As I recall, the way marigolds work on nematodes is that the worms enter the roots of the plant and are unable to complete their life-cycle; to be effective the whole field must be planted in marigolds for a season. Most of the ornamental types of marigold were not effective in this way; some types of Tagetes erecta were the best at it.
For those inclined to read more:
Effects of selected marigold varieties on root-knot nematodes and tomato and melon yields
http://apsjournals.apsnet.org/doi/pd....2002.86.5.505
Greenhouse studies on the effect of marigolds (Tagetes spp.) on four Meloidogyne species
http://www.ncbi.nlm.nih.gov/pmc/arti...344/pdf/62.pdf