Invisible pests are living in the soil of our vegetable gardens, and these microscopic round worms known as nematodes are problems for home gardeners and commercial farmers.

For many Louisiana gardeners, high population levels of nematodes can reduce the vigor and productivity of their crops. These microscopic worms also cause substantial plant and monetary losses for commercial farmers because they recur year after year.

It often is difficult to recognize nematodes as the problem, because they generally cannot be seen without magnification and because so many other health problems of plants cause symptoms similar to those seen with nematode damage. 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, since this pest does cause distinctive knots or galls on the root system of plants. Although you can't see the root-knot nematodes, you can see their damage.

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 the root-knot 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 a gardener to readily identify it. The only way for you to be sure that the problems with your vegetables are being caused by this nematode, 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 the fall and winter. Your parish LSU AgCenter Extension offiice can give you details on getting your soil analyzed for nematodes.

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

One effective way of reducing nematode populations and limiting damage is to plant resistant varieties. Some vegetables, notably the tomato, lima bean and southern pea, do have good resistant varieties against root-knot nematode, so it is a good idea to use a resistant variety when planting in a garden area where you have had problems with this pest.

Gardeners purchasing seeds from catalogs or local retail sources should pick varieties that have root-knot nematode resistance. Most tomato varieties have an F or N letter listed beside them on the label, indicating the type of resistance of the variety. The letter F is the abbreviation for Fusarium wilt, which is prevalent throughout the state. The N indicates that the variety has resistance against root-knot nematode. The root-knot nematode also interacts with Fusarium wilt to cause greater problems, so be sure to get varieties with resistance to both.

Gardeners also can take steps now to reduce losses in future crops. Turn the soil in empty beds several times between now and when you plant in the spring. This exposes nematodes to low temperatures and drying and can help reduce populations.

It also is a good idea to add compost, rotted manure or leaves to the garden to maintain or increase the organic matter content in the soil. Organic matter in the soil helps plants’ roots function more effectively and reduces the effect of the 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 the extent of nematode damage.

Another helpful method is to rotate where you plant different types of vegetables whenever you can. Plant different crops in different areas of your garden each year.

During summer, you also can plant vacant, heavily infested beds with a cover planting of French marigolds (Tagetes patula). Or you could cover the bed with clear plastic and solarize the soil with the heat of the sun during the summer.

In addition, there are products available, such as Clandosan, which 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 but 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.

Nematodes also attack ornamentals but generally don' t cause the problems they do in the vegetable garden.

In annual flower beds, however, it still is best to incorporate generous amounts of organic matter during bed preparation. Also, rotate your flower plantings just like vegetable gardeners rotate their vegetable crops.

I’ve seen nematodes build up and cause problems to impatiens and begonias planted in the same bed year after year. But dwarf gardenia is the only shrub I have seen with significant nematode problems.