Never heard of an oak with powdery mildew. Doesn't look like it from the picture, but could the compost have been piled around the trunk? No chemical fertilizers were used too close to the tree?

Scarlet oak is vulnerable to gypsy moth caterpillars, Chloris and oak wilt, but it doesn't look like either.

Fortunately I wouldn't think powdery mildew would kill an oak. A healthy oak can probably be defoliated and have to make a second crop of leaves every few years and shrug it off.

If we are confident it is a scarlet oak and powdery mildew, hosing down the tree might help. However, this would actually make some other fungus worse, and dessert adapted Californian species wouldn't like that.

The main thing is: on such a large tree, any disease will be hard or expensive to treat. It should improve once the consistently hot weather arrives. Weather conditions are favorable some years for these types of pathogens and there isn't a lot you can do about it. Powdery mildew is seldom fatal on a large tree.

Not sure but the "stipling" could indicate spider mites. The fairly heavy rain CA has gotten may have stimulated such pests. If so, really nothing to do.

We're going to take some of the infected leaves to our UC extension to see if they may be able to help with a diagnose and treatment, if any. I hope it's something that will go away with the warmer weather because I'm not a fan of pesticides/fungicides and this tree sits close to a few Japanese maples that would be harmed if we had to spray. Not to mention my neighbor keeps bees that frequent our yard and I don't want anything to happen to them. I'll post results, if we get any. Thanks GW!

Just a couple of comments but spider mites need very dry, warm conditions to flourish - not wet. They are very unlikely to be encouraged by excess rain :-)

And powdery mildew is usually considered just a cosmetic issue on larger woody plants. It will not kill them. And the conditions don't seem quite right for that either - that is usually a dry soil/high humidity situation and based on this year's rainfall in CA, dry soils seem unlikely.

If only a few leaves affected, I'd be very inclined to leave it all alone :-) We seldom need to intervene much and the tendency to do so often causes more problems than it solves.

4th image suggests eriophyid gall mites. If so, disfiguring but not serious.

Let us know what you learn from the Extension Service office.