Optimal timing of application of 2,4-D on ‘Navel’ sweet orange [Citrus sinensis (Osbeck)] reduces the size of the navel-end

Abstract

SUMMARY‘Navel’ sweet orange [Citrus sinensis (Osbeck)] fruit with large, open, or protruding navel-ends are predisposed to splitting and navel-end rot, and provide an entry point that hampers the control of insect pests. A reliable method to reduce the size of navel-ends, without compromising tree health or fruit quality, would provide multiple benefits. The objective of this study, was to determine the optimal timing and concentration of foliar spray treatments with the synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), to reduce the size of the navel-end on ‘Newhall Navel’ and ‘Washington Navel’ sweet orange fruit. The effects of 2,4-D treatment on fruit yield, tree health, and fruit quality, were also assessed. Treatments were applied to ‘Newhall Navel’ trees at a rate of 25 mg l−1 2,4-D at 2-weekly intervals, starting at full bloom (FB), and continuing until 4 weeks after 100% petal drop (PD). On ‘Washington Navel’ trees, 2,4-D was applied at 15 mg l−1, 25 mg l−1, or 35 mg l−1 at FB, or at 100% PD. The timing of the application of 2,4-D to increase the percentage of closed navel-ends, and to reduce their size, was more important than the concentration. Application of 2,4-D at FB significantly increased the percentage of fruit with closed navel-ends, and reduced their size in both ‘Newhall Navel’ and ‘Washington Navel’ fruit. Later treatments were ineffective. Application of 2,4-D at FB had no effect on fruit diameter, fruit yield, fruit number, or fruit quality parameters, except for a decrease in the fruit diameter:length ratio. In addition, successful 2,4-D spray treatment led to a slight decrease in fruit juice content. The application of a foliar spray of 2,4-D at FB, and at concentrations as low as 15 mg l−1, can be used to increase the percentage of closed navel-ends, reduce the number of culled fruit, and thereby improve the pack-out percentage for export quality ‘Navel’ sweet oranges.