Onion eelworm (also known as Stem and bulb nematodes) are small worm-like animals. The eelworm feeds within plant tissue often causing distortion and dieback. They are sometimes found on leaves but are mostly in stems, bulbs and tubers.
The main symptom on onions, garlics and leeks is that the bulbs tend to swell and distort. This is sometimes referred to as ‘onion blout’, ultimately the bulbs rot, crack and die.
There is no treatment for eelworm and the damaged plants should be disposed of – not on the compost heap – and no members of the allium family planted on the soil for at least 3 years.
A full description of this disease, its symptoms and what to do when you find it is given in the RHS website