Conquer Root-Knot Nematodes in Your Vegetable Garden!

Have you ever pulled up a plant and discovered knotted roots? If you did, you may have had root-knot nematodes. I was visiting my friend’s vegetable garden and she had noticed that her beans were going downhill quickly. She pulled up a plant and noticed some gnarly roots (Fig. 1).

Figure 1: Galls on roots from root-knot nematodes

What are Root-Knot Nematodes?

So what exactly are root-knot nematodes? They are unsegmented roundworms that are microscopic. Some nematodes, like root-knot nematodes, are parasitic to plants (Fig. 2) while others are beneficial, feeding on bacteria, fungi, and even insects.

Figure 2: Plant-parasitic nematode

Many plant-parasitic nematodes feed on plant roots; some feed only on the outside of the root while others burrow into the root. Root-knot nematodes go into the root when they are only juveniles and when a female matures, it becomes swollen and remain in one place. The nematodes inject hormones into the roots which causes them to form knots or galls. This symptom is easily recognized once you see it.

Since nematodes damage the roots, other pathogens such as fungi and bacteria can enter the plant causing a secondary infection. These secondary infections can cause wilts and root rot to form.

Where Do Nematodes Come From?

Nematodes live mostly in the soil, so if you have planted a bed above your native soil, then they can travel from up into the soil you placed in your bed. Nematodes are small enough that they can easily move through barriers that you may place for weeds like landscape cloth.

If you plant directly in native soil, then they may have already been there. Nematodes only move a few inches a year but they reproduce quickly in warm, moist conditions.

Nematodes can spread through contaminated soil on shoes, equipment, or infested plant material. They can move through the water or carried by animals or insects.

What Symptoms Do they Cause?

Nematodes reproduce quickly when the temperature is high and each year, if left uncontrolled, the problem can get worse. What you may see aboveground may include yellowing, wilting, and stunting.

The symptoms occur since the damaged roots cannot take up water and nutrients from the soil, causing them to decline. When my eggplants had nematodes, they didn’t produce many fruit and I couldn’t figure out what I did wrong until I removed the plants.

What crops do they affect?

The vegetables that are most commonly affected are tomatoes, potatoes, okra, beans, peppers, eggplants, peas, cucumbers, carrots (Fig. 3), field peas, squash, and melons. One person once told me that if you want to know if you have nematodes was to plant some okra because they just loved okra. I don’t know how true that is but I made sure that I planted my okra in a container when I knew I had nematodes.

Figure 3: Root-knot nematode galling on carrots.

How Do you Control Root-Knot Nematodes?

Plant Nematode Resistant Plants

So what can you do about root-knot nematodes once you know you have them? When I discovered that there were nematodes in my beds, I started looking for resistant cultivars to plant. I searched for cultivars online that were “nematode-resistant;” some packets will have a code of “N.” For example, I grew some ‘Beefmaster’ tomatoes from seed the next year which had nematode resistance and I had no nematode problems.

Soil Solarization

During the summer, we also did soil solarization (Fig. 4). This is the process of using heat from the sun to kill nematodes, as well as other pests. You cover the soil with a clear plastic tarp and bury the edges. Make sure your soil is moist but not wet first. You leave it on for at least 4-6 weeks, until you are ready to plant. You really want to cook the soil. This will only work in areas with full sun and it is best done in the summer.

Figure 4: Soil solarization on a raised bed.

Plant Cover Crops or Companion Plants

If the summer is far away, so what can you do for the spring crop? You can plant a cover crop between seasons. Cover crops such as hairy indigo, Pangola digitgrass, showy crotalaria, Sunn hemp, and velvetbean have all been known to have good control of nematodes. You would let the cover crop grow until you are going to plant, then mow it down and incorporate the residue as a green manure.

You may have heard about planting marigolds as a companion plant when you have nematode issues. Keep in mind that not all marigolds are as effective. The “Petite French” types are the most effective (Fig. 5). Also simply growing marigolds near your susceptible plants may not be as effective. You have to grow marigolds then plant your tomatoes or beans where the marigolds were the next season.

Figure 5: Petite French marigolds in garden bed.

Add Organic Amendments

Other things that can be done are added compost or manure to your garden. It mainly helps the plant survive the nematodes by improving the soil and increasing natural enemies rather than directly affect the nematodes.

Remove Infected Roots

Make sure you remove all the roots from the nematode-infested plants so they are not left to reproduce. Tilling can also help by exposing nematodes to sunlight which will dry them out.

Grow in Containers

Lastly, growing susceptible vegetable plants in containers is a sure way of preventing nematode problems. Just make sure you don’t use your native soil in your containers.

Key Takeaways

You often don't know that you have root-knot nematode until you have pulled out your plant. But once you know you have it, you can take steps to manage the problem through future seasons by planting resistant seeds, solarizing your soil, planting cover crops, or adding amendments. If it is too difficult to control the problem, you can always plant in containers to take the soil issue out completely.

If you have had a nematode problem, leave a comment and let me know how you handled it.