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Soil health RAGT

COVER CROPS AS A METHOD OF NEMATODE CONTROL

The negative effects of nematodes on important crops such as potatoes and sugar beet are well understood. Historically, these pests have
been controlled using nematicides such as Vydate but there is increasing interest in the use of crops which will interrupt the life cycle of
the nematode or to control them by biofumigation.
WHAT ARE NEMATODES?
Nematodes are a microscopic, worm-like, soil dwelling species. The damage they cause can be visible (e.g. malformed crops, leaf discolouration)
and invisible (e.g. yield reduction) and can also lead to secondary infections.
There are five families of nematodes which are widely recognised to
cause the most damage worldwide:

  • Cyst nematodes mainly affect potatoes and sugar beet. The nematode eggs are contained within a cyst that lives in the soil and when the eggs hatch the larvae invade the root tips, causing yield loss and in some cases deformation of the roots.

 

  • Root knot nematodes affect potatoes and carrots most severely but can also damage sugar beet and peas. Damage is caused when the nematodes invade the root cells, initiating cell death which then leads to the appearance of galls on the roots.

 

  • Lesion nematodes are migratory, meaning they move in and out of the root, laying eggs in the tunnels they make. The crops most at risk are carrots but potatoes, maize and peas can also be affected.

 

  • Stem nematodes cause damage by feeding on the contents of plant cells, causing those cells to die. Onions and potatoes are most commonly affected.

 

  • Stubby root nematodes infect the root systems of crops, causing stunting and deformation of the roots. Infected roots are also less able to supply water and nutrients to the plant. Crops affected include sugar beet and onion as well as potatoes, maize and winter OSR. In addition, these nematodes can also transmit the tobacco rattle virus.

COVER CROPS - HOW THEY WORK

Cover crops include species such as mustard, oilseed radish and rocket and can be planted as straights or as mixtures, depending on the target nematode population. Different crops have different methods of controlling the nematode population so it is vital to select the crop which has the most effect on the target nematode(s).

“Resistant” cover crops produce exudates which stimulate the hatching of eggs. However, once infected by the larvae, the plant will activate defence mechanisms to reduce any further multiplication by:

    • Either secreting a toxic substance that kills the nematodes, or

    • Producing more males than females, thus reducing the number of eggs produced in the next cycle. In this way they actively reduce the number of nematodes present in the soil.

  • “Non-host” varieties will not support the life cycle of the nematode; this means they will neither actively reduce nor actively increase the nematode population.

  • “Host” varieties, on the other hand, will support the feeding and reproduction of nematodes and could therefore exacerbate an existing problem. This highlights the importance of understanding exactly what species of nematodes are in the soil of each field in question.

It also follows that agricultural crops can act as hosts and non-hosts for different nematodes. It is therefore important to understand the impact of the rotation on nematode populations, not just how the nematodes affect the crops.

Cruciferous cover crop species such as oilseed radish, mustards and rocket can also have a biofumigation effect. This occurs when the crop is macerated and incorporated into the soil, releasing chemicals known as glucosinolates which can suppress soil borne pests and diseases, weeds and fungal pathogens.