Impact of Higher Atmospheric CO2 Concentrations on Tree Diseases

McElrone et al. (2010) studied the ways in which higher atmospheric CO2 concentrations, as well as variance in precipitation and temperature, are expected to affect plant diseases. The authors specifically analyzed Cercospora<!–[if supportFields]> XE “Cercospora” <![endif]–><!–[if supportFields]><![endif]–> leaf spot diseases affecting two deciduous trees: sweetgum (Liquidambar styraciflua<!–[if supportFields]> XE “Liquidambar styraciflua” <![endif]–><!–[if supportFields]><![endif]–>) and redbud (Cercis canadensis) trees. Their experiment was conducted at the Duke Forest Free-Air CO2 Enrichment (FACE) facility located in Duke Forest, North Carolina. Cercospora liquidambaris<!–[if supportFields]> XE “Cercospora liquidambaris” <![endif]–><!–[if supportFields]><![endif]–> and Cercospora cercidicola<!–[if supportFields]> XE “Cercospora cercidicola” <![endif]–><!–[if supportFields]><![endif]–> were found to be the leading pathogens infecting the redbud tree and the sweetgum tree respectively; these spot diseases affect the trees by causing lesions on the plants’ leaves. Moreover, the authors determined disease incidence and severity by analyzing a sample of random leaves. A chlorophyll fluorescence imaging analysis and leaf chemical analysis were also conducted to determine the affect of CO2concentration on these disease parameters. The results varied with the differing climatic factors. Because these diseases affect several crops that are economically significant, the results of the present study can be instrumental in implementing management strategies. —Daniela Hernandez
McElrone, A., Hamilton, J., Krafnick, A., Aldea, M., Knepp, R., DeLucia, E., 2010. Combined effects of elevated CO2and natural climatic variation on leaf spot diseases of redbud and sweetgum trees. Environmental Pollution 158, 108–114.

The researchers randomly sampled about 174 to 336 leaves per year from the different experimental plots. McElrone et al.’s experiment was conducted over the years 20002001 and the year 2005. The incidence of the diseases was found by analyzing the percentage of leaves infected. The severity of the diseases, however, was found by analyzing the percentage of leaf area affected and by calculating the lesion area. Observable lesions on the sample leaves accounted for disease incidence. Disease severity was analyzed by measuring the areas of the lesions using ASSESS: Image analysis software for plant disease quantification.
The Duke Forest FACE facility used in this experiment operates by surrounding experimental plants with rings of CO2emitters, which allow concentrations of CO2 within the rings to be controlled. The trees analyzed in the present study were planted in 1998 and placed into sub-plots of the FACE rings. For the purposes of this experiment, the trees were grown under both elevated CO2 and ambient conditions in the FACE facility. The natural precipitation and temperature in the site were left unaltered. The effect of CO2 levels on the plant diseases was determined through a chlorophyll fluorescence analysis and through a chemical analysis, using leaves from the plants to test this effect.
McElrone et al. found that lower than average temperatures cause a greater disease incidence in sweetgum. Additionally, the results showed that there was a greater Cerspora leaf spot disease incidence during wetter years particularly evident in the sweetgum tree. Conversely, the years that faced lower than average precipitation, demonstrated lower disease severity in the rosebud and a lessening of leaf lesions in both tree species. Taking into account the instances in which the plants were significantly affected, the authors found that both disease incidence and severity were favored under the higher CO2conditions.
However, the authors found that there was no significant change in the leaf chemistry among the plants grown under the differing CO2 concentrations. Through the chlorophyll fluorescence imaging analysis, McElrone et al. also concluded that although the plants might experience an increase in Cercospora<!–[if supportFields]> XE “Cercospora” <![endif]–><!–[if supportFields]><![endif]–> leaf spot disease under higher CO2 levels, the net effect is offset by the increase in photosynthetic activity by the uninfected leaf area. The authors suggest further research be conducted to analyze the effect that both the atmospheric CO2 concentration and the resulting climate changes have on these diseases.

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