Free-living nematodes and their impact on the yield and quality of field crops

Free-living nematodes (FLN) feed on the roots of susceptible crops, reducing crop quality and yield. They are also important virus vectors. Stubby root nematodes transmit Tobacco tattle virus (TRV) to potato resulting in a tuber disorder called spraing. FLN feeding on the roots of carrots, parsnips and sugar beet can cause multiple tap roots to form.

• FLN are generally most numerous in sandy and other light, open-textured soils
• Needle nematodes are found in a variety of soil types, but favour relatively undisturbed conditions
• Wet growing seasons and regular irrigation can increase the risk of damage because the nematodes require adequate soil moisture to move between plants
• In potatoes, most infections occur soon after tuber initiation
• Some potato varieties show spraing symptoms more readily than others

Stubby root nematodes life cycle

Scientific name: Trichodorus spp. and Paratrichodorus spp.

Stubby root nematodes have an extremely high rate of reproduction when soil temperatures are between 15°C and 30°C. They feed on various crop and non-crop species throughout the year.

Stubby root nematodes transmit Tobacco rattle virus (TRV) to potato, producing an internal disorder of the tubers called spraing. This does not affect yield, but reduces tuber quality and can make them unacceptable for sale. Although FLN can also transmit TRV to sugar beet, its effects are less serious than the direct damage.

Needle nematodes life cycle

Scientific name: Longidorus spp.

Needle nematodes multiply relatively slowly on various crop and non-crop species. Several generations occur each year, so both adults and juveniles occur together.

Crop damage

In many crops, feeding by stubby root nematodes causes a proliferation of thickened, ‘stubby’ roots. This causes poor top growth and reductions in yield and can make the plant more susceptible to drought stress and mineral deficiencies.

Spraing appears as chestnut-brown arcs, circles or lines through the tuber. Spraing-affected tubers are associated with short stems and crinkled, malformed and stem-mottled (discoloured) leaves. The tubers these produce can also contain brown flecks.

FLN feeding on taproots can cause the root tip to die and lateral roots to take over. In carrots, parsnips and sugar beet, this feeding can cause multiple taproots to form, known as ‘fanging’ (in carrots and parsnips) or ‘docking disorder’ (in sugar beet). Affected plants are stunted and healthy plants are often seen next to stunted plants, a symptom known as ‘chick and hen’.

Yield reductions can occur in most crops, but are most common in potatoes, carrots, parsnips and sugar beet. Yield losses of up to 17 t/ha have been estimated in sugar beet. Nematode feeding can also reduce root crop quality.

Non-chemical control

In potatoes, use of healthy certified seed or propagating stock can prevent the introduction of nematodes to a field. In areas at greater risk from spraing, use varieties with a lower susceptibility to TRV. Rotating potatoes with non-host crops and lengthening the rotations (growing potatoes no more than one year in six) will reduce nematode populations. Weeds can act as alternative hosts and a source of TRV.

Monitoring

Determine the number of FLN by soil extraction; a service offered by several accredited laboratories. Some can also detect whether or not the stubby root nematodes are carrying TRV. Transport soil samples carefully to protect nematodes from damage.

Thresholds

In sugar beet, severe symptoms may occur in soils with Trichodorus populations of more than 1,000 nematodes per litre of soil, or with Longidorus populations of more than 100 nematodes per litre of soil.

Insecticide resistance

None known.