Background
Leaf spot disease, which is characterized by chlorotic to necrotic localized leaf lesions, is caused by the Cercospora (a large genus of ascomycete fungi) that affect, in the words of the authors, "numerous economically important plant species around the world, including grapes, cereals, soybeans, peanuts, orchids, coffee, alfalfa and potatoes (Sinclair et al., 1987)," as well as redbud (Cercis canadensis) and sweetgum (Liquidambar styraciflua) trees, such as those growing at the Duke Forest Face Facility in Orange County, North Carolina (USA), where McElrone et al. studied the disease throughout the growing seasons of five different years.

What was done
For a period of five years (2000, 2001, 2002, 2003, 2005), the six scientists assessed how elevated CO2 (to 200 ppm above the ambient air's CO2 concentration) and natural interannual climatic variability affected the incidence and severity of leaf spot disease among the sweetgum and redbud trees growing in the several FACE rings at the Duke Forest experimental site, while in order "to determine how photosynthetic capacity surrounding pathogen damage was affected by CO2 exposure, the spatial pattern of photosystem II operating efficiency was quantified on C. canadensis leaves still attached to plants with an imaging chlorophyll fluorometer."

What was learned
McElrone et al. report that "disease incidence and severity for both species were greater in years with above average rainfall," while "in years with above average temperatures, disease incidence for Liquidambar styraciflua was decreased significantly." On the other hand, they found that elevated CO2 increased disease incidence and severity "in some years." However, they say that the "chlorophyll fluorescence imaging of leaves revealed that any visible increase in disease severity induced by elevated CO2 was mitigated by higher photosynthetic efficiency in the remaining undamaged leaf tissue and in a halo surrounding lesions."

What it means
Even in a situation where atmospheric CO2 enrichment was observed to sometimes increase the incidence and severity of leaf spot disease, the photosynthesis-enhancing effect of the extra CO2 was found to compensate for the photosynthetic productivity lost to the disease by enhancing productivity in healthy portions of diseased leaves and in leaves without lesions, for no net ill effect.

As for what has been observed in other studies of this nature, McElrone et al. report that disease incidence or severity has also been observed to be enhanced by elevated CO2 in four other "pathosystems" (Thompson and Drake, 1994; Mitchell et al., 2003; Kobayashi et al., 2006; Eastburn et al., 2009), that disease incidence or severity has been observed to be unaffected by elevated CO2 in another four pathosystems (Hibberd et al., 1996; Tiedemann and Firsching, 2000; Percy et al., 2002; Eastburn et al., 2009), and that the two disease parameters have actually been reduced by elevated CO2 in another seven pathosystems (Thompson et al., 1993; Thompson and Drake, 1994; Chakraborty et al., 2000; Jwa and Walling, 2001; Pangga et al., 2004; McElrone et al., 2005; Eastburn et al., 2009).

With respect to the conglomerate of pertinent studies that have been conducted to date, therefore, elevated CO2 has been found to lead, generally speaking, to (1) no net loss in the productivity of disease-infected plants in 31% of the studies, (2) a moderate increase in the productivity of disease-infected plants in 25% of the studies, and (3) a large increase in productivity in 44% of the studies, while the study of McElrone et al. (2010) suggests that concomitant warming may further enhance the productivity of the plants.

References
Chakraborty, S., Pangga, I.B., Lupton, J., Hart, L., Room, P.M. and Yates, D. 2000. Production and dispersal of Colletotrichum gloeosporiodies spores on Stylosanthes scabra under elevated CO2. Environmental Pollution 108: 381-387.

Eastburn, D., Degennaro, M., DeLucia, E.H., Dermody, O. and McElrone, A.J 2009. Elevated atmospheric CO2 and ozone alter soybean diseases at SoyFACE. Global Change Biology 10.1111/j.1365-2486.2009.01978x.

Hibberd, J.M., Whitbread, R. and Farrar, J.F. 1996. Effect of elevated concentrations of CO2 on infection of barley by Erysiphe graminis. Physiological and Molecular Plant Pathology 48: 37-53.

Jwa, N.S., Walling, L.L. 2001. Influence of elevated CO2 concentration on disease development in tomato. New Phytologist 149: 509-518.

Kobayashi, T., Ishiguro, K., Nakajima, T., Kim, H.Y., Okada, M. and Kobayashi, K. 2006. Effects of elevated atmospheric CO2 concentration on the infection of rice blast and sheath blight. Phytopathology 96: 425-431.

McElrone, A.J., Reid, C.D., Hoye, K.A., Hart, E. and Jackson, R.B. 2005. Elevated CO2 reduces disease incidence and severity of a red maple fungal pathogen via changes in host physiology and leaf chemistry. Global Change Biology 11: 1828-1836.

Mitchell, C.E., Reich, P.B., Tilman, D. and Groth, J.V. 2003. Effects of elevated CO2, nitrogen deposition, and decreased species diversity on foliar fungal plant disease. Global Change Biology 9: 438-451.

Pangga, I.B., Chakraborty, S. and Yates, D. 2004. Canopy size and induced resistance in Stylosanthes scabra determine anthracnose severity at high CO2. Phytopathology 94: 221-227.

Percy, K.E., Awmack, C.S., Lindroth, R.L., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F. 2002. Altered performance of forest pests under atmospheres enriched by CO2 and O3. Nature 420: 403-407.

Sinclair, W.A., Lyon, H.H. and Johnson, W.T. 1987. Diseases of Trees and Shrubs. Cornell University Press, Ithaca, New York, USA.

Thompson, G. B., Brown, J.K.M. and Woodward, F.I. 1993. The effects of host carbon dioxide, nitrogen and water supply on the infection of wheat by powdery mildew and aphids. Plant, Cell and Environment 16: 687-694.

Thompson, G.B. and Drake, B.G. 1994. Insects 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.

Tiedemann, A.V. and Firsching, K.H. 2000. Interactive effects of elevated ozone and carbon dioxide on growth and yield of leaf rust-infected versus non-infected wheat. Environmental Pollution 108: 357-363.