Elevated CO₂ drives nonlinear trait responses and reshapes growth-virulence trade-offs in a plant pathogen

Atmospheric CO₂ is rising at unprecedented rates. While its physiological effects on plants are well-documented, the direct evolutionary impact on plant pathogens remains unknown. We investigated how CO₂ interacts with other abiotic factors to shape the evolutionary potential of pathogens. We combined controlled CO₂ gradient experiments with quantitative genetics to assess the interactive effects of CO₂, temperature and geography on Phytophthora infestans. We measured intrinsic growth rate and aggressiveness, the two key pathogen traits, across seven geographically distinct populations and partitioned their phenotypic variance into heritability and plasticity components. Both intrinsic growth rate and aggressiveness showed a shared quadratic response to CO₂, peaking c. 2000 ppm before declining. Elevated CO₂ reduced phenotypic variance in aggressiveness under extreme concentrations. CO₂ also reconfigured the life-history correlation between growth and aggressiveness, shifting it from positive under ambient CO₂ to negative near the optimum. Heritability exceeded plasticity for aggressiveness in most populations, indicating that virulence responds primarily through selection. Trait responses were modulated by local climate and geography, with the strongest CO₂-driven increases occurring under thermal and geographic extremes. Our results demonstrate that rising CO₂ acts as a direct agent of selection, nonlinearly reshaping pathogen phenotypic architecture and life-history strategies in a geographically heterogeneous manner.