It is usually characterized by a complete collapse of either the entire plant, just one of the shoots, or just part of a shoot. The foliage will turn black and the veins take on a purple colour. Large-flowered cultivars are particularly susceptible.
A fungus. The spores are spread by water splash and the fungus can remain in the soil for many months in old infected plant tissue.
Leaves and shoots wilt and die back. Clematis wilt is rarely fatal to the plant. It might take up to a year, but the plant will usually sprout shoots from below soil level or from nodes beneath the wilted area.
Prune out the affected stems, cutting right back into healthy tissue – even cutting below soil level if necessary. There is no need to dig out the whole plant as new shoots may develop later in the season or the following spring.
Providing the Clematis is planted deep enough (you should plant it 15cm (6in) deeper than it was in the original container), there will be plenty of underground buds to produce new stems.
Wilt is always worse on plants under stress, so it's essential to plant the in good soil (loam based with plenty of humus) and keep the soil moist during dry weather.