Paul J. R. Cronje
Citrus Research International | Department of Horticultural Science, Stellenbosch UniversityOckert P. J. Stander and Karen I. Theron
Department of Horticultural Science, Stellenbosch University
Various citrus cultivars of ‘Navel’ and ‘Valencia’ orange [Citrus sinensis (L.) Osbeck], as well as mandarin and mandarin hybrids (Citrus reticulata Blanco) are prone to a preharvest physiological rind disorder, known as fruit splitting.
Similar disorders occur as fruit cracking and/or splitting in other commercially important horticultural crops, most notably in apple, apricot, cherry, grape, nectarine, prune, and tomato. Fruit splitting in citrus differs from other crops due to the unique morphology of a citrus fruit, consisting of the pulp and rind, which is made up of the spongy white internal layer, the albedo (mesocarp), and the external layer, the flavedo (exocarp).
Pressure applied by the rapidly expanding pulp during fruit growth leads to the formation of microcracks in the flavedo and initiation of fruit split at the stylar or navel end of the citrus fruit rind. This area of the rind is the thinnest and/or structurally weaker than other areas of the fruit rind. The episodic nature of fruit split is a good indication that certain environmental and/or cultural cues initiate the incidence of fruit splitting. Such factors include nutrient imbalances, specifically low Ca and K and high P, warm and humid climatic conditions, irregular water supply, and heavy crop loads.
The unique anatomical features of certain citrus cultivars, such as presence of a secondary fruitlet (navel), weaken the structure of the primary fruit. In addition, the thin rind of the reticulated mandarin and mandarin hybrids increases the susceptibility to fruit splitting. Current horti-cultural practices to reduce the incidence of splitting include a combination of crop load manipulation by thinning and GA3 application/girdling, as well as sufficient Ca, K, and P nutrition and consistent irrigation. There is, however, no single commercial technology to consistently reduce the incidence of this disorder.
Recent research on mandarin species reported significant reduction of fruit splitting through the application of a synthetic auxin, 2,4-D, as a foliar spray. However, results were erratic when applied to orange cultivars and negative effects on fruit quality, such as increased rind coarseness and reduction
in juice content, are some of the major drawbacks of 2,4-D application. This review aims to summarize the unique physiology of citrus fruit splitting, as well as the contributing factors and horticultural practices available to reduce the impact of this disorder.