Touch wood so far i have never had white rot, this could be for a number of reason. I don,t allow any one to walk on my garden
I don,t grow garlic in the ground, I believe that garlic is the main course of white rot.
Watering
This comes down to weather, i am not a lover of using tap water on the garden i find it bakes the soil, also there is nothing in it that will feed the plant. For me its an evil necessity so i will water after a week of no rain.
I have good drainage and prepare my onion bed in the Autumn giving it a good covering of lime.
My main types of onion are Sturon, Centurion from sets along with others from seed i don,t really pay any attention as to them being oval or flat.
The question was What liquid feed. I have no fancy for trying to give each onion an individual feed, my feed goes in when the bed is prepared. If i was to give them a liquid feed then it would be a watered solution of Fish Blood and Bone.
My thoughts on white rot.....do we really know what causes it. We associate rot with to much dampness, if you get it you cant grow onions for at least eight years, it spreads very easily, and its heart breaking.
As i stated i believe the cause is planting untreated Garlic bulbs into the soil.