Disease epidemics can profoundly shape the ecosystems of which they are a part, affecting productivity, nutrient cycling, community composition, and species interactions. Recent changes to global ecosystems can modify the effects of disease on organisms or ecosystems, but the nature and extent of these modifications are thus far little understood, particularly for plant viruses. Among the important drivers of complex changes in ecosystems are changing levels of atmospheric gases including the greenhouse gas carbon dioxide and the oxidizing agent ozone. I used open top chambers in the field to examine the effects of rising carbon dioxide and ozone levels on the spread and severity of barley yellow dwarf virus (BYDV), an economically and environmentally important pathogen of plants in the grass tribe. I examined the effects on viral fitness and viral spread in monocultures of the epidemiologically important host Avena fatua and in mixtures of A. fatua and Setaria lutescens, a poor host of the pathogen. In monocultures of A. fatua, within-host viral fitness declined with ozone concentrations, but addition of carbon dioxide to ozone restored within-host viral fitness to ambient levels. Despite reduced virus concentration in ozonated plants, transmission of the virus did not significantly decrease with gas treatment, suggesting that no abatement of BYDV epidemics is likely under future atmospheric conditions. Ozone also reduced host vegetative and reproductive biomass in monocultures. While adding carbon dioxide to the ozone restored vegetative biomass to ambient levels, it did not compensate for losses in reproduction. If this pattern extended to crop species, yield of major grain crops such as wheat, oats, and barley could be severely reduced. In mixtures of Avena and Setaria, within-host viral fitness and transmission iii rates in both Avena and Setaria were enhanced. In addition, when Avena and Setaria were grown in competition, carbon dioxide reduced the benefits to Avena of growing in mixtures across infection treatments. Although presence in mixtures significantly suppressed growth in Setaria, CO2 increased reproductive output in infected plants. These results highlight the potential importance of disease in plant competition under changing global atmospheres. Overall, these studies demonstrate the complex interactions between atmospheric conditions and community context that are likely to regulate both disease establishment and plant health under future atmospheric conditions. iv