Sensitivity of the U.S. Wheat Powdery Mildew Population to QoI Fungicides and Determination of the Complete Blumeria graminis f. sp. tritici cytochrome b gene

Authors: Hahn, Emily; Whetten, Rebecca; Cowger, Christina

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2021
Publication Date: 6/22/2021
Citation: Hahn, E.A., Whetten, R.B., Cowger, C. 2021. Sensitivity of the U.S. Wheat Powdery Mildew Population to QoI Fungicides and Determination of the Complete Blumeria graminis f. sp. tritici cytochrome b gene. Phytopathology. https://doi.org/10.1094/PHYTO-04-21-0132-R.
DOI: https://doi.org/10.1094/PHYTO-04-21-0132-R

Interpretive Summary: Wheat powdery mildew is a fungal disease that is managed mainly with resistant varieties and foliar fungicides in the United States. Quinone outside inhibitors (QoIs), also known as strobilurin fungicides, target the cytochrome b (cytb) gene in fungal mitochondria. European wheat powdery mildew populations have become resistant to QoI fungicides because of a mutation in cytb called G143A. We studied the level of QoI sensitivity of the U.S. wheat powdery mildew population. We collected samples of the fungus from 15 central and eastern U.S. states, and screened about 375 of them for sensitivity to two QoIs, pyraclostrobin and picoxystrobin. While we detected none of the three mutations in the cytb gene that cause resistance or lowered sensitivity to QoIs, we did observe a range of sensitivity to both fungicides in the U.S. wheat powdery mildew population. This suggested the population could be undergoing a gradual and so far mild loss of QoI sensitivity. The complete cytb gene sequence was identified for the first time in wheat powdery mildew.

Technical Abstract: Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is managed primarily with cultivar resistance and foliar fungicides in the United States. Quinone outside inhibitors (QoIs), which target the mitochondrial cytochrome b (cytb) gene, are one of the two main fungicide modes of action labelled for use on wheat in the United States. While European populations of Bgt are widely insensitive to QoI fungicides, largely due to the cytb mutation G143A, the QoI sensitivity of the U.S. Bgt population had never been evaluated despite years of QoI use on U.S. wheat. Approximately 375 B. graminis f. sp. tritici isolates collected from 15 states in the central and eastern U.S. were screened for sensitivity to QoIs pyraclostrobin and picoxystrobin. A modest range of sensitivities was observed, with maximum resistance factors of 11.2 for pyraclostrobin and 5.3 for picoxystrobin when compared to the QoI-sensitive control isolate JIW11. The G143A, F129L, and G137R cytb mutations were not detected in the U.S. Bgt population, nor were mutations identified in the PEWY loop, a key part of the Qo site. Thus, no genetic basis for the observed quantitative variation in QoI sensitivity of U.S. Bgt was identified. The complete Bgt cytb gene sequence was determined for the first time in the isolate 96224 v. 3.16 reference genome and, contrary to previous reports, an intron was found between codons 33 and 34 that appears to belong to intron group II, which is unusual in fungi.