Citation: Atallah, Z.K., Larget, B., Chen, X., Johnson, D.A. 2004. High genetic diversity, phenotypic uniformity, and evidence of outcrossing in sclerotinia sclerotitorum in the columbia basin of washington state. Phytopathology. 94(7):737-742. Interpretive Summary: Stem rot, caused by the fungal pathogen Sclerotinia sclerotiorum, is an important disease on potatoes in the Columbia Basin of Washington. Because of the lack of knowledge on population structures of the pathogen, the disease is poorly managed. In this study, samples of the fungus were collected from the major potato-growing region and used to determine the population structure. Using molecular markers, we found that the major variance of the pathogen was within subpopulations. Limited but statistically significant variance was found among different collection dates. Significant difference was not found in different years and locations. The data also suggested a potential for outcrossing, which also supported by the discovery of recombinant sexual spores of the fungus. The molecular groups were not correlated with the mycelial groups. Aggressiveness was not different among the fungal isolates in disease testing in the greenhouse. Testing with fungicides and temperatures did not yield significant difference among isolates. The findings in this study will be helpful in improving the disease management strategies.