This field trial investigated the effect on vineyard nematodes of organic soil amendments: poultry-litter (PL) biochar, composted cow manure, composted green waste and un-composted rice hulls. To investigate their effects on disease suppression, we chose a vineyard containing healthy grapevines proximal to grapevines with fungal root disease (caused by Ilyonectria spp.). Spring and winter surveys showed that nematodes did not interact with Ilyonectria root disease. Plant-parasitic citrus and ring nematodes predominated in deep soil (10–20 cm), whereas Rhabditis spp. (bacterial-feeder) and omnivorous Dorylaimidae (excluding plant-parasitic and predators) predominated in shallow soil (0–10 cm). After 2 years, the amendments generally decreased the total plant-parasitic nematode (TPPN) populations while increasing the total (non-plant-parasitic) free-living nematodes (TFLN), thus increasing the TFLN : TPPN ratios. PL biochar caused the greatest TPPN decreases (8.5- and 12.9-fold for diseased and asymptomatic grapevines, respectively). The changes caused by the organic amendments were less favourable in a drier season and for diseased grapevines, indicating the importance of seasonal conditions and initial disease status for interpretation of soil organic amendment trial results. This is the first vineyard investigation comparing the impact of PL biochar and other organic soil amendments on parasitic and non-parasitic nematodes.
