By Lauren Fessler Mathews, Sanju Kunwar, Ana Redondo and Ozgur Batuman
Based on numerous research studies conducted by scientists at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS), it has been proven that injections of oxytetracycline (OTC) can help to alleviate symptoms caused by huanglongbing (HLB) disease. This treatment is now widely practiced and crucial for preventing premature fruit drop and sustaining the citrus industry in Florida.
The first approved formulation of OTC for trunk injection, ReMedium TI®, was released in 2022 by TJ Biotech. The same company also manufactures FLexInject®, an injector designed explicitly for injecting mature citrus trees with OTC. In 2023, Rectify™, another injectable formulation of OTC, was approved. Both formulations have oxytetracycline hydrochloride as their active ingredient. OTC is a bacteriostatic compound that inhibits bacterial growth and replication. Like many antibiotics, OTC is sensitive to degradation under certain conditions, including high temperatures and intense UV radiation, which are inherent to Florida citrus groves. This degradation can be observed in the color change of the solution from bright yellow to dark brown (Figure 1).
In order to establish a threshold for exposure to field conditions under which OTC injections would remain effective at reducing HLB symptoms, a trial was conducted by injecting 10-year-old Valencia trees in the experimental grove at the Southwest Florida Research and Education Center (SWFREC) in Immokalee. FLexInject® injectors, made of a UV-protective material intended to prevent OTC degradation, were filled with 100 milliliters of freshly prepared ReMedium TI® (11,000 parts per million) and brought to the field. Trees were either immediately injected, or the filled injectors were left exposed to field conditions for several hours and then injected into trees.
In July 2023, trees were injected with water, fresh OTC and OTC that had been exposed for six, 24 or 48 hours. HLB management was evaluated monthly based on Candidatus Liberibacter asiaticus (CLas) titer quantification and visual disease ratings.
Four months post-injection in 2023, the freshly prepared OTC was the only treatment with a significant reduction in CLas titer compared to the water-injected control (Figure 1). This means that even as little as six hours of exposure did not provide sufficient HLB control in this grove.
This experiment was repeated in 2024 with the same treatments used in 2023 plus treatments with OTC that had been exposed for shorter intervals, including one and a half hours and three hours. Data from 2024 is currently being analyzed. The hope is that these additional intervals will be able to provide a more definitive threshold for the amount of time that filled injectors can be exposed to field conditions before failing to manage HLB. This threshold is important to establish so that this labor-intensive and costly process results in positive impacts and improved yields.
Commercial recommendations are to mix and use OTC within 48 hours, but it is clear that this does not take into account exposure to field conditions. Based on our current research, it is vital that growers inject oxytetracycline that is as fresh as possible to ensure maximum efficacy.
Acknowledgment: This project has been supported by funds from the Citrus Research and Development Foundation since 2024.
Lauren Fessler Mathews is a graduate research assistant, Sanju Kunwar is a post-doctoral research associate, Ana Redondo is a research coordinator, and Ozgur Batuman (obatuman@ufl.edu) is an associate professor — all at the UF/IFAS SWFREC in Immokalee.
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