Some nematodes harm plants, while others are useful against plant pests. We show how harmful nematodes can be controlled.
Nematodes, also known as roundworms, are extremely problematic pests for both hobby gardeners and farmers. While in the past it was still allowed to fight the nematodes living in the soil with very environmentally harmful nematicides, combating them with chemical pesticides is now prohibited. Of course, the nematodes did not disappear after this ban, which is why new, biological methods must now be used and tested so that it is still possible to control nematodes.
Harmful Nematodes
All relevant plant-damaging nematode species live in the soil and infest the roots of their host plants in different ways. What all species have in common is that they are microscopic and can occur in millions in one cubic meter of earth. Some nematode species remain in the soil, pierce root cells and feed on the exuding plant sap. Some other species, on the other hand, penetrate the plant roots and form typical root galls or cysts. The formation of cysts is one of the important reasons why nematodes are difficult to control. In these cysts, the roundworms are well protected and can survive for about 20 years without food.
Besides their stubbornness, the different nematode species are also capable of infesting almost any of our delicious garden plants. Whether carrots, celery, peas, spinach, cucumber, tomatoes or potatoes - the list could be extended as long as you like. In principle, no crop is safe from the eel-like pests. However, as you will see below, there are plants that have developed defense mechanisms against root-damaging nematodes over the course of evolution.
Detect Nematodes
A nematode infestation is not so easy to recognize. While the typical galls and cysts are easily spotted by digging up the rootscan be made, symptoms caused by other nematode species are difficult to assign. So that you can quickly see if your garden is infested, we have summarized the most important symptoms that indicate a nematode infestation:
- Rooted beard: Very many fine and strongly branched roots, which resemble a beard
- Root rot: Caused by subsequent infections that can penetrate through the punctured root cells
- An overall short root system, which has many lateral roots
- Growth depression, wilting and loss of yield due to damaged roots
- In larger vegetable beds, a nest-like occurrence of the symptoms described above is a sure sign of a nematode infestation
Nematodes on Potatoes
Especially in potato cultivation, the two potato cyst nematodes Globodera rostochiensis and Globodera pallida cause big problems. An infestation is easy to recognize by the small white-yellow, pin-sized cysts on the potato roots. Typical of infested potatoes is reduced growth, small leaves and increased formation of side shoots. Overall, the symptoms lead to severe yield losses. In addition to the potato, the two nematode species also infest other nightshade plants such as aubergines, tomatoes and nightshade weeds. Although the cysts of the nematodes can survive in the soil for about 20 years, a break in cultivation of 4 years is enough to starve out the majority of the nematodes. In addition to potatoes, no other nightshade plants should be cultivated on the bed during the break in cultivation. In addition, there are other practical ways to combat nematodes in the soil:
- Use resistant potato varieties in case of nematode infestation: Christa, Erstling, Gloria, Ukama or Quarta (maybe on the market soon: Trabant, Valdivia)
- During a break in the cultivation of nightshade plants, in addition to volunteer potatoes, nightshade weeds such as black nightshade or bittersweet nightshade must also be removed
- In the case of extreme damage caused by nematodes, so-called black fallow can help - you can find out what black fallow is in the chapter Fighting nematodes in the soil
- Tagtes do not help against the "potato nematodes"
NematodesTomatoes
Unfortunately, tomato roots are also one of the nematodes' favorite foods. In contrast to potatoes, however, tomatoes are preferentially attacked by gall-forming nematodes such as Meloidogyne hapla and Meloidogyne arenaria. Above-ground symptoms are often not properly recognized in tomatoes. However, if the yield from your tomato plants is declining year after year and the tomatoes are grown in the same location, gall nematodes may be to blame. Unfortunately, one of the tomato roots has to be dug up to check. The galls are located directly at the roots and appear like small (2 to 4 mm) ulcers. Against the nematode species M, which occurs frequently in the field. hapla unfortunately only helps with consistent crop rotation. For control, the 2- to 3-year cultivation of maize is recommended. Corn does not serve as food for the nematodes, so growing it consistently starves them. However, many other vegetables such as carrots, leeks and celery are host plants and therefore cannot be grown on the same plot.
