Benefits of reducing the size of the navel-end opening in ‘Navel’ sweet oranges (Citrus sinensis)

Abstract

‘Navel’ sweet orange [Citrus sinensis (L.) Osbeck] fruit are characterized by the presence of a secondary fruitlet (navel) located inside the stylar-end of the primary fruit. Fruit with large navel-end openings have increased susceptibility to develop navel-end fruit splitting and navel-end rot. Furthermore, large navel-end openings can provide refuge for insect pests including mealybugs. Foliar application of the plant growth regulator and synthetic auxin, 2,4-dichlorophenoxy acetic acid (2,4-D) during flowering time reduces the size of the navel, as well as that of the navel-end opening. The objective of this study was to determine if the closing of the navel-end opening by foliar application of 2,4-D could reduce the susceptibility of ‘Navel’ sweet orange fruit to mealybug [Planococcus citri (Risso)] infestation, Alternaria [Alternaria alternata (Fr.) Keissl] infection and the subsequent development of Alternaria black core rot (ABCR). Foliar application of 2,4-D at full bloom (FB) increased the percentage of fruit with fully closed navel-ends significantly. These results concur with previous research and were consistent across different production areas, cultivars and seasons, irrespective of the concentration and formulation of 2,4-D used. Apart from two anomalies, all 2,4-D treatments at FB resulted in a lower percentage of mealybug infested fruit at time of harvest and significantly reduced the percentage of fruit infected with ABCR. A combination treatment of 10 mg L−1 2,4-D and 0.20 mL L−1 tebuconazole applied at FB was most successful in reducing ABCR infection and therefore provides a novel control method for black core rot in ‘Navel’ sweet orange fruit.