Paclobutrazol sprays reduce vegetative growth and increase fruit production in young McIntosh apple trees

Abstract

Three years of study were conducted to examine the effect of paclobutrazol sprays applied in one growing season on young, non-flowering McIntosh/MM106 apple trees. PBZ treatments were able to reduce or stop terminal growth. By the end of the first growing season, terminal growth of the PBZ treated trees was from 35 to 80% of the control. PBZ treatments at rates of 200 and 50 mg L⁻¹ produced the least terminal growth in year of application and this effect was carried over into the following year. PBZ treatment rates of 50 and 12.5 mg L⁻¹ stimulated terminal growth in the second year to 125% of control. Highest rates of PBZ (200 and 50 mg L⁻¹) also increased flower cluster and fruit numbers in the second and third years following application. Tree trunk diameter growth in PBZ treatments 200 and 50 mg L⁻¹ was reduced to 72% compared to the control in the year of treatment. This growth at a reduced rate continued during the next 2 yr resulting in some treatments producing trunk growth as low as 65% of the control. The surfactant Tween 20 had little long-term effect upon the effectiveness of the PBZ treatments and was not effective in reducing the rate of PBZ required. Accurately timed foliar sprays of PBZ were found to be an effective management tool to reduce tree growth and induce fruit production in young, non-bearing McIntosh apple trees on vigorous rootstock. Key words: Apple, Malas × domestica, paclobutrazol