Plant Diseases Caused by Nematodes

Nematodes

• Nematodes are small, parasitic round worms which feed on plants. The majority of nematodes are too small to be seen by the naked eye. Adult nematodes fall within a range of .25 to 2 millimeters in size. Generally, the parasite lives in soil and feeds on roots, but some species invade other plant tissues. More than 1,000 nematode species feed on and cause disease in plants.

Nematode Feeding

• Nematodes feed on the contents of plant cells. The parasite injects an enzyme into the plant through a body part similar to a hollowed needle, called a stylet. The enzyme breaks up and liquefies the contents of the plant cells. The nematode sucks the liquid out of the plant through the stylet. Feeding in this manner leaves the plant more susceptible to disease-causing bacteria and fungi.

Symptoms

• Nematode-infested plants often look weak, wilted or as if they are suffering from drought. Common symptoms include yellowing of the plant, stunted growth and little or no response to water and fertilizer. Infested root systems start to decay, and are often discolored. The roots can either be overly branched or stunted and short. The injuries occur because of both toxic contents in the saliva of the nematode as well as feeding processes. Cells respond either by enlarging or degenerating cells; in some cases, both can occur.

Root-Infesting Nematodes

• Root-knot nematodes often cause heavy loss of plants, particularly in warm regions. This species of the parasite causes a knot-like swelling on the roots of the infected plant. Root-lesion nematodes travel through the various tissues while feeding on healthy cells. This type of feeding causes lesions on the plant, which allow any bacteria or fungi in the soil to attack the damaged tissues, leading to root rot.

Golden Nematode

• The golden nematode species attacks potatoes and is particularly problematic for European potato crops. Small dot-like golden cysts cover the roots of infected plants. The cysts are bodies of the female nematodes containing up to 500 eggs, which hatch for up to 17 years. Chemicals secreted by the potato plants encourage hatching. The disease is difficult to control due to these conditions.

Citrus Nematode

• The citrus nematode can heavily damage or fully destroy fruit crops, particularly on groves 15 years or older. The parasite is well distributed across citrus-growing regions, due to infested nursery crops. The burrowing nematode is a severe problem in tropical and subtropical regions. It attacks citrus fruits, bananas, tomatoes and more than 200 other important crops.

Control Methods

• Several methods can eradicate disease-causing nematode populations. Soil fumigants, or nematicides, can be used as pre- or post-planting treatment. Growth of resistant plants and use of certified nematode-free nursery soil are effective ways of controlling infestation. Typically, exposing plants and soil to moist heat (above 120 F) for 30 minutes is sufficient to kill most species of nematode as well as its eggs.