Netherlands Institute of Ecology, P0Box 40, 6666ZG Heteren, Netherlands

Background and objectives
The role of soil-borne pathogens in structuring natural vegetation is receiving more and more attention. This presentation concerns the role of soil pathogens in cyclic and directional succession. Effects of the plants on soil organisms and feedback processes will be discussed. The focus of the presentation will be on the population dynamics of endoparasitic nematodes, their life strategies and their possible interactions in the roots and root zones of dominant dune plants. The possible relationship between plant life histories and soil pathogens will also be discussed.

Soil-borne pathogens contribute to both the vigour of dominant coastal dune plant species and their succession [1]. Plant-parasitic nematodes and pathogenic soil fungi are assumed to be involved in these natural disease complexes [2]. The pathogen complexes of successive plant species are species-specific and change the competitiveness of their host plants [3]. Integration of these published results have led to some conceptual models that will be discussed.

Materials and Methods
The studies described were carried out in the laboratory, greenhouse and field. The vegetation succession at coastal foredunes serves as a model. In this system, dominant plant species succeed each other, providing relatively simple root zones and a sandy substrate which is poor in organic matter. These coastal foredunes are therefore convenient for these particular soil biological studies. Soil sterilization with gamma radiation, nematicides and fungicides have been applied, as well as soil inoculations with plant parasitic nematodes and pathogenic fungi (all cultured from strains collected from foredunes). Monthly soil samplings were made and nematodes were collected, identified and counted.

Results and conclusions
The specificity of successive soil pathogen complexes, as occurs in the root zones of the dominant plant species in coastal foredunes, is well correlated to endoparasitic nematode species. Every successive plant species has its own combination of cyst, root knot and root lesion nematodes. These nematodes seem to avoid each other in time but, as a consequence, put a constant parasite pressure on the plants. The exact role of endoparasitic nematodes in the successive soil pathogen complexes is now being studied. All dominant foredune plants are clonal. In a comparison with plants of other life history types, clonal plants showed a distinct response on soil pathogens.

1. van der Putten WH, Van Dijk C, Peters BAM, 1993. Nature 362, 53-56.
2. De Rooij-Van der Goes PCEM, 1995. New Phytologist 129, 661-669.
3. van der Putten WH, Peters BAM, 1997. Ecology 78, 1785-1795.