Have you ever pulled up vegetable plants such as tomatoes or peppers at the end of the growing season and noticed bumps along the underground portion of the stem or on the roots? Did you wonder what those were? Wonder no more.

You’ve got nematodes — and not the good kind.

Nematodes are tiny, worm-like multicellular animals adapted to living in water. There are an estimated half a million species of nematodes worldwide, including a group that lives in films of water that cling to soil particles. Nematodes are found in soils all over the world.

Some are good guys. Some beneficial nematodes help control cutworms, fleas, ants, termites, grubs, and other lawn insect pests. The nematodes that damage vegetable plants, though, are not a gardener’s friend. These are plant parasitic nematodes.

“It’s important to understand there are two different things going on here with nematodes and not get confused,” said Elizabeth L. Little, an assistant professor in the Plant Pathology Department at the University of Georgia. “Beneficial nematodes in the soil perform the work of a natural pesticide. The ones that leave bumps on vegetable roots are root-knot nematodes (Meloidogyne species). They burrow into the roots or the underground part of the plant stem, feed on the plant, and lay eggs. When the eggs hatch, the babies feed on the roots and, as a result, the plant is weakened, growth is stunted, leaves turn yellow and wilt, and production is reduced. In addition, plant-pathogenic fungi and bacteria can enter plant tissue through the wounds created by the nematodes.

How to prevent harmful nematodes

“The goal is to not get to this point,” stressed Little. “There’s no quick fix!”

That’s especially true in any current growing season when you have or suspect you have root-knot nematodes. Unfortunately, the only way to kill the nematode is to destroy the host, which in this case would be your vegetable plants.

“Nematode-killing chemicals are not available to homeowners,” said Little. “The only real control is to prevent plant parasitic nematodes from becoming a problem in the first place.”

There are several ways to do that, she said.

The number one prevention method, Little emphasized, is crop rotation. A general rule of thumb is to rotate to crops not related to each other. Rotating crops will not only help prevent nematodes, it also will help control plant diseases and harmful insects.

Another is to create a healthy soil environment by amending garden soil with organic matter such as compost.  Numerous biological control organisms thrive in a balanced soil ecosystem. These organisms will help reduce populations of harmful nematodes.

A third is to plant winter cover crops. When selecting cover crops, Little said to choose ones that are non-hosts for root-knot nematodes. By planting the correct cover crop, home gardeners can help suppress root-knot nematode populations, or at least not help them thrive, Little said. One way to determine a non-host cover crop would be to have your soil tested by a local Cooperative Extension Service. The test will determine if nematodes are present, and, if so, what kind. An Extension Service agent can then recommend a winter cover crop that will be effective in helping to reduce populations of harmful nematodes. Soil tests are most effective in the fall when nematode numbers are the highest.

A fourth method of prevention is to select vegetable varieties that are labeled as nematode resistant.

Carrots that have been infected with root-knot nematodes

Carrots that have been infected with root-knot nematodes as made clear by the Meloidogyne species's trademark bumps. (Photo: Wikimedia Commons)

How to encourage beneficial nematodes

Beneficial nematodes that help control certain types of lawn insects are commercially available for use by homeowners. To find a source, inquire at a nearby garden center, contact your local Extension Service, or look online.

Remember that when you receive them, nematodes are living organisms and must be handled carefully to be effective. They are supplied in a concentrated form, often on a wet sponge. Follow directions for application, which will involve mixing the sponge with water and applying the nematodes with a readily available type of liquid lawn applicator you can attach to a hose.

Some general tips for applying beneficial nematodes to the lawn:

  • Use them as soon as possible.
  • Follow package directions for storing them if you can’t use them right away.
  • Water them into the soil after application.
  • Be sure to follow any other package directions for maximum effectiveness.
You won’t see results immediately. The beneficial nematodes won’t kill insects that are present. It’s the next generation they will chomp!

It’s also important to remember that applying nematodes to your lawn isn’t a long-term solution to controlling annoying and harmful lawn insects. As with the prevention-is-the-best-cure approach to parasitic nematodes in vegetable gardens, maintaining good cultural practices that create healthy plant and soil environments that discourage insects is the best way to maintain a healthy lawn.

Related on MNN: