Migratory endoparasites can move into, through, and out from host tissues at any stage of development (except the egg). Migratory endoparasites generally live and feed in tender tissues such as the root cortex. They burrow through the tissue, breaking open many cells after feeding on them. Cells surrounding the feeding became necrotic by toxic materials liberated from the disrupted cells. The relatively large areas of dead cells usually turn brown to become small spots or lesions big enough to see, and are often easily colonized by fungi. Root rot diseases are often associated with infestations of migratory endoparasitic nematodes like Radopholus spp. The most important examples are species of burrowing and lesion nematodes. The nematode, Radopholus similis, causes the spreading decline disease of citrus. It is the subject of strict (and expensive) quarantine regulations for ornamentals, nursery stock, and other growing plants being exported and can severely limit growth of many ornamental plants. The foliar (Aphelenchoides spp.) nematodes feed on or inside the leaves and buds of ferns, strawberries, chrysanthemums, and many kinds of foliage ornamentals, and cause distortion or death of buds, leaf distortion, or yellow to dark brown lesions between major veins of leaves. Other nematodes which attack plants of above ground cause leaf or seed galls and still others cause deterioration of the bulbs and necks of onions and their relatives, but are not common here. Ectoparasitic and migratory endoparasitic nematodes generally deposit their eggs singly as they are produced, wherever the female happens to be in the soil or plant.

Sedentary endoparasitic nematodes include root knot (Meloidogyne spp.), cyst (Heterodera spp.), reniform (Rotylenchulus spp.), and citrus (Tylenchulus semipenetrans) nematodes. In most of these species, the second stage juvenile is the "infective" stage, which moves through the soil. Second stage juveniles locate host roots and enter them. They then establish a suitable feeding site within the root tissues. Once a feeding site is selected, the nematode injects growth regulating substances into the cells near its head, causing some of those cells to enlarge. These "giant" or "nurse" cells become specialized cells for feeding uninterruptedly by nematodes. At the same time, the nematode becomes immobile, and the body swells to a round, lemon, kidney, or ovoid form. Mature females of the sedentary endoparasitic nematodes generally produce large numbers of eggs which remain in their bodies or accumulate in masses attached to their bodies.

Nematicide treatment may be used to help diagnose the role of nematodes in some situations. Laboratory determination of the kinds and numbers of nematodes present is sometimes not sufficient to determine the role of nematodes in a growth problem, or to predict injury to a crop to be planted. Application of a nematicide, which is known to be effective against that pest, is sometimes the most clearcut way to determine their importance, or atleast the ability of an available treatment to remedy the situation. In such trials, it is best to apply the treatment to at least four areas of the suspect field, leaving adequate adjacent untreated areas for comparison. The nematicide product must, of course, be legal for application to the site and in the manner needed for the test. In a diagnostic nematicide trial always have untreated areas (front) and treated areas (back) for comparison.

Above Ground Symptoms

Since most plant nematodes affect root functions, most symptoms associated with them are the result of inadequate water supply or mineral nutrition to the tops: chlorosis (yellowing) or other abnormal coloration of foliage, stunted top growth, failure to respond normally to fertilizers, small or sparse foliage, a tendency to wilt more readily than healthy plants, and slower recovery from wilting. Woody plants in advanced stages of decline incited by nematodes will have little or no new foliage when healthy plants have substantial flushes, and eventually exhibit dieback of progressively larger branches. Gradual decline is typical of nematode injured turf and pasture. Plantings which are stunted by nematodes often have worse weed problems than areas without nematode injury, because the crop is less able to compete with weeds than it should be. The distribution of nematodes within any site is very irregular. Therefore the shape, size, and distribution of areas showing the most severe effects of nematodes will be highly irregular within the field. Nematodes move very few feet per year on their own. In the undisturbed soil of groves, turf, and pastures, visible symptoms of nematode injury normally appear as round, oval, or irregular areas, which gradually increase in size year by year. In cultivated land, nematode injured spots are often elongated in the direction of cultivation, because nematodes are moved by machinery. Erosion, land leveling, and any other force which moves masses of soil or plant parts can also spread a nematode infestation much more rapidly than it will go by itself. Nematode damage is often seen first and most pronounced in areas under special stresses, such as heavy traffic, excessive drainage because of slope or soil and dry areas outside regular irrigation patterns.