Identification and characterisation of naturally suppressive soils specific to ring nematodes (Criconemoides xenoplax)

Ring nematodes are one of the most common and abundant nematode species in vineyards. Understanding the crop management practices that modulate the suppressive activity of the soil microbiome towards PPN will lead to long-lasting and cost-effective crop production. This study will aim to detect, identify and characterise soils that maintain specific suppressiveness to ring nematodes.

Absolute and relative suppressiveness were determined by means of lab assays. Greenhouse pot trials were to assess nematode replication in different soils. Samples were also analysed for soil physicochemical properties. Different ecological indicators (nematode taxa abundance and microbially secreted enzymes) were used to correlate ring nematode suppression and abundance with properties of the soil food web by means of PCA analysis.

Sites that showed suppression as determined by the labassays and pot trials, also had high to very high numbers of ring nematodes, which seems to be the result of a density-dependent biological component present in these soils. Ring nematode numbers were negatively correlated with the soil enzymes indicating lower numbers of nematodes at higher levels of these soil enzymes. It seems that increased microbial activity in the soil, associated with higher levels of these soil enzymes could inhibit the reproduction of ring nematodes in these soils. High ring nematode numbers were positively correlated with the nematode-based index, EI, which parallels the intensity of nutrient enrichment in the soil. High values of EI indicate equivalent levels of food availability, which leads to an increase of the ring nematode population.

Overall, the presence of naturally suppressive soils against ring nematodes and the indicators of nematode suppression was not satisfactorily discovered in this study and future studies should rather be directed at the inducement of suppression in orchards. Stimulating or even creating suppressive conditions in the agricultural rhizosphere has been studied and applied to reduce PPN populations.

The presence of a biological factor in soils with high ring nematode numbers seems to be a dead end, since the presence of the organism(s) did not seem to be able to suppress the ring nematode population in those soils.

Naturally suppressive soils were not detected during this survey, but some trends were discovered in soils with low numbers of ring nematodes. Efforts should be redirected into inducing suppressiveness against nematodes in SA orchards