According to what I read, marigold's roots secrete alpha-terthienyl, a compound which has nematodal and anti-viral properties, and it's supposed to retards the hatching of nematode eggs.
Usually I get French Marigolds, but I can't seem to find them at my local nursery this year. Please excuse me while I rant a bit about the fact that my local nurseries don't carry the 6-pack of flowers anymore, only the 4" pots of individual plants. Arghhh!!! I know its all about the $, but not cool - not cool at all. And they only had these African Marigolds at both places, so I ended up with those this year. Not sure how they will work, but they are pretty. I just put a row in the middle of the six tomato plants in the main garden.
Next year I'm going to start my own from seed. That's the only way to get what you really want.