I planted cucumbers on June 19 and expected them to take off in the warm weather and soil, but they kept wilting in the noonday sun. So I watered more. They continued to wilt every afternoon. When they weren’t wilting, they were barely growing. Finally, I yanked one plant out of the ground to check its roots and it all made sense.
Swollen roots with knots here and there, a perfect example of damage from root knot nematodes.
What are root knot nematodes?
Root knot nematodes are microscopic worms that live in the soil and feed on plant roots. They insert a mouthpart called a stylet into a root cell and suck out nutrients. This causes swelling (“knots”) and impairs the root’s ability to take up water such that even if there is plenty of moisture in the soil, a plant will wilt.
The knots on the roots also impair the ability of the roots to transport nutrients up to the plant’s leaves. In a previous garden about ten years ago, I had a bed of peppers that were stunted and yielded poorly. They didn’t wilt; they just didn’t grow well. Initially, I guessed it was lack of soil fertility, but toward the end of the summer I pulled the plants out to discover the nematode knots all over the roots. So even in fertile soil, an infected plant won’t be able to use the soil nutrients like it otherwise would.
Root knot nematodes prefer warm, moist, and sandy soil, according to this page on nematode management guidelines from the University of California. That aligns with my experience. All of the nematode infections that I’ve observed have happened during summer and have been in sandy or loamy soil.
How to deal with root knot nematodes?
(See my more recent post, “Dealing with root knot nematodes in a vegetable garden.”)
What can you do about these pathogenic critters? First, look at this curious sight:
This is the vegetable bed where the infected cucumbers were. There were three plants that I removed near that blue bucket. Remaining in the bed are basil, carrots, green onion, kale, watermelon, and cosmos flowers in the back. Why aren’t they wilting? Are they not also being attacked by the root knot nematodes?
Non hosts
I dug around to check. The roots of the basil plants looked perfect, showing no swelling or knots. Some plants are not hosts for root knot nematodes. In other words, for whatever reason, the nematodes don’t feed on them. This page has a list of nematode host plants and basil is not listed.
Corn is the only other common summer vegetable that is not listed as a host, but corn is listed as a host on this page from North Carolina State University. I’m going to grow corn in a bed that I know has root knot nematodes to find out.
Resistant varieties
Right next to the cucumbers was an Arava melon plant that had also shown some wilting. I pulled it up to find lots of knots on its roots.
But next to this Arava melon was a Crimson Sweet watermelon plant which was growing fine. I dug around its roots and couldn’t see any knots.
It has been observed that some varieties of host plants are more resistant to nematodes than others. On a plant’s tag or seed packet you’ll find the letter “N” designating resistance to root knot nematodes.
Certain tomato varieties, for instance, might be known to be resistant to nematodes and more. The label might read “VFN.” That means resistance to verticilium wilt, fusarium wilt, and nematodes.
(Update summer 2021: Hoping to find a cucumber variety that would prove resistant to root knot nematodes, but finding none with such resistance labelled, I planted Alibi, Summer Dance, and Sweet-Slice. All were claimed to “exhibit good disease resistance” generally. Alas, all turned out to get highly infected with root knot nematodes. No luck.)
Plant early
The carrots in that bed haven’t grown as well as some of my other carrot sowings, so I wasn’t surprised to find some nematode knots when I pulled them up.
The carrots in this bed are one of my later sowings, done in April. It’s possible that had I sowed the carrots in January results would have been better. The soil is cooler in January than in April — and especially compared to here in July. And as that page from the University of California notes, root knot nematodes thrive in warm soil. In fact, it says that at soil temperatures below 64 degrees the nematodes cannot penetrate roots at all.
Fallowing
What to do now with this bed that is populated with root knot nematodes? Can I get rid of them? Doesn’t seem possible. But there are things that I can do to keep the population down and the future damage low. Planting the bed to non-host plants and resistant varieties will reduce the population because it gives the nematodes little to feed on.
An extreme version of this approach would be fallowing the bed — that is, planting nothing. Without any roots to feed on their numbers will diminish.
However: “Nematodes can survive for a year or more in soil as eggs or cysts,” says the California Master Gardener Handbook. So, even through fallowing, you’re not expecting to eradicate the nematodes, but only to reduce the population.
Turning the soil before or as you fallow is said to help. “Fallowing is more effective if soil is plowed and exposed to sun,” says the UC pest management guideline page for root knot nematodes.
