Rootstock effect on growth of apple scions with different growth habits

Apple scions with diverse growth habits were grafted on various size-controlling rootstocks and morphological characteristics were measured after 6 years of growth in the field. Scion had more influence than rootstock on monthly growth rate. Across all rootstocks, scions with spreading growth habits grew rapidly in April and May and achieved most seasonal growth earlier than scions with upright growth habits that grew slowly early in the season. In all growth habits and rootstocks, growth rate slowed appreciably but did not cease by August and growth did not terminate earlier for any one scion–rootstock combination. Across all scions, the dwarfing rootstock, M.9, consistently had the lowest and seedling rootstock had the greatest tree height and trunk diameter. However, no one size-controlling rootstock consistently influenced dates of bud break and full bloom, shoot elongation rate, or duration of growth. Tree growth form was not fundamentally affected by rootstock. Significant interactions indicated that effects of size-controlling rootstock on components of shoot growth will vary with apple tree growth habit. These effects on phenology and development can be significant to growers and may assist breeders in developing new apple cultivars.