While many garden pests are visible to the naked eye, root-knot nematodes lurk beneath the soil surface. These tiny worm-like creatures are small enough that gardeners often spot their damage before they locate the pests themselves.
And boy, is their damage unique! As their name suggests, this group of nematodes causes knotty growth on plant roots. While these sci-fi-esque growths are fascinating to look at, they are synonymous with weak and stunted plants. If left untreated, these pests can destroy most of your garden.
Before you raise your hands and declare the root-knot nematodes the victors, take note that there are ways to manage and control these pests. I’ll cover ways to determine if you’re dealing with root-knot nematodes, then share some prevention and control strategies.
What Are Nematodes?
The Nematoda phylum includes both beneficial and harmful species.
Nematodes are a phylum of small worms that contains about 25,000 individual species. While you may not see nematodes, tons of them are present in the world—some scientists think there are about 60 billion nematodes for each human on Earth! But don’t worry, not all these critters are harmful root-knot nematodes.
Nematodes can survive in diverse conditions and many different food sources. Some nematodes eat fungi, bacteria, or other microscopic creatures. Other nematodes feed on insect eggs or larvae, making them a welcome garden addition if you’re dealing with pests like cutworms and root maggots.
Another group of nematodes, including the root-knot nematode, feed on plant tissue and harm the targeted plants. This group of nematodes causes serious crop damage worldwide.
What Are Root-Knot Nematodes?
These harmful nematodes cause gall-like swellings on roots.
Root-knot nematodes all belong to the Meloidogyne genus. Although the species’ appearance and ideal habitat vary, they all dine on plant roots. As the pests feed, they cause the roots to develop small, rounded swellings known as galls. These “knots” are what give these nematodes their name.
This harmful microscopic pest impacts various plants, including edibles and ornamentals. The infected plants vary depending on the exact species of nematode. Some plants are susceptible to attack from multiple species. Others act as host plants for some species but not others.
Some vegetables susceptible to root-knot nematode damage include:
- Tomatoes
- Peppers
- Eggplant
- Beans
- Peas
- Onions
- Carrots
- Beets
- Broccoli
- Zucchini
- Cucumbers
These pests can also harm ornamental plants, including lilacs, snapdragons, and roses.
Life Cycle
These soil-dwelling pests have six life phases.
All species of root-knot nematodes go through six life phases: egg, four juvenile stages, and adult. Most of these nematodes overwinter in the cool soil as eggs. When the soil warms above 65°F (18°C), the eggs hatch and second-stage juveniles emerge. These tiny nematodes seek out plant roots, burrow in, and begin feeding.
The juveniles continue to feed on the plant roots, where they molt through the remaining juvenile stages and eventually turn into adults. Females then lay hundreds or thousands of eggs in a jelly-like mass outside the roots, and the process resumes.
Although eggs can overwinter in the soil, they can complete their life cycle in as little as a few weeks during the summer. These future generations of nematodes won’t travel very far on their own, but they can infect new areas as humans inadvertently move them with soil or plant material. Heavy rainfall and runoff can also help the nematodes travel across the land into new areas.
Common Species
Four species cause most plant damage in the United States.
While there are close to 100 different species of root-knot nematodes, four species cause the vast majority of plant damage in the United States. These species have different host plants and ideal conditions for growth and reproduction. Knowing which species are present in your garden can help you determine which plants you can grow without harm.
The four most common species are:
- Southern root-knot nematode (Meloidogyne incognita)
- Northern root-knot nematode (Meloidogyne hapla)
- Javanese root-knot nematode (Meloidogyne javanica)
- Peanut root-knot nematode (Meloidogyne arenaria)
How to Identify Root-Knot Nematodes
Since root-knot nematodes are only a few millimeters long, you won’t be able to spot the pests unless you grab a magnifying glass and take a close look. Instead, you should look out for various symptoms that signal the presence of these pesky pests.
Many of the above-ground symptoms closely resemble those present with other root issues like Fusarium wilt or root rot, so looking at the roots is often the best way to check for these parasites.
Stunted Growth
These pests limit root health, causing stunted growth in plants.
When root-knot nematodes chew on plant roots, they limit the health of the entire root system. Impacted plants can’t properly take up water and nutrients, which can lead to stunted growth. If you notice that a few plants or your entire garden are smaller than normal, these pests could be the cause.
However, don’t assume that stunted plants mean your soil is teeming with these harmful worms. Many other factors can lead to stunted growth, including underwatering, improper temperature, and lack of proper nutrients. It’s a good idea to check in on the overall health of your garden before using root-knot nematodes as the scapegoat for all your plants’ problems.
Wilted Tissue
Wilting leaves and stems may indicate infestation.
