Yellow starthistle (Centaurea solstitialis) is a major weed of western rangelands in the United States, causing severe economic and ecological harm. Efforts to control its invasions have not been successful to date, and the plant has evolved increased growth and seed production in these populations, further exacerbating the problem. We have found large differences in the microbial communities associated with this weed across its range, and evidence that the microbiome can alter weed performance. This research will explore the possibility that soil microbes from within the western United States could be used to hinder yellow starthistle invasions and simultaneously increase the success of valuable forage grasses, all while avoiding risks associated with importing biocontrol agents from other continents.Our work begins by forming collaborations with ranchers who are dealing with yellow starthistle invasions on the ground. We will work with these collaborators to identify sites in need of control and to collect seeds and microbes from these areas. These seeds and microbial collections will become part of our experiments. Our experiments will also include microbial communities that we have been selecting for their ability to reduce the growth of yellow starthistle. We will compare the effects of these different microbiomes on yellow starthistle performance, and on starthistle's competitive effects on a desirable forage grass. We will identify the microbes involved in different outcomes, and examine how the genetic makeup of the plants affect their performance in our experiments. Together, this work will advance our ultimate goal to understand whether microbes could be leveraged for control of yellow starthistle, and to lay a foundation for testing our solutions in the real world.?