Nematodes are small, round-bodied worms invisible to the eye. They live either in the soil or on stems and leaves of plants. A few varieties cause plant diseases, but most are an integral part of the soil foodweb. Earthworms, fungi, protozoa, anthropods and bacteria are also essential microorganisms that support nutrient creation in soil. Nematodes release nutrients in forms that plants can absorb.
Nematodes are microscopic, non-segmented worms typically 1/500 of an inch in diameter and 1/20 of an inch in length. The female can produce 500 eggs, which reach maturity in one month. They can move 2 to 3 feet upward in soil in one growing season and most species are not active in soil cooler than 65 degrees Fahrenheit. They are often divided into two groups; those who live inside roots and stems, and those who live outside root systems.
Plant-parasitic nematode species are harmful to crops. Symptoms of nematode damage include wilt, slow growth, chlorosis and reduced fruit production. Root knot nematodes cause root swelling, Nematodes inject saliva into host plants that can kill plant tissue or cause excessive cell creation.
Most nematode species are harmless to plant life and some are useful for controlling harmful insects. Insect parasitic nematodes are a biological insecticide, according to a Colorado State University Extension report and can be used to control harmful insects such as caterpillars and large beetles. Beneficial nematodes are used in Integrated Pest Management (IPM) systems in sustainable agriculture.
Nematode varieties function in different ways in the soil. "Agricultural soils generally support less than 100 nematodes in each teaspoon (dry gram) of soil," says soil biologist Dr. Elaine Ingham. Nematode distribution is in proportion to the number of bacteria and fungi in the same soil. Some feed on plants and algae, some are grazers that feed on bacteria and fungi and some feed on other nematodes.
There are several management strategies for controlling problem nematodes. The Cornell University Extension recommends replacing nematode-infested soil, using nematode-free plant stock, incorporating organic soil amendments such as compost into soil, and avoiding plants that are nematode-susceptible.