Since 2016, federal authorities have allowed Florida citrus growers to spray oxytetracycline solutions on their trees to combat citrus greening disease, but a newly published University of Florida Institute of Food and Agricultural Sciences study suggests that the bactericide would be more effective if it were injected into citrus tree trunks, a practice not currently approved under U.S. law.
Citrus greening disease, also known as huanglongbing or HLB, is caused by a bacterium transmitted by an invasive flying insect, the Asian citrus psyllid, said Michael Rogers, director of the UF/IFAS Citrus Research and Education Center in Lake Alfred. Although greening disease eventually kills citrus trees, growers have found that they can keep affected trees productive longer with supportive grove management.
Responding to industry requests, in 2016 the U.S. Environmental Protection Agency granted an emergency exemption allowing foliar applications of the bactericides oxytetracycline and streptomycin to Florida citrus groves. The move was intended to give growers another option for promoting tree health, and the exemption remains in effect.
“In concept, bactericide sprays support the health of affected trees by reducing populations of the greening bacterium within the tree, or at least by holding the populations in check,” Rogers said. “But one challenge is, the oxytetracycline needs to get into the tree tissues where the bacterial colonies are. Our researchers did not have much success on that front using foliar sprays, and saw no significant effects from the treatments. However, they did get good results when they applied oxytetracycline with trunk injections.”
The study was published online Aug. 1 by the journal Phytopathology. Authors included Nian Wang, a UF/IFAS microbiology and cell science professor, and colleagues Jinyun Li, Zhiqian Pang, Shuo Duan, Donghwan Lee and Vladimir G. Kolbasov, all with the UF/IFAS Citrus Research and Education Center.
In the study, researchers sprayed oxytetracycline solutions on the leaf canopies of greening-affected sweet orange trees, in two locations – a research greenhouse at the UF/IFAS center in Lake Alfred and an outdoor commercial citrus operation in Auburndale, Fla. The spray solutions contained a commercial oxytetracycline product applied at the maximum label-approved rate, plus an additive to help the oxytetracycline adhere to leaf surfaces. Control groups of trees were sprayed with water and used for comparison.
Leaves from trees in both locations were collected before and after treatment, then analyzed, Rogers said. The results indicated that, at two, four, seven, 14 and 21 days after application, oxytetracycline levels in the leaf tissues of sprayed trees were mild or undetectable. Other tests compared leaves collected before and after treatment, and showed little change in bacterial populations within the sampled trees.
“The researchers couldn’t cover every possible spraying scenario in one study, so it’s possible they would have seen different results if some of the variables were changed, such as the citrus variety, location, spray rate or the adjuvant used,” Rogers said.
The waxy coating found on the upper surface of citrus leaves may prevent absorption of oxytetracycline when it’s sprayed on trees, he said.
To assess the potential effectiveness of trunk injections, researchers conducted experiments in the same greenhouse and grove where the sprays were evaluated. In the greenhouse experiments, trees were injected with one of three solutions – one contained only water, the others contained water plus 0.025 grams or 0.05 grams of oxytetracycline.
Results showed that the injected greenhouse trees had significantly reduced bacterial populations just four days after the injection, whereas bacterial populations in the control greenhouse trees increased during the same time period. In the field study, oxytetracycline levels in leaf tissues increased steadily for two weeks after injection, reaching a peak concentration of .30 micrograms of oxytetracycline per gram of leaf tissue.
“In both the greenhouse and the field studies, when trunk injection was used, there was a strong correlation between the amount of oxytetracycline residue in the leaves and the concentrations of the greening bacterium found within the tree tissues,” Rogers said. “We believe that trunk injection is a promising delivery method for bactericides and plan to continue investigating its potential.”