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Nearly all the major crop pests in California–including plant parasitic nematodes–have been introduced on or in host plants or as contaminants on articles of commerce. There are, however, a number of nematode species of serious economic importance in other parts of the world which are not known to occur in California. The golden nematode is a serious pest of potato in Europe and on Long Island, New York; the sting nematode injures many crops in the southeastern states; the soybean cyst nematode, in the Mississippi Valley and other southeastern states, attacks soybean, snapbean, and other plants of the bean family; and the burrowing nematode, reported to be the cause of spreading decline of citrus in Florida, are some of the nematode species not yet established in California.

The many thousands of species of nematodes in the phylum Nemata are a group of animals commonly placed into four categories: parasites of man and animals; parasites of plants; species living in marine and brackish water; and free-living soil and fresh-water species. No single character or criterion can be used to distinguish nematodes from other similar animals, although one feature-not structural–sets nematodes apart, and that is the tremendous variation in size. Nematodes vary in length from the 1/125" ectoparasites of plants to the 25' long parasite of whales.

Nematodes of greatest economic importance are the several species of root-knot nematodes–Meloidogyne spp.–but pests such as the sugar-beet nematode– Heterodera schachtii–and the stem and bulb nematode–Ditylenchus dipsaci– are of major concern on specific crops.

Feeder roofs of grapes may be almost completely lacking in heavy nematode infestations.

Approximately 90% of the citrus trees in California are infested with the citrus nematode–Tylenchulus semipenetrans–which can reduce the yield of bearing trees and the growth of replants.

Nematodes impair the root systems of trees by releasing toxins and by introducing secondary bacteria and fungi. Sometimes root symptoms make nematode activity evident, but in many instances these symptoms do not differ from those of other root-debilitating agents.

The root-knot nematodes are a limiting factor in growing ornamental plants –roses, carnations, Shasta daisies, gerbera daisies, and many others. Three species·–Meloidogyne incognita acrita, M. javanica javanica, and M. hapla–are commonly found on ornamental plants in California. Steam sterilization of plant beds or preplanting treatments of soil with nematocides have been effective in their control.

The problems associated with nematode diseases of plants can be visualized as one part of a system consisting usually of three components: nematodes, plants and the linking medium, most frequently soil.

Nematodes are attacked by a variety of organisms including protozoa, fungi, and other nematodes. There are indications that some bacteria may cause reductions in nematode populations and, although no virus diseases of nematodes are known at present, some conceivably exist.

The degree of control required from nematocidal treatments, application methods and rates vary from crop to crop. Less than 10 nematocides are available commercially, and only three or four of those were introduced within the past 15 years. Some nematocides are distributed in the soil by diffusion in the gas or vapor state, by water solutions or emulsions, or by mechanically mixing the chemical into the soil. Whatever the method, the chemical must be dispersed through the soil to kill nematodes to the desired depth, without leaving a phytotoxic residue.