This is a disease that is often confused with bad culture, that is:
If the plant dies back, most people automatically think in terms of Clematis wilt, where in actual fact they might just be growing them under the wrong conditions.
For example; Clematis prefer moist fertile soil and shade at root/soil level.
Some varieties are more tolerant to the disease than others,for example C. montana.
Meaning selection of species might be the better option if ideal conditions are not available.
Clematis wilt is caused by a fungus that produces small spores that are spread by water from infected material.
These spores infect the leaves, causing the petioles (leaf stalks) to blacken, and then the fungus spreads to the stems and blackens the plant tissue resulting in the stems wilting and dying.
Prompt removal and destruction of infected stems may allow the plant to regrow from lower down the stem or below ground.
Prune the plant back to within 150mm (6") of its base, then spray the rest of the plant and soak the soil around the roots with a copper based fungicide.
As with many common garden diseases good hygiene can be the best form of control.
The fungus /spores can remain in dead plant material for many months so removal of this material may prevent reinfection.
There are no specific fungicides available to amateur gardeners for this disease, but the use of other fungicides may give some control.
Powdery mildew can strike in early autumn, this too can be controlled with a suitable fungicide.
Slugs and snails can also be a problem so treat accordingly.
Do not use these fungicides on the soil.