By Tripti Vashisth, Megan Dewdney and Lauren Diepenbrock
Citrus flowers profusely, but less than 2% of the flowers become harvestable fruit. In other words, 98% of the flowers seen during bloom will end up on the grove floor at some point during fruit development, whether as a flower, fruitlet, young fruit or mature fruit.
Profuse flowering allows trees to produce enough fruit to carry forward seeds. However, excessive flowering and fruit development are highly dependent on tree carbohydrate levels. Often, heavily cropped trees do not produce many flowers the following season because carbohydrates are depleted and there are insufficient reserves in the trees.
Another way trees adjust crop load to avoid carbohydrate depletion is to drop fruitlets in summer, commonly referred to as June drop. Interestingly, in Florida, June drop occurs from late April to May depending on the weather.
Developing fruit require lots of carbohydrates to provide energy for growth and development. These developing fruit compete with one another for carbohydrates. Chemical signals in the form of plant hormones in the developing fruit act as cues to the tree to continue carbohydrate export to the fruit. Weak fruit eventually abscise due to carbohydrate starvation, thereby adjusting crop load on the tree.
Often, when the fruit set is good due to favorable weather conditions in spring and a light crop in the previous year, followed by drought in April–June, a large June drop can be observed like this year. Since June drop is the natural way trees adjust their crop load, interfering with this process is not recommended. Reducing June drop may result in greater preharvest fruit drop later because of depleted carbohydrate reserves in trees and can affect next season’s crop (which is dependent on tree reserves). However, if excessive June drop is a concern, strategies such as frequent and increased irrigation during dry months (April–June) can be beneficial.
SUMMER AND SUMMER-FALL DROP
Other types of fruit drop are summer and summer-fall drop, which are common for navel oranges in Florida but can occur in other cultivars, too. Summer drop occurs during June–July when the fruit are about golf-ball size or slightly larger, whereas summer-fall drop of navels occurs September–October as fruit near maturity. Like June drop, summer drop and summer-fall drop are primarily induced by physiological rather than pathological factors.
In summer drop, the navel (secondary fruit) becomes yellowish, necrotic and dies, eventually leading to separation from the main fruit. Navel abscission is associated with increases in ethylene production (a plant hormone responsible for fruit abscission/fruit drop). Ethylene production at the stylar end (navel end) accelerates peel breakdown, resulting in a yellow-orange halo around the navel. Insects and pathogens can invade the weakened areas of fruit as secondary problems.
Navel summer and summer-fall drop may be reduced with the plant growth regulator (PGR) 2,4-dichlorophenoxyacetic acid (2,4-D), which should be applied six to eight weeks after bloom. 2,4-D application reduces secondary fruit abscission and thereby reduces fruit drop. Some studies reported reduced summer drop when gibberellic acid and 2,4-D were applied in combination. For details on PGR applications and physiological fruit drop, see https://edis.ifas.ufl.edu/hs1310 and https://citrusindustry.net/2020/06/10/fruit-drop-of-citrus-in-summer-months/.
Tripti Vashisth and Lauren Diepenbrock are assistant professors and Megan Dewdney is an associate professor, all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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