Reviewed November 1998
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Figure 1
Juvenile of root-knot nematode (endoparasite) penetrating a
tomato root. Magnified 1,800 times. Micrograph, courtesy of USDA-ARS.
The Plant Nematology Laboratory will analyze your soil for nematodes. Check with your MU Extension center about the fee for a nematode analysis, how to make payment and information on how to collect and submit a soil sample.
Most plants, either naturally occurring or cultivated, have nematodes associated with them. Some nematodes are endoparasitic, spending their lives within plants (Figure 1). Others are ectoparasitic, and only their stylets (hollow, protrusible spears used to puncture plants) enter the plant to extract plant nutrients (Figure 2).
Nematodes have a wide host range and are seldom considered a pest of just one plant species. Nematode injury may include:
Some endoparasitic nematodes cause tissue abnormalities known as galls or "root-knots," which block the flow of nutrients through the plant (Figures 3 and 4). Nematode feeding sites can provide entrance for other disease organisms and increase plant damage. Nematodes are a greater problem where conditions favor nematode growth, such as long growing seasons, sandy soil and plants under stress.
Most plant-parasitic nematodes get into the garden by infested soil or infested transplants. Once nematodes are present, they are almost impossible to eliminate, but their damage to plants can be reduced. Time is well spent inspecting roots of transplants before placing them in the garden whether they originate from a reputable dealer or a neighbor. Healthy transplants will outperform diseased transplants under any conditions and can prevent garden problems for years.
Figure 2
Dagger nematode (ectoparasite) feeding from cortex cells deep
inside the root of a plant. Photo by U. Zunke, Institut fur Angewandte Botanik.
Figure 3
Root-knot nematode on melons. Note malformed fruit. Fruit typically
ripens slowly or unevenly.
Figure 4
Root-knot galls on cantaloupe roots.
Soil usually contains many free-living (non-plant-parasitic) as well as plant-parasitic nematodes. Often several genera of plant parasites are present in the same soil although only one or two may cause major plant damage. When plant damage is observed, a trained person is needed to identify the nematodes present and determine if these species are capable of causing plant damage.
Root-knot nematodes are the most damaging plant-parasitic nematodes in the home garden, although other nematodes can also cause problems. Nematodes can severely restrict all the vital functions of plant roots, including the absorption and transfer of water and nutrients. Home gardeners, unlike commercial producers, have limited access to nematicides to reduce plant-parasitic nematodes (Figure 5). Because of the high toxicity level of these compounds, most gardeners would not choose a nematicide for home use. Therefore, it is important to prevent introduction of plant-parasitic nematodes if they are not present in a garden site.
Figure 5
Root-knot nematode infection of green bean plant on left. Note
galls on roots and lack of lateral roots. Green bean plant on right has been
protected from nematodes by a nematicide.
If nematodes are thought to be the problem, the soil should be analyzed by a reliable testing service. Collect a soil sample from several areas in the root zone and include some plant roots.
Nodules on the roots of beans and peas can be mistaken for early symptoms of root-knot nematode. Nodules are beneficial structures caused by certain bacteria that supply soil nitrogen to peas and beans. Nodules are attached to the side of roots rather than swollen parts of the roots or nematode cysts. Other plant pathogens can also cause growths on plant roots.
If nematodes are causing damage, one or more of the following management measures may be used:
Most vegetables can be attacked by root-knot nematode. Some of the most susceptible crops are:
Nematode damage also has been observed, but damage does not occur as often and is not as severe on:
These are fairly resistant to root-knot nematodes.
Other plants that grow well when root-knot nematodes are present are:
Plants related to each other usually are susceptible to the same diseases and should not be planted close to each other or follow each other in a rotation. Root crops should not be planted in the same area of the garden in succeeding years because most are susceptible to the same diseases.
Where space permits, choosing another garden site is a worthwhile consideration. However, moving the garden to a new site does not ensure complete escape from plant parasitic nematodes. They may be present in many areas because they can survive and sometimes thrive on the roots of certain farm crops, weeds and woody plants. If the new site has a population of root-knot nematodes, continuous gardening with susceptible crops can soon increase nematode numbers to damaging levels. This may necessitate another move or other measures. Do not continue to grow nematode-susceptible plants in an area with a history of nematode problems. Check seed catalogs for resistant varieties and adopt other management techniques to prevent nematode damage.
