Specificity, pathogenicity and population dynamics of the endoparasitic nematode Heterodera arenaria in coastal foredune

Key words : Heterodera , plant-parasitic nematodes, soil pathogens, Ammophila arenaria , occurrence, abundance, specificity, population dynamics, life history, pathogenicity, PCR-SSCP, molecular method, escape, sand burial, dispersal, migration, fitness, development time, survival, reproductive success, bottom-up, top-down. In natural ecosystems hardly any attention has been given to the population dynamics of plant-parasitic nematodes. In coastal foredunes, plant-parasitic nematodes are supposed to be involved in the degeneration and succession of the dominant sand-fixing grass Ammophila arenaria (Marram grass). The specificity, pathogenicity and population dynamics of the sedentary endoparasitic nematode Heterodera arenaria have been studied to determine if this species might be a key component of the soil pathogen complex of A. arenaria.H. arenaria was found to be specific to Elymus farctus and A. arenaria in the mobile area of the coastal foredunes. Colonisation of the newly deposited sand layer by H. arenaria corresponded well with the development of pathogenicity in a series of bioassays. However, direct addition of the nematode to A. arenaria did not result in growth reduction of the plant. So, H. arenaria behaves like a biotrophic parasite, which has a high specificity but is not aggressive. Therefore, H. arenaria did not seem to be directly involved in the degeneration of A. arenaria .Each year, the majority of the population of new H. arenaria cysts develops in the newly deposited sand layers. These layers are colonised by A. arenaria roots throughout the growing season. Migration to the new root layer may offer an individual nematode the benefit of early development and a larger potential offspring. The continuous release of juveniles in the field and their development in experiments indicate that release of juveniles from cysts is an ultimately determined process. Juveniles were found to emerge in November and many eggs or juveniles did not survive the winter period. The strategy of release, however, seems effective; the distance of migration could be too large to detect specific cues from the plant and the start of root formation in the field is highly variable. The emergence of juveniles late in the growing season could result in a second generation within the same year. The constant number of cysts per gram of roots suggests that the population density of H. arenaria is most likely a bottom-up directed process.