See the best times to use zinc and potassium applications to fight fruit drop in citrus infected by citrus greening, based on research.

The UF/IFAS Tip of the Week article maintains that fruit drop occurs normally in healthy citrus trees free of citrus greening, taking 10 to 15 percent of the crop; citrus greening can increase the rate of fruit drop to 40 percent of more of the total crop. Since regular applications of zinc sulfate (ZnSO4) and potassium sulfate (K2SO4) is used in healthy trees to “enhance auxin synthesis, improve fruit size, and increase citrus fruit retention,” researchers looked into using zinc and potassium applications to combat fruit drop in citrus greening-infected citrus. After getting positive results, researchers have done testing on the best timing of those applications. See the details from the article below.

Zinc and Potassium Applications to Combat Fruit Drop

Researchers used 50 grams of zinc and 60 grams of potassium per tree by foliar spray, and conducted the study for 2 years on Hamlin sweet oranges. The following results were found in studying zinc and potassium applications to combat fruit drop, according to the article:

  • “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…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.”
  • “Increase in yield was not significant in June- or July-only applications.”
  • “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.”

Researcher are conducting the studies this year as well, as well as conducting studies on Valencias.

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