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Sensitivity of Septoria glycines isolates to quinone outside inhibitor (QoI) fungicides

Danilo Neves: University of Kentucky

<div>Brown spot, caused by <em>Septoria glycines</em>, is a common foliar disease of soybean (<em>Glycine max</em>). Quinone outside inhibitor (QoI) fungicides have been effective in managing brown spot, but the risk of selecting strains with resistance to this class of fungicides is considered high. <em>Septoria glycines </em>isolates from Illinois and Kentucky were tested in vitro using azoxystrobin, pyraclostrobin, and trifloxystrobin amended media to determine the effective concentration in which 50% conidial germination was inhibited relative to a non-amended control (EC<sub>50</sub>). The mean EC<sub>50</sub> values of azoxystrobin, pyraclostrobin, and trifloxystrobin were 0.6583, 0.0962, and 0.1396 µg/ml, respectively. Isolates with high EC<sub>50</sub> values were identified, and sequencing of the cytochrome <em>b</em> gene from isolates with high EC<sub>50</sub> values showed they exhibited a transversion of a guanidine to cytosine at the second position in the codon 143, which is known as the G143 mutation. A single discriminatory dose of azoxystrobin, which was completely inhibitory for the QoI-resistant strains but allowed growth of resistant strains, was used to measure the sensitivity of 171 isolates. QoI-resistant isolates made up 29% of the <em>S. glycines </em>tested. Greenhouse experiments showed that QoI fungicides provided a high level of control of brown spot caused by QoI-sensitive isolates, but poor control of brown spot caused by QoI-resistant isolates.</div>