The use of plant growth regulators (PGRs) has been a popular method to treat HLB-infected citrus trees. The materials have continued to be applied in conjunction with trunk injection of oxytetracycline (OTC) to improve tree health.
Tripti Vashisth, an associate professor of horticultural sciences with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS), has been studying PGRs for six years and has shared that knowledge with growers.
INJECTING GIBBERELLIC ACID NOT ADVISED
Growers are eager for new alternatives to inject into citrus trees now that they’ve gotten a better handle on the application process. Vashisth says this has led to questions about whether gibberellic acid (GA) can be applied via trunk injection. GA has been the more popular compound among the PGRs to improve citrus tree canopies and fruit retention.
“We know whatever is injected into the tree is being delivered directly to the xylem, so the plant is taking up virtually everything that is being applied,” she says. “This is different from foliar or soil drench applications of GA, where some of that material is leaching away and not making it into the tree.”
The type of application will affect the rate at which the product should be applied for these very reasons. She cautioned that trunk injection of GA runs a significant risk of a high rate of phytotoxicity.
“When growers ask me what rate of GA they should inject into the tree, I tell them I have no clue because we’ve not studied that yet. My answer to them is we know that foliar and soil applications of GA usually work well, so there is no reason to inject it or risk phytotoxicity. And foliar and soil applications are going to be more economical options anyway.”
CHEMIGATING GIBBERELLIC ACID
Another question Vashisth has been fielding from growers has been whether GA can be applied through irrigation systems. This led her to do some research work on the question.
“Growers are interested in this because chemigation can be cheaper than foliar applications through savings on fuel for tractors and sprayers,” she says. “So, we created a trial looking at foliar alone, chemigation alone, foliar plus chemigation and a non-treated control.”
The treatments were applied to 14-year-old Valencia trees on Swingle rootstock. The foliar applications were made during their normal September through December timeframe at 10 ounces per acre. The GA chemigation rate was 30 ounces per acre in 10 gallons of water. The applications were made mostly from September to December.
“The trial revealed that the foliar plus chemigation application of GA provided the best results,” Vashisth says. “There was better canopy, reduced fruit drop and better yield. And there was also less dead fruit stuck on the tree, which we think was coming from the drought.
“We are not exactly sure why the foliar/chemigation is performing best but believe there is a synergistic benefit. And seeing less dead fruit on the tree, we think the chemigation is helping the roots with better water uptake.”
OTHER PGR TAKEAWAYS
Vashisth says other trends have become apparent over the years studying PGRs. One is where fruit drop is not a big problem, applications of 2,4-D don’t appear to provide any benefit.
“Every PGR works on different processes. We want to align PGRs based on tree phenology, so we are applying the right PGR at the right time,” she says. “For example, 2,4-D can reduce pre-harvest fruit drop, so if we apply it in the summer, it is not going to be very helpful.
“Growers need to think about their goals — boosting growth, reducing drop or reducing flowering — then apply the PRGs accordingly. But if a grower can only use one, then GA in the summer is by far the best option, because it does a little bit of everything — boosting growth and reducing drop.”
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