This is the eighth fact sheet in a series of ten designed to provide an overview of key concepts in plant pathology. Plant pathology is the study of plant disease including the reasons why plants get sick and how to control or manage healthy plants.
A number of genera and species of nematodes are highly damaging to a great range of hosts, including foliage plants, agronomic and vegetable crops, fruit and nut trees, turfgrass, and forest trees. Some of the most damaging nematodes are: Root knot (Meloidogyne spp.); Cyst (Heterodera and Globodera spp.); Root lesion (Pratylenchus spp.); Spiral (Helicotylenchus spp.); Burrowing (Radopholus similis); Bulb and stem (Ditylenchus dipsaci); Reniform (Rotylenchulus reniformis); Dagger (Xiphinema spp.); Bud and leaf (Aphelenchoides spp.); and Pine Wilt Disease (Bursaphelenchus xylophilus).
|Figure 1. Adult root-knot nematode. Photo courtesy G. S. Abawi, © The American Phytopathological Society.|
Nematodes are simple, multi-cellular animals—typically containing 1,000 cells or less. They are worm-like in appearance, but are taxonomically distinct from earthworms, wireworms or flatworms. They are bilaterally symmetrical, soft-bodied (no skeleton), non-segmented round worms. Most nematode species that attack plants are microscopic. The basic body plan of a nematode is a “tube within a tube.” Nematodes feed on other microorganisms and plants like bacteriovores, fungivores, omnivores, predators, and plant parasites. Some, however, are serious human, animal and plant pathogens. Those that attack animals or humans do not attack plants and vice versa. Heartworm in dogs and cats is an example of nematode diseases in animals and people.
Plant parasitic nematodes may attack the roots, stem, foliage and flowers of plants. All plant parasitic nematodes have piercing mouthparts called stylets. The presence of a stylet is the key diagnostic sign differentiating plant parasitic nematodes from all other types of nematodes. The bacterial-feeding nematode, Caenorhabditis elegans, is one of the best-understood animals on earth. It was the first animal to have its entire genome completely sequenced. The study of C. elegans has led to many new insights into animal development, neurobiology and behavior.
Signs and Symptoms
Typical root symptoms indicating nematode attack are root knots or galls, root lesions, excessive root branching, injured root tips and stunted root systems. Symptoms on the above-ground plant parts indicating root infection are a slow decline of the entire plant, wilting even with ample soil moisture, foliage yellowing and fewer and smaller leaves. These are, in fact, the symptoms that would appear in plants deprived of a properly functioning root system. Bulb and stem nematodes produce stem swellings and shortened internodes. Bud and leaf nematodes distort and kill bud and leaf tissue. In some cases, such as with SCN, yield loss may take place with no visible symptoms.
Parasitic nematodes are readily spread by any physical means that can move soil particles about—equipment, tools, shoes, birds, insects, dust, wind and water. In addition, the movement of nematode-infested plants or plant parts will spread the parasites.
|Figure 2. Adult lesion nematode. Photo courtesy Union Carbide, © The American Phytopathological Society.|
Various methods are available to reduce crop losses from nematodes:
1. Genetic Host Resistance
- Plant resistant species and cultivars. For example, in an area with soil heavily infested with the root-knot nematode, plant apricots, cherries, apples, pears or plums, which are resistant, rather than peaches or nectarines, which are highly susceptible. (A root-knot nematode-resistant peach rootstock called ‘Nemaguard’ developed by USDA plant breeders is available, thus permitting peach production even on infested soils.) Certain vegetable crops—sweet corn, asparagus, and cabbage—are resistant to root-knot nematodes whereas radishes are susceptible. Resistant ornamentals include the African marigold, azalea, camellia and oleander. In Long Island, New York, where the golden nematode is a serious problem for potato production, resistant cultivars are available. Similarly, soybean varieties resistant to soybean cyst nematode (Heterodera glycines) are also available.
2. Cultural Practices
- Use only nematode-free nursery stock for planting. In most countries, government nursery inspectors will condemn and destroy any nursery stock showing evidence of nematode infestation.
- In nursery operations, use benches raised off the ground and pot plants only into pasteurized soil mixes. Keep containers, bins, benches and flats clean. Fumigate outdoor growing fields where nursery stock will be grown.
- Rotate crops to control certain nematodes. Rotation is useful for types that have a narrow host range, such as sugar beets attacked by the sugar beet cyst nematode. Where the crop value is too low to justify large-scale soil fumigation, crop rotation is the only practical method of nematode control.
- Use cover crops that reduce nematode damage. Cover crops can improve soil structure and fertility, decrease soil erosion, be used as animal feed, and suppress weeds, insects and pathogens. Examples of cover crops that have been shown to suppress nematodes include cowpea, rapeseed, velvet bean and sudangrass.
|Figure 3. Aerial view of damage due to soybean cyst nematode. Photo courtesy G. L. Tylka, © The American Phytopathological Society.|
3. Chemical Applications
- Treat the soil area with a fumigant before planting. Soil mixes for container-grown plants can either be treated with a fumigant or steam-pasteurized at 82 degrees C (180 degrees F) for about 30 minutes. This method is too expensive for field crops other than commercial strawberry fields. The impending loss of methyl bromide may seriously affect the crops where it is used.
- Use nematicides in certain cases. All nematicides are poisonous and must be used carefully, following the directions on the containers exactly. Most such materials will injure or kill plants if applied too close to their root zones. As the number of commercially available nematicides decreases, greater emphasis has been placed on the development of alternative IPM practices.
4. Biological Control
• Although not widely available, scientists have explored the use of antagonistic fungi like Arthrobotrys dactyloides to trap and parasitize plant pathogenic nematodes. Pasteuria penetrans, a bacterial parasite, can also be used as biological control.
|Figure 4. White female soybean cyst nematodes on root surface. Photo courtesy R. D. Riggs, © The American Phytopathological Society.|
5. Government Regulatory Measures
• Avoid importing soil (or plants with soil on their roots) from areas that could be loaded with a dangerous nematode species new to the area. United States plant importation regulations forbid the introduction of plants with soil on their roots from other countries.
Introduction to Plant Disease Series
- Plants Get Sick Too! An Introduction to Plant Diseases
- Diagnosing Sick Plants
- Questions on Plant Diagnosis
- Keeping Plants Healthy: An Overview of Integrated Plant Health Management
- Viral Diseases of Plants
- Bacterial Diseases of Plants
- Fungal and Fungal-like Diseases of Plants
- Nematode Diseases of Plants
- Parasitic Higher Plants
- Sanitation and Phytosanitation (SPS): The Importance of SPS in Global Movement of Plant Materials
These fact sheets can be found at OSU Extension’s Ohioline website: ohioline.osu.edu.
Search for these Nematode disease fact sheets on Ohioline:
- Root-Knot Nematode: http://ohioline.osu.edu/hyg-fact/3000/3115.html
- Soybean Cyst Nematode: ohioline.osu.edu/factsheet/AC-39
- Lesion Nematode: apsnet.org/edcenter/intropp/lessons/Nematodes/Pages/LesionNematode.aspx
- Pine Wilt: apsnet.org/edcenter/intropp/lessons/Nematodes/Pages/PineWilt.aspx
- Sting Nematode: apsnet.org/edcenter/intropp/lessons/Nematodes/Pages/StingNematode.aspx