Did you see the white fungus on the leaves, stems, or fruit? Because I can't. But powdery mildew is a good guess especially in southern CA, the only 10a climate where one can grow grapes easily.

If it is mildew you may need a better fungicide. I don't think copper is the right choice for mildew. It could also just be dryness or low nitrogen since it seems mainly on the older leaves.

UC Davis has excellent info on this and any other pest of fruits, see below.

Here is a link that might be useful: powdery mildew