Seeking a Partner for OTC

Daniel CooperHLB Management

In September 2023, the USDA hosted a field day at its Picos Research Grove to present the initial screening trials of the Grove First project.

Few would dispute that the approval of oxytetracycline (OTC) for use as a trunk-injection therapy to treat HLB is one of the biggest breakthroughs in the nearly two decades of fighting the disease. Most Florida citrus growers believe the therapy has been a net positive and they are committed to continuing the treatment.

However, there’s a stipulation in the labels of available OTC formulations that has growers concerned. A one-year pause is required after applying the material two years in a row. As most growers finish their second round of treatments, that means next season is the year where a break is required.

This requirement has set off a conversation among growers and industry who are concerned that the break might impede the momentum gained over the past two years of reducing the HLB bacteria in trees. Some have called for a special exemption from the Environmental Protection Agency (EPA) to allow for a third application next year, but that request might meet resistance from the agency. The break is on the label to help avoid resistance to the treatment.


Rick Dantzler, chief operating officer of the Citrus Research and Development Foundation (CRDF), believes an exemption may be possible because of the application method.

“With injections, the bactericide is entering a closed system. There is no contact with non-target species, so resistance development is much less likely — certainly not after just three years,” Dantzler said. “I think we have a good argument and a shot.”


Setting a special exemption for OTC aside, researchers are seeking new compounds which might be approved in time for next year’s treatment window and beyond. With growers mastering the labor and workflow of injecting trees, it opens the door to a multitude of potential new treatments.

One that has been discussed is the use of streptomycin as a companion to OTC. This is despite a recent court ruling vacating streptomycin for use in citrus due to the Endangered Species Act. Some believe that legal hurdle could be overcome because the material would be applied via trunk injection versus a foliar spray that the court case focused on.

“We intend to have discussions with regulators and interested parties about the possibility of injecting streptomycin in combination with OTC because studies have shown this combination kills 99% of the bacteria that causes HLB,” said Dantzler. “The court ruling struck the use of streptomycin as a foliar spray primarily because an Endangered Species Act study had not been performed on pollinators. While injecting streptomycin as opposed to spraying seems qualitatively different from a scientific standpoint, we are still analyzing whether this changes things from a legal standpoint.”

The search for new materials is well underway by scientists at the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) U.S. Horticultural Research Laboratory in Fort Pierce. Studies are being funded by a grant from the USDA’s National Institute of Food and Agriculture (NIFA) and USDA-ARS. Michelle Heck and Randall Niedz of USDA-ARS lead the research team.

In June and July 2023, 88 chemistries were injected into HLB-infected trees at the USDA Picos Research Farm in Fort Pierce. The researchers were looking for big, noticeable responses from the injected materials indicating they might be reducing the HLB titer. The research has been dubbed the Grove First project because it starts in the grove.

This first level of screening turned up 20 treatment chemistries with effects comparable or better than the industry standard (OTC) for visual tree health and fruit yield in 9-year-old HLB-infected Valencia trees.


The 20 treatments then moved into a second-level screening process in commercial groves to measure different factors over the growing season, including tree response, yield, fruit weight, Brix, fruit drop and more.

“We took these top approximately 20 treatments and combinations and are partnering with growers to evaluate these at a larger scale,” Heck and Niedz noted in a written response to questions. “There are 1,600 trees so far in four commercial groves throughout the state. Two additional trials are planned for the spring. The second-level grower-validation phase is funded by CRDF, NIFA and USDA-ARS.”

While the research team awaits full results from the second-level grower-screening process, it is placing an emphasis on materials that could be used quickly in the regulatory process.

The CRDF board of directors requested chemistries with regulatory clearance that could be used by growers next year. This is CRDF’s highest priority since it will be an off year for OTC for many growers, and alternatives are needed.

“There are no alternatives, except for the chemistries Grove First identified in its ongoing first screening trial. To respond to CRDF’s request, we provided 10 chemistries to CRDF and Florida Citrus Mutual. A review of these chemistries by private regulatory consultants and the Florida Department of Agriculture and Consumer Services has been initiated,” Heck and Niedz noted. “The review’s purpose is to: 1) confirm that chemistries do not require additional regulatory clearance and are ready for use by growers and 2) initiate the regulatory path for chemistries to be cleared for use by growers. New chemistries identified by Grove First will be processed similarly, resulting in a continuous pipeline of chemistries to growers.

“Growers will test regulatory-approved chemistries statewide to validate their effects. To capture the effects of these chemistries across different citrus grower regions, citrus types (rootstock and scion) and management practices, our efforts through the Grove First project will track their performance.”


The USDA research team has emphasized the success of the project depends on grower collaborators. There is a simple and well-defined legal process to bring growers on board to help test new materials in their groves. For more information on the Grove-First program and/or how to become a collaborator, email Michelle.Heck@usda.gov or Randall.Niedz@usda.gov.

Share this Post

About the Author

Frank Giles


Sponsored Content