By Matt Gibson & Erin Marissa Russell
The microscopic pests known as root knot nematodes are tiny worm-like creatures that dwell within the soil and eat through the root systems of many garden plants. The feeding causes unsightly galls, or knots to form on the roots, which is where the pest gets its name. Root knot nematodes are one of several species of Meloidogyne and are also commonly called the southern root-knot nematode.
Some of the fruit and vegetable crops that are known to suffer severe damage from root knot nematode infestations are sweet potatoes, corn, pepper plants, okra, onions, cantaloupes, pumpkins, beans, peas, tomatoes, watermelons, eggplants, carrots, beets, lettuce, potato, radish, and squash. Root knot nematodes also affect ornamental plants and several fruit trees. The pests are also known to feed on and lay eggs on a wide variety of garden weeds, though infested weeds are rarely severely affected by their presence.
Female nematodes can lay upwards of 500 eggs at a time, allowing the pests to multiply and turn mild issues into severe infestations in a matter of days. Though the pests are incredibly small, large colonies of nematodes can cause severe root damage due to the large volume of nematodes feeding on the roots throughout the entire summer.
Root knot nematodes tend to be more damaging in sandy soil conditions. Preventative methods are essential to avoiding root knot nematodes in many cases, as the small eel-like worms are quite difficult to treat once infestations have taken place. Controlling root-knot nematodes can be especially challenging, as the pests spread out their reach from one garden to the next by traveling through the soil, and are carried from one garden to the next on tools, shoes, or on infected plants.
Root knot nematodes are not the only nematode species that can cause havoc in home gardens and landscapes, including the ring nematode species, root lesion nematodes, sugarbeet cyst nematodes, citrus nematodes, stem and bulb nematodes, and more. Root knot nematodes, however, are by far the most destructive garden nematode. They spend the majority of their life cycle feeding on various plant roots. Severe infections can lead to reduced yields and unsellable crops due to stunted plant growth.
The severity of root knot nematode infestations depend on the density of the nematode population, the nematode species and the host plant’s species and cultivar. Large nematode populations typically lead to larger knots, or galls. The northern root-knot nematode (M. hapla) produces much smaller galls than the southern root-knot nematode (M. incognita), which produces galls that are double the size of its northern cousin. The more nematodes that are present in the garden, the more galls that occur on each plant. The amount of nematodes in the area also increases the size of the galls on the infected plant’s roots.
How to Identify Root Knot Nematodes
The first symptoms you may notice are plants that are underperforming. Not every plant will react to infections in the same way and not all plants will be injured in the same manner. Some will be more affected than others. Symptoms include yellowing leaves, stunted growth, wilting during the hottest parts of the afternoon and bouncing back in the evening, plants producing less fruit, plants producing small fruit, and general underperformance. These symptoms usually appear during the summertime as the plants grow larger.
To confirm suspicions of root knot infestations, dig up a couple of the plants that are not growing well and examine their roots. If the roots appear knotted, but show no signs of rot, check a few other plants that appear to be suffering. In many cases, knotted roots, or roots covered in galls, will also show signs of rot. If you spot knotty roots and signs of rotting, it’s a safe bet that root knot nematodes are the culprit.
For just $10 you can get a diagnosis from the county extension office in your area, as well as a recommended treatment plan to help you rid your garden of these invasive roundworms. To check your plants for infection with your county Extension service, just put a few sick plants and a quart of soil from the area where you pulled up the struggling plants into a plastic bag. Tie the bag to keep the soil and root system from drying up and keep the bag in a cool, dark location until you can bring it to your county Extension building. Take care not to expose the bag to direct sunlight for any period of time, as the sunlight will cook the nematodes and kill them before you are able to get a diagnosis.
The Extension agent sends the plants and soil sample to a Nematode Diagnostic Lab for an assay which will reveal whether or not nematodes are the issue. The lab technician will tell you if nematodes are causing the problem you are experiencing and give you some experienced advice on how to handle the issue, if nematodes are indeed the cause.
