Milk may indeed inhibit mildew.. I do not know.
But plants cannot gain any nutrients from milk. They are plants, not baby cows.
The water in the milk is utilized as water.
Unless you have a dairy farm with lots of unsold milk to dispose of, this seems like a huge waste of money.
I think this technique is part science and part myth personally. For example - when I was in high school I worked in the meat & deli department of an IGA market, and every couple of months a Portuguese gentleman would come in and buy the fattiest piece of salt pork he could find.
To feed to his fig trees.
Logic dictates that all he was doing was fertilizing the ground with decaying meat, and that any commercial fertilizer would have a similar effect without making ones garden reek of death... But for whatever reason it seemed to work, as he managed to grow many many figs.
BTW, I can only imagine he had a greenhouse to grow them in - as there is no way in hell anyone in New England is going to grow any figs naturally.. That greenhouse must have smelled
awesome..