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Antibiotic Sensitivity In Vitro of the Mycoplasmalike Organism Associated with Citrus Stubborn Disease. J. W. Bowyer, Research Plant Pathologist, Department of Plant Pathology, University of California, Riverside 92502, Present address: Research Fellow, Department of Microbiology, University of Queensland, St. Lucia, Australia 4067; E. C. Calavan, Professor, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 64:346-349. Accepted for publication 14 September 1973. DOI: 10.1094/Phyto-64-346.

The mycoplasmalike organism associated with citrus stubborn disease was tested for susceptibility in vitro to 19 antibiotics, three systemic insecticides, and three systemic fungicides. Minimal inhibitory concn (MIC) and minimal biocidal concn (MBC) were determined for each substance by a dilution-broth method. The macrolide antibiotics, erythromycin and tylosin, were most active, each having MIC and MBC values of 0.2 and 0.8 µg/ml, respectively. Six members of the tetracycline group and carbomycin were inhibitory at low concns (0.1 to 0.4 µg/ml) but the biocidal levels were two to four times higher (1.6 to 3.1 µg/ml) than for erythromycin or tylosin. MIC values for oleandomycin, lincomycin, and filipin ranged from 1.6 to 3.1 µg/ml, and all were biocidal at 12.5 µg/ml. Chlortetracycline, chloramphenicol, and kanamycin were relatively ineffective, with MIC values of 12.5 to 25.0 µg/ml, and an MBC value of 50.0 µg/ml. Streptomycin and an antibiotic known as BP both had an MIC value of 50.0 µg/ml, and MBC values of 100 and 500 µg/ml, respectively. Penicillin and sulfanilamide were not inhibitory at 500 µg/ml, the highest concn tested. Of the systemic insecticides (cygon, furadan, and lannate) and systemic fungicides (benomyl, thiabendazole, and thiophanate M), only thiabendazole in 5% (v/v) dimethylsulfoxide had an MIC value (10 µg/ml) similar to that of the effective antibiotics. The results indicate that erythromycin, tylosin, and several members of the tetracycline group of antibiotics are potentially useful for control of stubborn disease. They contribute also to characterization of the mycoplasmalike organism associated with the disease.