Sensitivity of United States wheat powdery mildew (Blumeria graminis f.sp. tritici) populations to the demethylation inhibitor fungicides

Authors: MEYERS, EMILY - North Carolina State University; ARELLANO, CONSUELO - North Carolina State University; Cowger, Christina

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2019
Publication Date: 7/24/2019
Citation: Meyers, E., Arellano, C., Cowger, C. 2019. Sensitivity of United States wheat powdery mildew (Blumeria graminis f.sp. tritici) populations to the demethylation inhibitor fungicides. Plant Disease. https://doi.org/10.1094/PDIS-04-19-0715-RE.
DOI: https://doi.org/10.1094/PDIS-04-19-0715-RE

Interpretive Summary: Wheat powdery mildew, caused by the fungus Blumeria graminis f.sp. tritici (Bgt), is managed in the United States with cultivar resistance and foliar fungicides. However, fungicide sensitivity of U.S. Bgt has never been evaluated, even though cereal mildew populations in other countries have shown a tendency to become less sensitive to fungicides. Almost 400 Bgt strains were collected from 15 U.S. states over two years and their sensitivity to two demethylation inhibitor (DMI) fungicides, tebuconazole and prothioconazole, was tested. A large range of sensitivity to both DMIs was observed, with Bgt strains from the eastern U.S. (Great Lakes, Mid-Atlantic, and Southeast regions) being less sensitive and strains from central states (Plains region, Arkansas, and Missouri) being more sensitive. There was a correlation between tebuconazole and prothioconazole sensitivities: becoming less sensitive to one active ingredient was usually accompanied by reduced sensitivity to the other active ingredient (cross-resistance), although some strains did not follow this trend. This was the first survey of fungicide sensitivity in the U.S. Bgt population, and it indicated that Bgt in the eastern regions has become less sensitive to DMIs. This highlights the importance of rotating between chemistry classes to maintain the effectiveness of DMIs in U.S. wheat production. While cross-resistance was found, the inconsistency of it suggests there is not simply a single mutation that determines Bgt responses to these two DMI active ingredients.

Technical Abstract: Wheat powdery mildew, caused by Blumeria graminis f.sp. tritici (Bgt), is managed in the United States with cultivar resistance and foliar fungicides. However, fungicide sensitivity of U.S. Bgt has never been evaluated, despite high levels of insensitivity in other global cereal mildew populations. Almost 400 Bgt isolates were collected from 15 U.S. states over two years and phenotyped for sensitivity to two demethylation inhibitor (DMI) fungicides, tebuconazole and prothioconazole. A large range of sensitivity to both DMIs was observed, with more insensitive Bgt isolates originating from the eastern U.S. (Great Lakes, Mid-Atlantic, and Southeast regions) and more sensitive isolates from central states (Plains region, Arkansas, and Missouri). Cross-resistance was indicated by a positive association between tebuconazole and prothioconazole sensitivities at all levels of analysis (EC50 values, P < 0.0001), although correlation coefficients near 0.5 indicated some isolates deviated from this trend. There was weak evidence of a fitness cost associated with prothioconazole insensitivity (P = 0.0307) when analyzed at the state population level. This is the first assessment of fungicide sensitivity in the U.S. Bgt population, and it produced evidence of regional selection for reduced DMI efficacy. The observation of reduced sensitivity to DMI fungicides in the eastern U.S. underlines the importance of rotating between chemistry classes to maintain the effectiveness of DMIs in U.S. wheat production. While cross-resistance was demonstrated, variability in the relationship of EC50 values for tebuconazole and prothioconazole also suggests that multiple mechanisms influence Bgt isolate responses to these two DMI active ingredients.