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Managing an oomycete community: fungicide sensitivity and evolution of resistance to ethaboxam
Zachary Noel: Michigan State University; Alejandro Rojas: Duke University; Janette Jacobs: Michigan State University; Martin Chilvers: Michigan State University
<div>Seedling diseases caused by oomycetes pose a significant threat to soybean production. In 2011 and 2012, over 80 oomycete species were found to be associated with soybean seedlings. Over half of those species were found to be pathogenic. Evaluation of these species for fungicide sensitivity is important for management. Fungicide amended medium assays are slow, labor intensive and expensive. A high-throughput assay to evaluate fungicide sensitivity of many oomycete isolates at once was developed using optical density measurements of macerated mycelial fragments. Z’-factor was used as a quality control statistic. The assay was utilized to evaluate the sensitivity of 81 oomycete species to mefenoxam and ethaboxam. Of the isolates tested, 87.5% had an EC<sub>50 </sub>< 1 μg ml<sup>-1 </sup>and only one <i>Phytopythium </i>isolate had an EC<sub>50 </sub>> 10 μg ml<sup>-</sup>1 mefenoxam. For ethaboxam, 61.7% of isolates tested had an EC<sub>50 </sub>< 1 μg ml<sup>-1</sup>, whereas, species within <i>Pythium</i> clades A, B and E had EC<sub>50</sub> ≥ 20 μg ml<sup>-1</sup> ethaboxam. This suggested that reduced sensitivity to ethaboxam may be inherent and possibly related phylogenetically. Therefore, we investigated the evolutionary history and mechanism of resistance to ethaboxam. Phylogenies indicated that species with reduced sensitivity to ethaboxam followed a convergent evolutionary pattern and had evolved three separate times. Two different transversion mutations lead to the same amino acid change in the target gene of lineages with reduced sensitivity to ethaboxam.</div>

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