Effects of Elevated CO2 and Climate Change on Leaf Spot Disease of Redbud and Sweetgum Trees
McElrone, A.J., Hamilton, J.G., Krafnick, A.J., Aldea, M., Knepp, R.G. and DeLucia, E.H. 2010. Combined effects of elevated CO2 and natural climatic variation on leaf spot diseases of redbud and sweetgum trees. Environmental Pollution 158: 108-114.
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."
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.
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