Baseline sensitivity of Cercospora sojina and Corynespora cassiicola to pydiflumetofen

Pydiflumetofen is a new fungicide that belongs to the succinate dehydrogenase inhibitor (SDHI) class. This novel fungicide can help protect soybean plants against infection by foliar pathogens, including Cercospora sojina (causal agent of frogeye leaf spot) and Corynespora cassiicola (causal agent of target spot). A total of 88 single-spore isolates of C. sojina collected from soybean fields between 2007 and 2019 from Alabama, Georgia, Illinois, Indiana, Iowa, Kentucky, Louisiana, Michigan, Minnesota, Mississippi, Nebraska, South Dakota, and Tennessee and a total of 76 C. cassiicola single-spore isolates collected from soybean fields in Kentucky between 2017 and 2019 were evaluated for their sensitivity to pydiflumetofen. For this purpose, in vitro bioassays were conducted using different doses of pydiflumetofen (0, 0.001, 0.01, 0.1, 1, and 10 μg/ml). The effective concentrations of pydiflumetofen in which C. sojina and C. cassiicola isolates were inhibited by 50% (EC50) were calculated. The C. sojina EC50 values ranged from 0.335 to 4.335 μg/ml, the mean value was 1.616 μg/ml and median was 1.309 μg/ml. The range, mean and median of C. cassiicola EC50 were 0.037–0.248 μg/ml and 0.086 and 0.069 μg/ml, respectively. Establishing a baseline sensitivity is essential to identify any shifts in sensitivity of fungal populations over time. The findings in this current paper could be used to help monitor the sensitivity of C. sojina and C. cassiicola to pydiflumetofen over time.