I used a milk/water mix on one of my Crape Myrtles some time ago. Unfortunately, I then forgot to recheck if that mix helped until more than a year afterwards! LOL! Hee hee hee!

I haven't tried it but mine are suffering from powdery mildew... So I may need to give it a try. Will let you know if I do and if it works.

This is essentially the same as the Cornell Formula, a fungicide developed by a plant pathologist at Cornell University some 30-40 years ago. Unfortunately, via word of mouth and the Internet, the precise formulation has morphed into something other than the original and these 'revised' recipes may not have the same impact and can even become harmful to plants.

The correct formulation is 2-3 tsp. of baking soda (sodium bicarbonate) to 1G of water with 1 tsp. insecticidal or castile soap (not dish soap or detergent) and/or 1 tsp. horticultural oil (vegetable oil works as well). The baking soda actually alters the pH of the leaf surface, making it inhospitable to the survival of the fungal spores (milk accomplishes much the same process). The oil and soap act as surfactants, helping the mixture adhere to the foliage. You do not necessarily need to add both, however. It is as effective as any other commercial fungicide when mixed and applied properly

Newer research has indicated that potassium bicarbonate is somewhat more effective than the baking soda - baking soda just is a lot simpler to get one's hands on. And it is important to remember that like virtually all other fungicides, this is a preventative, not a curative. It can help to keep the disease from spreading to unaffected foliage but will not remove or eliminate any existing problems.

And it is always a good practice to combine a spray for powdery mildew together with good cultural controls that limit the incidence of the disease in the first place.