Answer: I can't really diagnose the problem without being able to see it and inspect the plant so I can only offer some general advise. Reddish leaves are usually showing signs of climate stress or a need for fertilizer; whitish coloring can either be sunscald or a fungal disease called powdery mildew. Clematis are subject to lots of different fungal diseases, especially if they are overcrowded or the weather is unseasonably cool. Since you've watered and fertilized there's not much to do in that department. You can take a few of the leaves to your local cooperative extension for positive diagnosis and control strategies. If it is a fungal disease any general purpose garden fungicide should help protect the unaffected leaves. Be sure to rake and remove any fallen leaves and plan on pruning your plants back next spring (or after they have finished flowering) to remove the damaged plant material. Best wishes with your collection of clematis.
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