Only specific nematodes are harmful to tomato roots. Other nematodes may infiltrate the soil around tomato roots without causing any harm. Most gardeners will never see nematodes, harmful or otherwise, unless they look through a microscope. A combined approach including crop rotation and planting of resistant tomato varieties is essential to controlling the kind that is harmful to tomato roots.
Over 50 kinds of nematodes exist, only some of which are harmful. Tomatoes are unfortunately very susceptible to infestation and infection by one of the latter kinds: the root knot nematode (Meloidogyne species). All nematodes are microscopic, and cannot be seen with the naked eye.
For positive identification of root knot nematodes on tomato plants, soil and root samples should be submitted to laboratories that specialize in soil analysis. However, gardeners may be able to identify the effects of root knot nematodes readily, without the aid of laboratory testing.
Root knot nematodes have no direct effect on above-ground portions of tomato plants. They burrow into the roots of tomato plants to lay and hatch their eggs. This interruption with a root system's normal functions causes the tomato plant to become stunted, either maturing noticeably early or late in the season. Foliage may also turn yellow, and the plant's overall vigor will decrease.
As root knot nematodes invade the roots of tomato plants, they create knobby protuberances on the roots called galls or knots. Stunted plants where root knot nematodes are the suspected culprits may be dug up, and their roots examined for this very visible sign of damage.
If many root knot nematodes continue burrowing into the same places in tomato plant roots, galls can grow as large as 1 inch in diameter. Most of the time, they do not get this large before tomato season has passed. The larger danger is that by continually creating wounds in the root system, root knot nematodes create convenient pathways for soilborne diseases and microorganisms to infect tomato plants.
Even if root knot nematode infestations are present, it may be possible to have a successful tomato crop. Root knot nematodes do not kill tomato plants on their own. If there is not a large root knot nematode presence, and tomatoes are still growing, a gardener may choose to do nothing for the duration of the season, then rotate tomato crops to a new area during the following tomato season.
Not all nematodes are bad, and not all nematodes harm tomato plant roots. Organic gardening specialists may sell beneficial nematodes to gardeners, which can be used to help control harmful insects in the soil at their larval stage. Cutworms, grubs and other harmful insect larvae are eaten by these types of nematodes—which do not do anything to harm tomato plant roots. Unfortunately, however, they are not known to attack root knot nematodes.
In the case of root knot nematodes and tomato plant roots, prevention is key. Nematodes are present throughout all gardens and yards, but only very large concentrations of harmful ones make themselves obvious to gardeners. Tomato planting spaces should be rotated every season, as should peppers, potatoes and other susceptible crops. This helps to avoid concentrating a root knot nematode population in one place.
Solarization of soil can help control root knot nematodes. In this process, soil is moistened, then covered with a clear plastic tarp for several hot, sunny days. When the soil hits 125 degrees Fahrenheit for 30 minutes, nematodes in all soil that has reached this temperature are killed. They will also die if soil hits 130 degrees Fahrenheit for just five minutes.
Planting of resistant crops is also advisable. Tomato varieties with the code VFN after their name are known to be resistant to root knot nematodes. Good sanitation practices (not moving plants or tools from known infected areas around) and leaving infected land fallow for a year can also aid in the control of root knot nematodes.