Can Zinc and Potassium Curb Fruit Drop?

The Florida citrus industry has been focused on finding solutions to the fruit drop problem that has hit groves hard in recent years. During the 2022 Florida Citrus Show in Fort Pierce, Fernando Alferez, University of Florida Institute of Food and Agricultural Sciences, assistant professor of citrus horticulture, presented findings from a research trial aimed at reducing fruit drop.

The three-year study looked at how foliar applications of zinc sulfate and potassium sulfate influence fruit retention and yield. In the study, various rates and timings of the applications were tested on Hamlin and Valencia oranges. The timing of applications studied were after fruit set (early June), fruit enlargement (July) and color break (September). The study shows application effectiveness is seasonal. For instance, the June application seemed to provide more benefit than the July one.

“We know from research literature and by observing other plants that zinc deficiency produces in plants two main things — oxidative stress and impairment of auxin signaling within the plant,” Alferez said. “We know in the case of several fruit crops, including citrus, that this produces fruit drop. In HLB-affected trees that are experiencing many stress factors, this deficiency can cause even more drop. We made applications to restore the zinc deficiency to improve oxidation and improve the auxin balance. The results show we could improve fruit retention with these foliar zinc applications. As for potassium, we observed improved fruit size and quality, and we know better-sized and quality fruit tends to hold on the tree better.”

While the first year of applications made in 2019 didn’t show much of an improvement when compared to a control, the following two years did show a trend toward improved yields and fruit retention, especially in Hamlin. 

“In the control plots, yields are very typical of HLB-affected trees at around 200 boxes per acre,” Alferez says. “We are seeing better yields in the zinc plus potassium application. In 2020, the application of zinc plus potassium improved yield by 22% and in 2021 by 31% over the control plot.” 

Because of this, the Citrus Research and Development Foundation is funding further research on this trial to better fine-tune timing and rates of zinc and potassium to improve fruit retention and yields.

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