Technical Abstract: Nearest neighbor spatial analysis was used to assess the effect of systemic rust caused by Uromyces scutellatus on stands density of Euphorbia esula/virgata, a highly invasive deep-rooted perennial weed of rangelands and natural areas in North America. ANOVA applied to nearest neighbor measurements within four pairs of stands in close proximity, with and without rust, in Hungary and Austria indicated that the stand densities of plants of E. esula/virgata in three of four rusted stands were less than companion stands with little or no rust. Using the nearest neighbor distance data, E. esula/virgata densities within stands where rust was prevalent were 48-73% of those with little or no rust. The fourth stand with rust was denser than a symptomless companion stand in 2004, but nearly all plants had symptoms. The same diseased stand was dramatically reduced in density when surveyed the following year and all plants observed in 2005 displayed rust symptoms. These findings indicate the potential impact of the microcyclic autoecious rust U. scutellatus should it be introduced as a biocontrol agent for E. esula/virgata in North America.