Growers have always been innovators, testing new production practices in their groves. When you add a disease like HLB, those experiments get ramped up in the effort to seek solutions. A good example of this was a discussion that began last year among growers that ascorbic acid (vitamin C) might be helpful in extending the effectiveness of the oxytetracycline (OTC) solution that is injected into citrus trees to reduce the HLB bacteria.
Studies have shown that even just six hours of exposure in the grove to heat and humidity can lead to OTC injections that no longer adequately suppress HLB bacteria populations. Growers experimenting with ascorbic acid noted the solution kept its color better than the label recommended solution. This created a good deal of discussion among industry.
These observations were taken to researchers at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) to evaluate ascorbic acid and OTC. Recently, researchers reported their findings in a Tip of the Week article published on CitrusIndustry.net. Following are some of those results:
RESEARCH RESULTS
As shown in Figure 1, the addition of ascorbic acid helps to slow the color change and sedimentation of OTC solutions. 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 ascorbic acid, 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 ascorbic acid. 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, ascorbic acid alone did not provide any measure of antimicrobial activity. Second, solutions that are amended with ascorbic acid did not have greater inhibition than those without ascorbic acid. Antimicrobial properties decreased with increased exposure time, regardless of whether ascorbic acid was present.
These preliminary results suggest that there may be no benefit to amending OTC solutions with ascorbic acid. But field trials in which OTC solutions, both with and without ascorbic acid, 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.
OTC WORKS
Ute Albrecht, UF/IFAS associate professor of plant physiology, has led much of the OTC research in Florida and was involved in the ascorbic acid study. During a recent citrus seminar, she noted it takes a lot of ascorbic acid to have any effect.
“OTC works perfectly fine, so just focus on doing it right by following the label instructions,” Albrecht said. “If you want to do it better, consider making injections on both sides of the tree to better distribute the OTC.”
For growers who want to experiment with different additions to the mix, Albrecht said to try it only on a small number of trees first. However, she warned that some of these things might do more harm than good.
Acknowledgment: The ascorbic acid Tip of the Week was authored by Lauren Fessler Mathews, a graduate research assistant; Ana Redondo, a research coordinator; and Ozgur Batuman, an associate professor — all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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