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Title: Phomopsis stem canker: a re-emerging threat to sunflowers (Helianthus annuus) in the United States

Author
item MATHEW, FEBINA - North Dakota State University
item ALANANBEH, KHOLOUD - North Dakota State University
item JORDAHL, JORDAN - North Dakota State University
item MEYER, SCOTT - North Dakota State University
item Castlebury, Lisa
item GULYA, THOMAS - Retired ARS Employee
item MARKELL, SAMUEL - North Dakota State University

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2015
Publication Date: 7/1/2015
Publication URL: http://handle.nal.usda.gov/10113/61915
Citation: Mathew, F.M., Alananbeh, K.M., Jordahl, J.G., Meyer, S.M., Castlebury, L.A., Gulya, T.J., Markell, S.G. 2015. Phomopsis stem canker: a re-emerging threat to sunflowers (Helianthus annuus) in the United States. Phytopathology. 105(7):990-997.

Interpretive Summary: Phomopsis stem canker caused by Diaporthe helianthi is an economically important diseases in some U.S. states on sunflowers as well as in other parts of the world. Phomopsis stem canker has increased 16 fold in the Northern Great Plains of the United States in recent years. To determine if Diaporthe helianthi was causing this disease, fungal cultures were isolated from diseased stems of sunflowers. Using DNA sequences and spore characteristics, we identified the two causal agents of the stem canker as Diaporthe gulyae and D. helianthi. Results indicated that both species were equally aggressive pathogens of sunflower. This research will be used by extension agents, plant breeders, and plant pathologists to implement management strategies for stem diseases of these three crop plants.

Technical Abstract: Phomopsis stem canker frequently causes yield reductions on sunflowers (Helianthus annuus L.) on several continents, including Australia, Russia, Europe and North America. Between 2001 and 2012, the incidence of Phomopsis stem canker has increased 16 fold in the Northern Great Plains of the United States. While Diaporthe helianthi was assumed to be the sole causal agent worldwide, a recently characterized pathogen, Diaporthe gulyae was found to be the primary cause of the disease in Australia. To determine the identity of Diaporthe spp. causing stem canker on sunflowers in the Northern Great Plains, 234 isolates were cultured from 275 infected stems collected during a three year (2010-2012) survey. Phylogenetic analyses of sequences of the ribosomal DNA internal transcribed spacer region, elongation factor subunit 1-a, and actin gene regions, in comparison with those of type specimens, confirmed two species (D. helianthi and D. gulyae) in the United States. Four methods were tested to assess the Phomopsis stem canker response using mycelial plugs of four D. helianthi isolates on a susceptible sunflower plant. Based on a higher mean recovery of D. helianthi isolates from the inoculated plants, the stem-wound method was adopted to compare aggressiveness between Diaporthe species and screening genotypes for stem resistance. The results demonstrated that D. helianthi and D. gulyae did not vary significantly (p=0.05) in their aggressiveness at 10-d and 14-d after inoculations.