Its good to water foilage but at certain times of the day. I know my maple can get that white mildew so i try not to.if u do it alot i would think its not good especially if water sits through the night. My other trees like chinese elm or my conifers i do more often but i make sure the water will evaporate by night .My satsuki azalea also benefits from watering the foilage. Im not a proffessional but ive been studying the last 3 years straight. I would say to leave the leaves on but i dont want to give wrong info. Maybe its even possibly to rub it off. I would spray maybe once a week till it goes away and let the trees drop the leaves cause fall is coming