McElrone et al. (2005) begin the report of their impressive work by noting that plant pathogens "drastically reduce plant growth in agricultural and natural ecosystems worldwide," and that "estimates of crop losses to all plant pathogens in the United States alone are ~$33 billion annually (Pimentel et al., 2000)," which indicates that what they did and what they found is of much more than mere academic interest; it cuts to the core of what the world will be like in a not-far-distant day.

So just what did they do? And what did they find?

McElrone et al. say they "assessed how elevated CO2 affects a foliar fungal pathogen, Phyllosticta minima, of Acer rubrum [red maple] growing in the understory at the Duke Forest free-air CO2 enrichment experiment in Durham, North Carolina, USA … in the 6th, 7th, and 8th years of the CO2 exposure." Surveys conducted in those years, in their words, "revealed that elevated CO2 [to 200 ppm above ambient] significantly reduced disease incidence, with 22%, 27% and 8% fewer saplings and 14%, 4%, and 5% fewer leaves infected per plant in the three consecutive years, respectively." In addition, they report that the elevated CO2 "also significantly reduced disease severity in infected plants in all years (e.g. mean lesion area reduced 35%, 50%, and 10% in 2002, 2003, and 2004, respectively)."

What underlying mechanism or mechanisms produced these beneficent consequences?

Thinking it could have been a direct deleterious effect of elevated CO2 on the fungal pathogen, McElrone et al. performed some side experiments in controlled environment chambers. However, they found that the elevated CO2 benefited the fungal pathogen as well as the red maple saplings, observing that "exponential growth rates of P. minima were 17% greater under elevated CO2." And they obtained similar results when they repeated the in vitro growth analysis two additional times in different growth chambers.

Taking another tack when "scanning electron micrographs verified that conidia germ tubes of P. minima infect A. rubrum leaves by entering through the stomata," the researchers turned their attention to the pathogen's mode of entry into the saplings' foliage. In this investigation they found that both stomatal size and density were unaffected by atmospheric CO2 enrichment, but that "stomatal conductance was reduced by 21-36% under elevated CO2, providing smaller openings for infecting germ tubes." In addition, they concluded that reduced disease severity under elevated CO2 was also likely due to altered leaf chemistry, as elevated CO2 increased total leaf phenolic concentrations by 15% and tannin concentrations by 14%.

Because the phenomena they found to be important in reducing the amount and severity of fungal pathogen infection (leaf spot disease) of red maple have been demonstrated to be operative in most other plants as well, McElrone et al. say these CO2-enhanced leaf defensive mechanisms "may be prevalent in many plant pathosystems where the pathogen targets the stomata." Indeed, they state that their results "provide concrete evidence for a potentially generalizable mechanism to predict disease outcomes in other pathosystems under future climatic conditions." And because of their insightful work, that future is looking particularly bright in terms of the never-ending struggle between earth's plants and the pathogens that prey upon them. Although elevated CO2 helps both sides of the conflict, it helps plants more and in more ways.

Sherwood, Keith and Craig Idso

References
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.

Pimentel, D., Lach, L., Zuniga, R., et al. 2000. Environmental and economic costs of nonindigenous species in the United States. Bioscience 50: 53-65.