Thesis (M.S., Natural Resources) -- University of Idaho, 2017 | Microbes that asymptomatically colonize the internal tissues and organs of plants are endophytes. Endophytes sometimes benefit their hosts by mediating responses to stress. Variation in endophyte interactions with hosts is observed depending on environmental factors. Here, variation in endophyte interactions is examined in wheat. First, functional roles of four Penicillium isolates were explored in wheat, cheatgrass, and cottonwood hosts. Next, demonstrated antagonists of Fusarium crown rot in wheat were taken to a field location to replicate antagonism; however, conditions were not conductive to disease development. Finally, Fusarium crown rot antagonists were applied to Fusarium head blight. Some inoculants were facilitators and antagonists of disease severity. Tolerance was observed in previously classified susceptible and moderately resistant. The disease facilitator and antagonists also contributed to tolerance as significant differences in disease levels were observed in inoculant treatments, but fecundity was not impacted. This may be the first report of endophytes as contributors to tolerance.