Effect of Oxytetracycline-Hydrochloride on Phytoplasma Titer and Symptom Progression of the 16SrIV-D Phytoplasma in Cabbage Palms from Florida

Lethal bronzing (LB) is a phytoplasma disease of palms in Florida, U.S.A. Historically, the use of the antibiotic oxytetracycline-hydrochloride (OTC-HCl) was optimized to treat for LY, and currently label rates for OTC-HCl developed for LY are being used to treat palms preventatively against LB. Because of the economic impact of LB, assessing OTC-HCl against LB is essential for developing and optimizing management options for this disease. In this study, Sabal palmetto palms declining from LB were injected with OTC-HCl to assess efficacy on LB. Four groups of palms were selected that represented healthy palms, early symptomatic palms, palms with moderate symptoms, and palms with late-stage symptoms. Within each group, treatment palms injected with OTC-HCl and a control group with no antibiotics were tested weekly by quantitative PCR for 1 year. For asymptomatic palms, treated palms never developed symptoms or tested positive, whereas one of the control palms did. The early symptomatic palms that were treated had similar levels of phytoplasma to the control group but had much slower symptom development. Palms with moderate symptoms had no difference in titer between the treatment and the control group, but treated palms had much slower symptom development. Palms with late- stage symptoms showed no difference in phytoplasma titer or symptom progression between treated and control palms. These results suggest that label rates of OTC-HCl appear useful as a preventative against LB, but once symptoms develop, label rates cannot cure palms. In the meantime, removal of symptomatic palms is recommended.