Optimize Zinc and Potassium Applications to Prevent Fruit Drop

By Fernando Alferez, Daniel Boakye, Murillo De Sousa and Pablo Orozco

After HLB was first detected in Florida, increase in disease-associated preharvest fruit drop in affected trees has significantly reduced grower returns. Fruit drop is normal in healthy citrus, accounting for 10% to 15% of the total crop. However, under endemic HLB conditions and depending on the variety, crop loss due to early fruit drop may be as severe as 40% or more of total fruit load. Despite many efforts made in the past decade to understand and minimize the problem, a solution remains elusive. The disease alters hormonal balance in the tree, affecting auxin homeostasis, ultimately reducing fruit retention and causing drop.

In trees free from HLB, regular applications of zinc sulfate (ZnSO₄) and potassium sulfate (K₂SO₄) are a common practice that may enhance auxin synthesis, improve fruit size and increase citrus fruit retention. Hence, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers assessed the effect of both zinc and potassium on fruit retention and yield of HLB-affected trees on Hamlin sweet oranges for two years. The rates used were 50 grams of Zn and 60 grams of K per tree by foliar spray.

Cumulative applications of both compounds in June, July and September increased yield by 30%. However, fruit drop greatly varied with time of application. To determine the best timing of application, with funding from the Citrus Research and Development Foundation, researchers started trials last year. Compounds were applied just once at selected dates: after fruit set and physiological drop (early June), at fruit enlargement phase (late July) or at fruit color break (around early or mid-September, dates vary with year).

An increase of 50% in yield was found when both compounds were applied in early September. Hurricane Ian in September 2022 prevented a later application in early October. This probably would have had a better effect on yield because at that time fruit drop is usually very high. Researchers are planning to perform that treatment this year.  

Increase in yield was not significant in June- or July-only applications. Taken together, this means that applications of zinc and potassium are most effective when the fruit starts to become ready to mature and to drop. This is usually indicated by peel color change. This may allow growers to optimize applications and to reduce materials and labor costs. Researchers are following the same approach for Valencia trees, but in this case, applications will be done until December.

Fernando Alferez is an assistant professor, Daniel Boakye was a graduate PhD student, and Murillo De Sousa and Pablo Orozco are graduate MS students, all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.

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