Optimizing Vegetative Growth and Yield in Apple Trees Through Split Applications of Prohexadione–Calcium, Ethephon, and NAA

Managing vegetative vigor is a critical challenge for apple production in subtropical regions, where high water availability often promotes excessive canopy growth. This study evaluated the effects of split applications of prohexadione–calcium (ProCa) combined with naphthaleneacetic acid (NAA) and ethephon on the vegetative growth and yield performance of ‘Maxi Gala’ and ‘Fuji Suprema’ apples during the 2022/23 and 2023/24 growing seasons. The experimental design consisted of six plant growth regulator (PGR) protocols: a commercial standard (Control) with two applications, and five protocols based on six split applications initiated when fruit diameter reached ~8 mm, with 10-day intervals. The treatments included ProCa; ProCa + NAA; ProCa + ethephon; ProCa + NAA + ethephon; and ethephon + NAA. The ProCa + NAA protocol demonstrated the highest efficiency in vigor control, reducing shoot growth by up to 38% in ‘Maxi Gala’ and 65% in ‘Fuji Suprema’ relative to Control. Furthermore, this treatment enhanced fruit skin coloration, increased the proportion of Category 1 fruit, and improved return bloom and overall yield. These findings suggest that split applications of ProCa associated with NAA provide a robust strategy to optimize apple orchard productivity under the edaphoclimatic conditions of southern Brazil.