate the ratio of cumulative crop (C) to scion weight (S). Although the regression equations varied somewhat among sites, at each location TCA explained from 94 to 97% of the variation in scion weights. Scion weights in North Carolina were approximately one-half of the mean for the other three sites. Compared to cumulative yield in Virginia, yields at the other three sites ranged from 45-55%. The C/S ratio, calculated by dividing the cumulative crop by scion weight, was more than twice as high in North Carolina and Virginia as in Illinois and Ontario. The treatments were ranked within each site from high to low for both C/S ratio and yield efficiency (YE). The relationships among the treatments were very similar at all sites using either the YE or C/S ratio as the basis of comparison. A logical conclusion is that the readily obtained and commonly utilized parameter YE is essentially as informative as the more destructive and difficult to obtain and therefore rarely measured parameter C/S ratio.