For Release On Or After 01/27/12

There is an invisible pest living in the soil of our vegetable gardens. For many Louisiana gardeners, high population levels of nematodes can reduce the vigor and productivity of their vegetable plants. These microscopic round worms are also a problem for commercial farmers, causing substantial crop and monetary losses as they reappear year after year.

“It often is difficult to recognize nematodes as the problem,” says Charles Overstreet, nematologist with the LSU AgCenter, “because they generally cannot be seen without magnification and because so many other health problems of plants produce symptoms similar to the damage caused by nematodes.”

Symptoms of nematode infestation include stunted growth, wilting, poor vigor and low production, all of which can be caused by numerous other disease organisms, environmental conditions or even the care plants receive.

The southern root-knot and reniform nematodes are the two nematode species that give gardeners the most problems.

Many gardeners are somewhat familiar with the root-knot nematode because this pest does cause distinctive knots or galls on the root systems of plants. Although you can’t see the nematodes, you can see the damage. The root-knot nematode is indiscriminate about the type of crop it feeds on. Just about everything you can plant in the home vegetable garden can be successfully attacked by this pest.

Okra, butter beans and susceptible tomato varieties are good indicator crops in the garden, since these plants may develop large and numerous galls on their roots in response to this nematode. Check the root systems of some indicator plants as soon as production ends in your vegetable garden. If there are a lot of galls, you can expect problems in the same garden area next year.

The reniform nematode doesn’t produce any distinctive symptoms on the roots to enable the gardener to readily identify it. The only way for you to be sure that your vegetable problems are caused by this or any other type of nematode is to have the garden soil checked in a laboratory. This can be done any time, but the best time is during fall and winter. Your parish LSU AgCenter agent can give you details on getting your soil analyzed for nematodes. The cost is $10.

It is virtually impossible to eliminate nematodes totally from the garden. A management program that will keep population levels low enough to prevent serious damage is generally the best way to deal with this pest.

One effective way to reduce nematode populations and limit damage is to plant resistant varieties. Considerable research has been done on many home garden crops looking for varieties resistant to root-knot nematodes. Unfortunately, it has had limited success because most crops don't have any source for genetic resistance to the pest.

Some vegetables, notably tomatoes, lima beans and southern peas, have varieties with good resistance to root-knot nematodes, so it’s a good idea to use a resistant variety when planting these crops in a garden area where you have had problems with the pest.

Gardeners purchasing seeds from catalogs or local retail sources should pick out varieties that have root-knot resistance. Most tomato varieties have a V, F or N letter listed beside them on the package or label. The letters indicate the type of resistance of the variety. The N indicates that the variety has resistance against root-knot nematodes.

Gardeners can take steps now to reduce losses in future crops. You can turn the soil in empty beds several times between now and when you plant them in the spring. This exposes nematodes to cold temperatures and drying that can help reduce populations.

It is also a good idea to add compost, rotted manure or leaves to the garden to maintain or build organic matter in the soil. Soil organic matter helps plants’ roots function more effectively and reduces the effect of damage nematodes cause by their feeding. It also encourages natural populations of fungus organisms that attack and kill nematodes. A fertile soil with plenty of available nutrients is always helpful in reducing nematode damage.

Whenever you can, rotate where you plant different types of vegetables from year to year. Don’t plant the same crops in the same spots each year. Plant very susceptible crops like okra behind less-susceptible crops, such as corn or greens, or where you have planted resistant varieties of vegetables like tomatoes or peas.

This summer, plant vacant, heavily infested beds with a cover planting of French marigolds (Tagetes patula). You could also cover the bed with clear plastic and solarize the soil with the heat of the sun during the summer. (Contact me at dgill@agcenter.lsu.edu if you want more information on how to do this.)

You also can buy products that contain chitin, the material that is a component of the shells of shrimp, crawfish and crabs. Like organic matter, the chitin increases populations of microorganisms that attack and kill nematodes. These products have shown encouraging results in some research studies and have been largely ineffective in others. Still, they would be worth a try if you are having problems, especially when used in conjunction with other management techniques.

Rick Bogren