The Tiny but Destructive Nematodes

Albert L. Taylor.

Nematodes differ from most of the other organisms that cause plant diseases in that they belong to the animal kingdom, not the plant kingdom.

The plant parasitic nematodes, or eelworms, are representatives of a large group of species that the zoologist considers quite different from the other kinds of animals called worms. That is, they are not closely related to the earthworms, flat worms, wire-worms, grubs, and cutworms, but are in a class apart. They have no close relatives.

Several thousands of species of nematodes are known. They differ in form, habits, and habitat. Some are parasites of animals and of man. Others live in the fresh waters of rivers, ponds, and lakes. Many live in the salt waters of the sea. A great number live in the soil. Most of those living in the soil can be classed as harmless and some even as distinctly beneficial, but several hundred species are known to feed on living plants as parasites and to be the causes of a variety of plant diseases.

Plant parasitic and free-living nematodes occur in enormous numbers in all kinds of soil in which plants can grow. A single acre of cultivated soil may contain hundreds of millions, but they are seldom if ever seen, even by the farmer who is constantly working with the soil. The reason is simply that, although they are thousands of times larger than bacteria, they are just a little too small to be easily seen with the naked eye, even when separated from the soil. The length of the full-grown plant parasitic nematode may be less than one sixty-fourth of an inch and seldom exceeds one-eighth inch.

Most are very slender, as the name eelworm suggests. Nevertheless, they have a highly complex organization. Their small bodies have muscular systems, specialized organs for feeding, a digestive system, a nervous system, an excretory system, and a well-developed reproductive system. Both males and females occur in most species, but reproduction without the males is not unusual.

The life history of plant parasitic nematodes is simple enough. Eggs may be deposited in the soil or in the plant on which the female feeds. In the eggs the immature forms, the larvae, develop and eventually hatch. If plants on which they can feed are available, they may begin to feed immediately, developing through several distinct stages. At the end of each of these, a molt takes place. After the last molt, the nematode becomes sexually mature and able to reproduce.

Most of the forms that have been closely studied have a minimum length of life cycle, from egg to egg-laying female, of several days to several weeks. The maximum time may be much longer, as sexual maturity is not will reached until the nematode begins to feed on a living plant. Until then it remains in the larval stage and lives on a reserve supply of food originally derived from the egg. The length of time this reserve food supply lasts depends on circumstances. In warm, damp soil, the nematode will be very active and use it up in a few weeks or months. In cool or dry soil, activity is less and the food supply lasts longer. The species found in cold climates can easily live over winter and are not killed by freezing of the soil. Some species are killed if subjected to drying, but others enter a dormant state in which they can remain alive for years and from which they can revive in a short time if moistened. The most remarkable of these are parasites of wheat, rye, and other grains and grasses. The wheat nematode has been revived after dry storage for 28 years. A species that parasitizes rye has been revived after 39 years. Certain species can live a long time outside in moist soil. Females of the golden nematode of potatoes and its relatives become transformed at death into highly durable cysts. Because few or none of their eggs are deposited, the cysts contain eggs with unhatched larvae, which may remain alive for 10 years or more.

As plant parasitic nematodes neither feed nor reproduce except on living plants, survival of the individual depends on its reaching a plant on which it can feed before its reserve food supply is finally exhausted. Unless hatched in a plant, that means that the nematode must travel through the soil in search of food. This movement seems to be more or less random wandering and, since nematodes are small, is confined to a small area of soil. Perhaps most nematodes never get more than a foot or two from the spot where they were hatched. Therefore nematodes spread very slowly by their own efforts.

Plant parasitic nematodes have many enemies in the soil. At any stage of their life they may be captured and devoured by other soil animals, such as insects or predatory free-living nematodes. Certain soil fungi have traps that seem especially designed to catch nematodes. Some are loops which close when a nematode starts to crawl through. Others have sticky surfaces to which nematodes adhere. In either instance, the fungus grows into the body of the nematode and kills it.

Even though the accidents of life take a large toll of the plant parasitic nematodes, a field population is seldom exterminated. Like other parasites, the nematodes manage to reproduce just a little faster than they can be wiped out. A single female root knot nematode may produce more than 500 eggs. If only a few of them survive to reproduce in turn, a great increase in the population of a field can take place during a summer when several generations follow one another.

