Citrus growers and researchers know that oxytetracycline (OTC) trunk injection can reduce the amount of the bacterium associated with huanglongbing (HLB) disease and improve fruit production and juice quality. However, new research shows the treatment also causes changes in microbial functions around citrus roots.
Researchers conducted a field study in a commercial Southwest Florida orchard containing 8-year-old Valencia sweet orange trees infected with Candidatus Liberibacter asiaticus (CLas), the bacterium associated with HLB. Ten trees received an OTC trunk injection while 10 untreated trees served as controls. Researchers examined leaves, bark, roots and the soil surrounding the roots at several times following treatment.
The injected trees had lower CLas levels in their leaves and showed improvements in fruit yield and juice quality. The researchers did not find an overall loss of microbial diversity or a detectable increase in antibiotic-resistance genes as a result of the treatment.
However, the effects below ground were more noticeable. OTC reduced the potential for several microbial processes involved in cycling carbon, nitrogen and phosphorus in the roots and surrounding soil. Some microorganisms associated with stress tolerance and responding to foreign chemicals became more abundant.
For citrus growers, the findings add another consideration to the use of OTC trunk injection. The treatment can provide measurable benefits to HLB-affected trees, but its effects extend beyond the pathogen and tree itself. Changes in root and soil microbial functions could have implications for nutrient cycling and long-term soil health.
The researchers emphasize that the changes were targeted rather than a widespread disruption of the citrus microbiome. More research will be needed to determine the long-term significance of these belowground changes, particularly with repeated OTC applications.
An article about the study — “Compartmental and functional responses of the citrus microbiome and resistome to the systemic delivery of oxytetracycline by trunk injection” — was published in Microbiological Research. The authors are Antonio Castellano-Hinojosa and Jesús González-López with the University of Granada in Spain; and Jasmine de Freitas, Deived U. de Carvalho, Brittney D. Monus, Sarah L. Strauss and Ute Albrecht with the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center.
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