P-121 - Corynespora cassiicola isolates from Kentucky soybean fields are losing sensitivity to pydiflumetofen

Target spot (TS), caused by Corynespora cassiicola, is a foliar disease of soybean in Kentucky that can reduce yields by up to 32%. Applications of fungicides that contain active ingredients from the succinate dehydrogenase inhibitor (SDHI) class, have been used to manage this disease. C. cassiicola isolates resistant to SDHI fungicides have been reported in a few other soybean-producing states but have not yet been reported in Kentucky. For this purpose, 64 single spore C. cassiicola isolates were obtained from soybean leaf samples with target spot symptoms collected from Kentucky soybean fields from 2020 to 2023. Isolates were tested to assess the effective pydiflumetofen concentration at which 50% of mycelium growth was inhibited relative to a non-fungicide control (EC₅₀). EC₅₀ values ranged from 0.0142 to 3.7725 µg/ml and the mean and median were 0.0247 and 0.0567 µg/ml, respectively. The resulting values showed five isolates with EC₅₀ values higher than 2 µg/ml. A fragment of subunits SdhB and SdhC from these 5 isolates and 2 isolates with lower EC₅₀ values were sequenced to identify any points of mutation associated with SDHI fungicide resistance. No mutation was identified at subunit SdhB. However, a change from asparagine (AAC) to serine (AGC) was identified at the codon 75 of subunit SdhC (C-N75S) from all isolates with higher EC₅₀ values. The frequency of isolates resistant to pydiflumetofen was 7.81%. This information will be valuable for developing a monitoring strategy to help manage TS and to reduce the selection and increase of SDHI resistance.