Sclerotinia sclerotiorum is one of the most destructive pathogens of sunflower in the United States and worldwide. Distinctive symptoms include mid-stalk rot, basal stalk rot and head rot from subsequent infection of the sunflower head. This fungal pathogen has a remarkably broad host range of over 400 dicot plants. However, little is currently known about the virulence strategies that allow S. sclerotiorum to successfully infect a wide range of plant hosts. The goal of our project was to identify S. sclerotiorum virulence determinants and effectors that contribute to disease development on sunflower. We evaluated a diverse collection of 232 S. sclerotiorum isolates for aggressiveness of mid-stalk lesion formation on two sunflower inbred lines. In addition, we performed genotyping-by-sequencing on 191 isolates to identify genetic markers for genome-wide association mapping to identify candidate genes associated with aggressiveness. A total of eight loci associated with variation in aggressiveness were identified.