Battling a hidden garden enemy: Nematodes

These tomato roots have galls, or swellings, caused by the root-knot nematode. This degree of galling will reduce the number or size of fruit the infected plant produces.

June is the time of the gardening year when a nasty pest awakes from its winter sleep (called dormancy) and makes its presence known.

The hidden enemy of many garden plants is the root-knot nematode, a tiny roundworm that lives underground. The individual roundworms are too small to be seen without a microscope, but the damage they do to susceptible plants is not.

Finding nematodes

The main damage on plants is to the roots. Round swellings appear all along the root system. These swellings are galls, a general term for a swollen mass produced on a plant by a pest.

When nematodes enter roots, they inject a protein that acts like a plant-growth hormone. The plant responds by making galls.

On less susceptible plants, such as dianthus or Shasta daisy, a few galls form along the roots, like beads on a chain. On very susceptible plants, like cockscomb or squash, galls grow so large they fuse together in a gnarly mass of what used to be root tissue.

Why does root-knot nematode trick plants into making galls? The galls are filled with plant sugars on which the nematodes feed. As the nematodes grow inside the galls, female nematodes also produce eggs there.

If the galls are large enough, they interfere with the plant’s ability to absorb water and dissolved nutrients, like nitrogen. Susceptible plants are shorter or smaller and may wilt, even when they have enough water. Fruiting vegetables, such as cucumber and tomato, produce small or misshapen fruit.

If a nematode-infested plant is pulled up, some of the roots and galls will come out of the ground with it. The roots of cool-season annuals that are susceptible to root-knot nematode, including larkspur and nasturtium, should be checked when they are removed from the garden at this time of year.

If galls are found, the roots (and galls) remaining in the soil should be removed with a shovel or trowel, as well as the soil around the roots.

Battling nematodes

Root-knot nematodes are killed by temperatures at 104 degrees in a good, hot, active compost pile. A home compost pile, however, may not reach this temperature. Galled roots should not be added to compost; rather they should be discarded with garbage. Nematodes do not invade the stem or leaves, so the above-ground portions of affected plants may be composted safely.

The best way to prevent problems with this pesky roundworm is to avoid growing susceptible plants during the summer and early fall when these pests are active. Cool-season annuals that are susceptible, such as pansies and snapdragons, should be removed by June 1.

Many gardeners like to grow parsley and dill for monarch butterfly caterpillars. Parsley, dill, carrots and cilantro, however, are very susceptible to root-knot nematode. Fennel is a good substitute, since it is mostly resistant to root-knot nematode. Although fennel is not quite as appealing to butterflies as parsley or dill, it is much safer for the garden in the long run. Alternatively, parsley and dill can be grown in pots.

Many ornamental plants were tested against root-knot nematode in the 1930s by C.C. Goff at the University of Florida. His lists of plants and their reactions are available online (http://edis.ifas.ufl.edu/in470). This, unfortunately, is the most current information available.

If galls are found, an easy way to rid garden soil of root-knot nematode is to plant French marigolds (Tagetes patula), African marigolds (Tagetes erecta), or their hybrids. Not only are marigolds immune to root-knot nematode, they eliminate most of the nematodes from infested soil. (I verified this in my yard by having soil analyzed for nematodes before and after planting marigolds.)

Marigold roots produce toxic chemicals that kill this garden villain. Moreover, any lingering nematodes cannot infect marigold roots so they starve to death.

When using marigolds to combat root-knot nematodes, it is crucial to plant them 6 inches apart in a solid mass. Planting marigolds among other plants does not work, because nematodes will avoid the marigolds and infect the other plants.

Checking roots for galls and making careful plant selections go a long way to preventing serious problems with this (no longer) hidden garden enemy.

Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at tknth@clemson.edu.