Authors: Tom Cadwell, and Thomas Osborne, MD
The majority of the data in this important article was compiled by my friend, the amazing Tom Cadwell. It represents a very comprehensive review of the challenges and options for dealing with this difficult infection as a home gardener.
A lot of people say that they have a brown thumb or black thumb for gardening. They tell me that no matter what they do, their plants look sickly and often die. In these cases, I often wonder if the problem might be hiding just beneath the surface of the soil in the form of the dreaded and secretive root knot nematode.
Root Knot Nematode Symptoms:
Plants have different amounts of susceptibility to the various species of root knot nematode (RKN). Some plants will show relatively little signs of infection if they are genetically adapted to deal with the buggers. However, when plants are not built to deal with this type of critter, they will look very sickly and can die when infected. Symptoms can mimic dehydration and/or nutrient deficiencies. There are no visible bugs above the soil surface and symptoms often persist or get worse despite taking action to improve watering and fertilization.
Root Knot Nematode Diagnosis:
If you are doing everything right with your water, soil and nutrients… and you still have sick looking plants, then it is time to get suspicious. A telltale sign of infection is a beaded appearance which is often near the tips of small ‘feeder’ roots, and gnarled, inconsistent, ‘cancerous’ looking growths on larger roots in severe infections. These focal root growths are called galls or knots. Some nematode species don’t show these knots in the roots, however, just about all of the significantly infected plants will have roots that look like they were thinned out and/or less robust. Definitive diagnosis of the specific nematode infection can be achieved by talking samples into your local state or county agriculture department.
What is a Root Knot Nematode?
A nematode is a microscopic roundworm that infects the roots of plants. This bug siphons carbohydrates away from the plant, damages the roots, and impairs the plants ability to get water and nutrients from the soil. Additionally, the nematode entry into the root creates a door for other soil borne pathogens to enter and infect the plant.
Root Knot Nematode Life Cycle:
Root knot nematodes go through 6 stages as they grow from an egg to an adult worm. In warmer months, nematodes go through these stages faster; in optimal conditions, many species can go an egg to an egg-laying adult in less than 3 weeks. Nematodes are less active when it is cold and when the temps are under ~10C they are inactive. Deep freezing temperatures can kill them.
Stages of a nematode life cycle (via UC California IPM):
- An egg stage
- 4 immature stages (the second-stage-juveniles are the ones that invade roots. This stage can survive in the soil for little over a year while they wait to infect roots)
- An adult stage. (The eggs are a very protected stage in the life cycle and eggs may stay viable for several years. However, consistently irrigating and planting helps to stimulate the eggs to pop-open so that the worms can be exposed and killed).
Once in the roots, the females stay inside protection of the root gall/knot while the male eventually leaves. The females lay the eggs as a jelly like mass that extends through the root into the adjacent soil.
Resistant & Susceptible Plants:
There are many, many types of root knot nematodes, and virtually any plant has some nematode that they are vulnerable to. However, the species most likely to cause problems in Southern California are Meloidogyne Incognita, Javanica, and Arenaria, which just so happen to have a very similar host range, destructive ability, and control measures. There are some plants that have a natural resistance to nematodes and other plants are quite susceptible.
Typically resistant plants:
Woody plants (trees and longer life cycle perennials) usually can at least tolerate root knot nematodes. Many tropical and sub-tropical plants (plants that die when it freezes) have an immunity or strong resistance to root knot nematodes. This is not surprising, because nematodes are typically killed off by temperatures that freeze the soil. Therefore, plants that live with this constant underground warm weather threat to nematodes have longstanding environmental pressures to evolve and deal with them.
Strawberries, garlic, garlic chives, some varieties of marigold, and several of the grain crops (wheat and barley, sudangrass) are immune or extremely resistant to RKNs. Citrus tend to be resistant to typical root knot nematodes, but there is a specific citrus attacking nematode variety that can do a lot of harm to citrus. Many plants have resistant varietals – tomato varieties with the code VFN (Verticillium, Fusarium, Nematodes) on the seed packet or label are resistant to common root knot nematode species but can still become infected at high soil infection levels (especially at temperatures over 90F, where the Tomato’s defenses work less well).
Pepper and Cowpea varietals resistant to nematodes are also available. Many plants are ‘tolerant’, which means what you would think, but also that nematodes can still reproduce on the plant (just at a slower rate). In our experience, tolerant is a bit of a coin flip – they are more tolerant than a non-tolerant varietal, but this might just be a difference between getting a decent yield out once vs not even being able to go to seed, and may or may not be worth growing in nematode infested soil… and, growing tolerant plants still increases the nematode density in the soil over time.
