Caused By: The Nematode Ditylenchus dipsaci
Onion Eelworms(Ditylenchus dipsaci) are microscopic worms that live in the soil and infects onion family of plants. This eelworm can also infect Narcissus, carrots, peas, potatoes, strawberry, alfalfa, oats, clover and probably man more plants and trees. This Eelworm are nematode worms that are microscopic, being are too small to see with the naked eye, but the damage they do is all too easy to identify. Infected Onions show distorted leaves, the bulbs are soft and may split. These Eelworms penetrate the base of stems or the bulb scales and feed on tissues. They inject toxic substances which produce distortion of the plant tissues.
An economically important feature of the stem and bulb nematode is its ability to survive repeated desiccation or drying. Dry nematodes can be reactivated by moisture when infested material is replanted or when a new crop is planted in soil containing old, infested material.
Apparently planting mustard as a green manure is supposed to produce a chemical in its roots that help prevent attack from eelworm. Using African Marigolds as companion plants help to deter the nematodes.
Introducing other nematode worms such as the one that attacks slugs and snails may help. It seems that if you increase the numbers of other beneficial nematodes the Ditylenchus dipsaci reduces in population.
Using a soil sterilant should help prevent attack.
Crop rotation should also be practiced.