Hamlin has long been Florida’s leading early season orange variety, but like many orange varieties in the Sunshine State, Hamlin is severely affected by HLB, with fruit drop being a major issue. Research using gibberellic acid to combat the effects of HLB has shown success with Valencia sweet oranges; the research results of September through January applications—which occur during Valencias’ floral induction and fruit growth periods—include a 30 percent average improvement in yield, a reduction in fruit drop, and an improved defense response by the citrus trees. However, Hamlins have a shorter fruit growth pattern than Valencias, and gibberellic acid applications on Hamlins have to stop by October or November so that fruit color and Brix development can occur. Despite these obstacles, recent field trial research into using gibberellic acid on Hamlins has also been very promising.
Using Gibberellic Acid on Hamlins
Researchers believe that one of the many effects of HLB on citrus trees is an increase in ethylene production; it’s a plant hormone responsible for natural fruit drop (abscission) in citrus. Gibberellic acid counteracts ethylene, meaning that gibberellic acid applications should reduce fruit drop and other HLB symptoms.
Researchers used two applications of gibberellic acid on a 10-acre Hamlin block, with another adjacent 10-acre block left untreated as a control. Applications were made on October 7th and November 5th. Ten days later, researchers observed that the trees in the block treated with gibberellic acid had a higher fruit detachment force (FDF) and lower fruit drop (abscission) than the control. Further inspection showed that pulled fruit were breaking at the junction of the calyx and stem in the gibberellic acid-treated block; fruit from the control block were coming off smoothly. Researchers believe this shows fruit was less likely to drop due to the gibberellic acid treatments.
Furthermore, researchers conducted a secondary test to simulate tree stress in mid-December as part of a field tour. Select trees from the same blocks as the first trial outlined above—both treated trees and trees from the control block—were sprayed with ethylene. After the ethylene sprays, the untreated trees from the control group dropped 30 percent more fruit than the trees that had received two applications of gibberellic acid.
While the effects of gibberellic acid treatment are promising, researchers maintained that they still need more time to study the effects of gibberellic acid on Hamlins.