Prohexadione calcium for suppression of vegetative growth in eastern apples

Prohexadione calcium is a plant growth regulator being developed jointly in the United States by the EASE Corporation and Kumiai Chemical Industry Co. Ltd. Prohexadione calcium inhibits the biosynthesis of gibberellin resulting in reduced internode length and vegetative growth. Foliar absorption is the only significant means of plant uptake. Uptake is generally complete within eight hours following application. Results indicate that acropetal movement within the plant is important while basipetal movement is limited. Applications of prohexadione calcium at rates of 63 ppm al to 500 ppm al on vigorous trees have provided excellent control of vegetative growth. The optimum rate of Prohexadione calcium required to provide effective control of vegetative growth depends on tree vigor, age, training system, crop load, and other factors that influence vegetative growth. Rates of 125 ppm al to 250 ppm al have typically been sufficient to provide effective vegetative control of vigorous trees. Earlier applications (petal fall) have provided more effective control of vegetative growth than later applications (second cover). Prohexadione calcium effectively reduces the level of gibberellin in the plant for tt;ree to four weeks following application. Prohexadione calcium does not persist in the plant or affect vegetative growth the following season. However, where the proper balance between vegetative growth and crop is achieved, one application may provide season-long control of vegetative growth. Due to its short-term effect and lack of persistence, Prohexadione calcium is a flexible tool for vegetative growth management that can be applied at a variety of timings and used to develop user-specific growth management strategies.