Nematodes are a diverse group of unsegmented round worms. Only some are parasites of plants, but they can do tremendous damage.
Read more about using cover crops to suppress plant-parasitizing nematodes:
Cover Crops for Managing Root-Knot Nematodes, by Harsimran K. Gill and Robert McSorley, University of Florida.
Gruver, L.S., R.R. Weil, I.A. Zasada, S. Sardanelli, and B.Momen. 2010. Brassicaceous and rye cover crops altered free-living soil nematode community composition. Applied Soil Ecology.