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Evaluation of Thymol as Biofumigant for Control of Bacterial Wilt of Tomato Under Field Conditions

Volatile plant essential oils thymol and palmarosa oil, used at a concentration of 0.7%, were evaluated under field conditions for control of bacterial wilt of tomato caused by Ralstonia solanacearum. The experimental fields were artificially infested with the bacterial pathogen. Two hours after infestation, the plant essential oils were applied, then the plots were sealed with plastic mulch for 3 or 6 days. Tomato seedlings were transplanted into the field 7 days later. In fall of 2002, 92.5% of tomato plants (cv. Equinox) wilted in the untreated control plots. Both thymol and palmarosa oil soil treatments reduced bacterial wilt incidence significantly. Thymol was more effective than palmarosa oil based on the final assessment, when 33.1 and 48.1% of the plants had wilted in plots treated with thymol and palmarosa oil, respectively. Soil treatment with either thymol or palmarosa oil produced significantly higher yield of tomato than the untreated control. In 2003, only thymol was evaluated. Thymol application significantly reduced bacterial wilt incidence on the susceptible cultivar Solar Set. Disease incidence in untreated plots reached 65.5%, while in thymol treated plots only 12% of plants wilted. Thymol treatment also increased yield of Solar Set significantly compared with the untreated control. This is the first report on the use of thymol for controlling a plant disease under field conditions, which indicated that this compound provided effective control of bacterial wilt on susceptible tomato cultivars when used as preplant treatment of soils. Because of its volatile property and broad-spectrum functions, thymol shows potential to be used as a soil biofumigant for the management of various plant pathogens.