Abstract: The aim of this experiment was to screen out the best bactericide for controlling Artemisia downy mildew. The pathogenic bacteria were isolated by the method of tissue identification and purification , which was identified as Bremia lactucae. Regel.The drug test used the growth rate of toxic medium to determine the inhibitory effect of seven fungicides on downy mildew. The results show: after 72h, EC₅₀ of Chlorothalonil was 1.46mg/L, the virulence index was the high, and the inhibition was most significant; EC₅₀ of dimethomorph and mancozeb were 4.31 and 5.63 mg/L, the inhibitory effects were significant; EC₅₀ of metalaxyl mancozeb was 9.19 mg/L, the inhibitory was good; EC₅₀ of propamocarb and sandofan were 26.31 and 101.32 mg/L, the inhibitory effects were common; EC₅₀ of sodium dichloroisocyanurate was the highest value, and the inhibitory was poor. The inhibitory effect was similar between 72 h and 96h.Therefore, among the seven fungicides, chlorothalonil had the best inhibitory effect.