Nematodes – What are They - Plant Damage They Cause

Nematodes, or microscopic eel worms, for some plants have assumed the role of Public Enemy Number One among the organisms that cause plant diseases. As far back as 50 years ago they seriously damaged one out of every ten acres of crops grown in the United States.

But farmers are not the only victims of damage by these pests. They attack all kinds of plants including ornamental and shade trees, annual and perennial flowers and even lawn grasses. How do they launch their attacks?

Nematodes have piercing mouth parts which are inserted into plant tissues to extract the juices, causing the plant cells to be invaded by fungi and bacteria. As a result they may soon die. In some cases the pierced cells may not die but may be stimulated to grow abnormally. In either situation the plant’s natural development is upset, and stunted growth, leaf blight, reduced size of fruit and flower… occasionally even death of the whole plant may occur.

Such nematode damage, however, is not easily recognized by most gardeners. Many cases of injury are incorrectly diagnosed as winter injury, unfavorable soil condition, or even lack of plant food.

It is because of their small size that they are not more generally recognized. Although nematodes are thousands of times larger than bacteria, they are just a bit too small to be seen without a hand lens. The plant-parasitic kinds range from less than 1/8 inch in length at maturity down to 1/125 of an inch. Thus, if a person plainly sees little white worms crawling over the soil or on plants, the chances are they are not seeing nematodes!

A second reason they are hard to diagnose is that many species of nematodes attack below-ground parts, particularly the roots, and their presence is not discernible unless the plants are lifted, the soil shaken or washed away, and the roots carefully examined. Of course, where large plants or trees are suspected of harboring nematodes, only a portion of a root can be dug up for examination.

The easiest type of injury to diagnose is perhaps that produced by the so-called root-knot nematodes belonging to the genus meloidogyne. They produce knots or galls on the roots, varying from a slight swelling, up to an inch or more in diameter. If such galls are pulled apart with two needles, pearly-white, pear-shaped nematodes will be just barely visible without a hand lens. The shape and size of this nematode vary greatly from most other species, which are much tinier and have long, slender, eel-like bodies.

Where no galls are present on the roots, and the gardener still suspects nematodes because of the generally unhealthy condition of the plants, the wise procedure is to submit adequate root specimens to the state pathologist. The so-called meadow nematodes, for example, do not produce galls on the roots but cause lesions on some roots and decay in others. Boxwood, iris and pin oaks are some of the plants subject to meadow nematodes.

Some nematodes, such as the root-knot nematode, can attack 1,000’s of different kinds of plants; others attack only one or two. One species infests the bulbs of narcissi and onions; another causes a shortening and twisting of the leaves and stems of perennial phlox, sweet Williams and hydrangeas; and still another blights the leaves of ferns, begonias and gloxinias.

The life cycle of nematodes is not a complex one. Eggs are deposited in the soil or in the plant on which the female feeds. From these eggs emerge the immature larvae which feed on the roots, stems or leaves, depending on the species involved. After several molts, the larva matures and is capable of reproducing. The entire cycle from egg to egg-laying female is completed in from several days to several weeks, depending on the kind of nematode and the surrounding conditions.

Some species can withstand long dry spells. They remain alive but dormant for several years and then, once they are moistened, become active in a short time. Nematodes found in the colder parts of our country are capable of surviving winters even in frozen soil. The gardener, receiving little help from nature in controlling some species, is forced to use special measures.

The measures to adopt depend on the kind of nematode involved, the crop affected and the size of the planting.

Though some reduction in injury is possible by crop rotation, this is not practical in small gardens as the entire garden is usually infested. Then, too, rotation is possible only where annuals are being grown.

The use of resistant varieties also offers a partial solution because nematodes, like other parasites, have plant preferences. Among the ornamentals that are very resistant to root nematodes are African marigolds, gaillardias, narcissi, tulips and zinnias. Resistant vegetables include corn and soybeans; resistant fruits are currants, goose-berries and certain varieties of grapes. Lilacs, peonies, snapdragons and violets are extremely susceptible to root-knot nematodes, as are such vegetables as carrots, celery, lettuce, peppers and tomatoes. Among the very susceptible vine fruits are muskmelons and some watermelons.

Fertilizing plants infested by root nematodes sometimes helps to overcome harmful effects caused by these same nematodes.

For leaf-infesting species of nematodes control measures may vary from keeping water off the leaves (where plants are grown indoors), to dipping pot-grown plants such as begonias and African violets into a highly poisonous chemical to the soil. Many of these chemicals are highly poisonous chemical and must be handled with care and it cannot be used on soils utilized to grow food crops.

Soil-infesting nematodes can be con-trolled in several ways. Small lots of soil can be heated to 180° for a half hour as a control measure. Larger amounts can be treated outdoors by injecting special fumigants into the soil where their volatile gases kill the pests.

Controlling nematodes is an ongoing problem which will continue to be studied and improved in the plant growing world.

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