Growth Regulators, Fruit Drop, Yield, and Quality of Navel Orange in Florida

Abstract

Abstract Fruit drop of navel orange [Citrus sinensis (L.) Osbeck], which occurs after June drop, was studied from 1980 through 1982 in 2 groves in north-central Florida. Summer drop, which occurs from June to August, and summer-fall drop, which occurs from late August through October, were the major periods of fruit drop. Fruit drop was significant in 1981, ranging from 63 to 200 fruit per tree. A dilute spray of 10 or 20 ppm 2,4-dichlorophenoxyacetic acid (2,4-D), alone or in combination with 20 ppm gibberellic acid (GA) and applied 5 to 9 weeks after midbloom, reduced summer fruit drop in all seasons. The same materials applied 13 weeks after midbloom were ineffective. An application of 2,4-D at 5 weeks after midbloom increased yields in 1981, but not in 1982, because summer drop was minor in that year. Midbloom application of GA + 2,4-D increased splitting and summer-fall drop in 1981, but generally these materials had no effect. Sprays of 2,4-D, GA, or the combination did not adversely affect internal fruit quality, although GA or GA + 2,4-D increased rind firmness when applied 13 weeks after midbloom.