|What is the best cure for root-knot nematodes? I have
heard that marigolds help. If so, what kind of marigolds? I
have also heard that certain kinds of mustard greens will
|Root knot nematodes are definitely destructive little pests, and difficult to eradicate. You can try crop rotation, planting resistant varieties, soil solarization (a technique using clear plastic to raise the soil temperature and drive the nematodes away), or letting the soil lay fallow for 2-3 years. You might also add organic matter to the soil, which will help retain moisture and get the balance of good guy type insects working on your in your favor to get rid of the bad guy nematodes.
The French marigold (Tagetes patula) is the only type effective in lowering nematode populations. They must be grown in a solid planting for a full season to suppress nematode populations. Burpee sells one that was the best in trials in Holland called 'Nema-gone'. You do need to grow them like a cover crop to get the full benefit.
I have no information on whether mustard greens would be as effective. Try the above strategies next year.