Another potential sign of root rot nematodes is wilted leaves and stems. Infected roots have a tough time taking up water and nutrients, which can lead plants to lose turgidity and look like they’ve had all the moisture sucked out of them. Even if your plant looks healthy and rigid in the morning, hot afternoon temperatures may cause them to wilt.
As with stunted growth, wilting plants aren’t a sure sign of nematode damage. Underwatering, overwatering, root rot, and other factors can also lead to wilting leaves. So make sure you examine your watering practices and the plants’ root health before you decide you’re dealing with root-knot nematodes.
Round Galls on Roots
They cause tell-tale knots on plant roots.
The most tell-tale sign of root-knot nematodes is the namesake knots they form on plant roots. These tiny swellings are true galls that are part of the plant tissue. Unlike with the nodules formed by nitrogen-fixing bacteria, you cannot rub these galls off of the roots.
The galls vary in size from a few millimeters to up to an inch. However, most of the knots are on the smaller side.
While it’s probably obvious, you need to be able to examine the plant’s roots to inspect them for knots. To check a row of carrots or beans, pull up a single plant and look for signs of damage. If this plant is infected, the other plants are also in danger.
If you don’t want to pull up a larger plant like a tomato or hibiscus, you can pull up smaller surrounding plants (including weeds) and check their roots for damage. However, since not all root-knot nematode species feed on all types of plants, this may not lead to an accurate diagnosis. Another option is to carefully dig around the edge of the plant’s root system until you find a piece of root you can inspect.
Nematode Test
Send soil and plant root samples for nematode testing to identify species.
If you think you’re dealing with nematodes but aren’t positive, you can send a sample of the soil and plant roots to a lab for a nematode test. This test will tell you what nematodes are present and alert you to the presence of other harmful nematodes like dagger nematodes and lesion nematodes.
How to Prevent Root-Knot Nematodes
Preventing root-knot nematode damage involves avoiding the pest’s introduction to your garden and creating an unfavorable environment. Here are some ways you can prevent infection and damage.
Inspect All Transplants
Prevention involves inspecting plants for knots before planting.
The best way to prevent an infestation is to keep this pest out of your garden in the first place. But since these pests are so small, this is easier said than done. Knowing how they enter your garden is the first step to keeping them out.
One of the most common ways these soil-dwelling pests enter a new area is via new plants. So before you plant the ‘Cherokee Purple’ tomato seedlings you purchased at the local garden center or transplant that catnip plant your friend divided for you, give the roots a good once-over. If you seed any small knots on the roots, avoid adding these plants to your garden.
Carefully Clean All Tools Before and After Use
Using tools across different areas spreads nematodes.
While you may not think much of grabbing a shovel to dig a hole in your backyard vegetable garden, and then using the same shovel to plant a rose bush in front of your house, this seemingly harmless task can spread detrimental nematodes. Moving soil by shovels, rakes, hoes, and other tools also moves the nematodes residing in the soil.
I’m not saying you should sanitize your shovel after each use. Rather, be mindful of how much soil your tools are moving. If you know one area of your land is heavily infested with root-knot nematodes, try to avoid carrying soil from this area to another area of your property.
How to Manage
Root-knot nematodes are difficult to remove, but beneficial nematodes can help tackle infestations.
If you determine you’re dealing with root-knot nematodes, there’s not an easy way to remove them from your soil. However, you can take steps to limit their spread as well as the harm they cause to crops.
The easiest way to treat root-knot nematodes is to apply beneficial nematodes to the areas they’ve infested. These are easy to find at your local garden center or hardware store. What matters most is your timing in applying them.
You’ll have to treat soil with beneficial nematodes twice for them to be effective. Apply two treatments two weeks apart in temperate seasons. Hit that sweet spot after the time root-knot eggs have hatched but before the heat of summer sets in. If you do this, you won’t have to worry about root-knot nematodes for some time.
Solarize the Soil
Solarizing soil with clear plastic will kill adults and eggs.
Both mature root-knot nematodes and their eggs cannot withstand temperatures above 130°F (54°C), so heating the soil above this range kills off the pests. Solarization is one way to heat the soil to this level. This practice involves moistening the infected soil and then laying a sheet of clear plastic over the ground for a few weeks in the summer. The plastic will trap heat and cause the soil temperature to rise.
Since solarization will only heat the top 6-12 inches of soil, it’s not a long-term solution for controlling root-knot nematodes. However, it will allow you to grow healthy annual vegetables and flowers.
Take Advantage of Cool Growing Seasons
Cold-tolerant plants grow before the pest becomes active in spring.
Since root-knot nematodes are inactive when the soil temperature is below 65°F (18°C), they won’t harm plants in early spring and late fall. Getting cold-tolerant plants like peas, lettuce, and kale in the ground in the early spring will allow the plants to grow before the harmful nematodes become active.
