G143A Mutation Identified in Diaporthe helianthi Causing Phomopsis Stem Canker in Sunflower

Phomopsis stem canker, primarily caused by Diaporthe gulyae and D. helianthi, is a significant disease of sunflowers ( Helianthus annuus L.), leading to yield losses of up to 40%. Foliar fungicides containing pyraclostrobin (quinone outside inhibitors [QoI]) are recommended for disease management; however, their widespread use poses a significant risk of selecting for resistant fungal strains. In this study, a cytochrome b (cyt b) gene-based polymerase chain reaction assay was developed to identify QoI-resistant strains of D. gulyae and D. helianthi. Given that the cyt b gene sequence for D. gulyae and D. helianthi was not available in the National Center for Biotechnology Information (NCBI) database, forward and reverse primers for the assay were designed based on the cyt b sequence of D. longicolla, a fungus within the Diaporthe genus. A total of 182 isolates of D. gulyae and D. helianthi, collected from six U.S. states, were cultured in potato dextrose broth, and genomic DNA was extracted. The cyt b gene fragments from the isolates were amplified, sequenced, assembled, and analyzed for mutations. Our findings confirmed a point mutation in the cyt b gene at codon 143 (GGT→GCT, G143A) in one isolate of D. helianthi recovered from diseased sunflower stalks in Polk County, MN, U.S.A., in 2019. The detection of the G143A mutation in a D. helianthi isolate emphasizes the need for monitoring QoI-resistant strains to maintain the effectiveness of QoI fungicides in disease management. [Formula: see text]