Fungicide sensitivity and non-target site resistance in Rhizoctonia zeae isolates collected from corn and soybean fields in Nebraska

Authors: GAMBHIR, NIKITA - Fmc Stine Research Center; KODATI, SRIKANTH - University Of Connecticut; Adesemoye, Anthony; EVERHART, SYDNEY - University Of Connecticut

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: The pathogen Rhizoctonia species is a complex group of seedling pathogens that cause yield losses in many crops. Rhizoctonia zea is a member of the group that was recently identified as the major pathogen in corn and soybean in Nebraska. 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). Isolates of Rhizoctonia zeae which had been collected during 2015-2017 from corn and soybean in Nebraska were evaluated to determine if they are sensitive and could be controlled by fungicides. Studies conducted in the greenhouse showed that azoxystrobin did not significantly reduce the disease severity or enhance the growth of soybean plants in the presence of the pathogen (P > 0.05). Genetic evaluation indicated that the pathogen, R. zeae did not have any mutation that may confer resistance to QoI fungicides, suggesting non-target site resistance of R. zeae to azoxystrobin. This is the first study to establish non-target site resistance of R. zeae to azoxystrobin and its sensitivity to commonly used seed treatment fungicides in Nebraska. This information will help to guide strategies for chemical control of R. zeae.

Technical Abstract: Rhizoctonia zeae was recently identified as the major Rhizoctonia 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 R. zeae to these fungicides, we isolated this pathogen from corn and soybean fields in Nebraska during 2015–2017 and estimated the relative effective concentration for 50% inhibition (EC50) of a total of 91 R. zeae isolates from Nebraska and Illinois. Average EC50 for prothioconazole was 0.219 µg/ml, fludioxonil was 0.099 µg/ml, sedaxane was 0.078 µg/ml, and azoxystrobin was > 100 µg/ml. In planta assays showed that azoxystrobin did not significantly reduce the disease severity or enhance the dry weight of inoculated soybean plants (P > 0.05). The cytochrome b gene of R. zeae did not harbor any mutation known to confer resistance to QoI fungicides, suggesting non-target site resistance of R. zeae to azoxystrobin. For prothioconazole, fludioxonil, and sedaxane, EC50 of isolates did not differ significantly among years of collection (P > 0.05) and their single discriminatory concentrations were identified as 0.1 µg/ml. This is the first study to establish non-target site resistance of R. zeae to azoxystrobin and its sensitivity to commonly used seed treatment fungicides in Nebraska. This information will help to guide strategies for chemical control of R. zeae and monitor sensitivity shifts in future.