In coastal foredunes, Ammophila arenaria (L.) Link grows vigorously when it is buried regularly by windblown sand and degenerates at stabilized sites. Nematodes and soil-borne fungi were found to be involved in its decline. In order to establish their role in the disease complex, seedlings of ^. arenaria were inoculated with several groups of potentially harmful soil organisms that were isolated from its root zone. Inoculation of single species of fungi did not reduce the growth of the seedlings, but combining all fungi that were commonly found in the Dutch coastal foredunes significantly redueed growth to about 80 % of that in sterilized soil. This indicates synergistic effects between commonly found plant pathogenic fungi. The addition of large numbers of the nematode Telotylenchus ventralis, the only species that could be successfully grown on A. arenaria in the laboratory, reduced plant growth to the same level as in non-sterile soil, but numbers were 80 times greater than in the latter soil. Inoculation with relatively large numbers of T. ventralis in combination with the commonly occurring fungi reduced plant growth in sterilized soil to a level similar to that in non-sterile soil. The involvement of other species of nematodes, such as Heterodera spp. or Meloidogyne maritima, in the decline of A. arenaria in non-sterile soil could not be proven in inoculation experiments with sterile soil, but it is likely that these species may also be involved. It is concluded, therefore, that several different combinations of soil organisms can be harmful to A. arenaria., so that natural decline is not caused by one simple well defined pathosystem.

A survey was carried out at nine locations in the Dutch coastal foredunes to identify the species of soil borne fungi and nematodes associated with Ammophila arenaria (Marram grass). Ammophila arenaria is a sand binding grass that is very important for the stabilization of coastal foredunes. Degeneration of the plants occurs at stabilized sites and is supposed to be caused by a combination of soil-borne fungi and nematodes. Canonical correspondence analysis (CCA) and two-way indicator species analysis (TWlNSPAN) were used to examine which fungal and nematode species usually coexist in the rhizosphere of vigorous and early declining stands of A. arenaria. In total, 47 species of fungi and 10 genera of plant-parasitic nematodes were found. According to CCA, the community of soil organisms of stands that were more than 10 years old was significantly different from recently established stands of 3 years old. Also, the community of soil organisms isolated from calcareous locations differed significantly from that of lime-poor locations. No relationship between the vigour of the plants (vigorous vs. early declining) and the soil borne species composition was found, although in roots of vigorous stands, the number of nematodes was higher than that of early declining stands. A relatively large group of soil organisms occurred generally. This group possibly contains an ubiquitous pathocomplex that cause the growth reducing effects of biotic origin which generally occur in A. arenaria. Analysis of this group of nematodes and fungi by TWINSPAN resulted in 9 different combinations of concurring soil organisms of which 5 combinations were present at all investigated locations. Two of the latter combinations contained both nematodes and fungi. The first contained three endoparasitic nematodes (Meloidogyne maritima, Heterodera spp. and Pratylenchus sp.) that concurred with the fungus Mucor hiemalis. The second group contained Heterodera spp., Telotylenchus ventralis, Filenchus sp. together with the potentially plant-pathogenic fungi Microdochium bolleyi and Fusarium culmorum, as well as the fungi Mortierella sp. and Trichoderma harzianum, all in relatively high numbers.It is concluded that both CCA and TWINSPAN are valuable exploratory techniques, especially when used in combination, to detect possible combinations of soil organisms which may be involved in the degeneration of A. arenaria. Further identifications of harmful organisms should be obtained from experiments.

Arnrnophila arenaria is a dominant sand-fixing plant species of the European coastal foredunes. It remains vigorous under regular burial conditions on seaward slopes, but starts to degenerate when sand accumulation diminishes. Several hypotheses have been put forward to explain this degeneration. In this study, we test the hypothesis that upward growth of plants following sand burial enables them to escape harmful soil organisms. Plants in a degenerating field stand of A. arenaria and potted plants grown in sterilized sand (outdoor pot experiment) were buried with sterilized or nonsterilized sand. Burial in both sterilized and nonsterilized sand resulted in stem elongation, increased numbers of living shoots, and increased shoot and root biomass. However, when plants were grown in and buried with sterilized sand, the numbers of shoots were significanlty higher than those buried with nonsterilized sand. The new root zone of buried plants was colonized by pathogenic soil organisms (nematodes and fungi) during the same growing season. It is concluded that by upward growth through pathogen-free sand, the plants benefit, at least temporarily, from escaping its pathogens and parasites.Rdsumd : L'Arnrnophila arenaria est une espkce de plante dominante capable de fixer les sables des dunes c8tikres frontales en europe. Elle demeure vigoureuse sous des conditions rCgulikres d'ensablement sur les pentes opposCes a la mer, mais commence 21 dtgCnCrer lorsque l'accumulation de sable diminue. Plusieurs hypothkses ont Ct C mises de I'avant pour expliquer cette dCgCnCrescence. Dans cette ttude, les auteurs Cvaluent l'hypothtse selon laquelle la croissance dressCe des plantes, suivant le recouvrement par le sable, rendrait ces plantes capables d'Cchapper aux organismes nuisibles du sol. Des plantes situCes dans une station en dtgtnCrescence de I'A. arenaria et des plantes produites en pot dans du sable stCrilisC (expCrience en pots a I'extCrieur) ont Ct C enterrCes avec du sable stCrilisC et du sable non-stCrilist. Dans les deux cas, l'enterrement avec du sable stCrilisC ou non-stCrilisC conduit a 1'Clongation des tiges, a I'augmentation du nombre des tiges vivantes ainsi que l'augmentation des biomasses des tiges et des racines. Cependant, lorsque les plantes poussent dans le sable stCrilisC et sont enterrCes avec du sable stCrilisC, les nombres de tiges sont significativement plus grands que chez les plantes enterrCes avec du sable non-stCrile. La nouvelle zone racinaire des plantes enterrCes est colonisCe avec des organismes tdaphiques pathogknes (nimatodes et champignons) au cours de la m&me saison de croissance. Les auteurs concluent qu'au moyen de la croissance dressCe a travers le sable sans pathogkne, la plante ont profitC, du moins temporairement, de I'Cloignement de ses pathogknes et parasites.

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