Soil solarization
There is one final practice that can reduce the population of root knot nematodes in that bed, which is soil solarization. This entails laying a plastic tarp over the soil to heat it up so much that it kills the nematodes. The plastic tarp should be left in place for 4 to 6 weeks during the hottest part of summer. Even then, it is effective only in the upper level of soil.
Summary
Keep root knot nematodes in mind this summer if you see that a vegetable plant is growing slowly and wilting often even though you think you’ve given it enough water, or if a plant is not growing and yielding as well as you think it should. Dig around to check its roots. If you see knots, then you’ve found the culprit.
What to do then? Plant non hosts or resistant varieties; plant in that area only at cooler times of year or plant summer vegetables as early as possible; fallow the area; and possibly solarize the soil.
By the way, no need to feel sorry for me and my diseased cucumber plants. Luckily, some Green Fingers in another part of the garden that I planted earlier are still producing well. The kids definitely aren’t starving.
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Hi Greg,
Do you have any advice how to keep the birds and rats from eating the fruits and blueberries?
Seems they get to the fruits always before me and eat at least part of them.
Hi Pirkko,
Have a look at this post: https://gregalder.com/yardposts/protecting-fruit-from-birds/
As for rats, I think you’ve got to kill them. I know of no other way. The classic, simple traps work well in my experience.
Hi Pirkko, I’m not sure I can help with the other fruits you have but I do have an idea for the blueberries that works great for me. I bought a dog cage from Home Depot. It’s black rod iron and stakes into the soil with the adjoining piece with studs slips into the slots to connect them. They come either in 2 piece sections or 3 piece sections. it stands about 3 feet tall. With that, I draped netting around the whole cage and held it down with palm size river rocks because they will get under the netting to get the prize. I also hold the netting in place around the cage with the old wooden clothes pins. The netting I use has small, maybe 1/2″- 1″ openings so the bees can still get in to pollinate. I do this just before they begin to ripen and take the netting off after harvest leaving the cage in place. This year I found a gofer snake had crawled somehow under the netting and was coiled up against the cage. It got tangled around its neck and I was lucky enough to save it and set it free. LOL I hope this helped you somewhat.
Nice article. Did your research find anything about this product? https://www.arbico-organics.com/product/monterey-nemtaode-control/root-knot-nematodes-control?gclid=Cj0KCQjwjer4BRCZARIsABK4QeWGFzZfwqnW-M52fZ7J2r6RvHt3fgF0liS2zaawDMhHIr5i-TCdybYaAoXHEALw_wcB
Thanks so much. I am going to try it really soon!
Hi Jane,
Hadn’t heard of or looked at that product. The University of California IPM page on nematode management says, “Currently no chemical nematicides or soil fumigants are available to home gardeners for nematode control in backyard gardens and home landscapes.” But maybe it’s not up to date? Or maybe there’s another reason for not listing it? If I find out anything, I’ll post here.
Does Serenade work for this? Have you tried it? I used it last year and it worked but I didn’t look to see what the problem was. Things wilted when they shouldn’t, so I used it. It is another expense though. I can’t help saying that this year has been lousy for me. I underfertilized some things and over watered some…with lots of experience, I have no excuse. I have limited space and I’m addicted to growing a lot of nightshades, which means rotation isn’t always an option. That is how I got started using Serenade. THANK YOU FOR YOUR NEWSLETTER, LOVE IT!
Hi Laurie,
Serenade isn’t listed as being effective against root knot nematodes. I’ve never used Serenade though so I don’t know anything about it other than what I read.
So timely. I swear my cucumbers are having the same issue. I’m pulling one up tomorrow to see.
Rabbits ate the leaves off my cucumbers this year before they ever had a chance to give 🙁 Here’s to next year Mr Green Finger, I’m building a cage around you and hope you survive (I already have a raised bed with a wire mesh on the bottom to keep the gophers away). Now I’m worried about this nematode things! I’m glad all the fruit trees I am growing are doing good or I’d be miserable! On a hillside in Whittier
Bummer, James. For whatever reason, rabbits have only nibbled on my cucumbers but allowed them to still grow well enough. Darn rabbits, you just never know what they’re going to decide to eat to the ground in one night. Glad they can’t get to your fruit!
Greg: I’m going to try drenching the soil overnight with white vinegar. It’s $2.00 / gal. if you buy two or more at S&F. I ‘ll just flush out the soil with water later.
