What was done
Tomato (Lycopersicon esculentum Mill. Cv Rutgers) plants were grown hydroponically for eight weeks in controlled environment chambers receiving atmospheric CO2 concentrations of 350 and 700 ppm.  At week five of the study, half of all plants growing in each CO2 concentration were infected with the fungal pathogen Phytophthora parasitica, which attacks plant roots and induces water stress that ultimately decreases plant growth and yield.

What was learned
Pathogenic infection reduced total plant biomass by nearly 30% at both atmospheric CO2 concentrations.  However, elevated CO2 increased the total biomass of healthy and infected plants by approximately the same degree (+30%).  In other words, infected tomato plants grown at 700 ppm CO2 had biomass values that were similar to those of healthy tomato plants grown at 350 ppm CO2.  Thus, atmospheric CO2 enrichment completely counterbalanced the negative effects of pathogenic infection on overall plant productivity.

What it means
As the air's CO2 concentration increases, it is likely that tomato and other agricultural crops that are susceptible to attack by the pathogen Phytophthora parasitica will exhibit enhanced protection against its growth-retarding influences.  Data from the present study, for example, demonstrate that the negative effects of pathogenic attack on final plant biomass were completely alleviated by a doubling of the air's CO2 concentration.  Thus, the agricultural industry can anticipate increased harvests in a future CO2-enriched world due to reductions in yield loss caused by this pathogenic fungal organism.