Information accumulated during the past century indicates that all of the crop and ornamental plants grown in the world can be attacked by plant parasitic nematodes. If there are exceptions to this rule, they must be very few, indeed. Probably most weeds and wild plants are also attacked.

That does not imply that any species of plant parasitic nematode can attack any kind of plant. All plant parasitic nematodes are more or less specialized, attacking some plants freely and others not at all, even when given every opportunity to do so and when no other source of food is available. So far as a given species of nematode is concerned, different kinds of plants may have varying degrees of suitability as food. On some they will not attempt to feed. On others they will feed, but seem unable to reproduce. Such plants are called immune to the nematode species concerned. On other plants reproduction is inhibited to various degrees; they are called resistant. Plants on which normal reproduction takes place are called susceptible. It should be emphasized that any such classification of plants can be taken as applying to only a single species of nematode. Plants immune to attack by one species of nematode may be highly susceptible to attack by others.

The species range from highly specialized (those that attack only a few kinds of plants) to polyphagous (those that attack a great many different kinds of plants). The reasons therefor are not known. In practice there seems to be no way of knowing which plants might be attacked by a given species except by experiment. Resistance to nematodes sometimes can be found in horticultural varieties of crops or in other species of the same plant genus. Advantage is taken of this fact in the development of nematode-resistant crop varieties.

Plants almost invariably become infected by nematodes that move into them from the soil. As would be expected, the underground parts of plants, roots, tubers, corms, and rhizomes are more apt to be infected than above-ground parts. Infection of stems, leaves, and flower parts is fairly common, however.

Damage to plants attacked by nematodes is due primarily to the feeding of the nematodes on the plant tissues. All the important plant parasitic nematode species have a special feeding organ, known as a stylet or spear. As seen in profile under the high-power microscope, the typical stylet resembles a nail with a thickened head, although close examination reveals that the head is composed of three more or less distinct knobs. Stylets, highly variable in size and shape according to the species of nematode, range from comparatively long to very short, and the knobs have a variety of forms, ranging from large and distinct to nearly absent. Those differences are useful in the identification of genera and species. The stylet really is more like a hypodermic needle than a nail. It is hollow, and the nematode uses it to pierce plant tissue or cell walls. With the stylet pushed into a cell, the nematode can suck out the cell contents. In preparation for this, it may inject a digestive secretion into the cell, evidently to liquefy and partly digest the food before it is ingested. Nematodes may enter the plant to feed, may feed from the outside, or be only partially embedded. Feeding habits vary according to species.

The feeding of a nematode may kill the cell or may simply interfere with its normal functioning. If the cell is killed, it often is quickly invaded by bacteria or fungi. If the cell is not killed, it and the adjacent cells may be stimulated to enlarge or multiply. Consequently the most common types of nematode damage are manifest as rotting of the attacked parts and adjacent tissue or the development of galls and other abnormal growths. Either can interfere with the orderly development of the plant and cause shortening of stems or roots, twisting, crinkling or death of parts of stems and leaves, and other abnormalities.

Those symptoms often are complicated by the presence of secondary invaders in the affected parts and, particularly in advanced cases, may present a confusing picture. Consequently the specialist depends only partly on symptoms for diagnosis and searches for nematodes in the plant or in the nearby soil.

The following representative nematode diseases are common and can be recognized fairly easily. It should be remembered that it is easy to mistake nematode diseases for those caused by some other organisms, and vice versa. Merely finding nematodes in diseased plant tissue or the soil is not conclusive evidence that they are the cause of the trouble. Nonparasitic types of nematodes often are found in great numbers in decaying plants and the soil always contains a variety of free-living nematodes. Positive identification should always be obtained before starting expensive or troublesome control measures. On the other hand, nematodes should always be considered as a possible cause of plant diseases when root systems are galled, shortened, or reduced by rotting; when the stems are shortened and thickened and the leaves do not grow normally; and some other abnormal growth is noted.

Probably the easiest of the nematode diseases to recognize is root knot, caused by nematodes of the various species of the genus Meloidogyne (formerly grouped under the name Heterodera marioni). They are called root knot nematodes. As the name implies, they cause the formation of knots or galls on roots of a great variety of crop and ornamental plants, including trees and shrubs. The typical simple galls are best observed on the younger roots, where they may look like beads on a string. Galls caused by at least one species of the genus commonly have several short, adventitious roots that rise from the upper part and produce a bushy appearance of the root.