Managing Phomopsis Stem Canker of Sunflower Using Improved Diagnosis and Quantification of the Causal Pathogens

Diaporthe helianthi and Diaporthe gulyae are known causal agents of Phomopsis stem canker of sunflower. Identification of these species of Diaporthe using morphological characteristics after isolation from infected plants is unreliable and inconsistent. In this study, two TaqMan qPCR assays were designed from the translation elongation factor region (EF1-α) of D. helianthi and D. gulyae. Assay specificity was validated by examining the assays with genomic DNA of various species of Diaporthe and other plant pathogens. The assays in this study were sensitive enough to consistently detect 10 ng to 1 pg of pure DNA of D. helianthi and D. gulyae. Both qPCR assays were highly capable of detecting D. helianthi and D. gulyae from infected sunflower samples, and these results were validated by traditional isolation of the pathogens onto potato dextrose agar (PDA). The qPCR assays were used for genotype screening for stem resistance to D. helianthi and D. gulyae under greenhouse conditions and 16 genotypes had lower pathogen DNA indicating resistance as compared to the susceptible check, cv. HA 288. The qPCR assays developed in this study will allow for improved diagnoses of Phomopsis stem canker and to breed for resistance to D. helianthi and D. gulyae in sunflower.