The effect of elevated atmospheric CO2 concentration on rice blast and sheath blight disease severity was studied in the field in northern Japan for 3 years. With free-air CO2 enrichment (FACE), rice plants were grown in ambient and elevated (≈200 to 280 µmol mol–1 above ambient) CO2 concentrations, and were artificially inoculated with consist of Magnaporthe oryzae. Rice plants grown in an elevated CO2 concentration were more susceptible to leaf blast than those in ambient CO2 as indicated by the increased number of leaf blast lesions. Plants grown under elevated CO2 concentration had lower leaf silicon content, which may have contributed to the increased susceptibility to leaf blast under elevated CO2