Rootstock effects on growth and fruiting of a spur-type and a standard strain of 'Delicious' over eighteen years

'Red Prince', a standard strain and 'Redchief' (Campbell strain), a spur-type strain of 'Delicious' apple were grown on several rootstock/interstock combinations for 18 years. The dwarfs were Malling 9 (M.9), M.26, M.9/Malling Merton 106 (MM.106), and M.9/M M.111. The semi-dwarfs were M.7, MM.106, and MM.111. Five three-tree replications were used. In-row spacing was varied from 1.8 to 5.5 m depending on the scion/interstock/rootstock combination; between-row spacing was 6.1 m throughout the experiment. Tree survival ranged from a high of 100% for five combinations to as low as 13% for both 'Delicious' strains on MM.106. In the dwarf group, trees of both strains on M.26 were the largest, those on M.9/MM.111 were intermediate, and those on M.9 and M.9/MM.106 were the smallest. The greatest numbers of rootsuckers were on trees on M.9/MM.111 and M.7. Crop density tended to be higher with 'Redchief' than 'Red Prince' and higher in the dwarf than semi-dwarf group. With both 'Redchief' and 'Red Prince', cumulative per-tree yields were greater on M.26 than on M.9 or M.9/MM.111. Trees of 'Redchief' on the three semi-dwarf rootstocks yielded similarly; 'Red Prince' trees on MM.111 out-yielded trees on M.7. With both strains, trees on M.9 and M.9/MM106 tended to have higher cumulative yield efficiencies than those on M.26 or M.9/MM.111. Cumulative yields (T/ha) for both strains were highest for trees on M.26 compared to all other rootstocks. Cumulative yields for the three semi-dwarf rootstocks differed little with either scion.