Most winter crops can be successfully grown because of nematodes reduced activity at cold temperatures. However, if allowed to grow too long into hot weather, these can end up harboring large nematode populations.
Additional info via Clemson University Cooperative Extension.
Typical susceptible plants:
In general, most deciduous trees are susceptible to infection. Among others, root knot nematodes will kill fig trees, and can significantly hurt Stone fruit, Mulberry and Pomegranate trees.
Non-resistant nightshades (Tomato, eggplant, pepper), legumes (beans, peas especially) and members of the squash family (Cucumber, zucchini, pumpkin melon, etc) seem to be especially prone to infection. Long season variants of the squash family like winter squash/pumpkin are especially troubled, which in part is related to their extended time in the ground.
If you know the species of the nematode doing the damage in your garden, then the UC Davis nematode search engine & database is useful to help you strategize your growing. If you aren’t sure, and are growing in a climate similar to Southern California then infection with the nematode, Meloidogyne incognita is a reasonable guess.
If you are diligent, you can graft your veggies onto resistant root stock. There are several resistant types of tomatoes that you can grow by themselves or grow as a root stock that you graft other tomatoes onto. However, the tomatoes with the nematode resistant Mi gene (astoma, Beaufort, Vigomax) seem to loose their resistance when it gets really hot.
This has the added benefit of possibly giving you other beneficial characteristics such as strong vegetative growth or resistance to other pathogens so that your finicky heirloom tomatoes produce at a volume similar to commercial varieties.
Prevention is critical because treatment and eradication of these worms is very difficult (we will get to that later in the article).
Quite often nematodes are brought into a garden or landscape from transplanted plants. Sometimes infected plants can come in from well-intended friends or from clueless nurseries. Carefully inspecting the roots of a plant before transplanting may help you to catch some infected plants. However, it is also easy to miss the visible signs if infection (galls/knots), just by looking at the exposed roots. Knowing that your plants come from a professional and or knowledgeable grower can help… but there are no guarantees here.
Planting grocery store bought items such as potatoes is a really bad idea and using a friend’s garden tools can also spread infection.
Diligent sanitation techniques is an often overlooked but super important component. In this way you have to think a bit like a medical doctor. Spreading infection from one place to the next is really easy because these harmful organisms are microscopic and can be present in large numbers in a just small bit of dirt. Therefore, you need to regularly sterilize your garden tools (and everything else that touches soil).
On that note, these bad soil dwellers can also be spread around on shoes or tires. If you have ever been to an island country such as New Zealand, you might have noticed that they are very interested in knowing about your recent hiking experience and footwear before they let you in the country. They are basically trying to defend their country against these types of microscopic foreign invaders that can wreak havoc on their isolated environment.
Home Soil Testing:
If concerned about the soil quality of outside plants, you can of do a home nematode test while quarantining the plants. However, this does take about a month. The method Tom Cadwell was taught by his arborist friend is to take several soil samples from the plant’s soil, mix it with “clean” soil in a 1 gallon pot, and then grow out melon seeds in a container filled with this inoculated soil. Then, after waiting a month, inspect the roots – if there are galls, nematodes are definitely present. If not galls/knots, then the soil is probably clean (at least clean of the type of nematodes that create knots). This method is suitable for trees that can wait a month before transplant. However, it is too slow for vegetables bought at the store.
Garden Tool Sterilization:
There are many ways to sterilize your garden tools; each with their own considerations.
Heat is a very reliable method. This can be done by using boiling water or an open flame. Of course, using an open flame is not recommended in areas that are at risk for fires. I got a culinary torch that I use to sterilize smaller tools and my wife uses in the kitchen.
A weak bleach solution will eradicate any nematodes it touches, (egg and juveniles). But, mixing buckets of the stuff is inconvenient and also corrodes metal tools quickly since it is an oxidizer. I personally like to use a cleaning bleach spray on larger tools just because it is so convenient to use.
There are reports that peroxide also works as a nematode disinfectant.
Pure alcohol (rubbing alcohol or ethanol) will dehydrate and kill juveniles reliably. However, alcohol won’t kill all the eggs and therefore it is not a good disinfectant of nematodes for this reason.
Good Growing Practices:
In addition to prevention methods, there are some good growing practices that will help.
Traditional crop rotation will protect somewhat against nematodes and is something you should always do with annual crops. However, due to nematodes extremely destructive traits and broad host range, it’s better to additionally utilize specific control mechanisms such as planting full crops of marigolds after a vegetable harvest (see discussion further down in the article).
Fallowing is basically the practice of leaving the soil bare for a period of time. This is usually done for a full year but is more effective when done for a longer period of time. When fallowing, it is important to keep the soil moist to induce the eggs to open. Once the worms are hatched, they will eventually die if there is nothing to feed on. Therefore, it is also important to control weeds at this time which nematodes may live on. Unfortunately, a small percent of juveniles will be able to survive almost a year after hatching in ideal conditions, so you may want to supplement this with additional soil treatment (see below).
