Fruit splitting is fairly common among citrus plants, particularity mandarin oranges. Typically, only a small percentage of fruit on affected trees will split. The exact cause is not known, but certain environmental and cultural stressors are known to contribute toward this annoying problem. Fortunately, it is not an indication of disease or pest infestation. Prevent fruit split from occurring by closely monitoring orange trees. Provide adequate fertilization and even watering, particularly if the weather is unseasonably hot during the fall season. Immediately remove or discard any split oranges from the tree and the ground. Split fruit breaks down rapidly. Decaying fruit will only attract disease, bacteria and unwanted pests. Split oranges are edible when ripe, but you will likely find that most split fruit will be green and useless.
Contributing Factors - Unseasonably warm temperatures combined with high humidity from September through November are highly associated with fruit splitting. It is believed that this sort of weather, when followed by relative dry periods, triggers splitting of some oranges. Essentially, drought-stressed orange trees start removing water from fruit to survive. If a lot of rain or irrigation follows drought, the previously removed water will return back from the roots to the fruit, but too quickly for appropriate processing. The rind of the dehydrated fruit may be unable to contain the intense water fluctuation - causing it to split.
Watering - Provide consistent irrigation during hot or windy weather to ensure that trees retain moisture in their roots. Certain orange trees, including young plants, dwarf varieties and trees planted in sandy or very porous soil, are less able to retain moisture in their roots. Consequently, these trees are especially susceptible to fruit splitting in the absence of adequate watering. Maintain consistently moist, but not soggy, soil under the orange tree canopy during hot weather.
Fertilization - Monitor fertilizer levels closely while fruit are on the tree. Depending on the cultivar, this can occur virtually any time during the year. While improper fertilization does not cause fruit splitting, low potassium levels can cause fruit peels to become thin and more prone to this phenomena. Check soil levels near orange trees for potassium if fruit splitting is observed and correct deficiencies. For best results, provide several small doses of fertilizer on a monthly basis during orange growing season.
References:
University of California Cooperative Extension Fresno County; Year-Round Gardening Questions Answered: Citrus Fruit Splitting
University of Florida IFAS Extension; Citrus Problems in the Home Landscape; Mongi Zekri, et al.; July 2010
University of California; Naval Orange Split; Pamela M. Giesel; 2001
University of California-Davis; Citrus Rind Splitting; October 2010
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