How to Prevent Root Knot Nematodes
As they say, an ounce of prevention is worth a pound of cure, and root knot nematodes are no exception. The prevention measures we outline here will help ease your mind because you’ll know your risk of facing an army of root knot nematodes has been drastically reduced. Familiarize yourself with these best practices, choose the ones that make the most sense for your garden (and choose as many as possible), and your garden will be ready to stave off root knot nematodes as much as is realistically possible.
- Choose seeds and seed potatoes carefully. For many types of plants, certified disease-free seeds are available, and planting these instead of standard seeds means you’ve taken the chance of introducing root knot nematodes to your garden with the new season’s planting down to almost nothing. (We say “almost” because certified disease-free seeds are not guaranteed to be free from the diseases or pests you’re looking to avoid. Instead, these seeds have been field tested and found not to carry disease. In the case of potato crops, examine your seed potatoes for the tiny surface-level bumps that can indicate the presence of root knot nematodes. If you find any suspicious seed potatoes, discard these (but not in your compost heap).
- Whenever possible, go for resistant varieties. For gardeners who are cultivating many different types of plants, resistant varieties are available on the market to drastically diminish the chance of a root knot nematode issue. If you’re growing one of these plants, seriously consider opting for a resistant variety so you can dismiss some of the stress and precautionary measures that go along with trying to avoid a root knot nematode infestation. A list of resistant varieties appears at the end of this article for your convenience.
- Clear your fields at season’s end. This control measure is both a prevention and a treatment, so make sure to use it whether you’re attempting to avoid a root knot nematode problem or stop one that’s already in process. After your crops have been harvested or flowers have faded, work quickly to pull up the plants, getting as much of the root system as possible. (If you have reason to believe that your garden may be housing root knot nematodes, don’t use these plants or soil from their habitat in your compost heap, or you could send the root knot nematodes back into the ring to battle your plants again.) Even if time gets away from you and you aren’t able to carry out this task right after harvest, don’t let plant stalks or debris remain in your garden during the winter. They give free lodging to root knot nematodes, offering them a place to hide out for the winter so they can pop back up in springtime.
How to Treat Root Knot Nematodes
Sometimes root knot nematodes are destined to strike regardless of how vigilantly a gardener has worked to prevent them. If this happens to you, you should be able to catch the problem before too much damage has been done because you’ve already learned how to identify the symptoms of infection. (And you’ve been doing regular checks to catch those signs as soon as they appear, right?) Should root knot nematodes strike your garden, here’s what you can do to put a stop to the invasion.
- Leave the field fallow, then break out the tlller: This technique should be executed in the summer season. First, allow the field where root knot nematodes have been identified to go fallow. Then till the field every 10 days, causing the nematodes to be turned up to the surface of the soil. Because the weather will be hot and dry during the summer, spending time on the surface of the soil will lead to the nematodes’ demise by letting them get dried out in the sun. Using this method can seriously cut down on the number of nematodes in your garden during a single summer.
- Solarize the soil: Like the previous approach, soil solarization calls on the heat and light of the sun to put a stop to a nematode infestation. To solarize the soil in an area where root knot nematodes have set up shop, start by tilling the soil in that area. Next, lay down drop cloths made of clear polyethylene to cover the ground in that area. Leave the drop cloths in place for six to 12 weeks. When you pick them up at the end of this period, the nematodes that were causing you problems will have been dispatched. Solarization works against root knot nematodes as well as some other pests and diseases because under the drop cloths, the soil heats up to a level that the nematodes cannot withstand.
- Crop rotation: Although root knot nematodes have so many different plants they can infest, many gardeners recommend rotating susceptible crops in an alternating schedule with resistant or less susceptible species. This single technique is, in some cases, enough to stamp out a root knot nematode problem. The question that determines whether or not this approach will work in your garden is whether or not you would like to grow any of the resistant plants. You’ll find a list of them below to help you decide. Crop rotation has the added benefit of reducing the risk of other insect infestations and diseases as well.
