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Free-living nematodes

Trichodorus, Paratrichodorus

Identification

Stubby root nematodes are migratory or free-living nematodes and are vectors of tobacco rattle virus (TRV) which causes 'spraing'. These organisms are not easily visible to the naked eye, being 1mm long, and a gram of soil may contain many thousands of individuals. The nematodes are most numerous in light, sandy soils.

Symptoms

The nematodes feed on daughter tubers and transmit tobacco rattle virus which causes spraing in potatoes. The symptoms of spraing are brown arcs or streaks within the tuber flesh. If high numbers of nematodes occur they can also cause direct feeding damage to the growing tips of the developing roots causing problems at crop emergence. Damage to the rapidly reproducing cells in the root tips leads to delayed crop emergence and loss of crop vigour. Damage to cells from wounding can also lead to invasion by pathogenic fungal diseases

Life-cycle

The nematodes lay eggs near host plants and after a few days the larvae wriggle out in search of food. Several generations occur each season and populations comprise a mixture of both adults and juveniles. Common weeds such as field pansy, knotgrass, groundsel, chickweed and shepherd’s purse can act as a host to the tobacco rattle virus.

Importance

Spraing is an important economic virus of potatoes and tubers infected with spraing are unmarketable. Crops most at risk are susceptible varieties, such as Pentland Dell or Russet Burbank, irrigated crops on light sandy soils and crops establishing in cold wet conditions. A number of other varieties do not develop spraing symptoms but these can suffer yield damage from nematode feeding.

Threshold

Other nematodes such as Pratylenchus spp. (root-lesion nematode) and Longidorus spp. (needle nematode) can also feed on the root tips of potato crops, the level of damage caused by these nematodes is directly linked to the soil population. A number of organisations in the UK, such as ADAS, SAC, Fera (The Food and Environment Research Agency) and NIAB, offer a soil analysis service to determine both the number and identification of free-living soil nematodes present in a soil sample. The free-living nematodes can also be tested for the presence of tobacco rattle virus as well as providing an assessment of the risk of spraing and feeding damage at crop emergence.

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