Peas And Root Knot Nematodes – A Guide To Pea Nematode Resistance

Peas And Root Knot Nematodes – A Guide To Pea Nematode Resistance

By: Mary H. Dyer, Credentialed Garden Writer

There are many types of nematodes, but root knot nematodes tend to be most troublesome, primarily because they attack such a wide range of crops. The worms are microscopic, but they cause big problems when they infest roots and prevent plants from absorbing nutrients and water.

To narrow it down even further, there are several species of root knot nematodes. The type in your garden may vary from your neighbor’s garden, depending on the vegetables you grow. This article discusses the pea root knot nematode.

Peas and Root Knot Nematodes

Are peas affected by root knot nematodes? Unfortunately, root knot nematodes of peas are common, especially in sandy soil. What can you do about peas with root knot nematodes? It’s not possible to eradicate the pests once they take up residence in your soil, but you can keep them under control.

Identifying root knot nematodes of peas is tricky because the symptoms – lumpy, swollen, knotty roots, are similar to nitrogen nodules, which occur naturally on the roots of peas and other legumes. The main difference is that nitrogen nodules are easy to pull off with your fingers; nematodes stick like glue and can’t be removed.

Other symptoms include poor growth and wilted or discolored leaves. If you still aren’t sure your local Cooperative Extension office can perform a soil test, usually at a nominal cost.

Controlling Root Knot Nematode of Peas

One of the easiest and most effective ways to control root knot nematode of peas is to grow nematode-resistant peas. Experts at a local greenhouse or nursery can tell you more about pea nematode resistance in your area.

Work generous amounts of compost, manure or other organic material into the soil and mulch pea plants well.

Practice crop rotation. Planting the same crop in the same soil year after year can produce an unhealthy buildup of nematodes. Plant peas as early as possible to get ahead of the problem.

Till the soil frequently in spring and summer to expose the pests to sunlight and air. Solarize soil in summer; till the garden and water it well, then cover the soil with clear plastic for several weeks.

Plant marigolds, which produce chemicals that are toxic to nematodes. One study indicated that planting an entire area thickly with marigolds, then plowing them under, provides good nematode control for two or three years. Interspersing marigolds among pea plants doesn’t appear to be as effective, but it may be worth a try.

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How to Fight Root Knot Nematodes (Meloidogyne incognita)

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.

Nematodes are microscopic worm like creatures that live in soil. In fact, they are the most numerous multi-celled creature on earth, found on every continent. The mere mention of them strikes fear into the heart of most gardeners in Perth – however the reality is that many nematodes are actually beneficial and in fact an essential part of healthy soil biology.

Many nematodes feed on bacteria, fungi, algae, small invertebrates and other nematodes. However there are a few varieties which feed on plant roots. They lay their eggs in the roots, and as they hatch and numbers grow, they deprive the plant of moisture and nutrients and the plant shows signs of yellowing, stunted growth, and may die.

Root knot nematodes thrive in sandy soils, and prefer warm conditions. Areas with long, dry summers and short winters suit them well. So does this sound familiar??

If you are concerned you MAY have root knot nematodes, the only way to be sure is unfortunately to rip out a plant and have a look! There will be warty lumps along the root surface.

Be aware that nitrogen fixing species (peas, beans, legumes) have nitrogen fixing nodules on their roots, which can sometimes be wrongly suspected of being nematodes.

The good thing is there ARE things that can be done.

If you have root knot nematodes in your garden, it is usually a symptom of the soil’s condition, and shows an imbalance in soil biology. Nematodes usually live in the top 30cms of soil, and eggs can survive for a few months in fallow ground. However if you plant in a crop which the nematode’s don’t feed on, and leave it for some months, the nematodes will starve.

The first thing to do is to incorporate more organic material into the soil. Lots of it. Animal manures are beneficial, especially chicken manure, because the nitrogen content stimulates conditions that are not ideal for the nematodes. There is also some evidence that raising pH (which fresh manure will do) will also help with nematode control.

Adding organic material also helps alter the balance of soil biology, and will encourage other nematode species to build up numbers that will then prey on the root knot nematode. Remember the organic gardener’s mantra feed the soil! Unfortunately this is never a ‘once off’, but your garden will benefit hugely from regular (at least yearly) generous top ups.

Photo to the right is root knot nematode damage.

Certain types of plants are particularly susceptible to root knot nematodes. Tomatoes are one. Many types of vegetables are susceptible (carrots, peas, beans, capsicum), as are grape vines, roses, and some stonefruit. The practise of crop rotation is beneficial. Brassicas (cauliflower, broccoli, kale, cabbage, mustard, kohl rabi, turnips, brussel sprouts, bok choi, radish, rocket, mizuna, collards) actually emit a substance into the soil that kills root knot nematodes, so if you have an infestation, plant out members of this family thickly.