Hi Dal,
Do you know that you have root knot nematodes? Do you know that the white vinegar will kill them? What else might the white vinegar kill?
I’ve heard the use of chitin (Found in crab meal) as a soil amendment and also compost tea can help.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2629878/
Hi Julia,
I hadn’t heard of the use of chitin, but it does seem plausible that composts and compost teas could help. The idea is broadly accepted among soil scientists that high populations and high diversity of soil life helps suppress the bad guys like root knot nematodes. This is said to happen through competition, through predation, and through other means.
Also, the above-mentioned page on nematode management guidelines from the University of California says, “You can add various organic amendments to the soil to reduce the effect of nematodes on crop plants. The amendments—which include peat, manure, and composts—are useful for increasing the water- and nutrient-holding capacity of the soil, especially sandy soils.”
Oddly, I had added an inch or two of my homemade compost to the bed where my infected cucumbers were about two months before I planted them. I had added the compost to cover the bases of the developing potato plants that were growing there before I planted the cucumbers. That compost didn’t seem to help much in this case. There are many possible reasons for that though.
I’ve been fighting RKN in tomatoes for years in San Diego. Cover crops seem to help a lot, but you need to plant a true cover crop, not just a few here and there as companion plants:
1. Trap crops. Nematodes infect arugula, but can’t reproduce in it.
2. Fumigation: grow mustard, chop it into small pieces (weed whacker) and immediately (within a few minutes) till it into the soil. Water the soil and then cover with plastic for a few days. The glucosolinates in the mustard kill the nematodes.
There are varieties of mustard (Pacific Gold) and arugula (Nemat) bred to be especially good at killing nematodes.
Bonus: either method gives you a lot of green manure to turn into your soil, and neither seem to be a target for other garden pests/diseases.
Good luck!
Hi Mike,
Sorry you’ve been fighting root knot nematodes too. Thanks for the contributions.
Here’s an article claiming to have found that Nemat arugula gets infected with a certain species of root knot nematode but doesn’t allow the females to produce many eggs (compared to infected tomato plants): https://www.researchgate.net/publication/233691170_Potential_use_of_arugula_Eruca_sativa_L_as_a_trap_crop_for_Meloidogyne_hapla
What I read is that the use of brassicas as a biofumigant can achieve some efficacy, but the effectiveness varies a lot. Maybe as it is tried more and more now that chemical fumigants are being outlawed, more effective and predictable results will be achieved.
Here are a few interesting articles on brassicas as biofumigants against root knot nematodes: https://www.sare.org/Learning-Center/Books/Managing-Cover-Crops-Profitably-3rd-Edition/Text-Version/Nonlegume-Cover-Crops/Brassicas-and-Mustards
http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Brassicas%20Factsheet%20Final%20August%202011.pdf
http://www.hort.cornell.edu/expo/proceedings/2016/Vine%20Crops%20DEMYSTIFYING%20BIOFUMIGATION%20WITH%20BRASSICA%20COVER%20CROPS%20ODea.pdf
What I always wonder about with methods like these are efficiency comparisons. For example, how much more nematode reduction do you get from all of the costs and work put into the brassica biofumigant practice compared to mere fallowing? I would hope to get a lot more.
For sure, it would be nice to get all of that green manure from the brassica though.
There’s not much $ cost for either method. Nemat arugula isn’t sold in small packages that I can see, but I bought 4 oz of bulk arugula seed and a few oz of Caliente 199 mustard seed fairly cheaply a few years ago. From the articles I read back then it’s more effective than fallowing, but it is a lot more work and water.
At least with arugula it should be effective even if you don’t turn it into the soil and cover it. Bonus: arugula germinates really reliably so it doesn’t take much seed to cover an area. It self seeds easily as well, so you can either cut it/pull it before it sets seeds or let it keep going and have companion plants later.
In contrast, I’ve had a lot of trouble getting many Nema-gone marigolds and french marigolds to germinate and grow. With just a few marigolds scattered around, it wasn’t much of a test.
Solarizing has never worked well for me, my garden beds are narrow and have a fence to the East so they don’t get much sun in the morning. The temperatures never really got that high, even in August. Also, while solarizing can kill the eggs, the juveniles (and adult males?) are mobile enough to move down lower in the soil and avoid the heat.
From the journal articles I read a few years ago:
– all of these methods (trap crop, biofumigation, fallowing, solarization) only provide partial protection for one growing season. If you go back to growing plants that are good nematode hosts, it only takes one summer to put you back at square one, and you will need to repeat the treatment.
