For best results;
Soak brown rice in water, remove rice and mix that water with milk, wait 3 days for milk to curdle and remove the curds.
Add molasses, mix and dilute with water and spray on anything with powdery mildew.

It's the lactic acid bacteria, they especially like to eat lactose so it's incredibly eat to breed them from milk. The brown rice is a good source of the bacteria and none of the other bacteria on the rice can eat the lactose so the lactic acid bacteria quickly outbreeds everything else in the mix.
Molasses adds trace minerals and more sugars for the bacteria to eat while its living on the plants.

You can just spray straight milk on plants, it works, just less effectively.