Background
The authors state that the root pathogen Phytophthora citricola is known to infect roots and trunks of European Beech (Fagus sylvatica L.) trees; but they say that several reviews of the effects of elevated atmospheric CO2 on plant parasite interactions -- including those of Manning and Tiedemann (1995), Chakraborty et al. (2000) and Garrett et al. (2006) -- conclude that "it is uncertain whether elevated CO2 favors or suppresses pathogens in herbaceous or woody plants."

What was done
In an attempt to obtain additional pertinent data about the subject, Fleischmann et al. grew well-watered Fagus sylvatica plants from seed in several 40-cm by 60-cm containers -- which were 30 cm deep and filled with natural forest soil -- for a period of four years within growth chambers that were maintained at either 400 or 700 ppm CO2 within a greenhouse. During this period, the trees received an adequate supply of all essential nutrients; but in the case of nitrogen (N), there were low N and high N treatments, where the high-N treatment received twice as much nitrogen as the low-N treatment. Last of all, half of the seedlings were infected with P. citricola in the early summer of the third year of the study; and half of the trees in each treatment were harvested at the ends of the third and fourth years of the experiment.

What was learned
The three German researchers report that "chronic elevation of atmospheric CO2 increased the susceptibility of beech seedlings towards the root pathogen P. citricola, while additional nitrogen supply reduced susceptibility." In fact, they found that 27% of the infected plants in the low-N high-CO2 treatment had been killed by the pathogen by the end of their study, while only 9% of the infected plants in the high-N high-CO2 treatment had died. In terms of the bigger picture, however, they found that surviving beech seedlings of the low-N high-CO2 treatment "managed to tolerate the root infection by (a) increasing their carbon gain, (b) improving their fine root functionality and (c) changing their allometric relation between below-ground and above-ground biomass."

What it means
As a result of the latter three phenomena, Fleischmann et al. were able to write that infected beech seedlings in the low-N high-CO2 treatment rose to the challenge presented by the pernicious pathogen and "enhanced [their] primary production rates in the second year of the experiment and increased above-ground biomass significantly as compared to control trees," thereby providing an exemplary illustration of the popular proverb that affirms that "whatever doesn't kill me makes me stronger."

References
Chakraborty, S., Tiedeman, A.V. and Teng, P.S. 2000. Climate change: potential impact on plant diseases. Environmental Pollution 108: 317-326.

Garrett, K.A., Dendy, S.P., Frank, E.E., Rouse, M.N. and Travers, S.E. 2006. Climate change effects on plant disease: genomes to ecosystems. Annual Review of Phytopathology 44: 489-509.

Manning, W.J. and Tiedemann, A. 1995. Climate change: potential effects of increased atmospheric carbon dioxide (CO2), ozone (O3), and ultraviolet-B (UV-B) radiation on plant diseases. Environmental Pollution 88: 219-245.