Large, protruding navel-end openings of navel oranges (Citrus sinensis (L.) Osbeck) are a major cull factor in the packhouse and make the control of some insects very difficult. Previous work on navel orange trees suggested that the synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), applied at full bloom at 20 mg·L-1 reduced the diameter of the navel-end opening. In a preliminary study to verify these results under South African conditions, similar results were obtained when 2,4-D was applied at 25 mg·L-1 at petal drop on ‘Palmer’, ‘Robyn’ and ‘Lane Late’ navel orange trees. To determine the optimal timing and concentration of application without any detrimental effects on fruit quality, 2,4-D was applied at 15, 25 or 35 mg·L-1 at either full bloom or petal drop to ‘Washington’ navel orange trees in the following season.

2,4-D applied at full bloom significantly reduced the average navel-end size and increased the percentage of closed navel-ends, irrespective of the concentration applied. In contrast, all petal drop applications had no effect on the average navel-end size or the percentage of closed navel-ends. None of the treatments (full bloom or petal drop) affected fruit diameter, internal fruit quality, fruit colour, colour at the navel-end, pedicel diameter or creasing incidence significantly. A weak relationship between fruit size and navel-end size was observed. The data suggest that the application time of 2,4-D to reduce the size of the navel-end opening of navel oranges, seems to be more important for optimal efficacy than the concentration applied.

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J.S. Verreynne, G. Mupambi, Department of Horticultural Science
University of Stellenbosch