Resistanceof<italic>Alternariaalternata</italic>tothiophanate-methyl,procymidoneanditsbaselinesensitivitytoprochloraz

Black spot, caused by Alternaria alternate, is one of the main diseases of Atractylodes macrocephala in China. In this study, resistance of 137 A. alternate isolates to thiophanate-methyl and procymidone was evaluated using mycelial growth rate method. The isolates were collected from Pan’an, Tiantai, Xinchang and Shengzhou in Zhejiang Province from 2015 to 2017. Results showed that the frequency of A. alternata population (n = 137) with high-level and medium-level resistance to thiophanate-methyl was 77.4% and 22.6%, respectively, and no sensitive isolates were observed. The resistance frequency to procymidone was 20.4%, wherein 21 isolates have low-level resistance and 7 isolates have medium resistance. Prochloraz showed the strongest activity against the mycelial growth of A. alternata among the 7 tested fungicides with the EC₅₀ value of 0.15 mg/L. Next, the baseline sensitivity of A. alternata to prochloraz was established. The EC₅₀ values for the tested population (n = 137) to prochloraz varied from 0.05 to 0.87 mg/L, mean value is (0.29 ± 0.11) mg/L, and their mean EC₅₀ value could be used as the sensitive baseline.