Most plant toxicology tests developed in support of environmental laws use a single stress applied to an individual plant. While tests using individual species or stresses require fewer resources and are easier to interpret, they are under increasing criticism for being unrealistic and missing important ecological interactions. The objective of this research was to increase our understanding of how plants and plant communities respond to a variety of stressors. Model plant communities of spring wheat (Triticum aestivum) and wild oats (Avena fatua) were planted at three densities and five proportions in the field. Puccinia recondita, the causal agent of wheat leaf rust, was inoculated on half of the plots. Disease severity was estimated as percent of wheat flag leaves covered by rust lesions. Plants were harvested at maturity and measured. Seeding density rarely had a significant influence on rust severity, probably because tiller density differed little as a result of compensation due to increased tillering at low seeding densities. In contrast, increasing the proportion of wheat in mixtures with wild oats consistently increased wheat leaf rust severity. There was no evidence to suggest that wild oats acted as a barrier to inoculum movement. Wild oats' effect on wheat leaf rust was probably through its competitive reduction of wheat tiller density. Both wheat and wild oats seed weight decreased as the proportion of wild oats increased in mixtures. This indicates that intraspecific competition was stronger in wild oats than was intraspecific competition with wheat in these mixtures. Wild oats generally did not respond to the presence of leaf rust on wheat, while wheat was negatively impacted. Thus, there was little competitive advantage to wild oats when its competitor (wheat) was diseased. A small subset of the field treatments was treated with ozone, because of the limited space available in the open-top ozone exposure chambers. Wheat height and aboveground biomass generally decreased with ozone exposure and with increasing disease severity in both years, while total grain weight decreased significantly only with disease and only in one year. There was no interaction between ozone and disease, regardless of cultivar, density, or plant response variable measured. There was little evidence that ozone exposure affected the severity of wheat leaf rust. In general, there seemed to be a lack of interactions among the different stressors and the results varied considerably depending on year and wheat cultivar. / Graduation date: 1998