War of the Nematodes! My battle to eliminate them from a planting bed.

After three years of hard work building up the soil in my main gardening bed I was shocked to discover plants dying on me left and right. As I dug them up the answer became apparent: root knot nematodes had invaded the bed.


Tomato roots completely taken over by nematodes.

Of the 200 varieties of nematodes vexing farmers and gardeners around the world, by far the most destructive is the root knot nematode (genus Meliodogyne.) It's a worm barely one-half a millimeter long that burrows into plant roots, feeds on them, lays eggs, and as these hatch forms large galls that interfere with the root's ability to absorb water and nutrients.

On the right: a sweet pea root totally taken over by root knot nematodes,
middle: a root with a serious infestation,
and left: a normal healthy sweet pea root showing a few external nodules
inhabited by beneficial nitrogen fixing organisms.

Starved by restriction of the roots, plants may wilt on hot afternoons and recover overnight or completely collapse. In this weakened state plants are more vulnerable to disease and insect attack.

Root knot nematodes thrive in warm, moist, well-aerated sandy soil and prefer areas with long summers and short winters. My high desert location is an environmental heaven for them.

They can build up to dangerous levels in soil after being imported on plants or soils brought into a bed. Once established, they can be almost impossible to eradicate.

That's what this page is about: my attempts to kill off these tiny demons and save my prime bed.

 
The First Battle: Determine the Enemy's Weaknesses

The best way to deal with root know nematodes is to live with them by growing plants that can tolerate them and may even suppress them. The problem is that most of the plants I intend growing in that plot are highly susceptible to them.

The only alternative is suppression: i.e., all-out war.

Several Internet searches turned a number of facts about nematode controls which, when coordinated together, suggest a battle plan.

1. Nematodes prefer warm sandy soil: therefore building up the organic matter in the soil and keeping it cool will help suppress them.

2. They move around in soils so even if all of them in a bed are eliminated they can reinfect the bed from soil in contact with the bed on either side. This migration can be blocked by installing barriers between the sides of the bed and the surrounding soil. Nematodes prefer the top 12 inches of soil so these barriers don't have to go deep.

3. Solarization can kill them, but it also kills off all the good microbes in the soil so this is an unattractive solution.

4. Root knot nematodes have predators like any other life form. In France there are melon fields that are heavily infected with nematodes yet they cause little damage. The reason is that fungi and bacteria that feed on nematodes are also present and keep them in check. To establish such a population of beneficial microbes I will work in a generous amount of shrimp meal. This slow release fertilizer contains a great amount of chitin, the same material found in nematode bodies and eggs. Incorporating this material into the bed should encourage microbes antagonistic to nematodes to grow. The nice thing about this technique is that it lets the good microbes move out into the surrounding soil. It's like carrying the battle into the enemy's homeland.

5. Leaving the plot fallow a year would starve any soil-born nematodes but then when replanted, new nematodes could migrate from the surrounding soil. This is a bad idea for a home gardener like myself because with so little space to work with I can't afford to put a bed into mothballs for a year.

6. While root knot nematodes don't like some plants, they really hate a few because they are toxic to them. The best of these are French marigolds. A variety called "Tangerine" is supposed to be the best but I couldn't find seed for it. (There are a couple of "Tangerine" French marigolds available but they have other names in their titles and it isn't clear which is the best. Care must be taken here because some marigolds may make the problem worse.) Fortunately the people at Park Seeds sell a French marigold named "Golden Guardian," which they claim has been tested and proven to be the most nemacidal marigold developed. Burpee also sells "Nema-gon" with similar claims but it's 4 feet tall while the Park's version is a more manageable 2 feet. (For a list of plant suseptability to root knot nematodes see: http://www.ces.nesu.edu/depts/pp/notes/Ornamental/nematodes/odin31_nematodes.htm.)

These marigolds work by trapping nematodes that invade their roots and killing them with plant secretions. The roots also saturate the soil with chemicals toxic to nematodes. The catch is that the plants have to remain in place at least three months.

7. While there are several chemical compounds proven effective against nematodes none of these are legal in the United States.

The Battle Plan:

I'm hitting the root knot nematodes in the bed with three simultaneous attacks:

First, I'll dig the bed up and clear out any infected roots. As I do this I'll incorporate a small mountain of organic matter in the form of peat moss and alfalfa meal.

Second, I'll dig in 10 pounds of shrimp meal (from Peaceful Valley Farm Supply) which I admit is ten times the recommended rate for the 100 square foot bed. This should really kick start the beneficial microbes.

Finally, the entire bed will be filled with Park's Golden Guardian French marigolds to kill off as many of the little monsters as possible.

 
I decided not to attempt installing a physical barrier between the bed and the surrounding soil. While there may be advantages to this I want to see if the controls I am employing have the ability to reach into the soil around the bed and reduce nematode populations there as well.

 
Assessing Success:

If all goes well the battle should be over around the end of summer. The problem then will be to determine if the war has been won. Estimating the density of totally infected sweet pea roots currently planted in the bed tells me that the infestation is widespread and extremely heavy. After the treatment I will plant the same sweet peas throughout the bed, let them mature for three months then dig them up and evaluate the number of root galls. If the number is zero or very low I will consider the war to have been won.

Please check back at the end of summer for the next update.

 
Final Thoughts:

I you want to find out if you have root knot nematodes in a bed and don't want to pay a fortune to have a soil sample tested, try planting nematode-susceptible plants and examining their roots after two to three months. If you see a lot of galls the infestation is bad. If there are none or just a few the problem is minor... for now. In time they will build up so be prepared for some intervention. The plants that are most susceptible to root knot nematodes are morning glory, larkspur, lobelia, helichrysum, amaranthus, calendula, blue lace flower, sweet peas, celosia and gourds.

It may not be possible to destroy all the nematodes. The best I can hope to do is reduce their numbers to the point where they don't represent a danger to plants. With this thought in mind, it will be necessary to carry out some anti-nematode maintenance every year. Perhaps the war can't be won but at least the enemy can be kept at bay.

 

 VICTORY!!!

After the Golden Guardian French marigolds had grown an entire summer, I removed them and planted the area of the that had exhibited the worse nematode infestation with a row of sweet peas. Five months later I dug them up and after carefully inspecting all their roots I could not find a single nematode nodule. I can't comment on which of the three steps taken was the most effective at eliminating the problem, but in concert they proved nematodes can be conquered. The question now is, "How long will this cure last?" The answer: One year. The following season had several plants die from the return of the nematodes. I suspect they crept back from the surrounding soil.

I have to warn anyone planning to use Golden Guardian French marigolds to control nematodes that these are not attractive plants and have tough stalks that are difficult to chop up so they can be dug into the soil.

 

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