Prohexadione-Calcium Controls Vegetative Shoot Growth in Apple

Abstract Prohexadione-calcium (Phd-Ca) (BAS 125W or 9054 W) applied at petal fall (PF) or within 10 days of PF to apple trees as a single spray or as multiple low-rate sprays reduced the current season's shoot growth. Sprays applied 2 to 3 weeks after PF were less effective. Timing of the initial spray was more important than rate in achieving early growth suppression, but rate was most important for maximum season-long growth control. Reduced shoot growth enhanced spray coverage and reduced dormant pruning time by as much as 23% over control trees. An effective cumulative dose (ECD) of 250 mg·L−1 (based on dilute or tree-row-volume equivalent) active ingredient Phd-Ca applied as a single spray or in several low-rate sprays generally produced season-long control of vigorous shoot growth. However, under some highvigor growing conditions a higher ECD (500 mg·L−1 or greater), applied in multiple low-rate sprays, was required to achieve an acceptable level of growth suppression. When shoots resumed growth in mid-season, a single spray (28 July) at 125 mg·L−1 provided additional suppression of shoot growth. During this 5-year study, there were no adverse effects on fruit quality and little or no effect on fruit size. There was no appreciable carryover effect on the next season's shoot growth or fruit size, and there was no additive effect from successive annual applications.