While most of the thousands of species of nematodes on Earth are not harmful, some nematodes parasitize and cause diseases in humans and other animals. Also, unfortunately, there are many that attack and feed on living plants.
What They Are
Often described as wormlike, nematodes are not closely related to true worms. They are multicellular animals with smooth, unsegmented bodies. The nematode species that feed on plants are so tiny that you need a microscope to see them. They are often long and slender, although some species appear pear-shaped.
Some nematodes feed on the outside surfaces of a plant, while others burrow into plant tissue. While soil-dwelling nematodes are the most common culprits, some species damage stems, foliage, and flowers.
No matter where they feed, these tiny creatures can seriously damage plants with sharply pointed mouthparts that they use to puncture cell walls. The real damage occurs when a nematode injects saliva into a cell through its mouthparts, and then sucks out the cell contents. The plant responds with swellings, distorted growth, and dead areas. Nematodes can also carry viruses and inject them into plants. The feeding wounds they make also provide an easy entrance point for bacteria and fungi.
Beneficial nematodes that live in the soil may feed on decaying material, insects, or other nematodes. For more information on these beneficial nematodes, see below.
What You See
Unlike most other disease-causing organisms, plant-parasitic nematodes seldom produce any characteristic symptoms. Most of the symptoms that do appear are vague and often resemble those caused by other factors, such as viruses, nutrient deficiencies, or air pollution. Nematodes feeding aboveground may cause leaves, stems, and flowers to be twisted and distorted.
If nematodes are feeding on roots, a plant may be yellowed, wilted, or stunted; infected food crops will usually yield poorly. If you suspect nematode injury to roots, carefully lift one of the infected plants and wash off the roots for easier inspection. If nematodes are causing damage, you may see small galls or lesions, injured root tips, root rot, or excessive root branching. For a positive diagnosis, contact your local extension office for information on where you can have your soil tested.
How They Spread
Whether they feed above- or belowground, most nematodes spend at least part of their life cycle in the soil. While they can’t move very far under their own power, they can swim freely in water, and they move more quickly in moist soil. They are also spread by anything that can carry particles of infested soil, including tools, boots, animals, and infected plants.
Nematodes as Pests
While most nematodes are beneficial (see Beneficial Nematodes below for more on their role in the garden), a few species of these translucent, unsegmented worms are plant parasites. Pest species are only about 1/50 inch long and cause root knots or galls, injured root tips, excessive root branching, leaf galls, lesions or dying tissue, and twisted, distorted leaves. Plants most commonly attacked at the roots include tomatoes, potatoes, peppers, lettuce, corn, carrots, and other vegetables. Plants that sustain leaf and stem injury include chrysanthemums, onions, rye, and alfalfa.
Slender, translucent, unsegmented worms, beneficial species of nematodes are 1/25 inch to several inches long. Their roles in the garden vary. Some are soil dwellers that break down organic matter and are common in compost piles. These decomposers are easily visible; they are about ¼ inch long.
Other nematodes (families Steinernematidae and Heterorhabditidae) attack and kill insects either by injecting bacteria (Xenorhabdus sp.) that kill the host within 24 to 48 hours or by entering the host, parasitizing, and feeding on it.
Beneficial nematodes are effective against a variety of pests, including weevils, clearwing borers, cutworms, sod webworms, chinch bugs, and white grubs. When purchasing and applying them, it is very important to select the right species of nematode, because different species are effective against different pests. In addition, nematodes require moist, humid conditions, and fairly warm soil to be most effective. Water application sites before and after spreading nematodes. When purchasing them, follow application directions carefully.