There seems to be confusion between wireworms and eelworms.
Mustard induces eelworm cysts to hatch and with no potatoes to live on they perish.
Eelworms are tiny nematodes that cannot be seen with the naked eye. The symptoms of nematodes are poor roots that grow into a birdsnest so that the plants wilt in hot weather. They lay their eggs in pinhead sized lemon shaped yellow cysts which can be seen attached to the roots of affected plants towards the end of Summer. Each cyst contains hundreds of eggs which hatch when potato roots, or mustard, are in the soil. Eelworms will gradually build up in soil if potatoes are grown too often. A break in the rotation of 3 or 4 years if sufficient to control them. Some farmers in Cornwall grow early potatoes on the same fields every year. They are able to do this because their crops are lifted before the eelworms lay their eggs.
Wireworms are the lavae of click beetles. The click beetles lay their eggs on grasses, which is why damage is worst in the first years after digging in pasture or rough , couch infested soil. Grass paths or badly cultivated plots also see infestations. The wireworms are between 5mm and 40mm long, yellow, shiny and difficult to squash, hence wireworm. Damage to crops is usually the roots of seedlings being eaten or small tunnels in root crops such as carrots and potatoes. Wireworm holes are often the entry point for slugs into potatoes.