Objective Tobacco black shank caused by Phytophthora nicotianae and bacterial wilt caused by Ralstonia solanacearum are important soil-borne diseases in tobacco, sometimes occurring in combination. Screening for efficient fungicide combinations that inhibit both P. nicotianae and R. solanacearum can provide an applied basis for developing compound formulations to simultaneously control both diseases.

Method The minimum inhibitory concentration method, mycelial growth rate method, and turbidimetric method were used to determine the in vitro inhibitory activity of nine tested fungicides, including tetramycin against P. nicotianae and R. solanacearum. Compound combinations with strong activity and synergistic effects were screened, and their control efficacy against tobacco black shank and bacterial wilt was further verified through pot experiments.

Result Among the tested fungicides, tetramycin, zhongshengmycin, ethylicin, and metalaxyl·hymexazol (active ingredient mass ratio of metalaxyl to hymexazol fixed at 1∶5) showed strong in vitro inhibitory effects against bothP. nicotianae and R. solanacearum. Their EC₅₀ values against P. nicotianae were 0.2567, 46.7807, 37.1316, and 4.2416 μg/mL, and against R. solanacearum were 0.0293, 2.8050, 0.7321, and 34.6220 μg/mL, respectively. Further, tetramycin and ethylicin were compounded with metalaxyl·hymexazol, respectively. The results showed that the mixtures of tetramycin with metalaxyl·hymexazol (active ingredient mass ratio of 1∶20) and ethylicin with metalaxyl·hymexazol (active ingredient mass ratio of 5∶2) exhibited good inhibitory effects on P. nicotianae, with inhibition rates of 71.33% and 59.89%, and toxicity ratios of 1.42 and 1.25, respectively, demonstrating synergistic effects. Pot experiments using these combination ratios showed that the control efficacy of the tetramycin + metalaxyl·hymexazol and ethylicin + metalaxyl·hymexazol compound formulations against tobacco black shank reached over 74.28% and 89.96%, respectively, significantly higher than those of tetramycin and ethylicin alone (40.79% and 38.69%, respectively). Their control efficacy against tobacco bacterial wilt reached over 52.52% and 78.72%, respectively, significantly higher than those of tetramycin and metalaxyl·hymexazol (34.46% and 20.26%, respectively).

Conclusion After being combined with metalaxyl·hymexazol at active ingredient mass ratios of 1∶20 and 5∶2, respectively, tetramycin and ethylicin demonstrated a significant synergistic effect on the inhibition of P. nicotianae and exhibited good control efficacy against tobacco black shank and bacterial wilt in pot experiments. They can be considered as potential chemical combinations for control of multiple soil-borne diseases in tobacco.