Sensitivity of Cercospora sojina to demethylation inhibitor and methyl benzimidazole carbamate fungicides

Cercospora sojina, the causal agent of frogeye leaf spot (FLS) of soybean (Glycine max), can cause significant yield losses. Foliar fungicide application is a common strategy used to control FLS; however, C. sojina strains with resistance to quinone outside inhibitor (QoI) fungicides have become widespread throughout U.S. soybean production regions. Because of QoI-resistance, demethylation inhibitor (DMI) and methyl benzimidazole carbamate (MBC) fungicides have become more important in managing FLS. Baseline sensitivity assays are important to monitor any shifts in fungicide sensitivity. Sensitivity of the DMI fungicides flutriafol and tetraconazole and the MBC fungicide thiophanate methyl were established for 145 C. sojina isolates collected prior to 2001(baseline isolates) and C. sojina isolates collected from 2007 to 2012 representing 12 states. Bioassays were conducted to assess effective fungicide concentration at which 50% of mycelium growth was inhibited relative to a non-fungicide control (EC₅₀) for each isolate-fungicide combination. Baseline EC₅₀ value means for flutriafol, tetraconazole, and thiophanate methyl were 0.268, 0.153, and 0.521 μg/ml. When means of EC₅₀ values for isolates collected from 2007 to 2012 were compared to baseline sensitivity EC₅₀ means, no significant difference (P < 0.05) was observed for sensitivities to flutriafol and tetraconazole. When this same comparison was done for sensitivity to thiophanate methyl, isolates collected in 2008 and 2012 were significantly more sensitive than the baseline isolates. Overall, these findings indicate no shift towards reduced sensitivity to flutriafol, tetraconazole, and thiophanate methyl. Based on these results, continued monitoring of C. sojina population sensitivity to DMI and MBC fungicides should occur.