ABSTRACT Elevated atmospheric carbon dioxide concentration ([CO2]) alters the physiology, growth, and development of plants. These changes in plant functioning are likely to impact the relationship between some plant pathogens and their hosts. This study examined the interactive effects of elevated [CO2] and the economically important, xylem-limited bacterial pathogen Xylella fastidiosa Wells et al. on the physiology and growth of Quercus rubra L. (red oak) seedlings. We hypothesized that growth at elevated [CO2] would ameliorate the negative consequences of infection on Q. rubra growth and physiology. Oak seedlings were inoculated with a natural strain of X. fastidiosa and grown under ‘‘ambient’’ (400 ppm) and ‘‘elevated’’ (1,000 ppm) [CO2] for 24 wk. Gas exchange, plant growth, and biomass allocation were measured to determine if elevated [CO2] alleviated the severity of X. fastidiosa–induced water stress. Xylella fastidiosa infection decreased photosynthesis, stomatal conductance, and transpiration, as well as leaf area and total biomass. Although photosynthesis increased in both noninfected and X. fastidiosa–infected plants grown at elevated [CO2], the lack of a significant [CO2]3infection interaction indicated that elevated [CO2] did not ameliorate the effects of X. fastidiosa infection on leaf-level physiology. Additionally, photosynthetic increases at elevated [CO2] did not translate to relief of infection symptoms at the whole-plant scale. The findings of this experiment did not support the hypothesis that growth at elevated [CO2] ameliorates the negative consequences of X. fastidiosa infection on seedling physiology and growth. Future studies should continue to explore the interactive effects of [CO2] and infection by X. fastidiosa on plant performance.