Martha, see the post I linked below. It talks about fungus/mildew on clematis and what has worked best for me. Just make sure the clematis gets plenty of water in the hot dry weather we have been experiencing. You don't need to wait to do something. If the plant is severely affected by mildew and it were my plant, I would cut it back and begin treatment with some sort of fungicide on the new foliage that emerges after you cut it back.

Here is a link that might be useful: Fungicide thread

Thank you for the thread and advice. It does have fresh leaves that look unaffected. I may cut the ones that have mildew and leave the heathy leaves and spray.
Thanks again