You can pick up a handful of soil almost anywhere and extract nematodes from it. Most kinds of nematodes escape notice, however, because they are so small that they cannot be seen without the aid of a microscope. Most soil-dwelling nematodes are of no concern to the gardener and are, in fact, a normal component of fertile soil. These innocuous nematodes feed on bacteria, fungi, algae, and even other nematodes.
Some nematodes are parasitic on insects and higher animals.
Others, however, are parasitic on higher plants and can present a serious problem in the home fruit garden. Because nematodes are out of sight and gradually multiply to damaging population levels, they have been referred to as the unseen enemy in agriculture. Nematodes are threadlike roundworms that move about in the thin film of moisture surrounding soil particles and root surfaces. The stylet is essentially a hollow spear, like a hypodermic needle, that the nematode uses to puncture cells and feed. The symptoms of nematode infection are commonly those of root impairment, such as growth reduction, increased wilting, mineral deficiency symptoms, decreased winter hardiness, and dieback in perennials.