Nematodes are unsegmented round worms that inhabit soil and water in great numbers. Some species are parasitic on plants. All nematodes develop from eggs and pass through a succession of larval stages (usually four) prior to adulthood. Reproduction may be sexual or parthenogenic. Nematodes are dispersed in soil, running water, and plant parts. Some species have a resting stage that will withstand desiccation.

Nematode feeding reduces plant vigor and induces lesions, rots, deformation, and gall and root knots. Infested fields appear uneven, usually with distinct patches of stunted plants.