Genetic and pathogenic diversity among isolates of Diaporthe helianthi causing Phomopsis stem canker on cultivated sunflower

Authors: Underwood, William

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2025
Publication Date: 3/12/2025
Citation: Underwood, W. 2025. Genetic and pathogenic diversity among isolates of Diaporthe helianthi causing Phomopsis stem canker on cultivated sunflower. Plant Pathology. 74:956-967. https://doi.org/10.1111/ppa.14061.
DOI: https://doi.org/10.1111/ppa.14061

Interpretive Summary: Phomopsis stem canker, caused by the fungal pathogen Diaporthe helianthi, is a destructive disease that limits production of cultivated sunflower globally. In the U.S., this disease has increased in prevalence over the past fifteen years, causing frequent yield losses. Although the disease has impacted sunflower production for decades, relatively little is known about the fungus that causes Phomopsis stem canker or the nature of sunflower resistance to the disease. In this study, Phomopsis stem canker lesions were collected from commercial sunflower fields across the major sunflower growing states of Minnesota, North Dakota, and South Dakota over two years. The fungus was isolated from disease lesions to determine the causal species and evaluate genetic and pathogenic variation among isolates of Diaporthe helianthi. Isolates across the three states were found to form a single population, with no apparent genetic differentiation among isolates collected in different states. However, isolates varied considerably in their ability to infect sunflower lines with known resistance to stem lesion formation. Most resistant sunflower lines exhibited isolate-specific resistance and were susceptible to some isolates. However, four sunflower lines exhibited strong or moderate resistance to all of the twenty isolates tested, indicating that these lines should be prioritized as sources of resistance in future sunflower breeding efforts.

Technical Abstract: Phomopsis stem canker disease of cultivated sunflower (Helianthus annuus L.) is most commonly caused by the fungal pathogen Diaporthe helianthi. This disease is characterized by brown stem lesions centered 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 two 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 behavior. To determine if sunflower resistance to stem lesion expansion is isolate-specific, fifteen sunflower lines previously determined to exhibit resistance to stem lesioning after inoculation with D. helianthi were evaluated for resistance to twenty D. helianthi isolates. Only four 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.