Although the harmful effect of disease on agricultural crops and livestock is well-known, scientists are just beginning to recognize the importance of disease in wild plants and animals. In addition to killing infected individuals and reducing populations of plants and animals, diseases can affect the species composition of ecological communities. For example, when species compete for resources, such as food and water, and also share a disease, the disease can prevent the better competitor from dominating and allow the species to coexist. Alternatively, the disease could wipe out the less competitive species more quickly. This study will investigate whether a fungal disease that kills grass seeds is contributing to the invasion and dominance of a harmful exotic grass species, cheatgrass, in the U.S. Intermountain West. The invasive grass degrades rangeland and promotes fire, but also spreads a fungal disease that kills seeds of native grasses as well as cheatgrass. The experiment will measure the disease in field populations in Utah, and calculate its impact on cheatgrass invasion and native plant persistence using mathematical models. The study will also manipulate fall rainfall to determine whether the disease's impacts depend on climate conditions. This research will improve scientific understanding of the effect of disease on a harmful grass invasion. The results will inform management decisions based on using disease to combat the invasive grass. Expanding scientific understanding of the impacts of wildlife disease will also aid conservation of other communities of interest such as tropical forests and coral reef fishes. In addition to its scientific impacts, this project will promote research experience for undergraduates. The research will also be incorporated into teaching materials for undergraduate courses. This project has already provided research experience for five undergraduates (four female), and it will result in the completion of a doctoral dissertation by a female mathematical biologist, an underrepresented minority in this field.