The most serious disease of clematis is commonly called clematis wilt- a stem rot/leaf spot disease caused by the fungus, Ascochyta clematidina, which mainly affects large-flowered clematis hybrids. The symptoms are very defining and include sudden stem collapse, often just as the flower buds are about to open. Within just a few days the stems and its leaves turn black. Leaf veins often develop a purple coloration. One or more stems of the diseased plant may be affected or all aboveground parts of the plant can be killed down to just below the soil level.

Often, an affected plant will recover after a year or two. Of course, prevention is the best medicine. Here's how:

1. Choose a favorable planting site with 6 or more hours of sun daily, rich, well-drained soil with pH about 7.0, and be relatively open to ensure good air circulation.
2. Select the most resistant plants. Species clematis and/or small-flowered hybrids seem to be more resistant than large-flowered forms.
3. Keep the clematis area free of plant debris on a year-round basis. Take special care to avoid injury to stem and roots since the wilt fungus requires a wound to begin its deadly work.
4. Avoid damaging the roots. Do not cultivate the soil around clematis plants and maintain good mulch.
5. Give good culture. Maintain good growing conditions to keep all clematis plants as stress-free as possible.
6. In the event that plants become infected, cut diseased stems just below ground level and destroy them. Spray any remaining, healthy parts of the plant and surrounding soil with a protective fungicide such as myclobutanil.

Source