Submitted to: Compact Fruit Tree
Publication Type: Trade Journal
Publication Acceptance Date: June 6, 2006
Publication Date: N/A
Interpretive Summary: Apogee is a very effective suppressor of shoot growth in apple trees. A potential negative side effect is a significant increase in fruit set and a corresponding decrease in fruit size. This paper reviews the literature on Apogee and its effect on fruit set and fruit size. Potential casual factors leading to increased fruit set are presented and suggested steps to ameliorate the problem are provided. This information is useful to apple growers who desire the use of Apogee to control shoot growth and suppress the shoot blight stage of fire blight, while avoiding problems with increased fruit set.
Technical Abstract: Apogee (prohexadione-calcium) plant growth regulator is a potent inhibitor of gibberellin biosynthesis that is recommended for reducing excessive vegetative growth and suppressing the shoot blight stage of fire blight in apple trees. Optimum response is achieved when an initial spray is applied at 250 ppm at the petal fall stage of development. At this concentration and time of application Apogee may increase fruit set. Increased fruit set can lead to an increased crop load at harvest and a reduction in fruit size. This paper reviews what is known about Apogee and its effect on fruit set and fruit size. While results have varied, most studies and observations indicate that sprays applied above 125 ppm may increase fruit set and reduce fruit size. An aggressive chemical thinning program has been shown to overcome problems with increased fruit set and reduced fruit size. Additional methods to lessen the impact of Apogee on fruit set and size are discussed. Recent studies indicate that under cool growing conditions Apogee may reduce fruit size independent of fruit set and crop load. These preliminary findings suggest further studies are warranted.