Nematodes: Hard to See or Eradicate


Question: I’m having difficulty with certain plants in my garden and I haven’t been able to determine the reason. The plants are stunted and grow slowly despite the fact that their sun exposure is correct and I am watering and feeding them properly. Do you have any idea what could be the problem?

A.B., Anaheim

Answer: Often plant problems as you describe arise from culture problems such as improper watering, unsuitable sun exposure, excessive or insufficient fertilizer, or crowding. However, it sounds like you’ve considered these causes of poor plant growth.

If your plants are growing more slowly than expected, wilt during the hot part of the day, have fewer and smaller leaves and fruit than expected, or just seem to be doing poorly without any clear reason, then you may have plant parasitic nematodes. This is especially true if you have sandy soil because nematodes are most common in warm, sandy soil containing little organic matter.


Nematodes are tiny round worms less than one-tenth of an inch long that are only visible through a microscope. There are many types, but root-knot nematodes attack popular vegetable crops such as tomatoes, peppers, cucumbers, carrots and cole crops.

These pests feed in the roots of plants, so signs of infestation are not visible unless you look below ground at the root zone. Infested roots are often stunted and have characteristic lumpy nodules and knots.

The nematodes feed on the roots and damage them by impairing their ability to take up water and nutrients, resulting in a general decline without readily recognizable symptoms.

Other kinds of less common nematodes attack the stems and bulbs of plants--most commonly garlic and leeks growing in cool, moist environments--but the damage caused by them is obvious on the stem and bulbs.

Generally, the only way to definitively diagnose a root-knot nematode infestation is to have a soil laboratory test a sample of plant root from the affected plants together with surrounding soil.

If you find that you have a nematode infestation, there is no way to cure the infected plants, but good cultural practices can slow the spread of the infestation to other garden areas and may control nematodes in areas already infested so that these locations may continue to be gardened.


To fight them, it’s important to know how nematodes spread. They move slowly in the soil and are usually transported by water, on the roots of plants from other places and on gardening tools.

In California, nurseries are rigorously inspected, so there is little risk that nursery stock will infest your garden. To prevent picking up nematodes from other gardens, it is best to take cuttings instead of entire plants. It is also advisable to use certified seeds and to promptly remove infected plants.

There are seven options for dealing with an established nematode population:

* Use nematode-resistant plants. There are varieties of nematode-resistant sweet potatoes and tomatoes on the market. Look for the designation VFN on seed packets. The “N” denotes root-knot nematode resistance.

Fruit tree varieties established on nematode-resistant rootstocks such as the Nema Guard rootstock may also be purchased. Some groups of plants are generally more resistant to nematodes than others.

For ornamental use, evergreens may be a good choice where nematodes are present because they tend to be more resistant to nematodes than other trees. Ask your nurseryman about resistant plant varieties.

* Leave the land fallow for a time, generally three to five years. This will reduce the nematode population by depriving the nematodes of food. Not only must nothing be planted in the fallow land, but the area must also be kept clear of weeds. Unfortunately, this will not yield a quick result.


* Rotate crops. Since most nematode species are attracted to small groups of related plants, it is possible to control them by rotating crops. Often nematode populations may be kept at low levels and can’t cause serious damage if the same type of plant is not placed in the same area more frequently than once every two or three years.

* Solarize the soil. In warm areas, soil solarization may also reduce nematode populations. The idea is to heat the soil for an extended period of time to kill the nematodes. This is done by leveling the area, clearing all weeds and debris, breaking large clods and covering the moist soil with a clear plastic tarp fitted tightly to the soil so that there are no air pockets. The plastic is left on the soil surface for four to six weeks.

Soil should not remain covered more than eight or 10 weeks, as this may result in oxygen depletion in the soil. The best time to solarize soil is during June and July, as heat peaks in mid-July. It is possible, though, to solarize from May through August in inland areas.

Solarization may not work in coastal areas where there are foggy conditions that prevent intense heating of the soil.

Soil solarization is not likely to result in complete eradication of nematodes because heat will not penetrate deep enough to destroy them below the top foot of the soil. Fortunately, many nematodes live near the soil surface, so this is an effective control method for home gardens and shallow crops.

The effects of solarization will last longer if care is taken to avoid soil movement that will bring nematodes to the surface from lower levels.


* Try experimental treatments. There are a number of other less predictable, more experimental methods for controlling nematodes, but none yields consistent results in scientific studies.

It is often said that planting asparagus, garlic and marigolds will deter certain species of nematodes. If these methods work, they do so only when plantings are dense and when the planted area is kept free of other plants.

The only type of marigold that appears to have some effect on nematodes is the species Tagetes patula. There is some evidence that dense plantings will reduce nematode populations. If you try this, consider your effort an experiment.

* Adjust planting dates. The effects of nematodes on plants can be minimized by adjusting planting dates so that plants are well established before nematodes become active.

If the planting of fall crops is delayed until soil temperatures drop below 64 degrees, most nematode species will be inactive. In the case of a few species, the soil temperature must drop below 50 degrees.

Similarly, early spring planting before soil temperatures rise will permit plants to become established before nematode activities begin, resulting in healthier plants. Early spring planting also permits crops to be harvested and soil cleared early in the warm season when nematodes are most active, which will limit available feed and further control the populations.


* Plant in containers. Planting in containers with commercial potting soil will enable you to avoid nematodes. Be aware, however, that you can still introduce the parasitic roundworms into the container through plant roots and by infected garden tools.

If a pot becomes infested with nematodes, replace the soil.

--Written by University of California Master Gardener Harold Pope of Irvine.

* Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners are here to help. These trained and certified horticultural volunteers are dedicated to extending research-based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or send e-mail to ucmastergardeners Calls and e-mail are picked up daily and are generally returned within three days. Please include your name and city.