Fungicide Sensitivity and Nontarget Site Resistance in <i>Rhizoctonia zeae</i> Isolates Collected from Corn and Soybean Fields in Nebraska

<i>Rhizoctonia zeae</i> was recently identified as the major <i>Rhizoctonia</i> species in corn and soybean fields in Nebraska and was shown to be pathogenic on corn and soybean seedlings. Fungicide seed treatments commonly used to manage seedling diseases include prothioconazole (demethylation inhibitor), fludioxonil (phenylpyrrole), sedaxane (succinate dehydrogenase inhibitor), and azoxystrobin (quinone outside inhibitor [QoI]). To establish the sensitivity of <i>R. zeae</i> to these fungicides, we isolated this pathogen from corn and soybean fields in Nebraska during 2015 to 2017 and estimated the relative effective concentration for 50% inhibition (EC₅₀) of a total of 91 <i>R. zeae</i> isolates from Nebraska and Illinois. Average EC₅₀ for prothioconazole, fludioxonil, sedaxane, and azoxystrobin was 0.219, 0.099, 0.078, and >100 µg ml^-1, respectively. In planta assays showed that azoxystrobin did not significantly reduce the disease severity on soybean (<i>P</i> > 0.05). The cytochrome <i>b</i> gene of <i>R. zeae</i> did not harbor any mutation known to confer QoI resistance and had a type I intron directly after codon 143, suggesting that a G143A mutation is unlikely to evolve in this pathogen. For prothioconazole, fludioxonil, and sedaxane, the EC₅₀ of the isolates did not differ significantly among the years of collection (<i>P</i> > 0.05), and their single discriminatory concentrations were identified as 0.1 µg ml^-1. This is the first study to establish nontarget site resistance of <i>R. zeae</i> to azoxystrobin and the sensitivity of <i>R. zeae</i> to commonly used seed treatment fungicides in Nebraska. This information will help to guide strategies for chemical control of <i>R. zeae</i> and monitor sensitivity shifts in the future.