Gibberellic Acid Not Recommended in Spring for HLB Trees

Jim Rogers HLB Management, Production, Tip of the Week

By Tripti Vashisth

Recent research has shown benefits of gibberellic acid (GA) application (monthly from September to January) in improving yield and canopy density of HLB-affected trees.

These findings have caught the attention of Florida citrus growers. Many have indicated that they are applying GA in their groves as per the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) recommendation. Many growers shared their encouraging observations of good flushing on trees following fall GA applications. As a result, many growers have contemplated use of GA in spring to promote vegetative growth on HLB-affected trees. However, use of GA beyond the recommended time of year needs careful scrutiny.

Note the new growth in gibberellic acid (GA)-treated trees and higher flowers and buds in untreated trees

GA is known to have multiple effects on citrus. Pre-HLB, GA research showed that postbloom GA application improved fruit set and aided fruitlet retention. Thus, a spring application can potentially increase fruit set and enhance fruit growth instead of enhancing vegetative growth. Due to limited carbohydrate translocation and availability, HLB-affected trees are often trying to adjust source (leaves) to sink (fruit, in this case). In simple words, trees are often trying to budget their resources. Therefore, a GA application in spring is likely to enhance fruit set and not promote as much desired vegetative growth.

On the flip side, it can be argued that good fruit set is the goal in citrus production. However, in the past few years, we have learned that due to HLB-caused stress, trees set more fruit than they can sustain. Thus, fruit drop can be seen all throughout the year but is more noticeable once the fruit attains maturity. Therefore, enhancing fruit set in absence of good canopy can result in unwanted fruit drop — in other words, a waste of tree resources.

Based on the current understanding and information, it is recommended not to use GA in spring until the trees have stabilized the crop load via June drop.

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

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