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Nematodes, also called nematodes or worms, belong to the species-rich and most species-rich group of animals in the world. It contains up to 20,000 different species. The worm-shaped animals are usually only a few millimeters large, white to colorless organisms. Nematodes include both free-living predatory species and those that parasitize on or in almost all species of terrestrial plants and animals. There are both useful threadworms and harmful ones.

For locomotion, nematodes serve a longitudinal musculature that extends from the head to the body end of the animals. By contracting muscles, they are able to move snaking or stretch a part of the body. The males are typically smaller than the females and visually distinguished by their characteristically curved tail. Free-ranging species feed on bacteria, algae, fungi, carrion and faeces, as well as predatory animals. Small projections on the mouth are used for food intake and for scanning the environment.

Damage to plants

Depending on the nematode species, different symptoms occur on the affected plants. By a centered arrangement of the animals, the plant stands often grow irregularly in an infestation with nematodes. There are different deformations of roots and leaves. After-effects are discoloration and wilting of the plants, which are caused by a lack of supply of water and nutrients. With a very strong soil contamination with nematodes also seedlings can be killed.
Symptoms of nematodes Ditylenchus, Pratylenchus or Meloidogyne include malignant growth, dead roots, discoloration of leaves or decreased number of flowers. The host plant spectrum is large: many fruit and vegetables such as apple, strawberry and cucumber, but also indoor plants such as hydrangea, hyacinth and begonia are attacked by nematodes.

Infection of plants

Infection with nematodes is usually via the soil or by already infected plants. The nematodes migrate with the help of the water film of the affected plants, the stems within plants and populate the leaf over the stomata. The entire developmental cycle from egg via larva to the adult often takes place within the plant.

Nematodes as pests

As many nematodes severely impair their metabolism due to their penetration into the root system of plants, they are considered pests in horticulture. In agriculture too, they often cause great damage - for example when growing potatoes. With the help of their mouth bites, the animals penetrate the plant tissue and release a saliva secretion, which leads to changes in the tissue. The activities of the herbivorous nematodes can in the worst case lead to the death of the plant.
A known harmful nematode species are the beet cyst nematodes (Heterodera schachtii). This species infests the roots of certain crops such as spinach, beets and rapeseed, causing enormous damage to agricultural and vegetable crops worldwide. The resulting beet exhaustion occurs especially when the plants are farmed too long on the same area. For example, in sugar beet, the phenomenon leads to increased lateral rooting and reduced beet growth. Enormous yield busses are the result of the disease.

Nematodes as beneficials

Nematodes can also act as beneficials. Thus, the worm-like animals contribute as so-called rotten inhabitants (Saprobionten) for composting and soil formation. The roundworms are also used against snails, black weevils, the larvae of the garden leaf beetle (also known as grubs) and other plant pests. Especially predatory nematodes of the genus Heterorhabditis win as beneficials more and more importance in the garden. The harmful larvae of weevils and garden gadwall beetles feed on the plant roots and severely damage the plants. The adult animals of the weevil and garden beetle can also weaken the leaves and flowers of plants in large populations by heavy feeding damage. The aim of the nematodes is to destroy the soil-living larvae of plant pests. They use the larvae as hosts for their own multiplication and kill them relatively quickly. However, nematodes do not control adult beetles as beneficial insects. If you want to get rid of annoying pests such as the garden gullet permanently, should spend two to three years in a row nematodes, since the use of the thread worms, the pest population with each year is lower.
For a 100 square meter area cost such biological pest controllers of the genus Heterorhabditis about twenty euros. These can be purchased in a garden shop or on the Internet. The useful nematodes are usually supplied in a clay granules, which is dissolved in water and then applied by watering can on the affected areas in the garden. How many nematodes you need depends on the affected area. As a guideline, you need about 500,000 roundworms per square meter. It is important that they quickly deploy the nematodes after delivery. If you do not get to it right away, the roundworms can be stored for a short time at four to eight degrees Celsius. In no case may the storage area be warmer, otherwise the nematodes would become active.

Dissolve the sound mixture

The resting nematodes are delivered in a ready-to-use clay mixture, which only has to be dissolved in water

Since nematodes require a moist soil to move around, you should water the soil well before it is released and keep it slightly moist for the first three weeks. Nematodes become truly active only from 12 degrees Celsius soil temperature, so they must not be applied too early in the year. Because of the preference for moist soil, but it may not be too hot, otherwise the nematodes dry up quickly and they do not do much service. Therefore, it is best to bring the roundworms out in cloudy weather or only in the evening hours.

Preventive measures

Alternating cultivation of different plants on one surface reduces the risk of infestation with nematodes. Susceptible crops should be put back on the affected area only after four years at the earliest. Through appropriate tillage and liming as well as a targeted, organic fertilization you improve the living conditions of the plants: Strengthened and healthy plants are less susceptible to diseases. Also the soil properties are stabilized by appropriate measures. Healthy soil contains helpful fungi and bacteria capable of killing harmful nematodes.
In recent years, specially resistant plant varieties have been bred against nematode infestation. For example, the tomato variety 'Dolcevita' and the potato variety 'Alexandra' are ideal for cultivation. The "Vigomax" dressing is suitable for tomato growing and is also resistant to many nematode species.

Control of nematodes

If you notice an infestation of nematodes on your plants, you should immediately remove the affected plants from the soil with tubers and roots and discard them in the organic waste. Disposal of the diseased plant parts on the compost is not recommended as the pests' eggs can survive for many decades and, once root exudates indicate the presence of a host plant, can hatch.
A fallow land with "enemy plants" such as tagetes, zinnias, calendula or mustard reduces the number of harmful nematodes in the soil, but the success depends on the nematode species, for example species of the genera Meloidogyne and Pratylenchus are only fought by a fallow This means that no plants need to be cultivated on the acreage over a period of months, and carefully remove all wild herbs, as the nematodes can multiply easily here as well. Steaming with steam for soil disinfection is effective against nematodes.

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