Most nematodes make a positive contribution by helping to break down organic matter and making nutrients available to plants, for example. A much smaller number of nematodes are what are known as plant-parasitic nematodes. These nematodes can cause considerable damage to many of our crops.
With the naked eye and based on the damage pattern, it is almost impossible to tell with certainty which nematode is involved. Microscopic examination is the sole and necessary solution. The appropriate measures are taken only once it is clear which nematode population is involved. Without this knowledge, steps may be taken too late or incorrectly, and the damage may worsen.
Within the plant-parasitic nematodes, we generally distinguish five main groups: root-knot nematodes, root-lesion nematodes, free-living nematodes, stem nematodes and cyst nematodes. Given below is an overview and the most important features. A complete overview and more information can be found at Kennisakker and www.aaltjesschema.nl (Dutch)
This group of nematodes causes the characteristic nodules on the roots of plants. The nematodes can reproduce rapidly and on many different crops. The damage results in considerable economic losses, especially due to rejections because of quality and in some cases yield losses. This always depends on the type of nematode and the crop. For example, the Meloidogyne species M. chitwoodi and M. fallax in potatoes are a quarantine disease.
There are several Pratylenchus species, of which P. penetrans is the best known and most damaging. The nematode penetrates the core of the root, leaving behind the characteristic brown spots. A heavier infestation can lead to root rot. The root-lesion nematode must be considered in the crop plan, as it can multiply on a large number of crops.
Besides the direct damage caused by the nematode, the entire root system becomes more susceptible to fungi, among other things. For example, it can increase the effect of scab (Rhizoctonia) and early wilting disease (Verticillium) in potatoes.
Most of their active life is spent above ground in the plant parts. That includes the stems, but also the flower buds, for example.
A short life cycle and rapid multiplication mean that the level of infection can increase rapidly during the season.
The nematodes can survive for a long time in the soil, sometimes up to 10 to 20 years, without a host plant. Onions are particularly susceptible to stem nematodes. But sugar beet and potatoes can also suffer a lot of damage. Flower bulbs with stem nematode infection are rejected.
The free-living nematodes are the species that dwell outside the plant and superficially attack the roots. It is mainly the Trichodorus variants that can cause considerable damage. Even with smaller populations, especially under colder, wet conditions with slow crop growth, the effects can be seen. Characteristically, there is a lot of growth of the root and the sprouts. The damage this causes can be considerable, especially when the nematodes attack the growing point of the root or sprout. The free-living nematodes can also transmit viruses, such as the tobacco rattle virus (TRV)
Cyst nematodes can occur anywhere and are often highly specialised in certain crops. They are generally well-controlled with crop rotation and resistances. However, this is subject to the condition that the correct population can be identified and the right measures taken. Cyst nematodes are characterised by the cysts that remain in the soil containing the eggs. They partly emerge spontaneously during the growing season, even without a host plant in the vicinity. As a result, without host plants, the population gradually decreases. If there is a host plant, the attractants will cause many larvae to hatch in a short time. The larvae enter the root and disrupt the hormone balance there, damaging the root system.
Cyst nematodes can generally be controlled well with resistant varieties, for example. However, the nematodes constantly adapt through selection, which is why a variety selection test can be useful in the case of potatoes. This gives us an insight into the virulence of the population and the effects of varietal resistance against it.