By Lauren Fessler Mathews, Ana Redondo and Ozgur Batuman
Trunk injections of oxytetracycline (OTC) have been proven to give huanglongbing (HLB)-affected citrus in Florida a boost in productivity and quality. However, previous studies have shown that even just six hours of exposure to heat and humidity in the grove can lead to OTC injections that no longer adequately suppress CLas populations. This short window of time sparked interest in determining if any additive could be used to extend the duration during which OTC could maintain its full level of bacterial inhibition. One suggested additive was ascorbic acid (AA), also known as vitamin C.
As shown in Figure 1, the addition of AA helps to slow the color change and sedimentation of OTC solutions. University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers wanted to determine if this correlated with maintaining the antimicrobial properties of OTC.
The first step was to conduct assays in the lab with a proxy bacterium (Xanthomonas citri subsp. citri, causal agent of citrus canker) to test the level of bacterial inhibition for OTC solutions, both with and without the addition of AA, and later being exposed to high temperatures for a range of time periods. For these tests, researchers prepared solutions of 11,000 parts per million (ppm) OTC from commercially available formulations for trunk injection with acidified water. Half of these solutions were amended with 11,000 ppm AA. All solutions were then placed in an incubator set to 86 degrees Fahrenheit for 0, 1.5, 3, 24 or 48 hours. The level of bacterial inhibition of each of these solutions was then tested.
Initial tests yielded the following results (Figure 2): First, AA alone did not provide any measure of antimicrobial activity. Second, solutions amended with AA did not have greater inhibition than those without AA. Antimicrobial properties decreased with increased exposure time, regardless of whether AA was present.
These preliminary results suggest that there may be no benefit to amending OTC solutions with AA. But field trials in which OTC solutions, both with and without AA, are exposed to environmental conditions and then injected are ongoing. UF/IFAS researchers hope to have the yield data from this trial soon, which should help to solidify recommendations.
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, Ana Redondo is a research coordinator, and Ozgur Batuman (obatuman@ufl.edu) is an associate professor — all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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