Abstract: The toxicity of Dimethachlon, Mancozeb and their mixture for the Alternaria alternata was tested in the laboratory respectively by control spore germination. The results showed that the control rate of the bacterial spore germination increased obviously by the mixture of Dimethachlon and Mancozeb at proper proportion. The cotoxicity coefficient (CTC)of the mixture was 385.0819 and indicated that it had great synergy for control of the bacterial spore germination. The field experiments also illustrated that the tobacco brown spot could be controlled effectively by application of the mixture.