Different formulations of zinc applied by foliar spray increase fruit yield by reducing fruit drop in Hamlin and Valencia oranges, according to horticulturist Fernando Alferez. In some cases, he said, potassium enhances the beneficial effects of zinc.
Alferez is a University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) associate professor at the Southwest Florida Research and Education Center in Immokalee. He presented his findings during the recent Citrus Nutrition Day event at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
HLB is a major cause of increased preharvest fruit drop. Other possible causes include loss of root density and off blooms, which are also related to HLB, Alferez reported.
His hypothesis is that auxin levels must be maintained at high levels to avoid fruit abscission triggering and fruit drop. Alferez thinks zinc may modulate fruit physiology and retention by maintaining auxin levels. He added that potassium increases fruit quality.
Key summary points from his presentation were:
- For Valencia fruit yield and quality, particulate zinc treatments increased yield by 53% compared to trees treated with zinc sulfate. Brix was not significantly affected, but the Brix-to-acid ratio was increased.
- A significant increase in Hamlin fruit yield was observed with Ferti-Zinc and Nu-Zinc as compared to zinc sulfate. Hamlin fruit drop was reduced by about 30% in all treatments until December 2024.
- Overall tree health was improved by foliar application of zinc. Particulate zinc formulations (Ferti-Zinc and Nu-Zinc) improved tree health better than zinc sulfate.
Alferez and his team are now assaying zinc applications in oxytetracycline (OTC)-treated trees. Because such trees are in healthier shape, researchers expect better fruit retention and more fruit yield as a result of OTC and zinc treatments.
See also Key Takeaways From Citrus Nutrition Day and Citrus Nutrition Day Explores Benefits of Silicon.
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