Distribution of G143A mutations conferring fungicide resistance in Virginia populations of Parastagonospora nodorum infecting wheat

Authors: KAUR, NAVJOT - Virginia Tech; Mehl, Hillary

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2021
Publication Date: 1/13/2022
Citation: Kaur, N., Mehl, H.L. 2022. Distribution of G143A mutations conferring fungicide resistance in Virginia populations of Parastagonospora nodorum infecting wheat. Plant Health Progress. 23(1):28-32. https://doi.org/10.1094/PHP-05-21-0082-RS.
DOI: https://doi.org/10.1094/PHP-05-21-0082-RS

Interpretive Summary: Fungicides are an important tool for controlling crop disease including Stagonospora nodorum blotch (SNB), a foliar disease of wheat. There are several different classes of fungicides with different modes of action, and quinone outside inhibitors, commonly known as strobilurin fungicides, were highly effective for control of a broad range of crop fungal diseases when they were first introduced. Unfortunately, development of resistance to strobilurin fungicides has been documented for numerous plant pathogens. Strobilurin resistance in populations of Parastagonospora nodorum, the causal agent of SNB, was recently documented in Virginia, and this was the first report of strobilurin resistance for this pathogen in the U.S. However, the study was based on a relatively small number of fungal isolates from a limited number of locations. The objective of the current study was to conduct a state-wide, two-year survey of P. nodorum populations in Virginia wheat and quantify the occurrence of strobilurin resistant isolates using a DNA-based method that detects the mutation that causes resistance to this class of fungicides. Based on the presence of the mutation, strobilurin resistance was common and widespread in populations of the fungal pathogen that causes SNB in wheat. Thus, strobilurin fungicides may be losing their effectiveness for control of SNB in Virginia and the surrounding region.

Technical Abstract: Stagonospora nodorum blotch (SNB) caused by Parastagonospora nodorum is an important leaf spot disease in the mid-Atlantic United States. Disease management approaches include use of resistant varieties, cultural control, and foliar fungicides. Frequent use of foliar fungicides can select for fungicide resistance within pathogen populations. Recently, the first report of quinone outside inhibitor (QoI) fungicide resistance in the United States was made based on a relatively small collection of P. nodorum isolates from Virginia. The objective of this study was to conduct a statewide, 2-year survey of P. nodorum populations in Virginia wheat and quantify frequencies of the target-site mutation that confers QoI resistance. A total of 318 isolates of P. nodorum were obtained from wheat collected at seven locations distributed throughout the wheat-growing regions of Virginia in 2018 and 2019. A previously designed pyrosequencing assay that detects the G143A substitution in the cytochrome b gene of P. nodorum was used to screen isolates for the presence or absence of the target site mutation. The G143A substitution was detected in all sampled fields. Among locations and years, frequencies of the mutation in P. nodorum populations ranged from 5 to 32% (mean = 19%). Thus, the QoI-resistance-conferring G143A mutation was widespread in P. nodorum populations in Virginia, and it occurred at a relatively high frequency. Results suggest that fungicides containing QoI active ingredients may not be effective for controlling SNB in Virginia and the surrounding region, and application of standalone QoI fungicides for disease control in wheat is not recommended.