Ina,
The HDRA did trials on the milk water mix a couple of years ago and found that it was effective :D - of course thanks to the EU, no one is allowed to use milk and water mix on their plants to prevent mildew >:( ???
If, on the other hand, you choose to spray a 10:1 mix of water and milk on to the leaves of mildew prone plants as a growth stimulant, they can't touch you for it!!!!
;D