Evaluation of fungicide efficacy against <i>Cercospora sojina</i> and selection for QoI-fungicide resistance in soybean

Frogeye leaf spot (FLS) of soybean is caused by Cercospora sojina. In 2010, resistance to the quinone outside inhibitor (QoI) fungicides was reported. Since then, evaluating FLS for QoI-resistance has been of particular interest in Tennessee and other soybean-producing states. In order to determine alternative fungicide options, fungicides with solo and combination mode of action were tested in 2013-2014. The objectives were,1) to evaluate fungicide efficacy for disease control and yield protection, and 2) evaluate selection pressure for QoI-resistance. Treatments included the following fungicide groups: QoI, DMI, MBC, Chlorothalonil, QoI+DMI, SDHI+QoI, SDHI+QoI+DMI, and DMI+MBC. QoI and Chlorothalonil treatments failed to control FLS. Any product with a QoI-component listed as an active ingredient exerted greater selection for QoI-resistance than products lacking QoI-components. Combination-QoI treatments provide better disease control than solo strobilurin treatments, but still exhibited selection pressure for resistance. Chlorothalonil and SDHI+QoI treatments were not as effective as alternate modes of action at controlling FLS when there was a high proportion of resistance in a field. The four fungicide groups with the greatest efficacy were the solo-DMI, solo-MBC, combination DMI+MBC, and combination SDHI+QoI+DMI treatments. The 3^rd objective was to assess C.sojina epidemiology in Tennessee and whether or not it varied across the state. This was accomplished by sampling non-treated portions of farmer’s fields in 2014 and 2015 across west and middle Tennessee, referred to as sentinel plots. The sentinel plot samples revealed that selection for QoI resistance and progression of FLS disease was less favorable in middle Tennessee compared to west Tennessee. Furthermore, upper canopies of soybeans were more likely to harbor greater levels of QoI-resistant C.sojina than lower canopies.