Always spray before you see any mildew or rot. Once you see it, it's too late.

The sprays are preventative........they stop mildew from forming, but once it's there, they don't stop or kill it.

Also, spray before a rain, not after, for the same reason. You want the plants to be coated with spray (protected) before the rain comes.

UC Davis is the expert connection on all things fruit pest related in CA.

Here is a link that might be useful: grape PM