-you need to grow A LOT of cover crop, chop it finely, and bury it quickly for it to have a chance at being effective.
-The main effect of compost isn’t to lower the population of nematodes, instead it makes the plants roots healthier, so they are still able to do their jobs even while under stress from nematodes.
For me: I’m doing more crop rotation now (my garden is pretty small for fallowing for a year or more), growing tomatoes in pots, and growing a lot of arugula when a bed is free for a few months.
These days I’m mostly fighting spider mites!
Mike,
You’re so generous! Thanks for taking the time to share all of that.
I fight spider mites too. They took down a few potato plants over the past month. Seems to happen this time of year, every year.
Cover crops such as planting rye grass in late summer, then pulling them out in winter like a chop-n-drop method, may reduce nematodes for the following growing season. You may also try planting marigolds or certain legumes between your crops. There are several articles that suggests this, link below:
https://bexar-tx.tamu.edu/homehort/archives-of-weekly-articles-davids-plant-of-the-week/control-garden-nematodes-with-cereal-elbon-rye/
https://edis.ifas.ufl.edu/in892
Hi Warren,
Thanks for sharing these. I wish I had two areas infected with root knot nematodes so I could try a cover/trap crop on one and fallowing on the other. And I wish these studies compared the nematode reduction obtained with the cover/trap crop to fallowing or solarizing.
Are the surface roots of avocado trees at risk of nematode infestation?
Hi Barbara,
The UC IPM page linked to above lists avocado as being infected by root lesion nematodes but not root knot nematodes. Here’s the page again: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7489.html
I’ve personally never seen evidence of root knot nematode damage on any avocado roots.
This morning I read your post and thought it was a relief to know I haven’t been the only one struggling with cucumbers and melons this year. I remember thinking before how weird it is that my tomatoes and eggplants are thriving, yet the cucurbits planted in the same soil look terrible. So I went and pulled out my dying cucumber plants, which I’d been meaning to do this week anyway… lo and behold, it was the same sight captured in your photos above.
Sad. But at least now we know.
Wow, Arden! I was thinking of someone like you when I wrote this post. You’re wondering why the plants are struggling, wondering if it’s something you’re doing or not doing, but the answer is hidden under the dirt. We both know now.
Nice timing on the subject of pests! I just found the weirdest bugs covering… and I mean covering, my barbados cherry. I neemed them and they dropped off, I thought they were egg sacs at first but I pulled one off and looked at it under a jewelers glass and it had tiny little legs. Bugs freak me out so when I find one of my plants covered with them, rather unsettling lol… take a peek, anyone know what these are? I tried looking around the net, honestly I did.
https://ibb.co/6gKth00
Thanks for sharing your experience. There isn’t too much out there that is super-helpful.
I, too, have had a really bad infestation this year of RKN in San Diego. I lost all my summer squash and ended up planting new seedlings in containers to avoid them.
Yesterday I pulled a german johnson tomato plant that had stopped producing and seemed to really be struggling. It had HUGE knots that looked like dreadlocks. I have so gnarly pictures if you want to see. About half of the tomato plants I pulled in a 3×12 bed were infected but I did get a small harvest.
WHAT I’ve tried:
–I applied beneficial nematodes (SF), about 5 million, over a one week period in July. I’m confident it slowed down the RKN, especially on my eggplant. They started producing fruit only a few weeks after application. Unfortunately, a month later, the plants are wilting in the sun again. I will try another application but after pulling up the dreadlocks plant, I’m not holding out too much hope.
–I planted marigolds around the area where I pulled up affected cucumber plants. They are supposed to have produce a substance that kills the RKN so we’ll see.They are now about 2 ft high and, strangely, have no flowers.
–I read that brassicas are not as susceptible so I’ll plant my cool weather seedlings in those knotty beds and see what happens.
–Fallowing and solarization will be the absolute last resort!
This is a whole lot of “we will see if this works” for a first year gardener but I’m determined to keep up the new hobby I have fallen in love with.
Hi Kelli,
Thanks for sharing this. Sorry you’re dealing with this too.
According to the list of host plants linked to in the post, brassicas are susceptible to root knot nematodes. However, I’ve never noticed them showing infection in my garden. A possible explanation is that the RKN are not as active in the cooler soil during the time of year that brassicas are grown in here. I’ll pay closer attention to this in the coming months.