Remove plants after harvest:
The longer plants are in the soil, the longer they allow the nematodes to breed and lay eggs. Therefore, pull out and remove your veggies as soon as they have finished their production.
Inoculating plants with symbiotic root fungus or bacteria will help to prevent infection. Some of these beneficial organisms may also kill nematodes. Products such as Root Shield and Actinovate have a lot of the good bugs that can help to protect your plants.
Sometimes trees can be infected without any signs of infection. Therefore, tree roots can be a hidden reservoir of disease. It is really amazing how far tree roots can travel so this can be a real challenge in the ongoing battle against nematodes. For this reason, some diligent vegetable gardeners advocate removing all nearby trees before starting a vegetable patch.
At the very least it is a good idea to cut/remove the roots that encroach into the garden zone, which will also reduce water and nutrient competition that vegetables will have with the tree roots. However, the roots will likely grow back over time.
Unfortunately, home composting can be a way to spread nematodes. Uncooked root scraps (carrots, potatoes, beets, etc) can harbor highly resistant nematodes from commercial farms. Composting is great though – just stick to cooked stuff, fruit and leaf and stem scraps.
Water runoff from infected areas can spread the disease.
Avoid planting what they love:
Avoid growing plants that nematodes love is one technique that is unsatisfactory in my opinion. At the very least, removing weeds that may harbor the nematodes is also really important – their short reproductive cycle in midsummer can get ahead of you very quickly.
Root Knot Nematode Soil Treatment:
There are a lot of treatment options for nematodes. However, these are fastidious creatures and difficult to completely eliminate. Therefore, dealing with them usually means ongoing treatment and not a cure.
A lot of gardeners know that marigolds are great for gardens but fewer people know why. As it turns out, marigolds produce chemicals that are toxic to a lot of bad bugs and these marigold chemicals can also protect other plants.
One of the easiest and effective organic control methods for root knot nematodes is to grow marigolds as a solitary crop between other crop plantings for at least 2 months. However, doing this alone will only control the numbers of nematodes and is unlikely to reduce the numbers to a point that is low enough to grow longer season crops such as squash.
Importantly, not all species of marigolds will work and not all species of nematodes (like the northern root knot nematode) are adversely impacted by marigolds. Many resources say that French Marigolds are the most effective against nematodes (some of the many french varieties include Nemagold, Petite Blanc, Queen Sophia, and Tangerine). Others varieties reported to protect against nematodes include Tagetes Patula and Marigold Mexican.
Some species of marigolds are completely ineffective and nematodes will actually feed on them (such as signet marigolds, T. signata or tenuifolia). Additional reading on the topic available via the article about marigolds in the Journal of Nematology.
There are important limitations to the effectiveness of marigolds to note. Marigolds don’t have as powerful of a nematode killing (nemacidal) effect in the winter for two reasons – 1). nematodes are not active in cold parts of the year and when the temps are under 10c they remain inactive. Therefore, if they are in this hibernation like state then they wont be roaming around and getting caught up in the death trap of the marigolds. 2) Marigolds tend to go to seed when days are shorter than 12 hours. Therefore, in the colder months, their roots wont grow as deeply, thus reducing the maximum nemacidal effect.
As a result, the best time to use Marigolds is as a spring pre-plant if warm enough, in between a double crop some time in summer, or right after you cull in late summer/early fall but before you’ve reached the equinox by at least a month.
The active ingredient in shrimp meal is chitin. If you put a lot of chitin in your soil (in the form of shrimp meal) it will foster the growth of chitin eating bacteria. Since the nematode eggs have a chitin outer coat, this method basically breads a hostile biological killing environment for the nematode eggs. The research shows low effect until there is a LOT of shrimp meal, so to be effective, you likely have to use it as your primary nitrogen fertilizer – but it is very effective when it starts working!
Unfortunately, using shrimp meal over time can eventually result in a Calcium buildup in the soil. Early on this can actually be beneficial t your plants. However, like anything, too much of a good thing can eventually be bad. Too much Calcium can make it difficult for plants obtain other soluble minerals from the soil. Therefore, it is probably a good idea to do a soil test after using shrimp meal for a few years just in case, and to use with caution if you have already added a lot of lime or gypsum to your soil.
Shrimp meal also has a lot of nitrogen in it that is released into the soil after about 1-3 months ((Shrimp is 6-6-0 fertilizer). Therefore, using 35 lbs per 1000 feet may cover your yearly nitrogen needs for most crops. Regardless, modest compost supplementation preseason and midseason is almost always be beneficial.
