Summary of the NC-140 Apple Physiology Trial: The Relationship Between 'Golden Delicious' Fruit Weight and Crop Density at 12 locations as Influenced by Three Dwarfing Rootstocks

An experiment designed specifically to evaluate the influence of rootstock on average fruit weight of 'Golden Delicious' apple [Malus x sylvestris (L.) var. domestic(Borkh.) Mansf] was established at 12 locations in North America. Trees on three dwarfing rootstocks (G.16, M.26 EMLA and M.9 NAKBT337) were allowed to fruit for the first time in the third growing season. Over a 5-year period whenever initial fruit set was adequate, trees were hand-thinned to one of five crop densities (CD) ranging from 2 to 14 fruit per cm(2) of trunk cross-sectional area (TCA). Yield and number of fruit harvested per tree were used to calculate average fruit weight. Analysis of covariance was used to evaluate the effects of rootstock on fruit weight when CD was added to the linear model as a covariate. The interaction for site, rootstock, year, and crop density was significant, so data were analyzed by site. At 8 of the 12 sites, CD interacted with year and/or rootstock, so an unequal slopes model was fitted to those data. Where the CD x rootstock interaction was significant, least squares means for fruit weight were estimated at three levels of CD for each rootstock within each year and slopes for each rootstock were compared. In general, the slopes were most negative for trees on M.26 EM LA and least negative for trees on M.9 NAKBT337, indicating that fruit weight was most affected by CD for trees on M.26 EMLA. Fruit weight, regardless of CD, was generally lowest for trees on G.16 and highest for trees on M.9 NAKBT337. These results substantiate previous reports that rootstock can influence fruit weight, independent of CD, and that trees on M.9 NAKBT337 produce relatively large fruit.