Studies have shown good results when vulnerable crops are preceded with the African marigold cultivars Crackerjack (Tagetes erecta ‘Crackerjack’) and Flor de Muerto (Tagetes erecta ‘Flor de Muerto’) or the French marigold species Bonita Mixed (Tagetes patula ‘Bonita Mixed’), Gypsy Sunshine (Tagetes patula ‘G’), Lemon Drop (Tagetes patula ‘Lemon Drop’), Scarlet Sophia (Tagetes patula ‘Tangerine’), Single Gold—which is sold under the commercial name: Nema-Gone—Tangerine (Tagetes patula ‘Tangerine’). The dwarf varieties of French marigold Petite Gold (Tageted patula ‘Petite Gold’) or Petite Harmony (Tagetes patula ‘Petite Harmony’) are also proven to be good choices.
Cluster marigolds where the greatest numbers of nematodes have been spotted (or where the most severe nematode damage was sustained), or you may choose to plant swaths of marigolds several feet wide that stretch across the entire garden. Make sure the marigolds are arranged so they create a solid mass, with no more than seven inches of room in between plants. Let them grow for at least two months, then churn them into the soil with your tiller when you mow them down. As long as you’re struggling with nematode issues, it’s a good idea to plant marigolds every other year.
- Boost the microbial activity of your soil by mixing in amendments. The larger the population of microorganisms in your soil is, the less likely you are to face a root knot nematode influx. Both chitin fertilizers and organic material that contains chitin will have the desired effect. Partner your chitin amendment with a fertilizer that offers potassium so your plants (and microorganisms) get a well-rounded diet.
Plants Varieties Resistant to Root Knot Nematodes
- Bell peppers, only certain varieties—Charleston Bell (Capsicum annuum ‘Charleston Bell’), Carolina Wonder (Capsicum annuum ‘Carolina Wonder’)
- Hot peppers, only certain varieties—Carolina Cayenne (Capsicum annuum ‘Carolina’), Charleston Hot (Capsicum annuum ‘Charleston Hot’)
- Lima beans, Nemagreen (Phaseolus lunatus ‘Nemagreen’)
- Marigold (Tagetes)
- Salvia (Salvia officinalis)
- Southern Peas/Cow Peas/Field Peas, only certain varieties—Charleston Nemagreen (Vigna unguiculata ‘Charleston Nemagreen’), Clemson Purple (Vigna unguiculata ‘Clemson Purple’), Colossus (Vigna unguiculata ‘Colossus’), Hercules (Vigna unguiculata ‘Hercules’), Mississippi Purple (Vigna unguiculata ‘Mississippi Purple’), Mississippi Silver
- Sweet Potatoes, Jewel variety only—(Ipomoea batatas ‘Jewel’)
- Tomatoes, only certain varieties—Better Boy (Solanum lycopersicum ‘Better Boy’), Celebrity (Solanum lycopersicum ‘Celebrity’), Classica Solanum lycopersicum ‘Classica’), Goliath (Solanum lycopersicum ‘Goliath’), Park’s Whooper (Solanum lycopersicum ‘Park’s Whooper’), Small Fry (Lycopersicon esculentum ‘Small Fry’: cherry tomato), Sweet Million (Lycopersicon esculentum ‘Sweet Million’: cherry tomato) Viva Italia Solanum lycopersicum ‘Viva Italia’)
- Zinnia (Zinnia elegans)
There are chemical control measures available to fend off root knot nematodes, but those reduce populations of beneficial pollinators as well, and many gardeners choose not to use them for a spectrum of reasons. If you’re watching your plants carefully for symptoms of root knot nematodes so you can jump into action once you spot the signs, your treatment should begin before the nematodes can do too much damage. Quicker diagnosis leads to less severe infestation, and in cases like those, the biological and cultural methods of prevention and treatment we’ve outlined in this article should suffice to put a stop to the root knot nematode invasion.