Your article and included links are the most clear information I have found in the past two days since realizing I have Root Knot Nematodes in at least two of my raised veg beds, very likely more of them as well as possibly throughout the yard. It explains a lot about why certain veggies just never thrived. I would see funny looking beets, carrots and even potatoes one year but just thought it was a grub or some other pest. This year my green beans faltered and I knew something was up. I know legumes can have nitrogen fixing nodules on them and I was hopeful but since reading up on it, I realize the nodules are from RKN not nitrogen fixing nodules. So I have two questions: 1) The nitrogen fixing nodules would rub off easily from the root of the green beans right? If it sort of snaps off or can’t be removed it’s RKN? 2) how common is Root Knot Nematode? I am a pretty experienced gardener and really did not realize it was such a common problem among home gardeners in Southern California and beyond. There is not a lot of clear direction about identifying it and managing it. You’ve provided some good resources but it just seems like it’s not talked about very much. is it becoming more common recently? Any thoughts on that? Thank you so much.
Hi Sharon,
It’s been a while since I’ve had a close look at the nodules on roots made by nitrogen-fixing bacteria, but as I recall they are as you describe and they are more like little balls whereas the galls made by root knot nematodes are often varying in shape, sometimes just swellings, and they’re well attached to the root. The RKN galls are often numerous too whereas I don’t recall the nodules of nitrogen-fixing bacteria often being as numerous on a single plant’s root system.
As for how prevalent root knot nematodes are, I can only say that I’ve had them in both of my last two vegetable gardens, and both of these have had sandy or sandy loam soils. I read that RKN damage is worst in sandy soil.
Because RKN damage is to the unseen roots, my guess is that more people have plants that suffer from them than know. I’m trying to be more diligent about checking the roots on plants that don’t grow as well as I expect, but it takes effort and it takes mindfulness that a root disease could even be the culprit.
I have been battling root knot nematodes for about eight years. I spray the beneficial nematodes in early spring before the beds warm up. Water them in well and they will be hatched and hungry by the time the root knot nematodes hatch. Make sure what you use has Steinernema feltiae nematodes.
Last year (and this year) I sprayed twice (once early winter/spring and again spring/summer). At the end of last year I had barely any damage to my tomato roots and none on my squash which is a major change. Usually the heirloom tomatoes and summer squash roots are covered in knots, and grow and produce poorly. I use Biologic Scanmask and am having success with it.
I have used crab shell meal but didn’t notice much of a difference, I had better success with the marigolds for nemotodes when mid season I cut them into small pieces and turned the ground over and let them rot. Adding lots of compost helps but as I read in an earlier post, just can’t make enough of it!
Thanks for sharing this, Joan. I can’t seem to find anywhere that lists the species Steinernema feltiae as controlling root knot nematodes. But I’m interested in the potential of using such beneficial nematodes.
This year I’ve chosen all of my tomatoes (and other veg) based on their claim to be RKN resistant. We’ll see how that goes. I’m also trying to get everything in the ground as early as possible before the soil warms up enough to be very hospitable to RKN. (But I got too eager with my first round of tomatoes and lost many to a frost last week!)
I pulled all my sad tomato plants yesterday and didn’t notice the roots having that look but I’ll be looking for this problem as well. I mostly lost everything to spider mites this year. They even took out my mint, catnip, basil and cucumbers. I don’t have much veggie garden space because of my crowded avocado tree obsession so unfortunately I have to plant the same vegetables year after year in the same areas. I wonder how deep the harmful nematodes go in the soil. My solution is to empty out the soil and fill again with new compost and soil but its obviously expensive. I bought every spider mite spray I could find yesterday and am going to wage my war on those bastards though. Thanks for all this information. Our figs got the dreaded black fly larvae this year too. There is a new pest to discover every year 🙁
Hi Bita,
Please update me on how those sprays work for mites. Sorry you have the black fly in your figs. I read that Pasadena was one of the first places it was discovered in California this year: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=49051
Thanks for the link. They are ugly buggers. I am cutting open the infested figs and dunking them in a tub of bt solution to kill them. That article says to bury the fruit and Ive heard of freezing them but I don’t really want them in my freezer.
I find a lot of great fertilizers and pest control at my local hydroponic stores and they gave me a sample of something to use to control caterpillars. I will update about the spider mite sprays which I also got there. The cannabis industry has to keep things on the safe side for smoking so I trust the things they use over some toxic looking things at garden stores.
Hopefully people control the fig flies in their yards so the problem doesn’t get worse.