Prevention and Detection of Fungicide Resistance Development in Rhizoctonia zeae from Soybean and Corn in Nebraska

Abstract

Thegoalofthisresearch was to advance the foundational knowledge required to quantify and mitigate fungicide resistance in Rhizoctonia zeae, the seedling disease pathogen ofsoybean and corn. In vitro sensitivity to azoxystrobin, fludioxonil, sedaxane, and/or prothioconazole was determined for 91 R. zeae isolates obtained mostly from soybean and corn fields in Nebraska. Isolates were sensitive to flu-dioxonil, sedaxane, and prothioconazole (EC50 <3 µg/ml) and had a positively skewed EC50 distribution. Isolates were not sensitive to azoxystrobin in vitro (EC50 > 100 µg/ml) or in planta. Application of azoxystrobin did not significantly decrease disease severity or improve total dry weight of the soybean plants (P > 0.05). The risk of resistance developmentin R. zeae was estimated by characterizing its population structure. Eighty-one R. zeae isolates were genotyped using six microsatellite markers. Results showed that the population has a mixed mode of reproduction and is structured according to geographic region, suggesting limited dispersal. These population characteristics suggest that R. zeae has an intermediate risk of resistance development. Overall, this research establishedthecurrent status of fungicide sensitivity in R. zeae in Nebraska and estimated its risk of resistance development, which can inform fungicide resistance management for R. zeae.