Species

Root-knot nematodes (Melioidogyne spp.) are common pest species of nematodes that can cause problems in home vegetable gardens and landscapes. Nematodes are tiny roundworms that live in the soil.

Many species of nematodes are beneficial organisms and do a great job of controlling soil insect pests. See Parasitic Nematodes.

Host/Site

Nematodes live in the soil and feed on plant roots. They can be found on a wide variety of fruit and vegetable plants, annuals, perennials and woody landscape plants.

Identification

Indicators
Root-knot nematodes cause swellings called galls. They are usually the size of a pea, but can grow to 1 inch in diameter. This root damage reduces the ability of the plant to take up water. There will usually be fewer roots on a nematode infested plant. Roots may also be a darker color. Root damage can force the plant to create side roots, giving root structures a whispery appearance.

Above ground damage often appears as wilting and water stress. This includes stunted growth, yellowed leaves and decreased leaf and fruit size. Damage is first confined to one area, but will spread slowly in subsequent years if control is not taken.

Life Cycle

Root-knot nematode eggs hatch into the first immature stage in the soil. This juvenile travels through the soil looking for a suitable root. Once a root is found, the nematode invades the root, often causing a gall growth as the juveniles mature. After mating, the male leaves the root while the female remains within the root, producing an egg mass on the root surface. Root-knot nematodes can develop from egg to adult in 21 to 28 days in warm summer weather.

Natural Enemies

Naturally occurring soil organisms attack nematodes. These include many of the fungi, bacteria, and other nematodes found in healthy soil.

Monitoring

Above ground symptoms are wilt, yellowing and stunting. Below ground symptoms include the galls described above. If the indicators explained above are noted with no other apparent cause, suspect nematodes. Monitoring can also consist of removing several samples of soil or roots of infested plants, and bringing them to your local cooperative extension specialist. Your extension specialist may be able to identify nematode species and give you a population density.

Action Threshold

Action is required to stop the spread of nematodes when symptoms such as wilting, yellow leaves or stunted growth are noted. Once you have determined that nematodes are the cause of your problem begin taking action to get the infestation under control.

Physical/Mechanical Controls

Healthy soil is the best defense against and control of nematodes. The addition of organic compost to landscape and vegetable garden beds will increase the moisture holding capacity, nutrient content, and tilth of the soil. When incorporated into nematode-infested beds, organic composts not only may reduce stress and stimulate root development but also offset the impact of nematode feeding on plant vigor. Also, higher organic matter content may also stimulate the activity of pests and parasites of root-feeding nematodes, which may reduce their populations in the soil. Good irrigation, fertilization, and soil care will help minimize damage caused by nematodes.

Soil amendments containing chitin, made from shells of crustaceans, have been shown to be effective at reducing nematode populations. It serves as a food source for chitin eating fungi and the shells of nematode eggs are also made of chitin so the fungi attacks them as well.

When an infestation is detected, one of the best methods for control is plant isolation or removal. Nematode infestations travel at a rate of 3-6 feet per year, so if an infestation can be localized and treated, surrounding plants will not be harmed.

Plant choice is another important part of a nematode control program. Do not plant known susceptible species in areas known to be infested. For example, if a nematode infestation is determined one season, grow a different plant during the next.

Soil solarization may also be effective. Solarization is a technique effective against many fungi, nematodes and weeds in which a clear plastic tarp is left on the soil surface for 4-6 weeks during the hottest part of the season. This method is not practical for all growers.

Biological Controls

Commercially available beneficial Parasitic Nematodes can be used to control root-knot nematodes. As with all beneficial organisms, follow directions included by the supplier.

Chemical Controls

Not recommended. More effective methods of nematode control include planting nematode-tolerant plants or planting nematode susceptible plants in soils without nematodes.

References

Flint, Mary Louise. Pests of the Garden and Small Farm: A Grower's Guide to Using
Less Pesticide. University of California, 1999.

Dreistadt, S. Pests of Lanscape Trees and Shrubs, 2nd Ed. Berkeley: University of California Press, 2004.