Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2012
Publication Date: 1/1/2013
Citation: Bolton, M.D., Rivera-Varas, V., Secor, G. 2013. Identification of the G143A mutation associated with QoI resistance in Cercospora beticola field isolates from Michigan, United States. Pest Management Science. 69:35-39. Interpretive Summary: Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the most serious foliar disease of sugarbeet worldwide. Disease control is mainly achieved by timely fungicide applications. In 2011, CLS control failures were reported despite application of quinone outside inhibitor (QOI) or strobilurin fungicides in several counties in Michigan, USA. Samples were collected from fields with CLS control problems and strobilurin resistance was confirmed in the laboratory testing using a spore germination assay. We sequenced a portion of the cytb gene, which encodes the enzyme cytochrome b that is targeted by strobilurin fungicides. All resistant isolates had a mutation that caused an amino acid exchange in cytochrome b. This amino acid exchange is likely to be involved with the resistance mechanism in strobilurin-resistant strains of C. beticola. We designed a highly sensitive and high throughput molecular assay that detects strobilurin-resistant strains from sensitive strains. Taken together, the G143A mutation is associated with strobilurin resistance in C. beticola. Monitoring of strobilurin resistance will be essential to ensure that strobilurins can play a role in CLS management.
Technical Abstract: BACKGROUND: Cercospora leaf spot (CLS), caused by the fungus Cercospora beticola, is the most serious foliar disease of sugarbeet (Beta vulgaris L.) worldwide. Disease control is mainly achieved by timely fungicide applications. In 2011, CLS control failures were reported despite application of quinone outside inhibitor (QOI) fungicide in several counties in Michigan, USA. The purpose of this study was to confirm the resistant phenotype and identify the molecular basis for QOI resistance of Michigan C. beticola isolates. RESULTS: All isolates harvested in 2011 exhibited EC50 values > 0.92 µg ml-1 to pyraclostrobin and trifloxystrobin and harbored a mutation in cytochrome b (cytb) that led to an amino acid exchange from glycine to alanine at position 143 (G143A) compared to baseline QOI-sensitive isolates. Microsatellite analyses suggested that QOI resistance emerged independently in multiple genotypic backgrounds at multiple locations. A real time PCR assay utilizing dual-labeled fluorogenic probes was developed to detect and differentiate QOI-resistant isolates harboring the G143A mutation from sensitive isolates. CONCLUSION: The G143A mutation in cytb is associated with QOI resistance in C. beticola. Accurate monitoring of this mutation will be essential for fungicide resistance management in this pathosystem.