Back in the early days of the HLB fight, it was observed that what is going on underground in citrus tree root systems is just as important as what is happening aboveground. In 2013, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) and U.S. Department of Agriculture-Agricultural Research Service researchers published that 27% to 40% root loss due to HLB occurs before symptoms show up in the tree aboveground.
There are some hopeful signs that stability or improvement in root mass might be occurring with applications of oxytetracycline (OTC). Aboveground, growers have reported mixed results with the new trunk-injection therapy. While most growers agree it has been a net positive, yield and quality improvements have not been as dramatic as hoped for. But what about the roots?
HUGE DATASET
To answer the question about roots, there is a huge dataset available to look for trends. In 1992, Syngenta began a Citrus Soil Assay service to help growers determine if they had treatable levels of phytophthora in their groves. It is one of the largest datasets in citrus and the largest phytophthora root assessment for any crop.
While the service was originally designed to help preserve the lifespan of Ridomil fungicide, it has evolved over the years to provide additional information to growers and researchers who track the data. It also became a way to benchmark root health impacts caused by HLB. The data has demonstrated that HLB and phytophthora interact and cause even more damage to citrus roots.
“The program really has helped us understand phytophthora, but over the last 10 to 15 years, it has become even more important because we’ve been able to track that relationship between it and HLB and their impact on root loss,” says Kendra McCorkle, Syngenta agronomic service representative. “Now, with OTC injections, we believe it is even more important that we help growers understand what’s happening below the ground with the roots.”
As of May 1, more than 500 soil samples had been pulled this year. The five-year average of samples collected is 3,188. It is a big job, and for years Syngenta has utilized a summer internship program to help pull the bulk of these samples. Syngenta representatives also pull samples throughout the year.
1:1 RATIO
Jim Graham, retired UF/IFAS professor of soil microbiology, helped develop the Syngenta soil assay and thresholds for treating phytophthora. He’s tracked the data over the years and continues to consult with Syngenta and the university. Graham also was the lead researcher that demonstrated the amount of root loss happening before HLB symptoms showed up in trees aboveground.
“After we discovered HLB causes root loss, it became apparent that growers really needed a measurement of their root mass, because that affects all things related to nutrient and water uptake in trees,” Graham says. “There is a 1:1 ratio between root loss and fruit loss, meaning if you lose 50% of your roots, you are going to lose 50% of your fruit.”
Research has shown that ratio holds true for other root ailments including phytophthora and nematodes.
“This was all greatly reinforced by the root loss we observed from the more recent hurricanes. There was a massive slippery slope of fruit loss starting in 2018 after Hurricane Irma the previous year,” Graham says. “This made us even more confident in the relationship between root loss and yield. So, the root loss resulting from Irma was not recovered the following year or even five years later. Then we had Hurricane Ian in 2022.”
SAVING ROOTS
With what is known about HLB, hurricanes and root loss, Graham says the ability to inject OTC the year after Hurricane Ian was a life saver.
“I am pretty certain of this,” he says. “Had we not had OTC treatments, it could have put these trees in the economic gutter after Ian. Why am I so sure of this? After Irma, we suffered root loss with no real recovery. These trees could not have absorbed another level of root loss like that after Ian.”
But after OTC treatments began in 2023, the soil assay data showed things were different following Ian. Graham says there’s a high level of confidence that growers who are utilizing the Syngenta service also are using trunk-injection therapy. This allows for observations on how the treatments affect roots.
“Where we measured the massive root loss after Hurricane Irma the following year, we see that root mass (statewide) after Ian is essentially the same,” Graham says. “That’s good news. So, why didn’t we see overall increases in root mass after the first treatment of OTC? Because some of the groves that took a big hit from Ian won’t recover or at least not in the first year. The fact that statewide root mass is holding steady is a great sign.”
In areas that did not take the brunt of Hurricane Ian’s wrath, the data is even more promising. Root assay sample data pulled this year at two groves south of the hurricane’s path shows a 25% and 30% increase in root mass.
“When you see that kind of massive increase in root density in one season, it is a huge turnaround for tree health,” Graham says. “This is unprecedented, and I believe turning these trees in the right direction for the first time since HLB began impacting these groves.”
McCorkle sends growers and industry stakeholders monthly reports that provide results on soil samples. The report breaks down data into four production regions in the state — East Coast, Ridge, South Ridge and Flatwoods/Southwest Florida. The analysis shows that root mass is up year-over-year everywhere except in the Ridge and South Ridge, which took hard hits from Ian.
With proper application, OTC is taken up well by the roots provided the root system is not too far gone. When OTC hits the root system, it controls the HLB bacteria better than in shoots and leaves. This suggests that OTC should help roots survive and support the tree above.
“Research has confirmed that root turnover in a tree before HLB was anywhere from six to twelve months. In the case of HLB, it is every six to 12 weeks,” Graham says. “Even HLB-infected trees can regenerate roots; they just can’t sustain them to survive. And roots are very expensive for the tree to replace, so that is taking away from everything aboveground. It is catastrophic to the tree to lose roots.”
The latest data coming out of the Syngenta soil sampling program suggests OTC is killing the HLB bacteria and improving root mass and density. Graham says this is a great sign and should provide encouragement for growers who plan to continue trunk-injection treatments.
It is worth noting that more roots equal more opportunity for phytophthora, so monitoring and treating for the disease will continue to be important in the coming years.
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