Background
According to Chakraborty and Datta, "changes in plant physiology, anatomy and morphology that have been implicated in increased resistance or can potentially enhance host resistance at elevated CO2 include: increased net photosynthesis allowing mobilization of resources into host resistance (Hibberd et al., 1996a.); reduced stomatal density and conductance (Hibberd et al., 1996b); greater accumulation of carbohydrates in leaves; more waxes, extra layers of epidermal cells and increased fibre content (Owensby, 1994); production of papillae and accumulation of silicon at penetration sites (Hibberd et al., 1996a); greater number of mesophyll cells (Bowes, 1993); and increased biosynthesis of phenolics (Hartley et al., 2000), among others."  Now, the authors of this important new study describe yet another way in which atmospheric CO2 enrichment may tip the scales in favor of plants.

What was done
Chakraborty and Datta studied the aggressiveness of the fungal anthracnose pathogen Colletotrichum gloeosporioides (Penz.) Penz. & Sacc by inoculating two isolates of the pathogen onto two cultivars of the tropical pasture legume Stylosanthes scabra Vog. (Fitzroy, which is susceptible to the fungal pathogen, and Seca, which is more resistant) over 25 sequential infection cycles at ambient (350 ppm) and elevated (700 ppm) atmospheric CO2 concentrations in controlled environment chambers.

What was learned
It was determined that the aggressiveness of the pathogen population was reduced at the twice-ambient level of atmospheric CO2, where aggressiveness is defined as "a property of the pathogen reflecting the relative amount of damage caused to the host without regard to resistance genes (Shaner et al., 1992)."  Specifically, the authors report that "at twice-ambient CO2 the overall level of aggressiveness of the two [pathogen] isolates was significantly reduced on both [host] cultivars."  In addition, they say that "as shown previously (Chakraborty et al., 2000), the susceptible Fitzroy develops a level of resistance to anthracnose at elevated CO2, but resistance in Seca [which is more resistant at ambient CO2] remains largely unchanged."

Simultaneously, however, pathogen fecundity was found to increase at twice-ambient CO2.  Of this finding, the authors report that their results "concur with the handful of studies that have demonstrated increased pathogen fecundity at elevated CO2 (Hibberd et al., 1996a; Klironomos et al., 1997; Chakraborty et al., 2000)."  How this happened in the situation they investigated, according to Chakraborty and Datta, is that the overall increase in fecundity at high CO2 "is a reflection of the altered canopy environment," wherein "the 30% larger S. scabra plants at high CO2 (Chakraborty et al., 2000) makes the canopy microclimate more conducive to anthracnose development."

What it means
In view of the opposing changes induced in pathogen behavior by elevated levels of atmospheric CO2 - reduced aggressiveness but increased fecundity - it is difficult to know the ultimate outcome of atmospheric CO2 enrichment for the pathogen-host relationship.  More research, especially under realistic field conditions, will be needed to clarify the situation; and, of course, different results are likely to be observed for different pathogen-host associations.  What is more, results could also differ under different climatic conditions.  Nevertheless, the large number of ways in which elevated CO2 has been demonstrated to increase plant resistance to pathogen attack (see Background) suggests that plants may well gain the advantage as the air's CO2 content continues to climb in the years ahead.

References
Bowes, G.  1993.  Facing the inevitable: Plants and increasing atmospheric CO2.  Annual Review of Plant Physiology and Plant Molecular Biology 44: 309-332.

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

Hartley, S.E., Jones, C.G. and Couper, G.C.  2000.  Biosynthesis of plant phenolic compounds in elevated atmospheric CO2.  Global Change Biology 6: 497-506.

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

Hibberd, J.M., Whitbread, R. and Farrar, J.F.  1996b.  Effect of 700 µmol per mol CO2 and infection of powdery mildew on the growth and partitioning of barley.  New Phytologist 134: 309-345.

Klironomos, J.N., Rillig, M.C., Allen, M.F., Zak, D.R., Kubiske, M. and Pregitzer, K.S.  1997.  Soil fungal-arthropod responses to Populus tremuloides grown under enriched atmospheric CO2 under field conditions.  Global Change Biology 3: 473-478.

Owensby, C.E.  1994.  Climate change and grasslands: ecosystem-level responses to elevated carbon dioxide.  Proceedings of the XVII International Grassland Congress.  Palmerston North, New Zealand: New Zealand Grassland Association, pp. 1119-1124.

Shaner, G., Stromberg, E.L., Lacy, G.H., Barker, K.R. and Pirone, T.P.  1992.  Nomenclature and concepts of aggressiveness and virulence.  Annual Review of Phytopathology 30: 47-66.