When the soil rises above 65°F, the nematodes will come out of their winter slumber and start to feed on plants. However, your early spring crops may be ready to harvest by this point.
Choose Resistant Plants
Plant resistant species and varieties.
Since root-knot nematodes can be difficult to control and prevent, it’s always a good idea to choose resistant species or varieties. Although these pests impact a wide variety of plants, some species are resistant to attack. Some types of floss flowers, salvia, and marigolds are resistant to numerous root-knot nematode species. These can be great companion plants or parts of your crop rotations.
Along with planting resistant species, you can also look out for varieties that are resistant to attack. Let’s look at tomatoes, for example. While varieties like ‘Brandywine’ and ‘Sungold’ are susceptible to nematode attack, hybrids like ‘Sakura’ and ‘Mountain Merit’ can withstand these microscopic worms.
Remove Roots and Weeds
Pull roots from the soil to slow the spread.
Since these nematodes feed on plant roots, limiting the number of roots in the soil helps slow their growth and spread. While I generally recommend leaving roots in the soil to increase the amount of organic matter, pull up any roots infected with root-knot nematodes. If you have an active compost pile that heats up to above 130°F (54°C), you can toss the plants in the pile. Otherwise, it’s best to put the infected roots in the trash.
Along with removing infected roots, keep weeds down. Many weed species are suitable hosts for these nematodes, so sometimes more weeds equals more nematodes.
Leave the Ground Fallow
Let infected areas rest or plant nematode-resistant cover crops instead.
Since root-knot nematodes feed on plant roots, they won’t survive if they don’t have any suitable roots to feed on. If you have the time and ability to let an infected area of your garden rest for a year, the nematode numbers will drop.
One option is to let your garden go completely fallow. This means you won’t grow anything in this area for a few months or even a year. Since nematode eggs can survive up to a year in the soil, a longer fallow period will have a bigger impact than a shorter period. If you decide to let a piece of ground go fallow, I recommend covering it with a piece of silage tarp to prevent soil erosion.
Another option is to avoid planting susceptible ornamental and edible crops for a year and plant nematode-resistant cover crops instead. I prefer this method over fallowing since these plants will continue to feed beneficial soil microbes and prevent erosion without supporting the harmful nematodes.
Some cover crop options for dealing with root-knot nematodes include cowpea, sorghum-sudangrass, and rye. Since these plants grow best at different times of the year, you can grow multiple rounds of nematode-resistant cover crops in a single year.
Rotating your crops is one way to prevent root-knot nematodes as well. You can choose to leave an area fallow after planting something that would attract nematodes, or you can plant nematode-resistant plants. By planting something different in an area, you prevent their buildup.
Plant French Marigolds
French marigolds help limit root-knot nematodes in the garden.
If you’ve been around the plant world for any period of time, you’ve probably heard people tout marigolds for their ability to repel everything from mosquitos to deer. While not all of these claims are rooted in truth, French marigolds are proven to help slow the spread of root-knot nematodes.
Planting French marigolds in the garden can help suppress these parasitic nematodes. The marigolds won’t completely eradicate the pests, but they will help limit damage and slow their spread. This prevention, however, occurs in the season after you plant marigolds. That means next year’s tomatoes get the benefit.
Maintain Proper Irrigation and Fertilization
Regular irrigation and fertilization improve plant resistance.
Just like eating a healthy diet and getting enough sleep can help humans fight off infections, proper irrigation and fertilization can help plants ward off root-knot nematodes. We’re not saying that healthy plants will repel these parasites, but they will receive less damage.
Root-knot nematodes cause more damage to drought-stressed plants than they do to non-stressed plants. Irrigating your plants on a regular basis is one way to limit drought stress. You can also mix in compost to improve a soil’s water-holding capacity and mulch the top of the soil to limit evaporation and conserve moisture.
Frequently Asked Questions
Some bacteria and fungi species feed on root-knot nematodes. Many Trichoderma species of fungi can help control nematodes. Beneficial nematodes feed on root knot nematodes as well.
Root-knot nematodes grow and reproduce most rapidly when the soil is moist and warm. Therefore, they are the most active during moist periods during the spring through fall.
While humans may ingest root-knot nematodes when eating root crops like beets, carrots, and potatoes, the nematodes won’t infect humans. Instead, they’ll travel through the digestive tract and come out on the other side.
Final Thoughts
Root-knot nematodes can cause serious damage to many crops, but staying on top of these pests can help limit their adverse effects. Remember that solarizing the soil and planting resistant cover crops can help eliminate these pests, and planting resistant crops can help you grow healthy crops even when the nematodes are present.
