Effect of Adjuvants on Oxytetracycline Uptake upon Foliar Application in Citrus

Abstract

Recently in Florida, foliar treatments using products with the antibiotics oxytetracycline and streptomycin have been approved for the treatment of citrus Huanglongbing (HLB), which is caused by the putative bacterial pathogen ‘Candidatus Liberibacter asiaticus’. Herein, we assessed the levels of oxytetracycline and ‘Ca. L. asiaticus’ titers in citrus trees upon foliar applications with and without a variety of commercial penetrant adjuvants and upon trunk injection. The level of oxytetracycline in citrus leaves was measured using an oxytetracycline ELISA kit and ‘Ca. L. asiaticus’ titer was measured using quantitative PCR. Low levels of oxytetracycline were taken up by citrus leaves after foliar sprays of oxytetracycline in water. Addition of various adjuvants to the oxytetracycline solution showed minimal effects on its uptake by citrus leaves. The level of oxytetracycline in leaves from trunk-injected trees was higher than those treated with all foliar applications. The titer of ‘Ca. L. asiaticus’ in the midrib of leaves from trees receiving oxytetracycline by foliar application was not affected after four days and thirty days of application, whereas the titer was significantly reduced in oxytetracycline-injected trees thirty days after treatment. Investigation of citrus leaves using microscopy showed that they are covered by a thick lipidized cuticle. Perforation of citrus leaf cuticle with a laser significantly increased the uptake of oxytetracycline, decreasing the titer of ‘Ca. L. asiaticus’ in citrus leaves upon foliar application. Taken together, our findings indicate that trunk injection is more efficient than foliar spray even after the use of adjuvants. Our conclusion could help in setting useful recommendations for the application of oxytetracycline in citrus to improve tree health, minimize the amount of applied antibiotic, reduce environmental exposure, and limit off-target effects.

Development of Europium-Sensitized Fluorescence-Based Method for Sensitive Detection of Oxytetracycline in Citrus Tissues

“…The injection solution was prepared by dissolving 100.0 g (equivalent to 17.3 g of pure OTC) of Fireline 17 WP (Agro Source, Tequesta, FL, USA) in 200 mL of water. Each tree was injected with a 20-mL aliquot of the Fireline solution (equivalent to 1.7 g OTC) using a 20-mL Chemjet manual injector (Healthy Tree PHC, Inc., Norfolk, VA, USA) as described previously [ 7 ]. The injector was left overnight until all the solution was taken by the tree.…”

“…A trace amount of OTC (202 µg/kg) was detected in fruits nine months after treatment [ 6 ]. Recently, we showed that trunk injection of OTC was more efficient than foliar application [ 7 ]. The C Las titer was significantly reduced in oxytetracycline-injected trees thirty days after treatment, whereas it was not affected by foliar application.…”

“…The C Las titer was significantly reduced in oxytetracycline-injected trees thirty days after treatment, whereas it was not affected by foliar application. The addition of various adjuvants to the OTC solution did not enhance its uptake by citrus leaves [ 7 ].…”

Citrate Mediated Europium-Based Detection of Oxytetracycline in Citrus Tissues

“…The presence of OTC in the phloem beyond the girdling indicated that OTC could be translocated from the xylem to the phloem during acropetal movement [ 8 ]. Recently, we studied the uptake of OTC by citrus trees in the field after trunk and foliar application [ 9 ]. Our results showed that foliar spray of OTC was less effective than trunk injection [ 9 ].…”

“…Recently, we studied the uptake of OTC by citrus trees in the field after trunk and foliar application [ 9 ]. Our results showed that foliar spray of OTC was less effective than trunk injection [ 9 ]. Higher levels of OTC (~5 µg g −1 FWT) were found in trunk-injected trees compared with those treated using the foliar application (~0.1 µg g −1 FWT) [ 9 ].…”