Nematodes of the genus Pratylenchus, known as meadow or root lesion nematodes, are another common type of root parasites. They feed in the cortex of roots and destroy the cells on which they feed. Fungi then attack the dead tissue. In the early stages the only visible symptom of attack is a small, reddish-brown lesion on the root. The lesion later enlarges, often girdles the root, and eventually severs it. Heavily attacked plants have greatly reduced root systems; most of the feeder roots are destroyed late in the season. The same sort of damage is also caused by other nematodes and by other types of soil organisms; positive diagnosis depends therefore on identifying the meadow nematode.

Stubby root nematodes (species of the genus Trichodorus) and sting nematodes (Belonolaimisgracilis) are external parasites, which apparently feed mostly on root tips. The feeding causes the root tip to stop growing and turn brown. Parts of the root may then die, probably because of attacks of secondary invaders. The final result is a reduced root system with many short root stubs. The attacks are particularly damaging to seedlings. Being external parasites, these nematodes will be found only in the soil.

Bulb and stem nematodes, species of the genus Ditylenchus, cause more or less localized deformations of stems and leaves. Stems are shortened and thickened; leaves are twisted, shortened, and otherwise distorted; and bulbs, such as narcissus and onion, become soft. In the later stages there may be rotting of the infected tissues. Nematodes can be found in large numbers in the affected parts, but are very slender and difficult to see without a microscope.

The wheat nematode, Anguina tritici, causes deformation of leaves of wheat and other grains in the early stages of growth. The nematode later invades the developing ear, causing the formation of galls in place of grain. The galls are shorter than normal wheat grains and look much like smut balls. But smut balls are soft enough to crush with the fingers and nematode galls are hard. If the gall is cut open and placed in a little water, the contents spill out and can be seen under the microscope to be thousands of minute worms, the larvae of the nematode. If still alive, they will start active movement in a few hours.

THE VARIOUS KINDS of nematode damage interfere with the growth of plants. Reduction in the size of the root system by rotting or galling restricts its efficiency in obtaining the food and water the plant must get from the soil.

Root knot galls distort the tissue that has the function of conducting food and nutrients to the upper part of the plant. Damage to stems and leaves also interferes with normal growth. Consequently the yield of crop plants is reduced. Crippled plants cannot produce a high-quality crop. With some crops, such as carrots and white potatoes, galls and rot caused by nematodes can make culls out of what would otherwise be salable produce.

The general appearance of a crop heavily attacked by nematodes that damage roots gives the impression that it is suffering from lack of fertilizer and water, even when they are available in the soil in abundance. The color is a lighter or more yellowish green than normal. Nematodes are seldom evenly distributed in the soil, so the growth of plants is uneven and patches of stunted plants appear here and there. Heavily infected plants may die prematurely, because of rotting of the roots, while clean or lightly infected plants are still growing normally.

The relation of nematodes to other soil-borne diseases, such as fusarium wilt of cotton, is not clearly defined. It is certain that plants attacked by both nematodes and bacterial or fungus diseases often suffer severe injury and that control of nematodes has often resulted in fair control of the other disease. The usual theory is that nematodes, by damaging the plant tissue, prepare the way for infections by bacteria and fungi, which would not occur otherwise, but it has been difficult to demonstrate this relationship experimentally.

I have no reliable estimates of the amount of damage nematodes do to crops in the United States each year, but there is general agreement that it is at least several hundred million dollars. The use of soil fumigants for nematode control during the past several years has often produced dramatic proof that nematodes in the soil can make the difference between a good crop and one not worth harvesting. Yield increases of 25 percent to 50 percent after soil fumigation are common. Experiments with soil fumigation have also made it evident that severe nematode damage can occur in any part of the United States on a great variety of crops, including tree crops. It is also evident that severe nematode damage is not confined to the farm. Home gardens and ornamental plantings in city yards also are often damaged; there may even be damage to the flower pots on the window sill. New kinds of plant parasitic nematodes are constantly being found as are new locations for the more familiar species.

Little is known of the origins of the plant parasitic nematodes, but information-as to their distribution in 1953 indicates that man has been largely responsible for their multiplication and spread from place to place. Being so very small, they are often unnoticed contaminants of plants, roots, bulbs, and tubers used for planting. It is probable that some of the worst of the nematode pests have moved from country to country around the world with such material. Soil moved from place to place, purposely or accidentally, may also contain plant parasitic nematodes. They are transported over long and short distances in soil adhering to farm implements and vehicles or to the feet of men and animals. Drainage water carries them from field to field. Certain species may be blown about by the wind.

After plant parasitic nematodes are introduced into a field, it may be a long time before their presence is noted. This is partly because increase from a small number is a slow process. Even when some damage to the crop is noted, the trouble may be attributed to declining soil fertility or to a succession of unfavorable growing seasons.

ALBERT L. TAYLOR is a member of the division of nematology investigations of the Bureau of Plant Industry, Soils, and Agricultural Engineering. He has done experimental work on soil fumigation and, while employed by the Shell Chemical Corporation from 1946 to 1949, did research and development work on the soil fumigant D-D.