The influence of exogenously applied 2,4-dichlorophenoxyacetic acid on fruit drop and quality of navel oranges (Citrus sinensis L.)

Orchard experiments were carried out in Botswana with the objective of evaluating the effect of 2,4-dichlorophenoxyacetic acid (2,4-D) on reducing premature fruit drop. Different concentration levels of the 2,4-D (8, 16 and 20 mg/L were applied exogenously to mature fruit trees of sweet orange (Citrus sinensis L.) in the 2004/2005 season. In the 2005/2006 season the 2,4-D treatments ranged from 20 to 40 mg/L concentration. There appeared a general increase in fruit drop for the month of October in all treatments but a decrease in fruit drop was observed in the fruit trees with 16 and 20 mg/L 2,4-D concentration, that is, from November through February; with the latter showing the least number of fruits that dropped throughout the execution of the experiments. The application of 20 mg/L 2,4-D significantly reduced fruit drop by more than 50% but higher concentration levels of the plant growth regulator significantly increased fruit drop. It was also evident that, small sized fruits were more susceptible to fruit drop than larger fruits. These findings suggested that, 2,4-D can be an effective tool to control fruit drop by enhancing retention, as well as improving the quality of navel oranges under dry climatic conditions.