Allelochemicals Biologica... - Name

THE ALLELOPATHIC POTENTIAL OF GINSENOSIDES 169
5.2. Bioactivity of Ginsenosides at Ecologically Relevant Concentrations
As emphasized in Section 1.3, before ginsenosides can be considered allelochemicals,
it has to be demonstrated that they are in fact present in the rhizosphere soil at biologically
active concentrations. Therefore, after demonstrating the presence of
ginsenosides in the soil, in vitro bioassays were conducted using ginsenosides at an
ecologically relevant concentration of 0.06%. At this level, ginsenosides were shown
to remain bioactive. That is, the growth of Py. irregulare was significantly greater
than control, while that of T. hamatum remained unchanged (Nicol et al., 2003).
5.3. Plant-Fungal Allelopathy in Ginseng Gardens and Implications for Disease
The discovery of ginsenosides in ginseng root exudates and rhizosphere soil at biologically
active concentrations suggests that plant-fungal allelopathy could occur within
this crop. This in turn could influence disease levels as the ginseng secondary chemicals
potentially, and differentially, influence the growth of different groups of soilborne
organisms (i.e., pathogens and antagonistic saprotrophs).
Naturally-occurring soilborne antagonists can play a role in preventing or reducing
disease levels in crops (Azad et al., 1987; Miller et al., 1989; Larkin et al., 1993).
However, the applied use of Trichoderma spp. as a biocontrol agent often does not
exert the desired level of disease control. One reason is that isolates of these biocontrol
fungi often suffer from poor rhizosphere competence (Papavizas, 1982; Chao et al.,
1986). Consequently, poor (natural) Trichoderma spp. competence in the rhizosphere
of ginseng could lead to increased disease levels in the plant. Interestingly, in an
attempt to address the issue of the lack of Trichoderma spp. rhizosphere competence,
it was found that exudates from healthy plant roots did not support the growth of T.
harzianum, but did in fact support that of Py. ultimum (Green et al., 2001). More
evidence for the involvement of a soil factor in preventing successful biocontrol by
Trichoderma spp. is found in the work of Hong et al. (2000) where several Trichoderma
isolates were shown to inhibit C. destructans in vitro, but this biocontrol effect
failed to materialize when applied to potted ginseng plants. Again, the specific mechanism
involved in both of these reports was not identified, but our results suggest that
plant-fungal allelopathy is one possible explanation.
Ginsenoside-mediated stimulation of several major pathogens is likely to be
involved in the eventual establishment of ginseng diseases. Three important root
pathogens were repeatedly observed to grow better after the addition of ginsenosides
to their growth medium (i.e., C. destructans, P. cactorum and Py. irregulare).
Therefore, it can be hypothesized that the chemical environment of the ginseng
rhizosphere favours fungal root pathogens over potential antagonists of these pathogens
and that this has a direct consequence on the year-to-year disease levels in ginseng
gardens. We are still working on the exact mechanism of growth stimulation in
pathogens, but our preliminary results lead us to believe that in one case (i.e., Py.
irregulare) the activity is extra-nutritional. For Py. irregulare, increased growth may
be due to the ginsenosides or transformed ginsenosides acting as sterol analogs. Also,