Root Knot Nematodes, Meliodogyne spp.

Root knot nematodes are my current sworn enemy in the garden. They are very frustrating because unless you know what to look for, you may never know you have a problem. Nematodes are microscopic soil dwelling roundworms. There are many different kinds of nematodes and not all are garden pests. However, the root knot nematode is a very annoying pest indeed. Above ground, plants are stunted. Below ground, the little guys are sucking on the plant’s roots and robbing it of nutrients. This weakens the overall root system, starves the plant and allows entry points for fungus and disease. Bad stuff.

I have had plants that mysteriously won’t grow. No amount of fertilizer, water or sunlight seems to make them happy. Then, I pull out the plant and find the tell-tale sign of root knot nematodes- galls on the roots. The roots are stunted and distorted. They look like they are covered in tumors.

According to my California Master Gardener Handbook, plant parasitic nematodes (including the root knot type) can form complexes with pathogenic fungi in the soil. The pathogens and nematodes work together to damage the plant. Susceptible plants are damaged more than would be predicted from each pathogen alone. They are in cahoots! What is a gardener to do?
Nematodes are known to affect many valuable crop plants. There are chemical treatments available, but for a home gardener who wants to be organic, the options are more limited. I read somewhere that adding organic matter can help by encouraging beneficial soil microbes. I tried that. Plants were still stunted. So I bought nematodes to kill the nematodes. They are supposedly beneficial nematodes that prey on root knot nematodes. Of course, you can’t see nematodes so I felt like I could be the Emperor with no clothes. I have no idea whether there was something in the package or not, much less what the invisible material may do in the garden. But I was desperate so I shelled out the money to give it a try. I applied the beneficial nematodes in my garden and at a client’s house. So we have two experimental plots. I’ll let you know how it goes.
Of course, you can always grow plants that are resistant or not affected by parasitic nematodes. I have noticed that new zealand spinach is totally immune. While other plants are stunted and won’t grow, new zealand spinach flourishes. So if all else fails, I’ll always have that.

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  1. I toured the organic garden of a local landscape architecture professor. He had just discovered root knots on many of his plants, and he beleived that he had them for years but the plants survived because he used lots of biologically active compost. Of course, if your compost heap doesn’t get hot enough, it could spread the buggers everywhere.

    It seems to be working for me- I had nematode problems last year, but barely any this year. I compost like a maniac.

    Bermuda grass is nematode resistant, and marigolds kill them. That makes for a slow but easy way to get rid of them. Crab shell based fertilizers are said to get rid of them. Nematode eggs are coated in chitin, which is what crab shells are made of. When lots of chitin is present, fungi arise to eat it. Neptune’s Best makes a product like that, but it is expensive.

  2. I’ve read that saprophytic fungi prey on nematoded when they are starved of nitrogen. You might consider blending in wood chips or driving in stakes or building hugelkultur beds as a way of culturing predatory fungi.

    (Fungi, by the way, are also rich in chitin, so increasing the proportion of fungi to bacteria may also help the way crab shells do.)

    It seems, from the paper I linked to above, that soil has to be nitrogen-starved in order for wood to help with nematodes, so this strategy might only be good in legume beds or other places where scarce N is OK.

  3. Here in Australia I’ve read of two other possibilities.

    The first is growing mustard as a green manure crop. http://www.abc.net.au/gardening/stories/s2534493.htm
    I am now growing in beds that have had the mustard treatment, but its early days so I’m not sure of how well its worked.

    The other remedy is molasses – ‘half a litre to two litres of water and spread over one and a half square metres of affected garden area’.
    I’ve yet to try this.

  4. I’ve used a ground sesame seed product that seemed to be effective in lessening the damage.

    And lately I’ve been doing chop-and-drop with (specifically) Mexican Marigold (Tagetes minuta), based on reports that the leaves are deterrents to nematodes. No results to report yet.

  5. I’ve read about the marigolds too, so I plant marigolds (organic plants of course) where there is an infestation. I had nematodes in the Cuke beds this summer, planted marigolds and the problem seems to be gone. We shall see next spring for results though.

  6. I had the same problem. Marigolds didn’t work for me. I solarized a couple of beds this summer and that has seemed to help considerably. I also reintroduced beneficial fungi and bacteria from Stamets (didn’t noticeably help when I used it on infested soil, but after solarization I wanted to make sure they were in there).

  7. One more possibility: Sunn hemp (not related to actual hemp…therefore, legal…).

    It’s a legume, used as a fiber crop, forage, and green manure.

  8. Joel,

    Excellent suggestion-noticed that groworganic.com has sunn hemp for sale in bulk. I’ve also heard about the use of mustard and, of course, certain strains of marigolds (Seeds of Change sells a nematodal marigold). Was reading that it’s important to figure out the species of nematode causing the problem in order to grow the correct cover crop. The university of Florida has a $25 nematode test–don’t have the URL, but if you ask Mr. Google you can find out.

  9. How did the beneficial nematodes work out? I’m dying to find out whether or not I should invest in these.

    I am also battling the evil RKNs. With limited funds and a quickly passing planting window, I took the route of sterilizing the soil in my raised beds with boiling water. Sadly, I had to kill my earthworms and whatever else was doing good.

    I haven’t checked the roots on my new plants, but they are already looking much healthier than the last batch in same soil. Root knot nematodes are the herpes of the gardening world and I hate them!

    I was recently given a tomato plant by a neighbor. Normally I would have popped it into the soil without thinking twice. With my newfound suspicion I checked the roots. Completely covered with galls. I lopped it off and stuck the cuttings into the garden, tossing the infested roots. Fingers crossed.

  10. One more idea, related to the sesame flour:

    Growing actual sesame plants. Apparently one of them covers about a square foot, they produce a lot with little irrigation, and the roots repel nematodes and leave the soil in great shape.

    I bought some bulk-bin black sesame because I’ve heard it’s more popular in the Middle East…I’ll see how well it grows this spring. And I’m certain what I don’t plant will be tasty.

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