Effect of Various Antibiotics on Development of Lethal Yellowing in Coconut Palm

Fifty coconut palms exhibiting early symptoms of lethal yellowing were selected for treatment with oxytetracycline-HCl, tetracyclineHC1, streptomycin nitrate, potassium penicillin-G, or griseofulyin. Two grams of antibiotic in 0.2 or 1.0 liters of water were pressure injected into the trunk of each tree except for the griseofulvin treatments which were applied as a paste into holes bored in the trunk. In addition, applications of 20 and 40 g oxytetracycline-HCl in 20 liters water were made to the soil around several af fected trees. Remission of symptom development was obtained only with tetracycline antibiotics directly injected into the palms. Of these, oxy tetracycline-HCl produced a more rapid remis sion and was more likely to prevent further symp tom development than was tetracycline-HCl. The ability of the tetracycline antibiotics to effect remission, contrasted with the non-efficaciousness of penicillin and streptomycin, further supports the possibility of a mycoplasmal etiology of lethal yellowing. Remission of symptom development in lethal yellowing affected coconut palm (Cocos nucifera L.) treated with tetracycline antibiotics has been reported by McCoy (2) in Florida and has also been achieved in Jamaica (P. Hunt, personal com munication). Without treatment, this disease us ually kills palms within 4-5 mo. of the initial ob servation of symptoms. Affected palms may die in as short a time as one month. Symptoms of lethal yellowing usually begin with premature nutfall, followed by inflorescence necrosis. A yellow flag leaf may also appear in the middle of the crown at this time. As the disease progresses, the lower fronds become yellowed; the yellowing moves upwards until all fronds are af fected. Eventually the spear or bud leaf becomes Florida Agricultural Experiment Station Journal Series No. 5173. I wish to thank Mr. George Gwin, Florida Department of Agriculture, Division of Plant Industry, for his assistance in locating and diagnosing trees used in this study. I also wish to thank the city of Coral Gables, Florida, for providing municipally owned trees to be used in this test, necrotic, the meristem dies, and the crown will topple from the tree leaving a telephone pole-like stump. A ten step scale of disease severity has been devised and is presented in Table 1. Mycoplasma-like bodies have been observed in the phloem of lethal yellowing affected coconut palms from both Jamaica (1,4) and Florida (M. V. Parthasarathy, personal communication). The presence of these bodies in conjunction with the effects of tetracycline treatment suggests that they may be the etiologic agents of lethal yellowing. Since the Mollicutes as a class are insensitive to the penicillins and yet are highly sensitive to the tetracyclines, a test comparing the therapeutic value of these antibiotics was established in Feb ruary, 1973. In addition, the antibiotics strepto mycin and griseofulvin were evaluated. Materials and Methods Aqueous solutions of the test antibiotics were introduced into diseased palms through direct trunk injection at a pressure of 7 kg/cm2, or in the case of griseofulvin, by inserting an aqueous paste into 2 holes bored in the trunk. In addition, oxytetracycline-HCl soil drenches were attempted in two treatments. The pressure injection utilized a compressed air tank containing the antibiotic solution connected by hose to a hollow lag screw inserted into a hole bored in the trunk of the tree (after Tarjan (5)). The hose was connected to the screw by means of a quick connect coupling to allow rapid setup of the apparatus. Two formulations of oxytetracycline-HCl (terramycin) were used: i) 22.6% active bulked with sucrose and stabilized with tartaric acid; and ii) Table 1. Coconut lethal yellowing severity rating scale. primary 0 Healthy or incubating 1 Nutfall only2 2 1 necrotic inflorescense* 3 2 or more necrotic inflorescenses2 4 Yellowing in lower fronds only yellowing 5 Yellowing in lower and middle fronds 6 All fronds yellowed, spear leaf good 7 Spear leaf dead, some green fronds left dying 8 Spear leaf dead, all fronds yellow 9 Palm totally dead (telephone pole) Zinay or may not have one yellow flag leaf in center of crown 504 FLORIDA STATE HORTICULTURAL SOCIETY, 1973 20% active, bulked with citric acid for stability. The stabilizing agents were necessary to prevent hydrolysis of the antibiotic solution. TetracyclineHC1 (achromycin), potassium penicillin-G, strep tomycin nitrate, and griseofulvin were injected in distilled water without any bulking agents or stab ilizers. Two grams of each antibiotic were injected per tree (penicillin = 3.2 x 106 units) in 1.0 liters distilled water except for the griseofulvin treat ment which was 4 g in aqueous paste. The terramycin-tartaric acid formulation was also evaluated at 2 g in 0.2 liters water to determine the feasibil ity of injecting highly concentrated solutions, thereby reducing the time required for treatment. The terramycin soil drenches consisted of either 20 or 40 g a A. Agricultural Terramycin (Pfizer, Inc.) in 22 liters water injected into the root zone at 3 sites spaced equidistantly around each tree. Fifty mature coconut palms exhibiting early symptoms (stages 1 to 4) of lethal yellowing were selected for treatment in the vicinity of Coral Gables, Florida. Disease readings were taken at the time of treatment in mid-February, 1973 and at monthly intervals thereafter. Results Remission of lethal yellowing, as judged by nut set and retention, and by production of healthy new growth including clear new inflorescences, was induced only by the tetracycline antibiotics terra mycin and achromycin. Table 2 lists all treatments, Table 2. Initial and final mean lethal yellowing severity ratings, number of trees treated, and number of disease remissions obtained for each antibiotic treatment. Treatment Mean Severity + std. dev. Number No. trees remissions2