First report of Diaporthe caulivora causing Phomopsis stem canker on sunflower (Helianthus annuus L.) in Minnesota

Authors: HENNING, SAMUEL - Oak Ridge Institute For Science And Education (ORISE); Underwood, William

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2024
Publication Date: 2/17/2025
Citation: Henning, S., Underwood, W. 2025. First report of Diaporthe caulivora causing Phomopsis stem canker on sunflower (Helianthus annuus L.) in Minnesota. Plant Disease. 109:502. https://doi.org/10.1094/PDIS-10-24-2068-PDN.
DOI: https://doi.org/10.1094/PDIS-10-24-2068-PDN

Interpretive Summary: Phomopsis stem canker disease of cultivated sunflower is characterized by brown stem lesions, often resulting in plant death or lodging and causing substantial yield losses in years where weather conditions favor disease. This disease is of global importance and can be caused by at least 15 different species of fungi in the same genus but is typically caused by a species that specifically infects sunflower. Surveys of Phomopsis stem canker disease lesions were carried out in 2018 and 2019 in the major sunflower growing states of North Dakota, South Dakota, and Minnesota to obtain pathogen isolates and determine the causal species. Two fungal isolates obtained from disease lesions collected in a field near Crookston, MN exhibited unusual appearances when grown in culture and were considered unknown species. DNA sequencing revealed that these isolates were a species normally associated with soybean stem canker disease. Subsequent experiments confirmed that the isolates of the soybean stem canker species were able to infect sunflower when artificially inoculated onto the plants. This is the first report of this species causing Phomopsis stem canker on sunflower in the United States. These findings suggest that monitoring of this potential causal species is warranted, and further research may be needed to evaluate conditions favorable for infection by this species and determine if sunflower lines resistant to other Phomopsis pathogens are also resistant to the soybean stem canker species.

Technical Abstract: Phomopsis stem canker is a major disease limiting production of cultivated sunflower in all regions of the world where this crop is grown. In the United States, this disease has increased markedly in prevalence over the past fifteen years. Phomopsis stem canker is primarily caused by the fungus Diapothe helianthi. However, other Diaporthe species, including D. gulyae and at least thirteen additional species have been reported to cause the disease in some sunflower producing regions. Disease surveys were conducted during the 2018 and 2019 growing seasons to sample Phomopsis stem canker disease lesions from commercial sunflower fields across the major growing region comprised of North Dakota, South Dakota, and Minnesota. A total of 363 fungal isolates were obtained from survey samples. Among these, two isolates from a field near Crookston, MN exhibited morphological features distinct from either D. helianthi or D. gulyae, the most common causal agents of Phomopsis stem canker. Sequencing of the ribosomal DNA internal transcribed spacer region, the EF-1a (translation elongation factor 1a) gene and the ß-tub (ß tubulin) gene identified the unknown species as D. caulivora, a species normally associated with soybean stem canker. Greenhouse inoculations confirmed the pathogenicity of the D. caulivora isolates on sunflower. This species has previously been reported to cause Phomopsis stem canker on sunflower in Argentina. However, this is the first confirmed report of D. caulivora causing Phomopsis stem canker on sunflower in the United States. Further research on and monitoring of this species as a sunflower pathogen may be warranted.