There are varieties that are resistant to heat-loving gall nematodes, which prefer to attack tomatoes in greenhouses and under foil, for example:
- Caprese, Corianne, Ducati, Dolcevita, Matias, Picolino and Zebrino.
And more good news for lovers of other tomato varieties: With the rootstock Vigomax you can in principle make all tomato varieties resistant to the heat-loving gall nematode. You can find more tips on pests on tomatoes here.
Fight Nematodes
When combating nematodes, the focus is not on completely eliminating all pests. This goal is utopian and cannot be achieved with practically any countermeasures. Rather, it is about reducing the nematode population to such an extent that the small number of roundworms no longer causes any visible damage. In the next few years, it is very likely that new biological control measures will come onto the market, which are currently being researched. The scientific focus is primarily on fungi, which capture the nematodes with microscopically small lasso-like threads and then render them harmless, or infest the eggs of the small roundworms.
Nematodes in the soilfight
Nematodes can usually be controlled very effectively by starving them out - but how does it work? In principle, it is only necessary to ensure that no host plants are available for the harmful nematodes to feed on a bed infested with nematodes. However, that is easier said than done. Since it is not possible to determine the individual nematode species in practice, it is usually difficult to choose the right crop rotation. Because the different species can infest a large number of plants and also use a wide range of plant species as hosts. So if you are not sure which nematode species affects your plants, then we recommend black fallow. With this biological control measure, the bed is kept free of plants over a longer period of time. This measure is particularly effective if you observe the following points:
- For nematode control, the bed should lie fallow for at least 4 - 5 months
- Nematodes are particularly dependent on food in summer, so starvation tactics work particularly well during this period
- Weeds should not grow in the field during this period either, so rake them out regularly
- Disadvantage: The open ground is unprotected during this period and threatened by wind erosion
Thermal control measures are also available to horticulture. Like all soil dwellers, nematodes are very sensitive to high temperatures. Using special steam devices, the soil can be heated to such an extent that even deep-seated nematodes are effectively combated. However, this method is very expensive and unfortunately not practical for us hobby gardeners.
Chemical control of nematodes?
Our garden soil is inhabited by an almost innumerable number of microorganisms. In addition to its sheer mass, the variety of species is almost inexhaustible. All of these soil creatures take on essential functions in the nutrient cycle. Plants decompose and the nutrients released as a result can be reabsorbed by other plants. Microorganisms such as earthworms literally till the entire soil and thus ensure good ventilation of the plant roots. If pesticides against nematodes in the soil were used on a large scale at this point, everyone can imagine what the consequences would be. The long-term damage to other organisms would be difficult to calculate, as would the risk of groundwater contamination.
Nematodes used to be fought with toxic gases, among other things, which inwere introduced to the ground. However, these control measures also had significant side effects on other organisms and sometimes even on humans. For this reason, the use of chemical pesticides, so-called nematicides, is generally prohibited throughout Germany.
Tages against nematodes
As mentioned at the beginning, there are plants that form antibodies against nematodes and can thus fight them. One plant that has perfected this mechanism is marigolds. The plants form so-called terthiophenes, which have a toxic effect on nematodes. If a nematode gets to a marigold plant and wants to absorb the plant sap at the roots, it will be killed by the toxins it contains. And this is exactly where the limitation in the effectiveness of marigolds lies. Only species of nematodes that regard marigold plants as host plants and puncture their root cells are controlled. For this reason, gall and cyst nematodes are almost never combated by marigolds. Free-living nematodes from the Pratylenchus genus can be recorded all the better. These free-living nematodes like to attack vegetables such as carrots, celery, beans, lettuce, onions, peas and leeks and can become a major nuisance in the vegetable patch. You can best combat large accumulations of nematodes with an area-wide intercropping of marigolds. Do this as follows:
- Use a 1:1 mixture of Tagetes patula and Tagetes erecta; these two strains are very effective in nematode control
- Note that marigolds are not frost hardy and should be left on the bed for at least three months to be effective.
- In addition to caring for the marigold plants, as many weeds as possible must be removed regularly
This biological control method can reduce a nematode population by up to 90% in the long term, making it an effective biological measure against free-living nematodes such as Pratylenchus.
Nematodes can do a lot more than just damage your plants: some of the roundworm species also fight other pests. Here you will find all information on the use of nematodes as beneficial insects in the garden.