Combined Toxicity Test of Complex Agents of Metalaxyl and Other Fungicides against Phytophthora parasitica var. nicotianae

[Objective] To overcome the resistance of Phytophthora parasitica var. nicotianae against metalaxyl and effectively control its damage, new efficient complex agent of metalaxyl was studied and developed. [Method] The toxicities of nine fungicides against P. parasitica were measured using growth rate method. On this basis, the fungicides with good effects were selected to compound with metalaxyl, and the optimum complex ratio was confirmed. [Result] The toxicity of metalaxyl was the strongest with EC50 of 2.130 5 μg/ml; followed by carbendazim, mancozeb and dimethomorph with EC50 of 2.357 9, 2.639 8 and 2.778 8 μg/ml. The effect of cyazofamid was the poorest with EC50 of 6.278 8 μg/ml. The optimum complex ratios of dimethomorph, carbendazim and mancozeb with metalaxyl were 40∶60, 30∶70 and 20∶80, and their co-toxicity coefficients were 138.80, 124.25 and 115.00, respectively. [Conclusion] The complex agents had application and promotion value, which could be used to carry out further field trials.