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 Figure 1. Root knot nematode damage to tomato.

Figure 2.  Galls on tomato roots.

Disease

Several species of root knot nematodes cause a stunting and wilting of many vegetables and ornamentals, including tomato, potato, eggplant, sweet potato, radish, geranium, and peony. The disease typically appears in roughly circular patches in a field or garden, but may become more uniformly distributed when susceptible plants such as tomato are continuously planted in an area. Diseased tomato plants are stunted and have pale green leaves.

Diseased plants also may show symptoms of nitrogen, potassium, or phosphorus deficiencies even though adequate levels of these minerals are in the soil. Severely infected plants wilt during hot daytime temperatures, then recover at night. The most characteristic symptoms of the disease occur below ground. Roots develop multiple small tan galls; these galls often fuse to cause extensive swelling and distortion of the root system. The roots also become much shorter and bushier than on healthy plants.

Cause

Root knot nematodes (Meloidogyne spp.) are small, eel-like worms, which overwinter in the roots of perennial weeds or as eggs and larvae in soil or infected root debris. In the spring, larvae penetrate the roots and excrete a substance that induces the formation of root galls. Injury to the roots affects the uptake of water and minerals and eventually results in the development of above-ground symptoms described previously. Infection by the root knot nematode also may "break" disease resistance to Fusarium and Verticillium wilt.

Control

Root knot may be prevented by avoiding the introduction of the pathogen into the garden or field. Carefully check and discard any transplants showing swelling or galling of the roots. Never introduce soil into the garden from areas where root knot is known to be a problem. Rotation periods of 3 to 5 years with corn or other non-host plants will reduce nematode populations in the soil. Several varieties of tomato are available which have resistance to the root knot nematode (see Table 1).

Root knot nematode populations in the soil may also be reduced by a process called soil solarization. Solarization is usually done in mid-summer to maximize soil heating effects. It is the heating process that kills eggs of the nematode. Prepare the soil by tilling and irrigating, then cover the soil with a clear plastic film. The film should be left on the soil for at least 2 weeks to ensure sufficient heating.