Effects of an unspecialized soil pathogen on congeneric plant species with different geographic distributions.

This study investigated the potential of a root rot causing fungal pathogen to affect the geographical distribution of plant species. A glasshouse experiment was conducted that compared the responses of congeneric plant species of contrasting range types to the presence or absence of a soil borne fungal pathogen at three different moisture levels. Five congeneric plant species pairs were selected, each with one species present and one absent in humid western Europe: Centaurea scabiosa–C. stoebe, Dianthus deltoides–D. carthusianorum, Inula conyzae–I. hirta, Potentilla neumanniana–P. cinerea and Scabiosa columbaria–S. ochroleuca. The oomycete Pythium ultimum was selected as the root rot causing soil borne pathogen because of its wide geographical distribution, wide host range and reported preference for high soil moisture. The response variables measured included relative growth rates of above- and belowground biomass, leaf number and the shoot-root ratio. Two plant species pairs (Dianthus and Scabiosa) showed a significant interaction of distribution type with presence of the pathogen. The species with a continental distribution suffered a greater reduction in growth due to the pathogen than the species with an oceanic distribution. However, across all species and genera the pathogen’s effect was not dependent on range type or soil moisture and affected only RGR of leaf number. The study revealed that unspecialized fungal pathogens might have an effect on the performance of some plant species and might affect continentally distributed species more than oceanic ones, which might put them at disadvantage when combined with, for example, competition. Nevertheless, this effect was not recorded for all genera tested. Given the little knowledge on the effects of unspecialized pathogens on wild plant species, further studies with different soil borne fungal pathogens causing root rot and an enlarged set of plant species are recommended.