Influence of agronomic practices on the development of soil suppression against cyst-forming plant-parasitic nematodes

Cyst-forming nematodes are a major group of plant pests of economic importance worldwide. There are well-documented cases of suppressive soils where plant-parasitic nematodes are maintained at very low population densities by one or more microorganisms that co-exist in the same environment. The role of agronomic practices in the establishment and support of soil suppressiveness offer new possibilities in the management of these pests. To test if this specific suppressiveness would evolve under a five-year monoculture of susceptible potato in soil infested with Globodera pallida Pa3 populations Chavornay and Delmsen, a microplot study was conducted. Final populations of total eggs remained similar for Delmsen but oscillated for Chavornay. Proportions of diseased eggs increased dramatically at the first year after harvest and oscillated until the fifth year, especially for Chavornay. This suggested a predator - prey relationship of the nematode and a suppressive factor. Yields of total marketable potato tubers increased in Chavornay but decreased in Delmsen. In a greenhouse experiment, females developing in these soils were examined for inherent microbial communities through PCR-DGGE and pyrosequencing. Candidate microorganisms that possibly affected the egg health status of G. pallida were identified. The dominance of the fungus Malassezia, especially M. restricta, known for its pathogenic abilities on human and animal skin, a tissue that shares structural proteins with the nematodes cuticle, warrants further investigation. To examine if susceptible crops were crucial for the successful establishment of suppressive transfer, population dynamics of Heterodera avenae and H. filipjevi were monitored in microplots amended with a nematode suppressive oat monoculture soil and cropped to susceptible cereal hosts during four years. Although no effect of the amendment was detected on H. avenae or H. filipjevi population densities, or in plant growth and yield parameters, percentage of diseased eggs increased up to 65.5% under the third cropping season, and yield was almost not constrained compared to the non-infested control plots. This suggested that the monoculture of susceptible crops was sufficient to induce and support the establishment of suppressive factors. In a climate chamber experiment, the suppressive potential of the oat monoculture and two soils from intensive cereal cropping systems (one from conventional, one from organic farming) was tested against H. avenae female populations. Results of this experiment indicated a suppressive potential of the monoculture and intensive cereal cropping soils. Furthermore, a high similarity in suppressive potential and microbial communities between organic and conventional farming soils suggested a stronger effect of the cropping rather than the farming system. It was tested if tolerant sugar beet cultivars permit soil suppressiveness to establish by allowing some reproduction of the primary parasite, necessary for the increase of antagonists. Population dynamics and egg health status of Heterodera schachtii Schach0 and Schach1 were monitored under a four-year sugar beet monoculture of susceptible, resistant and tolerant genotypes. Pasteuria nishizawae and Dactylella oviparasitica were included in this study, as representative model organisms for an obligate bacterial parasite and a facultative antagonist, respectively. The ability of D. oviparasitica and P. nishizawae to suppress the populations of H. schachtii was inconsistent, compared to the corresponding controls. However, irrespective of the treatment and the cultivar, eggs became soon diseased, so by the third cropping cycle, diseased eggs represented over 90% of the total eggs. As expected, in each cropping season the tolerant genotype yielded the highest in both pathotypes. The presence of nematode-controlling factors in this monoculture of susceptible, resistant and tolerant sugar beet cultivars, that may be responsible for the dramatic increase of diseased eggs, was suggested. Findings of these studies proportionate baseline data that complement the evidence of the potential of agronomic practices to induce and support the establishment of soil suppressiveness. In this study, monoculture or intensive cropping of susceptible host crops, and cultivation of resistant and tolerant host genotypes, supported the development of soil suppressive factors against different cyst-nematodes mostly in increased proportions of diseased eggs.