Genome-Wide Association Mapping in Sunflower ( Helianthus annuus ) Reveals Common Loci and Putative Candidate Genes for Resistance to Diaporthe gulyae and D. helianthi Causing Phomopsis Stem Canker

and cause Phomopsis stem canker of sunflower ( L.) in the United States. Because Phomopsis stem canker did not gain importance until the disease epidemic in 2010, limited studies were conducted to understand the genetic basis of sunflower resistance to and . The objectives of this study were to evaluate the United States Department of Agriculture cultivated accessions for resistance to and as well as to utilize genome-wide association studies (GWAS) to identify quantitative trait loci (QTLs) and putative candidate genes underlying those loci common to both organisms. For each fungus, 213 accessions were screened in a complete randomized design in the greenhouse and the experiment was repeated once. Six plants per accession were inoculated with a single isolate of or at four to six true leaves using the mycelium-contact inoculation method. At 15 days ( ) and 30 days ( ) postinoculation, accessions were evaluated for disease severity and compared with the susceptible confection inbred PI 552934. GWAS identified 28 QTLs common to the two fungi, and 24 genes overlapped close to these QTLs. Additionally, it was observed that the resistance QTLs derived mainly from landraces rather than from wild species. Seventeen putative candidate genes associated with resistance to or were identified that may be related to plant-pathogen interactions. These findings advanced our understanding of the genetic basis of resistance to and and will help develop resources for genomics-assisted breeding.