Identification of sunflower (Helianthus annuus) accessions resistant to Diaporthe helianthi and Diaporthe gulyae.
Phomopsis stem canker is an important disease on sunflower (Helianthus annuus) in Minnesota, North Dakota, and South Dakota. To date, Diaporthe helianthi and D. gulyae have been reported as the common and primary causal agents in this region. The objectives of this study were to compare aggressiveness of the two species and evaluate the USDA cultivated sunflower germplasm collection for resistance to the two pathogens. Five D. helianthi isolates and five D. gulyae isolates were compared for their aggressiveness on the USDA susceptible confection inbred 'HA 288' using the stem-wound inoculation method. The results of this study demonstrated that D. helianthi and D. gulyae isolates did not vary significantly (P>0.05) in their aggressiveness. Using a representative isolate of D. helianthi and D. gulyae, 49 accessions were screened for Phomopsis stem canker using the stem-wound method in the greenhouse. Among the 49 accessions, 13 and 4 accessions were less susceptible to D. helianthi and D. gulyae, respectively, compared with HA 288. Only one accession (PI 552939) was observed to be significantly less susceptible to both D. helianthi and D. gulyae when compared with HA 288.