Cultivar, planting density, and plant growth regulator effects on growth and fruiting of tissue-cultured apple trees

Tissue-culture (TC)-propagated 'Gala' and Triple Red 'Delicious' apple trees grown at three planting densities were not treated (CON) or treated with plant growth regulators (PGRs) starting the third or fourth season to control tree size and maximize fruiting. 'Gala' and 'Delicious' trees budded on M.7a rootstock (BUD) were also included as controls 'Gala' trees were larger than 'Delicious' after the first three growing seasons but 'Delicious' were larger than 'Gala' at the end of 9 years. BUD trees were larger than CON trees the first few seasons but final trunk cross-sectional area (TCSA) of CON trees averaged 43% greater than BUD trees. Paclobutrazol and uniconazole treatments more readily controlled the growth of 'Gale' than 'Delicious' and uniconazole was more effective than paclobutrazol in controlling tree size. Daminozide + ethephon sprays (D+E-S) did not influence tree size. Tree size of both cultivars was inversely related to planting density and both triazole PGRs were more effective in controlling tree size as planting density increased. The trees had fewer flowers as planting density increased and BUD trees generally had more flowers than CON. Triazole PGRs had little effect on the flowering pattern of 'Gala' trees but tended to stimulate flowering of young 'Delicious' TC trees, although the increases were not sustained. The D+E-S treatment increased flowering of 'Gala' trees the last 3 years of the experiment and consistently increased flowering of 'Delicious' TC trees. Fruit yields were higher for young 'Gala' compared to 'Delicious' trees and the final cumulative yield per tree for 'Gala' was also greater. Yield per tree decreased as tree density increased and was the same for BUD and CON trees. D+E-S increased cumulative per tree yield of 'Delicious' but not of 'Gala'. Cumulative yields per tree for triazole-treated TC trees were the same as, or significantly lower than, CON trees. Increasing tree density did not increase yield/ha. Yield efficiency of 'Gala' trees was increased by three, and of 'Delicious' trees by one, of the triazole treatments, because they reduced TCSA proportionally more than they reduced per tree yield. There was less bienniality with 'Gala' than 'Delicious' and no difference between BUD and CON trees. Bienniality indices were higher for paclobutrazol-treated 'Gala' trees compared with CON 'Gala' but only uniconazole applied as a trunk paint increased the bienniality index of 'Delicious' trees.