PGRs: Multipurpose Tools for Citrus Improvement

Daniel CooperPGRs, Tip of the Week


By Tripti Vashisth

Canopy health of HLB-affected citrus has been directly related to disease severity, fruit drop and overall fruit production. Some success has been found with the use of certain plant growth regulators (PGRs) such as gibberellic acid (GA) and 2,4-dichlorophenoxyacetic acid (2,4-D) to achieve healthier trees with reduced fruit drop and increased production.


Application of these PGRs to Hamlin in various combinations and weather conditions has given some promising results for building resilience in the HLB-endemic era. Grower-collaborated field trials evaluating the use of GA on HLB-affected Hamlin were initiated in Polk and Hardee counties in August 2021 and repeated in 2022. GA was applied two to four times from August to November at 45-day intervals. Yield results from year one showed that GA-treated trees retained on average 34.4 pounds more fruit than the control trees. In year two, after Hurricane Ian devastated the Hardee County site, the GA trees still retained on average 50 pounds more fruit than the control trees.


As a hurricane-recovery measure, 2,4-D was applied to a subset of GA-treated trees in Hardee County two weeks after Hurricane Ian. The treated trees retained 33.3 pounds more fruit than control trees at harvest. On top of Hurricane Ian, Hardee County also sustained a significant freeze in early February 2022. The control trees lost 17% of their canopy while the GA trees only lost about 4%. This suggests that GA treatments may also help with freeze-damage mitigation. Results show that the use of PGRs such as GA and 2,4-D may help the tree’s ability to mitigate HLB disease symptoms while also protecting against Florida’s unpredictable weather patterns. 

  • GA applications on HLB-affected Hamlin increased yield and reduced fruit drop. 
  • GA applications mitigate negative response to abiotic factors such as hurricanes and hard freezes. 
  • Use of 2,4-D can reduce further fruit drop caused by damage from hurricanes. 

Acknowledgment: Funding for this project was provided by the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Initiative funded by the Florida Legislature and the U.S. Department of Agriculture-National Institute of Food and Agriculture.

Tripti Vashisth is an associate professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.

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