Fungi and bacteria can be found within plant roots, shoots, and leaves, some of which can cause disease or be plant growth promoting. The interactions of fungi and bacteria can change whether and how much a single microbe can be beneficial or detrimental to a plant, impacting the yield of crops and health of forests. Recently, many plant-associated fungi were found to have bacteria living inside of them; these "endohyphal" bacteria (EHB) can impact whether the fungus can make toxins or reproduce. However, there have been no broad screens for the presence or consequences of EHB in pathogenic Fusarium spp., despite the major diseases caused by these fungi across many crops in the United States, from cereals to vegetables. Building on ecological EHB and genomics expertise at the University of Arizona, the PD proposes to survey twoFusarium spp. that infect small grains and lettucefor the presence of EHB and determine if EHB contribute to pathogenicity. Additionally, the PD will identify bacterial genes necessary for fungal infection using RB-TnSeq, a high throughput transposon mutagenesis technique. Making agriculture more sustainable and resilient relies on improved tool development to overcome mounting disease pressure, changing climate, increasing public opposition to chemical usage, and pathogen resistance to synthetic chemicals. A more thorough understanding of the prevalence, impact, and genetic underpinnings of interactions between destructive fungi and EHB could provide new avenues for control, which is of increasing importance given decreasing efficacy of reliable antifungal treatments.