It is not a very efficient way to fight PM.

I heard good reviews on Spectracide Immunox fungicide.

PM fungus is not only on the surface, it gets inside the leaf tissue. Definitely remove blossoms and the most affected leaves.

Thanks for the fungicide suggestion, I will look into that for the future.

I agree, milk is probably not as effective as other things.

I did spray once, and things looked better. Perhaps it just made them look better.

I did a second spray to follow up, at about 2 weeks.

But the milk leaves a residue on the leaf. I have bathed the plants under running water, gently rubbing with my fingers. But the milk stains are still there.

Couple of points - powdery mildew is a surface issue only. It does not penetrate plant tissue and on smooth leaved plants, you can actually wipe or wash it off. But if left in place, it can alter photosynthesis and transpiration, which can cause both foliage and flowers to wither and die off.

And milk has been shown to be an effective fungicide for control of powdery mildew in a number of scientific studies. Part of the effectiveness is attributed to lactic acid which alters the pH of the leaf surface to a degree that is inhospitable to the development of the mildew spores. It also has antibacterial and antiviral properties that seem to be effective against a range of plant pathogens.

But like most fungicides, especially those addressing PM, it will work best as a preventative, spraying before you see the symptoms. Virtually all fungicides, even home remedy ones, are prophylactic in nature, not curative. As commercial chemical fungicides tend to be some of the more toxic home pesticides, I don't care to use these indoors (I seldom even will resort to them outdoors!!). If you don't find the milk working for you - and it should, remembering that it is only preventive - then try something like neem oil