Occurrence of quinone outside inhibitor (QoI) resistance in Virginia populations of Parastagonospora nodorum infecting wheat

Navjot Kaur, Chase Mullins, Nathan Michael Kleczewski, Hillary Laureen Mehl

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Stagonospora nodorum blotch (SNB) of wheat, caused by Parastagonospora nodorum, is managed using cultural practices, resistant varieties, and foliar fungicides. Frequent fungicide use can select for fungicide resistance, making certain chemistries less effective; this may in part explain increasing severity of SNB in the mid-Atlantic U.S. Quinone outside inhibitor (QoI) resistance has been documented for a diversity of fungi, but it has not been reported for P. nodorum in the U.S. The objectives of this study were to 1) evaluate QoI sensitivity of P. nodorum from Virginia wheat fields, 2) screen P. nodorum for QoI target site mutations in the cytochrome b gene, and 3) develop a molecular assay to detect target site mutations associated with QoI resistance. Sensitivity of 16 isolates to pyraclostrobin and azoxystrobin was evaluated with radial growth assays, and the cytochrome b gene was sequenced. One isolate was insensitive to both fungicides, and it had the G143A mutation in the cytochrome b gene. For azoxystrobin, 10 isolates without target site mutations had reduced sensitivity. Additional isolates (N=74) were sequenced, and seven had the G143A mutation; all seven isolates with the mutation had reduced sensitivity to pyraclostrobin and azoxystrobin compared to a sensitive control isolate without the mutation. A pyrosequencing assay targeting G143A was developed as a rapid method to screen P. nodorum for the QoI resistance-conferring mutation. To our knowledge, this is the first report of QoI resistant P. nodorum in the U.S. Overall resistance frequency was low, but resistance management practices are needed to maintain the efficacy of fungicides for SNB control.

Original languageEnglish (US)
JournalPlant disease
StateE-pub ahead of print - Oct 1 2020

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