POWDERY MILDEW is common on certain landscape trees and shrubs.
NORWAY MAPLES, especially the red or reddish leaved varieties are particularly susceptible. Crimson King and Schwedleri Norway maples are commonly affected.
This is a great article about powdery mildew on Norway maples.
1. This problem is primarily cosmetic, not harmful to health of maples.
2. The fungus that causes powdery mildew on Norway maples is not contagious to other species. However, the same conditions that promote mildew on Norway maple will promote powdery mildew on other plants
3. The spores are ubiquitous, so no measures to control will prevent it from affecting other Norway maples.
If someone has this every year and wants to try to control it, fungicides in June and July may prevent it. But that does not prevent it from going to other trees or plants, so spraying trees that are affected would not protect nearby trees (see 2 and 3 above)
Small plants with mildew can be sprayed with small sprayers and several different home remedies.