Managing Nematodes in the Garden

Charles Overstreet, Ferrin, Donald M., Mcgawley, Edward C.  |  1/30/2006 11:08:36 PM

Galling of tomato roots by root-knot nematode.

Carrot showing symptoms of root-knot injury.

The unusual symptom (blisters) caused by root-knot nematode on Beauregard sweet potato.

Nematodes are common pests in Louisiana gardens. Their small size means that most people have never actually seen them. There are dozens of different types that feed on the roots of vegetables, but the root-knot nematode is one of the most destructive types found throughout our state. Since this nematode has been found in 20% of the gardens examined by the Nematode Advisory Service, as many as 80,000 Louisiana gardens could contain this pest. It prefers a light, sandy soil and is generally found wherever these soils are present. Unfortunately, it is readily spread in infested soil or on plants and can easily be introduced into your garden.

Most people discover how destructive the root-knot nematode can be when symptoms occur. Mild symptoms include stunting, loss of vigor, wilting and reduced yields. Severe symptoms include early plant death and complete loss of production. The galling produced by root-knot nematodes are such a distinctive symptom that it can be recognized by anyone. Galls are small to large swelling of the roots in the areas where nematodes have entered. Small galls may have only a few nematodes present in them while large galls may have a dozen or more. Beauregard, a commonly grown sweet potato variety, produces an unusual symptom (blisters) on the storage root.

Okra, tomato, cucumber, butter beans, squash, and melons are some of the most vulnerable plants. The greatest damage occurs on plants that are grown during the summer months when soil temperatures are warm. Root-knot nematode isn’t active in the soil when temperatures are below 65Fº and vegetables grown in late fall, winter, and early spring suffer little if any injury.

Managing nematodes may involve using one or more techniques that have proven to reduce nematode populations. Low numbers of nematodes at the time of planting can often mean growing a crop without a lot of damage. High numbers, however, can be disastrous. Because of the quick life cycle of root-knot nematode, levels quickly rebound and cause problems to the next crop.

Resistant varieties: Several vegetables have root-knot resistance. Tomato varieties such as Big Beef, Celebrity, Champion, Better Boy, Monte Carlo, First Lady, Carmello, Spring Giant, Bingo, Carnival, Floramerica, and Terrific have been specifically bred to resist the pest. Some southern pea varieties such as Magnolia, Mississippi Silver, and Mississippi Purple are also very resistant to the root-knot nematode. Plant any of these resistant varieties in areas where root-knot nematodes have begun building up or prior to planting a very susceptible crop. These varieties have worked fairly well for us and help keep the levels fairly low. Southern peas are fairly cheap and can be readily used as a summer cover crop to lower populations of the root-knot nematode. Unfortunately, the effects of planting a resistant crop are short. Nematodes will multiply rapidly once a susceptible crop is planted, but if the population levels are low enough at the time of planting, a susceptible crop will usually not be seriously damaged.

Crop rotation: Most of the problems with nematodes arise because susceptible crops are grown in the same area each year. Gardeners need to consider rotating crops within the garden and never plant very susceptible crops consecutively. Since most vegetables are usually considered susceptible, finding a suitable crop can be difficult. The vegetables that are not good hosts for the root-knot nematode include mustard, collards and turnips. Strawberry and peanut are considered immune but are not often grown in the garden.

Escape planting: Root-knot nematodes are not active until soil temperatures reach 65°F in the spring. Many of our late fall and early spring crops are grown when temperatures are too low for the root-knot nematode to be a problem. If these same crops are planted when temperatures are high enough for the nematode to be active, then the crop may be severely damaged. A good example of escape planting is the Irish potato. It is normally planted in January or early February.

Fallowing: Allowing the land to lay idle without a crop has been a practice used for thousands of years. Clean fallow, which involves keeping the soil free of weeds or grass, has been used to starve nematodes of a food source and reduce the populations. The only major drawback to clean fallowing is that it is detrimental to soil (both through erosion and in structure) and should be used only infrequently. A cover crop is preferable, assuming the cover crop will not build up the nematode.

Fertilization: Most of the influence of fertilization is indirect. The damage caused by a light infestation of nematodes may be reduced by higher rates of fertilizer. Since you are improving only the growth of the plant, nematode levels may be much higher on these plants at the end of the growing season. If any nutrient is already low in the soil, damaged roots by nematodes will certainly make it much more difficult to obtain. Conduct soil tests to make sure adequate nutrient levels are present. However, excessive amounts of fertilizer can cause salt buildup or pH problems.

Organic mulches: Adding organic amendments to the soil is an effective method of reducing damage by nematodes. These amendments may have an effect on the nematode population or plants in several ways. They may stimulate microorganisms in the soil that attack nematodes. One kind of microbe in particular, called the nematode trapping fungi, has been of great interest because of its unique ability to form trapping structures to catch and kill nematodes. Organic amendments may also improve soil structure, water holding capacity and plant nutrition which makes for better growing conditions. Plants growing in a good soil environment may tolerate nematode injury before symptoms begin to develop.

Sanitation: Included under this category are such things as weed control, crop residue destruction and disinfection of equipment. Since many weeds are hosts of nematodes, it is important that management practices include a good weed control program. Plants should be tilled up or removed as soon as they are through producing to prevent any further nematode development. Plants that have badly galled roots should also be removed from the garden. Don't add them to a compost pile unless you are sure that the temperature will get high enough during composting to destroy nematodes. Nematodes can be killed by heat when temperatures reach 111-118°F. Nematodes in the infected roots can also be killed by exposure to the sun. Equipment can spread nematodes from one area to another. If nematodes are a problem in one area, it may be a good idea to wash off equipment before moving to another location.

Site selection: If sufficient land is available, gardeners with nematode problems should try to rotate their plant site every few years. Try to select areas which have been in pasture or grasses. Many gardens or ornamentals can be planted in only one area. Even if you cannot move the site, plan to use a careful rotation of the crops that you plant.

Trap Crops: The idea of trap crops is to plant a susceptible crop for a short period, allow the nematodes to attack it, and then till it up before the nematodes have a chance to reproduce. There is high risk in using susceptible crops. Southern root-knot nematodes can develop egg-laying females within 20 days of entering a root during warm conditions. The life cycle is longer when temperatures are lower. If the timing of crop destruction is off, then you may end up with a higher population of nematodes for the main crop. The best trap crop to use is the French marigold or one of the resistant cowpeas.

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