Rootstock tolerance to apple replant disease for improved sustainability of apple production

In 2006, a multi-location field experiment of 12 apple rootstocks with and without pre-plant soil fumigation was established at 9 locations in the USA, Canada and Mexico by the NC-140 rootstock research group. Rootstocks were B.9, M.9T337, M.9Pajam 2, M.26, M.7, Geneva® 11 (G.11), G.16, G.30, G.41, G.210, G.935 and CG.4210. 'Royal Gala' was the scion cultivar. Over the 4 years of the project there was a strong interaction of rootstock and location on tree growth and yield. Generally, B.9 rootstock exhibited the weakest growth while M.7 had the most vigorous growth. There were no significant differences in tree size between the two clones of M.9 (T337 and Pajam2) and the two dwarf Geneva stocks (G.11 and CG.4210). G.41 and G.16 were similar in size to M.26 while G.935 was significantly larger than M.26 while G.30 and G.210 were slightly larger than G.935 but smaller than M.7. Trees on G.935 had the greatest cumulative yield followed by G.210, G.30, G.16, G.41, G.4210, G.11, M.9Pajam2, M.9T337, B.9, M.7 and M.26, which had the lowest yield. Cumulative yield efficiency was greatest for B.9 followed by G.935, G.41, CG.4210, G.11, M.9T337, G.16, M.9Pajam2, G.210, G.30, M.26 and M.7, which had the lowest yield efficiency. Rootstock tolerance to replant disease at each site was assessed by comparing the percentage improvement in growth and yield in fumigated plots to un-fumigated plots. G.16, G.41, CG.4210 and B.9 showed consistent tolerance to replant disease across sites while M.26 was the most susceptible. Other stocks, which showed some tolerance to replant disease were G.30, G.935, G.210, G.11 and M.9T337. There were large differences among locations in the effect of fumigation (severity of replant disease). At some locations growth and yield were greatly improved by fumigation while at other locations there was little effect of fumigation.