A quick look at plant disease caused by nematodes
Only a few of the huge number of nematode species in our soils cause plant disease.
Nematodes are roundworms, one of the most ancient and diverse groups of animals on earth. Most nematodes are microscopic, free-living and feed on bacteria, fungi, protozoans and other nematodes, but about 15 percent of the known nematode species are parasites of plants. Bacterial-feeding nematodes play a crucial role in cycling plant nutrients in soil and are highly beneficial. Of the plant-parasitic nematodes, many are of concern to Michigan farmers including: root-knot, cyst, root-lesion, lance, dagger, needle, stubby-root, stunt, ring, spiral, pin, stem, bulb, bud and leaf nematodes.
Plant parasitic nematodes are simple animals, often with less than 1,000 cells. They possess a stylet mouthpart, used to pierce plant tissues, extract juices and secrete material that helps parasitize the plant. Nematodes occur in a variety of shapes and sizes. Typically, they are long and slender, but when mature, can appear swollen and not very worm-like. Nematodes all molt, similar to insects. Even though these organisms are very small, they have a sophisticated nervous system and sensory organs, allowing them to find their host plants, locate the specific plant cells they need, mate and reproduce.
Some plant parasitic nematodes spend most of their lives inside plant tissues. These nematodes are “endoparasites” and have the advantage of protection from predatory organisms living in surrounding soil that would gladly kill and eat them. Others live mostly in the soil without the benefit of protective plant tissues and are called “ectoparasites.” Ectoparasites have a reduced risk of dying when the host plant dies, but are more likely to be attacked by a predator or pathogen.
Nematodes are also susceptible to death from harsh environmental changes, like temperature and water availability. Some, not all, types of nematodes can enter into a state of metabolic inactivity (sort of a suspended animation) during these times of environmental stress. If they can do this, they can often survive for years waiting for more favorable conditions to trigger their revival.
Some nematodes move similar to snakes. However, most can move no farther than a meter or so within their lifetimes. The spread of nematodes from field to field usually involves farm equipment, contaminated plants or seeds, soil or mud transferred on the feet of people or animals, or surface water movement.
Various nematodes feed on all plant parts: roots, stems, leaves, flowers and seeds. They specialize in the use of their stylet, depending on their style of feeding. Most plant parasitic nematodes are root feeders and live in the soil. The damage caused to the above-ground plant is general in nature and related to root damage. Nutrient deficiency, wilt, stunting, yield depression and sometimes plant death can result. The main diagnostic signs and symptoms of nematode infestations are root cysts or root galls, and “nematode wool” on bulbs and corms.
There are four main tactics used to control plant parasitic nematodes: biological, cultural, chemical and genetic.
Biological control includes use of natural predators or pathogens or introducing those organisms. Biological control of nematodes is generally much more effective in the laboratory than the field.
Cultural control consists mostly of crop rotation with non-host plants. This is a very effective method to limit nematode population growth and can reduce nematode levels below damage threshold in the years when non-host crops are grown. However, some nematodes, such as root-lesion, are difficult to manage using crop rotation due to their wide host ranges.
Chemical control consists of economically viable products that kill nematodes in soil. Fumigants and liquid or solid non-fumigants are available. Most fumigants have been banned by EPA because of environmental hazards, but a few are still available. The non-fumigant nematicides work similarly to insecticides. These products can reduce nematode populations, but are not as effective as fumigants. Because of the expense of nematicide use, they are practical only on high-value crops.
Genetic control typically consists of using resistant varieties.
Good nematode control depends on an integrated pest management approach using a combination of control methods. Michigan State University Extension educators can provide assistance to growers who suspect nematode problems in crops.
The following articles provide more detail on plant parasitic nematodes and their control: