Marigold versus Nematodes
Marigold flowers are a natural help in the battle against nematodes, and some marigold varieties are much more effective than others. The French (Tagetes patula), African (T. erecta), and South American (T. minuta) marigold species are most effective. You can read more about the effectiveness of specific varieties on the following two web sites:

Nema Notes
Nematode Suppressive Crops

Marigolds often supply the greatest nematode suppression when they are turned into the soil at the end of the year. This suppression is caused by chemicals in the marigolds that prevent nematode larvae from laying their eggs in marigold root knots, and by deterring nematode feeding in the surrounding soils. It’s been written that these suppressive effects can last up to three years after incorporation of the marigolds!

Other biological controls for root knot nematodes include:

1) solarization—warming the soil to temperatures that kill the nematodes, often accomplished with black plastic;

2) mixing chitin or crushed shell into the soil— kills nematodes by cutting them;

3) drenching the soil with a liquid fish emulsion— repels nematodes and feeds the plants;

4) drenching the soil with a neem liquid—repels the nematodes, too;

5) dressing the plants with a leaf mold compost— hosts and supports beneficial fungi that help suppress nematodes, or;

6) introducing beneficial nematodes—kills harmful nematodes by parasitization.