Nematodes are microscopic round worms. Root knot nematodes attack a variety of plants including tomatoes. They feed on small roots and cause the tissue to grow abnormally producing galls. The damaged roots absorb less water and nutrients from the soil. They are also more susceptible to infection by pathogenic fungi.
Once nematodes have been introduced into a garden, there is no practical way to eradicate them. You can grow tomatoes in nematode infested soil by planting resistant varieties to root knot nematodes. The letter 'N' after a variety name indicates it is resistant to root knot nematodes. This information is provided in most seed catalogs and on the plant label of some tomato transplants sold at garden centers.
There is no chemical treatment that can be used in a home garden to get rid of nematodes. If you want to grow tomatoes that are not nematode resistant, plant them in another area of your yard. Be careful not move soil or plants from areas infested with nematodes to other areas of your yard. Also, thoroughly clean garden tools after they are used in an infested area.
Root knot nematodes are not very active when the soil temperature drops below 60 degrees F. They usually cause little damage on cool season crops grown during winter and early spring. Nematodes are most active when the soil temperature is above 70 degrees F and they often damage warm season crops from summer to early fall.
Susceptible crops will grow better if nematode populations are reduced before planting. This can be done with a variety of methods including fallowing and cultivating soil to eliminate all plant growth for a few months in the summer or by covering prepared and irrigated soil with a sheet of clear plastic 4-8 weeks during warm, sunny weather or by growing certain marigolds as a cover crop for several months.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, September 2012
For more information, see UC Pest Note 7489 –Nematodes http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7489.html