Commonly known as Eelworms, these minute creatures are an important part of the soil microfauna (there may be 30 million per square meter). Most nematodes are harmless or beneficial (some break down organic matter, others control insect pests), but a few suck juices from plant roots. In small numbers nematodes don’t do much damage (though disease can enter through their lesions and they sometimes transport viruses). In a healthy soil, nematodes are usually kept under control by fungi, insects and other predatory nematodes. However they sometimes build up in the soil to the point where the soil becomes "sick." Plants growing in such soil show signs of nutrient deficiency because of serious root damage. The simplest way to deal with nematode infestations is to plant resistant varieties when available. Crop rotation also helps.
The most commonly encountered harmful nematode species include Potato Cyst Eelworm, Bulb Eelworm and the Root Knot Nematode.
Image: Scott Bauer, USDA Agricultural Research Service, Bugwood.org