Soil-borne plant pathogens of Ammophila arenaria in coastal foredunes

Ammophila arenaria (Marram grass) is the most dominant sand-fixing plant species in the Dutch coastal foredunes. This species has a natural ability to emerge from being buried and is therefore used to stabilize the coastal foredunes. On seaward slopes where plants are buried regularly with windblown sand, plants retain their vigour, but start to degenerate when sand accumulation diminishes. One of the factors that may cause degeneration at stabilized sites is the infection of roots by nematodes and fungi. Burial by fresh windblown sand may enable the plants to overcome these harmful soil organisms. In the present study, the nature of the soil-borne disease and its relationship with sand deposition is investigated. In a field survey, a wide range of nematodes and fungi were isolated from the root zone of A. arenaria . Subsequent inoculation-experiments showed that adding single fungal species did not reduce the growth of seedlings whereas combining all commonly found fungi together did, thus indicating synergistic effects. Adding 80 times more individuals of the semi- endoparasitic nematode Telotylenchus ventralis than present in natural soil reduced the growth of seedlings to the same extent as in natural soil. Several groups of soil organisms, especially those groups that include plant-parasitic nematodes, have shown to affect the growth of A. arenaria Burial with unsterilized root zone sand was less beneficial for plant growth than burial with sterilized or beach sand. This implies that plants are able to escape infection by soil organisms through upward growth following sand accumulation. Fungi colonized the freshly deposited layer of sand faster than plant-parasitic nematodes. Furthermore, it could be shown that in windblown soil numbers of fungal propagules and nematodes were reduced. Rejuvenation of stands along the accumulating edges of blowouts can, therefore, be explained by the reduced inoculum pressure of plant-pathogenic organisms in the deposited soil. The amount of sand and the time when sand is deposited are important components in the chances of A. arenaria to escape infection by soil organisms.