Answer: Clematis can be host to both fungal and bacterial leaf spots, which you can control by removing the affected leaves before the spores or pathogens move to other areas of the plants. I'd do that first to see if it helps. If not, the problem might be Clematis wilt.
Clematis wilt is a fungus which causes wilting and die-back of shoots in the large-flowering varieties of clematis. It affects plants just above ground level, and young leaves are affected first - eventually the whole stem turns brown and wilts. Stems can't recover, but it rarely affects the roots.
There is no cultural control, aside from cutting an affected stem down to soil level. Chemical control - treat with a systemic fungicide such as Benomyl two or three times at 10 day intervals as soon as the problem is noticed, and a new stem will grow.
Best wishes with your clematis!
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