Onion White Rot
This fungus causes the leaves to wilt and turn yellow. Under wet conditions, the plants may not wilt but will become loose in the soil. If you lift the plants, you will see a white fluffy growth on the base.
Onions, garlic and leeks causing wilting and yellowing foliage. It may persist in the soil for many years.
Throw out any infected plants, and don’t grow onions, garlic or leeks in that spot again for at least eight years. This is a very persistent fungus that survives in the soil for a long time. There is no chemical control.