Build Resilience in HLB-Affected Hamlin Trees

Josh McGill HLB Management, Tip of the Week

By Taylor Livingston and Tripti Vashisth

Fighting HLB alongside weather disasters like freezes and hurricanes is leaving growers with little options for sustaining yields. The Hamlin sweet orange variety is known for higher susceptibility to HLB symptoms which cause rapid tree decline, including increased pre-harvest fruit drop and canopy loss.

Post-freeze trees treated with gibberellic acid dropped less fruit.

Production strategies that target fruit drop and canopy health improvement could fill the gap needed to keep production going. Canopy health has been shown to be directly related to HLB disease severity, pre-harvest fruit drop and fruit production. The use of select plant growth regulators (PGRs), such as gibberellic acid (GA) and 2,4-D (a synthetic auxin), has been shown to improve the production of HLB-affected citrus and is a tool that growers can utilize immediately.

These PGRs have also been shown to improve durability of Hamlin in the wake of inclement weather such as hurricanes. Grower-collaborated University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) field trials evaluating the use of GA on HLB-affected Hamlin were initiated in Polk County and Hardee County in August of 2021 and repeated in 2022. GA was applied two to four times from August to November at 45-day intervals.

Two weeks after a freeze hit Hardee County in February of 2022, canopy density was measured in an experimental trial. Trees receiving GA lost an average of 4% of their canopy. Untreated control trees (UTCs) lost an average of 17% of their canopy (see photo).

Yield results from year one of the study showed that GA-treated trees retained an average of 34.4 pounds more fruit than UTCs. In year two, after Hurricane Ian impacted both sites, the GA-treated trees still retained an average of 50 pounds more fruit per tree than the UTCs.

As a hurricane recovery measure, 2,4-D was applied to a subset of GA-treated trees in Ona, Florida, two weeks after Hurricane Ian. The treated trees retained 33.3 pounds more fruit per tree than the UTCs at harvest. Significant improvements in canopy density of the GA-treated trees are also evident at both sites. Results from this trial suggest that use of PGRs on Hamlin can improve canopy health and reduce pre-harvest fruit drop caused by HLB and inclement weather.

Taylor Livingston is a biological scientist, and Tripti Vashisth is an associate professor, both at the UF/IFAS Citrus Research and Education Center in Lake Alfred.

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