PGRs for HLB-Affected Trees: An Update

Using the plant growth regulators (PGRs) cytokinin in spring, gibberellic acid (GA) in summer and 2,4-D in late fall can be helpful for HLB-affected trees, horticulturist Tripti Vashisth reported.

Her PGR update came during a Feb. 21 OJ Break. Multi-county citrus Extension agent Chris Oswalt hosted the event at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

Vashisth, who works at the center as an associate professor, said the combination of cytokinin in spring and GA in summer increased yield by approximately 90 pounds per tree in a grower trial. Vashisth noted that the grower in that trial has very little fruit drop. She noted that 2,4-D doesn’t seem to be helpful when fruit drop isn’t an issue.

GIBBERELLIC ACID

Repeated applications of 10 ounces per acre of GA plus an adjuvant reduces fruit drop and improves yield and canopy growth, Vashisth said. She added that GA will keep fruit green and said a 2- to 4-month gap should be left between the last application and harvest. When an appropriate gap is given between the last application and harvest, GA doesn’t reduce Brix, she said.

Vashisth recommended GA applications be made on Hamlin oranges from August to November and on Valencia oranges from September to December. GA applications should be avoided from January to May. Vashisth said spring (March to May) GA applications enhance fruit set, and enhanced fruit set is not desirable with current tree conditions.

2,4-D

Repeated applications of 2,4-D counteract the abscission signal, thus enhancing fruit retention, Vashisth reported. She emphasized that proper timing of applications is critical; applying 2,4-D too early or too late won’t help.

BEST OF BOTH WORLDS

Combining GA and 2,4-D gives the “best of both worlds in reducing fruit drop,” Vashisth reported. She said small fruit drops first, and 2,4-D can stop small fruit from dropping “whereas GA keeps fruit growing longer, thus reducing the drop.” If severe fruit drop is a problem, it’s recommended to use 2,4-D plus GA. “Otherwise, a summer-fall spray of GA is recommended,” Vashisth stated.

Learn more from Vashisth about PGRs for HLB-affected trees.

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