As always, follow the instructions on the label. However, using 35 lbs per 1000 square feet is probably a good number to follow. Important note: before the shrimp meal breaks down (1-3 months) it may actually suppress seed germination. Therefore, apply the shrimp meal at least 1 month before your planned planting time.
Additional reading available from the journal of Nematropica and the International journal of environmental research and public health.
Actinovate is a commercially available mix of natural beneficial microscopic Microorganism. After applied, these “good bugs” protect the plant and attack all kinds of harmful disease causing organisms. It seems that using Actinovate with shrimp meal provides added benefit. Heres a reference from the University of Hawaii that covers some of the organic options.
Trap and remove nematodes:
Growing susceptible plants with a quick crop cycle (such as chinese broccoli, or radish) can attract the nematodes to migrate to that host plant. If you are able to pull that plant out of the ground before they complete their lifecycle then you have trapped them and you can removed them from your garden before they can release their eggs.
You can use this strategy with any plant type so long as you harvest it before the nematodes reproduce. However, radish and Chinese broccoli have worked well.
If the typical outside temperature is cool (around 15C (59F) or lower):
Seed Radish directly into the soil and ensure you harvest within 45 days.
Prestart Chinese broccoli in small pots to get them going. When they are about 1-2 weeks old, plant them in the garden and pull them up 28 days later.
If the temperatures are warmer:
Since a nematodes lifecycle is accelerated in warmer conditions, there is a rough formula you can follow to determine the optimal timing for this technique when it is hotter outside.
Specifically, you will have to add up the number of days with degrees over 10C, multiplied by the number of days the plant is in ground. If that number is under 300, they have likely not reached the egg laying stage.
For example, if the daily temperature was fairly constant at 25C (77F) for 18 days then your calculation would be [15 x 18 = 270]. Since 270 is less-than 300, then the nematodes probably haven’t reached the egg laying stage yet.
However, if the temperatures were typically 30C (86F) for the same 18 day time period then the calculated number is [25 x 18 = 450]. Since 450 is larger-than 300 then at this point, you missed the boat and unfortunately you now have a lot more nematode eggs in the soil.
Solarization is the process of using natural sunlight to disinfect the soil with heat. To do this, first soak the soil with water. Then place a clear or semi opaque plastic Visqueen sheet over the soil and hold it down sight with rocks or soil. This will produce a greenhouse effect that will cook everything inside.
The drawbacks of this method is that it tends to only treat the top part of the soil and the plastic can degrade from UV sunlight over time. As you can imagine, best results are in the summer months when it is hottest. Additional reading on the topic available via The Journal of Nematology.
There are beneficial nematodes that you can buy that kill juvenile root knot nematodes and other soil dwelling bad guys.
Nematodes tend to survive better in sandy soil. Therefore, augmenting the soil with organic material may help a tiny bit.
Some have advocated using neem oil as a soil treatment. I like to use neem oil as an insecticide leaf spray, but I am personally not a big fan of this technique because it can indiscriminately kill the good bugs and it can miss the bad bugs that are hiding in chunks of soil that do not get exposed to the neem oil. It also suppresses nitrifying bacteria, which is undesirable for legumes. This may be a good rotation alternative if you are at an excessive calcium level from shrimp meal. Additional information on the tipic available via the journal Nematologia Mediterranea and Journal of Agriculture and Rural Development in the Tropics and Subtropics.
Root Knot Nematode Eradication:
Complete elimination of root knot nematodes from the home garden is very difficult. If you are a commercial farmer, effective but toxic chemicals are available to treat the soil. However, you can’t get these high powered nematode pesticides as a home gardener (and you shouldn’t try to – they range from ‘very toxic’ to ‘carcinogenic’ to ‘chemical relative of nerve gas’).
None the less, with years of planting resistant crops, mixed with mini-seasons of Marigolds between, shrimp meal and promoting good bugs… you may potentially eradicate the nematodes (if you’re lucky). At the very least, you can achieve management of them and successfully grow some vegetables.
Best of luck and happy gardening!
Mr. Tom Cadwell is a self-proclaimed science geek with gardening as his primary hobby, other than video games – which are his actual career. He leads the game design organization at Riot Games, and was the original design director of League of Legends. Tom Cadwell on LinkedIn: https://www.linkedin.com/in/tom-cadwell-404b49
Dr. Thomas F. Osborne is the creator of this website. He is a medical doctor with a a known gardening problem. He is a Harvard trained Radiologist and Neuroradiologist. Dr. Osborne is also the Director of Medical Informatics at vRad and Director of Medical Research at MEDNAX. Dr. Thomas F. Osborne on Linkedin https://www.linkedin.com/in/tomosbornemd
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