How Long Should Apple Rootstock Trials be Conducted?

Apple rootstocks are usually evaluated for 10 years, but there is an economic incentive to shorten the evaluation period. Data from the 2003 NC-140 Dwarf Apple Rootstock trial were used to determine the number of years required to separate rootstocks based on trunk cross-sectional area (TCA).  ‘Golden Delicious’ trees budded on 11 dwarfing rootstocks were planted at 12 locations with 2 trees on each rootstock randomized within four blocks in a generalized randomized complete-block design. Annual TCA data from each location were analyzed with a repeated measures ANOVA with SAS’s  Proc Glimmix and the slicediff option  was used to compare rootstock lsmeans within each year. Data for only 4 locations have been analyzed, but the relationship between TCA and year was linear in British Columbia and quadratic in Pennsylvania, Utah, and Wisconsin. Rootstocks were significantly different after 2 years in Utah, but 3 years were needed to separate rootstocks in the other locations. Depending on the location, the number of years to detect a difference between B.9 vs. M.26 was 3 or 4 years, 4 to 6 years for B.9 vs. T.337, 5 to 8 years for T.337 vs. M.26, 7 to 8 years for M.26 vs. Pajam2, and 6 to >10 years for  T.337 vs. Pajam2. Although other characteristics may take longer to sort out, it appears that rootstock trial require at least 8 years to separate M.26 and M.9 Pajam2 into different vigor classes.