You won't see individual nematodes because they are so small, but their damage is obvious.

Various species of these microscopic worms are found all over North America, but they are a more severe problem in the South. They feed on the roots of a wide variety of plants, including tomatoes, celery, beans, and spinach. Infected plants are stunted and yellow, may wilt in hot, dry weather, and can die if badly infested. Other symptoms include roots with many small, round nodules on them, and taproots that develop many small side roots such as in the image at left. Nematodes are spread via infected soil, water, tools, and plants. Damage is similar to that caused by other stresses that injure roots; have your soil tested for nematodes to verify that they are the culprit.

Plant resistant varieties. Keep the soil's organic matter level high to encourage nematode antagonists. Tilling a rye cover crop into the soil produces a substance toxic to nematodes. Leaving fields fallow and weed-free for 1 to 2 years usually produces an 80- to 90-percent per year reduction in root-knot nematode populations.

In warmer areas, soil solarization (using the sun's heat to kill the seeds) is effective. Till soil, water thoroughly, and cover soil with sheets of clear 2- or 4-mil plastic. (Using a double layer, separated slightly by a garden hose or wooden battens, increases effectiveness.) Seal the edges with soil or stones. Sunlight passes through the plastic and heats the soil, which stays warm. The goal is to raise the temperature in the top 3 to 6 inches of soil by 10 degrees F. Depending upon the amount of sun and how hot it is, the process can take 6 to 8 weeks.

Photo courtesy of USDA Agricultural Research Service