Abstract: 【Objective】Isolates of Phytophthora infestans on tomato in Guangxi province were determined for the sensitivities to metalaxyl, cymoxanil and dimethomorph in order to give the basic information for integrate management of this disease.【Method】Sensitivities were determined by measuring radial growth on agar medium amended with fungicide, compared with the floating-leaf-disk method. 【Result】 Two hundred and thirty-nine isolates were collected from eight tomato growing areas during 2000 to 2006. The results of determine showed that the frequencies of sensitive, intermediate and resistant isolates to metalaxyl were 42.26%, 35.98% and 23.53%, respectively, while the frequencies of sensitive, intermediate isolates to cymoxanil were 37.24% and 62.76%, respectively, and that to dimethomorph were 54.81% and 45.19%, respectively. However, no isolate resistant either to cymoxanil or to dimethomorph was detected. The baselines of sensitivity to metalaxyl, cymoxanil, and dimethomorph were 0.003599µg/ml, 0.07156µg/ml and 0.0627µg/ml, respectively. Variations in sensitivities among isolates from different areas or different years were low for cymoxanil and dimethomorph, but very high for metalaxyl. All isolates from Tianlin and Wuxuan were sensitive to metalaxyl. But metalaxyl-resistant isolates predominated in Tianyang, with the frequency of 51.35%. The EC50 values of some isolates from Tianyang were higher than 500µg/ml and their resistance levels were over 100000 folds. The comparison with the floating-leaf-disk method indicated both techniques provided equivalent results.【Conclusion】 These studies suggested that metalaxyl could still be used in Tianlin, Wuxuan and Nanning, where the resistant isolates had not been found, but in other areas, the use of metalaxyl should be reduced or stopped, and cymoxanil or dimethomorph was recommended.

Key words: Phytophthora infestans, metalaxyl, cymoxanil, dimethomorph, sensitivity