Powdery Mildew, most common, is a fungus and is carried on the wind. It can stay on the ground and be splashed onto the bush.
What can you do?
Don't water at night. If you get the foliage wet, make sure it has time to dry.
Give your bushes adequate ventilation by pruning so the center stays pretty clear for air to circulate.
Depending on the weather and where you live, spraying might be the only answer.
ck at your local garden center. I like systemic, because it lasts longer
Spray the underside of the leaves from the bottom to the top then give the top a shot.
I like to spray the area around the bush.
Good luck