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The effect of soil pathogens on plant communities was investigated using four old-field perennial plant species and five isolates of a pathogenic oomycete in the genus Pythium. These Pythium strains were isolated from the roots of two of the plant species, Danthonia spicata and Panicum sphaerocarpon, used in a previous experiment on the consequences of changes in the soil community on plant growth. In this previous experiment, Danthonia and Panicum changed the soil community in a manner that reduced their growth relative to that of a third plant species, Anthoxanthum odoratum. In the current experiments, we found that inoculation with Pythium reduced overall plant mass and root:shoot ratios, but Danthonia and Panicum were more susceptible to the presence of Pythium than the other two plant species, Anthoxanthum and Plantago lanceolata. In addition, Pythium accumulates at different rates on different plant species, with a greater than tenfold higher population observed in association with Panicum compared to Anthoxanthum. The results of these experiments suggest that the accumulation of species-specific soil pathogens could account for the previous observation of negative feedback on plant growth through changes in the soil community. As negative feedback may act to maintain plant species diversity within a community, these results suggest that soil pathogens may themselves contribute to the maintenance of plant species diversity.
Mills et al. (Wed,) studied this question.