Green manure seeds contain members of the brassica family and have the added benefit of adding a large amount of organic matter once it has been slashed and returned to the soil. We sell green manure seed packs @ GLSC.

African and French marigolds both exude a substance into the soil which will kill root knot nematodes. However they must be planted thickly (one or two flowers dotted around your vegie garden simply won’t be enough) and left in the ground for at least three months to be effective.

Using approx. 2 tablespoons of molasses per litre of water (dissolve when the water is warm), apply this to the nematode infected soil with a watering can. (Note: I have read varying quantities recommended – even up to 1:4 molasses to water ratio!) Molasses is available in many forms. Look for the least refined and most organic possible. We do sell it in small tubs, or you could try health food stores or stock feed suppliers, but you may need to buy larger amounts.

Molasses works by increasing the food source for bacteria in the soil. It changes the balance of bacteria/fungi/nematodes in the soil biology, and this may not necessarily be a good thing. Earthworms can be adversely affected also. Use this treatment sparingly and as a last resort.

(Note: Molasses is advocated by some as a regular soil additive. For plants that prefer a bacterial dominant soil biology (eg. Turf/grasses) this may be the case. It would be beneficial when planting a new lawn, or when trying to revitalise a struggling one for instance. However we wouldn’t recommend it for shrubs and trees as woodier plants benefit from a more fungal dominant soil biology.)

Photo to the right is nitrogen fixing nodules. (Not nematode damage.)

Leaving bare soil covered with plastic weighted down at the edges will work to ‘cook’ the soil and kill nematodes. Of course, it will also affect beneficial soil biology the same way. For bad infestations, turn through the soil every few weeks to ensure the maximum exposure to sun & heat throughout the top 30cm layer. Areas left like this for a few months, particularly over summer, will have less problems in future seasons. Just remember to re-inoculate the soil with lots of healthy biology – use lots of compost to dig through before re-planting.

What to do with infected plant material

Firstly, ensure all roots are dug up and not left in the soil. Very hot composting methods will destroy root knot nematode eggs, but if you are uncertain that your composting methods will do the job, then cut off the roots and dispose of by burning. The tops of infected plants are fine for composting.

Nematodes don’t move a huge distance on their own – it is thought only a metre or so. Practising good hygiene (being careful not to distribute infected soil or root material on garden tools or by careless digging) and selective planting, crop rotation and soil improvement methods, you should be able to control their numbers so they do not have such a devastating effect on your crops.


Most effective nematicides for the control of root-knot nematodes are banned, which demands a better understanding of the plant-nematode interaction. Understanding how gene expression in the nematode-feeding sites relates to morphological features may assist a better characterization of the interaction. However, nematode-induced galls resulting from cell-proliferation and hypertrophy hinders such observation, which would require tissue sectioning or clearing. We demonstrate that a method based on the green auto-fluorescence produced by glutaraldehyde and the tissue-clearing properties of benzyl-alcohol/benzyl-benzoate preserves the structure of the nematode-feeding sites and the plant-nematode interface with unprecedented resolution quality. This allowed us to obtain detailed measurements of the giant cells' area in an Arabidopsis line overexpressing CHITINASE-LIKE-1 (CTL1) from optical sections by confocal microscopy, assigning a role for CTL1 and adding essential data to the scarce information of the role of gene repression in giant cells. Furthermore, subcellular structures and features of the nematodes body and tissues from thick organs formed after different biotic interactions, i.e., galls, syncytia, and nodules, were clearly distinguished without embedding or sectioning in different plant species (Arabidopsis, cucumber or Medicago). The combination of this method with molecular studies will be valuable for a better understanding of the plant-biotic interactions.

Keywords: BABB CTL1 Meloidogyne spp. clearing confocal microscopy giant cells nodules syncytia.

Conflict of interest statement

The authors declare no conflict of interest.


The fixation and clearing method…

The fixation and clearing method allows the complete sectioning of a gall induced…

The fixation and clearing method…

The fixation and clearing method allows the neat visualization of the plant-nematode interface.…

Galls and giant cells, are…

Galls and giant cells, are clearly observed at any stage of development. (…

Altered expression of CTL1 yields…

Altered expression of CTL1 yields a decrease in the GC’s size. ( a…

Other biotic interaction can be…

Other biotic interaction can be visualized after fixation and clearing. ( a )…