Potato Cyst Nematodes (or eelworms)

From: http://www.sasa.gov.uk/seed-ware-potatoes/nematology/potato-cyst-nematodes-pcn

To understand the importance of Potato Cyst Nematodes in potato cultivation it is helpful to know a little of the life history of these pests.

The nematodes (or eelworms) survive as cysts in the soil. Each cyst, about the size of a pinhead, will contain up to 400 juvenile nematodes each enclosed within a thin egg shell. When a potato crop is planted in infested soil, substances released from the growing roots stimulate a high percentage of juvenile nematodes to hatch. After hatching these juveniles swim through the moisture surrounding the soil particles and cut their way into the potato roots to feed. The damage they cause produces a stunted root system and the plant responds by producing many additional lateral roots. Severe root damage may result in patches of poor top growth but such damage is unlikely in seed crops.

The young nematodes develop in the roots, feeding on the growing plant. Males remain ‘worm-shaped’ and swim off into the soil when mature. Females grow and swell within the root to erupt through the root surface, whilst the head remains embedded in the root. When mature they release a chemical to attract the males. After mating the body wall of the female starts to tan and harden, forming the resistant cyst which protects the fertilised eggs within. Eventually the female dies and falls of the roots, or is dislodged when the tubers are lifted.

The potential of the pest to increase its population size depends upon the ability of each of the females within the 400 eggs to develop into a new cyst in the following years. Each spring a small proportion of the eggs within the cyst will hatch spontaneously, but unless the juveniles find a potato plant they soon die. Long rotations will therefore keep PCN population levels down. However, survival of some eggs for over 20 years makes this pest extremely difficult to eradicate completely.

From: http://www.sasa.gov.uk/seed_potatoes/nematology/pcn.cfm