If you mean you have leaf blight, which is a botrytis, the only way to treat it is to use general cleaning up methods, like removing and destroying tops and leaves, and not growing any other members of the onion family, like shallots, garlic, chives, leeks etc. nearby.
Then you will need to apply a fungicide to the soil and the growing onions at regular intervals or have a break from growing any members of the onion family for two or three years.
However, if you mean onion white rot, this is far more serious and cannot be eradicated.
All you can do in this case is grow onions in containers into which you have put clean soil or compost from another site.