Translocation of Oxytetracycline in Citrus Plants after Root Drench and Stem Delivery

Huanglongbing (HLB) is the most destructive disease to the citrus industry. In North America, HLB is caused by Candidatus Liberibacter asiaticus (CLas) and is transmitted by the Asian citrus psyllid, Diaphorina citri. Recent studies showed that antibiotics such as oxytetracycline and streptomycin were effective against the CLas pathogen in planta. The objectives of this study were to investigate the uptake, translocation, and stability of oxytetracycline in citrus seedlings. Oxytetracycline was delivered via root or stem. The level of oxytetracycline in treated plants was monitored using high-performance liquid chromatography (HPLC) and/or enzyme-linked immunosorbent assay (ELISA). The HPLC and the ELISA methods showed similar results at high concentrations; however, the ELISA was more sensitive than the HPLC method. The highest level of oxytetracycline after root incubation was found in roots, followed by stem-xylem, stem-phloem, and in leaves. On the other hand, the level of oxytetracycline in the xylem and phloem was higher than that found in the root when delivered via stem. Oxytetracycline was still detectable in all tested tissues thirty-five days after treatment, indicating that oxytetracycline was relatively stable in citrus plants and could inhibit CLas growth for a few months in the field