Abstract: In this study, the sensitivity of Alternaria alternata population (n = 103) to pydiflumetofen, a novel succinate dehydrogenase inhibitor (SDHI), was determined by the mycelial growth rate method. The inhibitory activity of this fungicide on spore germination and its protective and curative effects on black spot were evaluated. The results showed that the EC₅₀ value of pydiflumetofen against A. alternata population (n = 103) was between 0.027 µg/mL and 1.175 µg/mL, and the average EC₅₀ value was (0.236 ± 0.101) µg/mL. The sensitivity frequency distribution showed a unimodal curve, and the average EC₅₀ value could be used as the sensitivity baseline of A. alternata to pydiflumetofen. Further analysis showed that no amino acid mutation was detected in succinate dehydrogenase subunit B/C/D (SDHB, SDHC, SDHD) subunits of the strains with sensitivity differences. Pydiflumetofen also had strong inhibitory activity on spore germination and its protective effect was superior to the curative effect. These results provide a basis for the promotion and scientific application of pydiflumetofen in the management of cherry black spot and other diseases.