Time Required for Classifying Rootstock Vigor in Multi-location Rootstock Trials

Data from two multi-location NC-140 dwarf apple rootstock trials were used to determine the number of years required to detect significant differences between rootstocks based on trunk cross-sectional area (TCA) to place them in appropriate vigor classes. The 1994 trial consisted of 18 rootstocks at 19 locations and the 2003 trial consisted of 23 rootstocks at 11 locations. Data for each location were analyzed separately because a repeated measures analysis indicated a significant three-way interaction for location x rootstock x year. As part of the repeated measures analysis for each location, all rootstocks were compared to each other within each year. In general an additional year was required to detect significant differences in the 2004 trial compared to the 1994 trial, possibly due to differences in scion cultivar or the number of replications. At most locations, extremely vigorous rootstocks, such as PiAu 56-83, and low-vigor rootstocks, such as M. 27, were statistically different from the standard rootstocks (M. 26 and M. 9 NAKBT337) four years after planting. Detecting statistical differences between rootstocks within the range represented by M. 9 NAKBT337 and M. 26, required six or seven years at most locations. At most locations rootstocks with similar vigor, such as M. 9 NAKBT337, G. 935 and G. 16 were not significantly different after 10 years. Most cooperators in future multi-location rootstock trials should be able to properly classify rootstock vigor after six or seven years.