The foliage wilts turning black and limp, followed by die back of the whole plant. Clematis are particularly susceptible.
Various fungi which are either soil borne or which have entered the plants through pruning cuts or other wounds.
There are no chemical controls. Affected plants should be dug up and destroyed. With clematis you can try to prevent damage by planting the rootball about 8cm (3in) below the level they were in the pot. This means that there are still some buds buried underground that remain viable after a wilt attack and can grow even if the rest of the plant has died. Cut back the wilted clematis to ground level and wait for new shoots from below ground.