What was done
In a study conducted within the BioCON (Biodiversity, Carbon dioxide, and Nitrogen effects on ecosystem functioning) FACE experimental setup located at the Cedar Creek Natural History Area in east-central Minnesota, USA, the authors evaluated the effects of an approximate 190-ppm increase in the atmosphere's daytime CO2 concentration on leaf photosynthetic rates of the common Stiff Goldenrod herb (Solidago rigida) growing in monoculture for two full seasons, together with its concomitant effects on the incidence and severity of leaf spot disease on the herb.

What was learned
Although elevated CO2 had no significant effect on plant photosynthetic rate in their study, Strengbom and Reich report that "both disease incidence and severity were lower on plants grown under elevated CO2." More specifically, they found that "disease incidence was on average more than twice as high [our italics] under ambient as under elevated CO2," and that "disease severity (proportion of leaf area with lesions) was on average 67% lower under elevated CO2 compared to ambient conditions." Also, diseased leaves of both CO2 treatments had only half the photosynthetic rates of healthy leaves.

What it means
In this most interesting study, Strengbom and Reich discovered that the "indirect effects from elevated CO2, i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher CO2," which as noted above was negligible. Hence, the two researchers conclude that "it may be necessary to consider potential changes in susceptibility to foliar diseases to correctly estimate the effects on plant photosynthetic rates of elevated CO2."

On another note, Strengbom and Reich report that the plants grown in CO2-enriched air had lower leaf nitrogen concentrations than the plants grown in ambient air, as is often observed in studies of this type, and that their results "are, thus, also in accordance with other studies that have found reduced pathogen performance following reduced nitrogen concentration in plants grown under elevated CO2 (Thompson and Drake, 1994)." In addition, they conclude that their results are "also in accordance with studies that have found increased [disease] susceptibility following increased nitrogen concentration of host plants (Huber and Watson, 1974; Nordin et al., 1998; Strengbom et al., 2002)." It is possible, therefore, that the ongoing rise in the air's CO2 concentration may help many plants of the future reduce the deleterious impacts of various pathogenic fungal diseases that currently beset them and thereby increase their productivities above and beyond what is typically provided by the more direct growth stimulation resulting from the aerial fertilization effect of elevated atmospheric CO2 concentrations.

References
Huber, D.M. and Watson, R.D. 1974. Nitrogen form and plant disease. Annual Reviews of Phytopathology 12: 139-155.

Nordin, A., Nasholm, T. and Ericson, L. 1998. Effects of simulated N deposition on understorey vegetation of a boreal coniferous forest. Functional Ecology 12: 691-699.

Strengbom, J., Nordin, A., Nasholm, T. and Ericson, L. 2002. Parasitic fungus mediates change in nitrogen-exposed boreal forest vegetation. Journal of Ecology 90: 61-67.

Thompson, G.B. and Drake, B.G. 1994. Insect and fungi on a C3 sedge and a C4 grass exposed to elevated atmospheric CO2 concentrations in open-top chambers in the field. Plant, Cell and Environment 17: 1161-1167.