MIldew, fungus everywhere on trees, wooden fences!
No. There are many different varieties of fungus, some are of concern, but most are not.
You may be seeing some lichens on trees and fences, and they are completely harmless and a sign of good air quality.
Here is a page about them: https://extension.umd.edu/hgic/lichens-trees-and-shrubs
For your vegetables, refer to these pages on vegetable plant profiles, and see the "common problems" for each of them: https://extension.umd.edu/hgic/plants/vegetable-crops
Japanese maples can have many problems. While you may see some powdery mildew, it's likely there are other problems that are of greater concern. If you'd like to send us photos of what is going on, we'd be happy to take a look.