Fortunately, many nematode-resistant tomato varieties are now available . Tomato varieties that have nematode resistance will have an "N" designation on the seed packet. If they are resistant to Fusarium (a wilt causing fungus), they will be designated as "F." Some tomato varieties are also resistant to Verticillium (a wilt causing fungus) and will have the designation "V." Multiple pathogen resistance in the seed is designated as "VF" or "VFN."
Plant-parasitic nematodes often interact with other soil pathogens, causing more plant damage than either pathogen would cause alone. The interaction may render plant resistance ineffective.
The use of nematode-resistant varieties is a good management choice because it involves minimal effort and expense. However, resistant varieties are not available for all vegetable crops, and in some cases, resistance is incomplete. Incomplete resistance means that the nematode levels will build up if host plants are grown but the number of nematodes will increase more slowly than if a susceptible plant is grown.
Lettuce, onions, radishes, leafy greens, green peas, early beans, and cabbage (and related plants) can be planted early and escape serious nematode damage. These spring-planted crops grow when temperatures are too cool for nematode reproduction and activity. The plants are harvested before nematode damage becomes serious. However, late summer plantings of some of these crops for fall production can be severely damaged by nematodes. Nematode damage permanently retards growth and development of these plants.
Destroy crop roots as soon as the plants are no longer wanted or productive in the garden. Nematodes continue to feed and reproduce on root fragments in the soil and build up for the next crop, causing damage in susceptible crops. The effects of root-knot nematode damage are most evident in full-season root crops (Figures 6, 7 and 8).
Figure 6
Root-knot galls on parsip root systems. Photo by J. D. Eisenback,
Virginia Polytechnic Institute and State University.
Figure 7
Root-knot galls on carrot roots. Photo by C. Overstreet, Louisiana
State University.
Figure 8
Sweet potatoes infected with root-knot nematodes. Note the characteristic
cracking.
High soil organic matter helps retain moisture and adds to the available plant nutrients. Increased water and nutrients help plants fight nematode attack. Organic matter in the form of peat, manure or compost all will increase the decay of organic matter, releasing nutrients through microbial action. The increased level of microbes in the soil favors the build-up of organisms that feed on all soil microbes, including nematodes. Care should be taken when adding compost that the compost does not include partially decomposed roots containing plant-parasitic nematodes or other soil-borne pathogens.
Organic matter can also be introduced into the soil through planting a green manure crop such as the legumes, clover, vetch or non-legumes such as rye. These crops when used as a green manure crop are planted in the fall or early spring and tilled into the soil before planting. There is some evidence that the incorporation of this green manure crop produces compounds that are toxic to nematodes.
Soil-borne pathogens such as nematodes can be killed in the upper layers of the soil by soil solarization. This process traps the heat from the sun shining through clear plastic and kills the plant pathogens. Soil solarization needs to be done during the summer when air temperatures and solar radiation are high and in an area where no plants are growing. Moist soil improves efficiency of the kill. Cover the area with thin polyethylene film and leave it in place for at least three months. To be effective, soil temperatures should be maintained between 98 and 126 degrees Fahrenheit for several months.
The most accurate way of diagnosing a nematode problem is a laboratory soil analysis. A good sample taken between late May and early September is essential for accurately assessing the presence of nematodes, the species involved, and their relative abundance.
A good sample consists of several subsamples taken from different areas of the garden surrounding the roots of symptomatic plants. Garden soil containing roots should be randomly sampled if the area has a history of nematode infestation. Each subsample should represent the upper 7 to 8 inches of soil and an area of about 100 square feet. A hand trowel makes an adequate sampling tool if the soil is soft or has been tilled recently. Make a trowel-size hole 7 to 8 inches deep, and then remove a slice of soil from the side of the hole. This is a subsample.
Collect the subsamples in a container and mix thoroughly. From this mixture, remove about 1 pint as the sample. The sample should be promptly sealed in a plastic bag and submitted for analysis. If shipment is delayed, keep the sample out of the sun and reasonably cool.
G6204, reviewed November 1998