Luke,
I get powdery mildew on my roses because of the humidity and water hitting the leaves. I have used milk to get rid of it. Not 100% effective but it helped a lot.
Try it but apply it early in the morning or late at night with a spray bottle. The focus is not to apply it under the sun. Need it to dry off on the leaves without the help from the sun.
Wash it off after a week or so but early in the morning so the leaves will dry off before the sun gets too hot.
Good luck and hope this will help.
Edit: Greenfig was right. Not mildew on Lukes. Martins,,,,???