Root nematodes: Root-feeding nematodes are attracted to substances exuded by plant roots. Some of them remain on the outside of the root while feeding; others penetrate the root tissue, and live inside the root. Roots may be damaged from the punctures made during feeding, but the most significant damage occurs when nematodes inject "saliva" into the roots. This saliva contains a toxin that causes cells to collapse or disintegrate, resulting in dark lesions and dead areas along the roots. In some cases, the toxin stimulates rapid cell growth or enlargement, resulting in numerous dark, bushy roots, or galls or swellings on roots. Nematode damage limits the ability of the root system to supply the aboveground plant parts with water and nutrients, causing plant wilting, discoloration, stunting, and in severe cases death. Some nematodes indirectly injure plants by rendering them more susceptible to root-rot fungi. Although the nematodes do not directly transmit fungi, they can transmit certain virus diseases while feeding. Nematodes can move through the soil by themselves, but they move slowly, traveling at most 3 feet during a season. They are spread more rapidly by infested soil and transplants, contaminated equipment, and irrigation water.

Foliar nematodes: Foliar nematodes live and feed mainly inside plant stems and leaves. They spread through the plant by swimming through moisture on wet leaves. The foliage of infested plants is stunted and distorted, and brownish black blotches develop on the leaves.