Characterization of Genetic Resistance to Sclerotinia sclerotiorum and Epidemiology of the Disease in Brassica napus L.

This dissertation contains three research chapters conducted on Sclerotinia stem rot (SSR) of canola (Brassica napus L.). This disease is caused by the fungus Sclerotinia sclerotiorum and is considered endemic in canola-producing areas of North Dakota. The first research chapter presents results of a study that evaluated the role of eight phenotyping scoring systems and nine variant calling and filtering methods in detection of QTL associated with response to SSR. The study, conducted on two doubled-haploid mapping populations, showed that using multiple phenotypic data sets derived from lesion length and plant mortality and imputing missing genotypic data increased the number of QTL detected without negatively affecting the effect (R2) of QTL. Nineteen QTL were detected on chromosomes A02, A07, A09, C01, and C03 in this study. The second research chapter presents results of a work that assessed the role of temperature regimes and wetness duration on S. sclerotiorum ascospore germination and ascosporic infection efficiency. This study showed that optimum ascospore germination occurred at 21 ?C while it significantly decreased at 10 and 30 ?C. Infection efficacy experiments indicated that extreme temperatures and interrupting wet periods were detrimental for the disease development. A logistic regression model with 75% accuracy was developed for the disease perdition. The third research chapter presents results of a study that evaluated the role of temperature on mycelial growth of 19 S. sclerotiorum isolates collected from different geographical regions and on SSR development on plant introduction (PI) lines with different levels of resistance. Mycelial growth and disease development peaked at 25 ?C. While lesion expansion on resistant cultivars and the susceptible check was negatively affected at 30 ?C, the disease developed significantly on the PI with a high level of susceptibility. Results of these studies provide insights into integrated management strategies of SSR