Nematodes are also called Eel Worms. They are part of the Roundworm family just like the ones in your garden but a lot smaller in size. 0.5 to 1.5 mm long.
The females can just be seen with the naked eye but males are too small to be seen without magnification. Some scientists classify animal roundworms (eg Barber’s pole, Pimply Gut worms of sheep & cattle) as being Nematodes but most people only think of Nematodes as being plant problems.
Like Bacteria and Fungi, there are good & bad Nematodes & ones of no importance. The good ones are free-living ones that break down organic matter, kill other Nematodes or trap & kill some Fungi, Bacteria, Algae, Actinomycetes, Protozoa and other Nematodes.
The good Nematodes do not attack living plants and are sometimes used to control Chafers, Lucerne weevil, cucumber beetle, some grasshoppers, & earwigs. There is some evidence that some good Nematodes will weaken cotton Boll weevil by forcing the weevil to emerge too early & starve.
Parasitic and non parasitic nematodes
Then there are the disease-causing ones that most people think of when they think of Nematodes.
Bad Nematodes or parasitic Nematodes need a living plant to infest and thus reproduce. They attack the roots of some plants and a few do attack the above-ground plant parts.
The Nematodes that most people know about are those that attack Tomato plants. They pierce the roots of the plant which responds by forming a gall that restricts the movement of water & therefore nutrients. This means that the plant is less productive from the time it is attacked, so produces less fruit.
Nematodes have a very limited ability to move within the soil but are readily moved when infested soil, roots, bulbs, machinery, shoes, water etc are transported from one place to another. They survive either attached to plant roots or as cysts containing their eggs that then survive until they can emerge and infest some other host plant roots.
Parasitic Nematodes prefer light, sandy, well-aerated soil so tend to be a serious problem in the better types of soil that most gardeners aim for.
Nematodes are hard to control – Yes I did say control, not eliminate, as any vacuum in nature is quickly filled with some other organism good or bad – so control is the object.
1. Toxic Chemicals or fumigation – usually Methyl Bromide but this has been banned in Australia for a few years now & is only allowed to be used by qualified people within Commonwealth Quarantine.
2. Crop Rotation – Do not plant the same plant (or the same family of a plant) in the same space for at least 3 years (or more if you have the space). This is so the Nematodes die out
3. Use plants that repel Nematodes such as – Asparagus, Dahlia, African & French Marigold (they have an effect for up to 3 years in the soil), annual Salvia, Calendulas, radishes, White & Black mustard, turnip, onions, corn, some cowpeas, and velvet beans. The cover crop of Crotalaria discourages Nematodes also.
4. Spray with Neem oil. It contains active ingredients like azadirachtin, Nimbin, picrin, and sialin. It is said to affect the natural hormone balance of the worm and other bugs so they behave unnaturally and die. However it is best to use as a spray as drenching the soil is apparently not good for tomatoes, cabbages, and onions. Mix 1 tsp of neem oil to 1 liter of water and add 1/2 tsp of liquid dish soap for extra punch and spray on leaf surfaces http://npic.orst.edu/factsheets/neemgen.html
5. Solarise the soil. This is done by digging the soil and breaking up any clods (This is important as the clods will not get hot enough to give control to the Nematodes), smooth the soil surface & water it thoroughly. Cover the soil with a sheet of thin, clear plastic making sure the edges are firmly held down to trap the sun’s heat. The plastic sheet is left in place for 5-6 weeks in Summer when temperatures of up to 55o C can be reached and for longer in the cooler months.
Nematodes are usually killed at 50o C. The soil is ready to be used again as soon as the plastic sheeting is removed.
6. Plant plants that are resistant to Nematodes such as Broccoli, Brussels sprouts, chives, cress, garlic, leek. Others that have some resistance include; Globe artichoke, Jerusalem artichoke, horseradish, parsnip, rhubarb, spinach, sweet potato & turnip. Highly susceptible crops include Cucumber & tomatoes.
7. Another control is the use Organic amendments eg. some saw-dusts, that as they decompose give of some volatile fatty acids & phenols that deter Nematodes.
Today’s Did You Know…?
Queensland Department of Agriculture (now called Department of Primary Industries) used to have a world authority on Nematodes – Dr. Bob Colbran. He used to identify about 6 or 8 new Nematodes each year but has now sadly retired.
“…In 1976, Bob Colbran was awarded the Agricultural Science Medal by the Australian Institute of Agricultural Science for his services to agriculture. He finished his career as Director of the Plant Pathology Branch of the Queensland Department of Primary Industries, retiring in 1986. When asked in 2007 what aspects of his work gave him the most satisfaction, he replied without hesitation: ‘finding and describing new nematode species and developing nematode control measures that were useful to farmers’.
Excerpt from CSIRO history of plant and soil nematology in Australia and New Zealand