Objective On the basis of the optimum mycelial growth temperature investigation of Coryneum populinum Bres, the pathogen was measured for susceptibility to five fungicides. The approach provides a theoretical reference for disease control of poplar gray spot and fungicide acting mechanism against the pathogen.

Method The mycelium growth rate method and conidia germination method were used to determine the antibacterial effects of five tested fungicides on C. populinum mycelium growth and conidia germination, and the toxicity regression equation was constructed and the EC50 value was calculated. The morphological changes of mycelia and conidial germination were observed after fungicide treatment.

Result The optimum mycelial growing temperature of C. populinum was 20 ℃ with the colony diameter of 4.25cm after 25 days, and the conidial germination rate was 79.00% after 12h on PDA. The mycelial growth and conidial germination were both inhibited at above 30 ℃. The inhibitory effects of the five fungicides on mycelial growth of the pathogen showed the trend of Difenoconazole > Tebuconazole > Carbendazim > Prochloraz > Thiophanate-methyl, with the average EC50 values of 0.0048, 0.024, 0.047, 0.25 and 0.49μg·mL ⁻¹, respectively. The virulence of the five fungicides to the pathogen’s conidial germination showed the pattern of Difenoconazole > Prochloraz > Thiophanate-methyl > Carbendazim > Tebuconazole, with the average EC50 values of 85.07, 101.23, 123.28, 254.80 and 327.78μg·mL ⁻¹, respectively. The hyphal surface of the pathogen became rough with enlarged branches, and no spore was produced after application of Difenoconazole. The conidial germination became morphologically abnormal with branching and enlargement or no enlagement of germ tubes.

Conclusion The fungicide difenoconazole has high virulence to C. populinum, and it shows a potentially effective candidate agent for management of popular grey spot.