Objective Poplar gray leaf spot caused by Coryneum populinum is an important fungal disease of poplar. Evaluating the susceptibility of C. populinum to fungicides will provide data support for the screening of high-efficiency fungicides and compounds for the prevention and control of poplar gray leaf spot.

Method The susceptibility of C. populinum to 5 fungicides (carbendazim, mancozeb, tebuconazole, chlorothalonil and prochloraz) and 3 compounds (tebuconazole ∶ chlorothalonil, carbendazim ∶ chlorothalonil and carbendazim ∶ prochloraz) was determined by mycelium growth rate method and spore germination method, and the microscopic morphology of mycelium and spore germination were observed.

Result Tebuconazole and carbendazim had the strongest inhibitory effect on mycelial growth with EC₅₀ (median effect concentration) of 0.025 and 0.048 μg/mL, respectively. Chlorothalonil and prochloraz were the strongest inhibitor to spore germination with EC₅₀ of 95.13 and 100.67 μg/mL, respectively. The compounds of carbendazim ∶ prochloraz (4∶1) had a synergistic effect with co-toxicity coefficient (C_CT) of 150.75, and the most virulent with EC₅₀ value of 0.038 μg/mL to mycelial growth. The compounds of tebuconazole ∶ chlorothalonil (1∶4) had an additive effect with C_CT of 83.72, and the most virulent with EC₅₀ value of 132.31 μg/mL to spore germination. The hyphal surface of C. populinum became rough with enlarged branches after tebuconazole treated. The conidial germination was severely malformed with branching or without germination. Germ tubes were produced from the top, base or middle sites of conidia. The top of germ tubes was slightly swollen or without elongation.

Conclusion The combination of tebuconazole with chlorothalonil or prochloraz, and carbendazim ∶ prochloraz (4∶1) with chlorothalonil or prochloraz were recommended agents for preventing poplar gray spot. Alternate use of fungicides could delay the development of pathogen resistance.