Ginseng saponins (ginsenosides) were isolated from soil associated with the roots of commercially grown American ginseng (Panax quinquefolius L.), identified via LC-MS and quantified via analytical HPLC. The ginsenosides, including F(11), Rb(1), Rb(2), Rc, Rd, Re and Rg(1), represented between 0.02 and 0.098% (average 0.06%) of the mass of the soil collected from roots annually between 1999 and 2002. The same ginsenosides were also isolated from run-off of undisturbed plants grown in pots in a greenhouse using a root exudate trapping system. To investigate (1) whether these saponins could influence the growth of pythiaceous fungi pathogenic to ginseng, and (2) whether soil levels of ginsenosides were sufficient to account for any effects, bioassays were completed using a crude saponin extract and an ecologically relevant concentration of purified ginsenosides. Thus, when cultured on media containing crude saponins, the colony weight of both Phytophthora cactorum and Pythium irregulare was significantly greater than that of control, indicating a strong growth stimulation by ginsenosides. The growth of Pythium irregulare was also significantly stimulated after addition of an ecologically relevant, low concentration (i.e. 0.06%) of purified ginsenosides to culture medium. By contrast, growth of the saprotrophic fungus Trichoderma hamatum was slightly (but not significantly) inhibited under the same conditions. These results imply that ginsenosides can act as allelopathic stimulators of the growth of pythiaceous fungi in the rhizosphere, and this may contribute to the disease(s) of this crop.