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Investigation into the mechanism of resistance to azoxystrobin in Cercospora sojina, the causal agent of frogeye leaf spot
J. STANDISH (1), M. Tomaso-Peterson (1), T. W. Allen (2), S. Sabanadzovic (3), N. Aboughanem-Sabanadzovic (4). (1) Mississippi State Univ, Mississippi State, MS, U.S.A.; (2) Delta Research and Extension Center, Stoneville, MS, U.S.A.; (3) Mississippi State University, Mississippi State, MS, U.S.A.; (4) Institute for Genomics, Biocomputing & Biotechnology, Mississi

Frogeye leaf spot (FLS) is a foliar disease of soybean caused by <i>Cercospora sojina</i> Hara. Circular to angular lesions develop and may coalesce to form larger, irregular spots. When lesions cover more than 30% of the leaf surface blighting occurs leading to potential yield loss. Quinone outside inhibitor (QoI) fungicides are often used to manage FLS, but resistance to this class of fungicides has been documented in various phytopathogens. Resistance to QoIs is generally associated with the presence of amino acid substitutions within the cytochrome b (<i>cyt b</i>) gene. A glycine to alanine substitution at position 143 (G143A) is known to confer complete resistance to QoI fungicides. Two other notable substitutions that confer moderate resistance are phenylalanine to leucine at position 129 (F129L) and glycine to arginine at position 137 (G137R). The objective of this study was to evaluate <i>C. sojina</i> isolates collected from Mississippi soybean fields for resistance to QoIs by identifying point mutations within <i>cyt b</i>. Genomic DNA was extracted from <i>C. sojina </i>isolates collected during the 2013 growing season as well as from a baseline isolate collected prior to the release of QoI fungicides. A 238-bp fragment of the <i>cyt b</i> gene was amplified using<i> C. sojina</i>-specific primers that were developed in our laboratory. Nucleotide sequences of <i>C. sojina </i>isolates from Mississippi were compared to a known sensitive baseline isolate to identify potential point mutations.

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