Genetic and Pathogenic Diversity Among Isolates of <i>Diaporthe helianthi</i> Causing Phomopsis Stem Canker on Cultivated Sunflower

ABSTRACT Phomopsis stem canker disease of cultivated sunflower ( Helianthus annuus ) is most commonly caused by the fungal pathogen Diaporthe helianthi . This disease is characterised by brown stem lesions centred around petioles accompanied by destruction of the pith, often resulting in plant death. In this study, Phomopsis stem canker disease lesions were collected from commercial sunflower fields in Minnesota, North Dakota, and South Dakota over 2 years in 2018 and 2019. The Diaporthe species associated with the disease lesion was isolated and identified for 363 total samples, and D. helianthi was isolated from 93% of lesions, with the remaining lesions primarily associated with D . gulyae or both D. helianthi and D . gulyae . Population analyses of 280 D. helianthi isolates, genotyped using genotyping‐by‐sequencing, revealed no significant differentiation among populations across the three states and indicated primarily outcrossing reproductive behaviour. To determine if sunflower resistance to stem lesion expansion is isolate‐specific, 15 sunflower lines previously determined to exhibit resistance to stem lesioning after inoculation with D. helianthi were evaluated for resistance to 20 D. helianthi isolates. Only five sunflower lines were strongly or moderately resistant to all isolates, while the remaining lines exhibited isolate‐specific resistance. These results will be useful to facilitate breeding efforts aimed at improving sunflower resistance to Phomopsis stem canker.