Background
As a prelude to their work, the authors write that obligate pathogens "have a more intimate relationship with their host and must have the host to survive," whereas facultative pathogens "live saprophytically and generally result in disease (or tend to be more severe) under conditions of plant stress such as low nutrition or water."

What was done
Well watered and fertilized seedlings of loblolly pine (Pinus taeda) and northern red oak (Quercus rubra) were grown out-of-doors in open-top chambers constructed within large soil bins located at the USDA-ARS National Soil Dynamics Laboratory in Auburn, Alabama (USA), where they were exposed to atmospheric CO2 concentrations of either 360 or 720 ppm with or without being infected by the fusiform rust fungus (the obligate pathogen Cronartium quercuum f.sp. fusiforme), and where the pines were also grown with or without being infected by the pitch canker fungus (the facultative pathogen Fusarium circinatum) for various lengths of time that ranged from weeks to a full year, with each of the three experiments being conducted twice.

What was learned
With respect to the pine Fusarium rust study, Runion et al. report that "percent infection was not significantly affected by CO2 concentration," but that in spite of this fact "the percentage of loblolly pine seedlings which died as a result of rust infection was generally significantly lower under elevated CO2 in both runs of the experiment." With respect to the oak Fusarium rust study, they say "the percent of oak seedlings with uredia was consistently lower for seedlings exposed to elevated CO2 in both runs," and that "the percent of oak seedlings with telia was significantly lower for seedlings exposed to elevated CO2 at the 16 and 19 days evaluations in both runs of the experiment." Last of all, with respect to the pine pitch canker study, the four researchers indicate that "the percent of loblolly pine seedlings which developed cankers following inoculation with the pitch canker fungus was consistently lower for seedlings grown under elevated CO2 in both runs of the experiment ... with infection in elevated CO2-grown seedlings remaining about half that of ambient-grown seedlings."

What it means
Runion et al. state that the results of their study suggest that "disease incidence -- regardless of pathogen type -- may be reduced as atmospheric CO2 concentration continues to rise," which phenomenon should significantly benefit the two species of trees in the high-CO2 world of the future we seem destined to inherit.