Growth regulation of apple trees by Prohexadione-Ca application in a nordic climate

Prohexadione-Ca, an acylcylohexadione, is a plant growth retardant jointly developed by BASF, Germany and Kumiai Chemical Industry, Japan. In 2012, a trial on 11-year-old 'Aroma'/'M 9' apple trees growing on the experimental farm at NIBIO Ullensvang, western Norway was initiated. Trees were treated with prohexadione-Ca (trade name Regalis^®, 10% prohexadione-Ca as active ingredient) 10 days after full bloom (June 6) at concentrations of 125 or 250 g ha^-1; other trees were also sprayed 10 days after full bloom and again one month later (July 5) at the same concentrations and compared against an untreated control. Treatments were applied to individual whole trees in a randomized complete block design with four replications. Prohexadione-Ca treatments significantly reduced shoot growth of treated apple trees. Two treatments with 125 g or one treatment of 250 g reduced growth by ~80%. Prohexadione-Ca had little effect on yield and fruit quality. Shoot growth was also retarded in the year following application and this is attributed to a carry over effect of the prohexadione-Ca application(s).