Today 
Clematis wilt is caused by a fungus that enters the plant through a wound made by an insect or an abrasion, such as rubbing from a plant tie. It's spread by water splash, and blocks the uptake of water in the stems, causing instant collapse. Infected foliage must be disposed of immediately, as the fungus can survive on the plant if left lying on the ground. The early, large-flowering varieties are most prone to attack, with smaller-flowering species (eg, C. montana) being the most resistant.
Solution
Organic
Cut back affected stems to healthy growth, even if this means to below ground level, and the clematis should send up new shoots. Bin the infected material, don't compost it. If the problem recurs, replant in rich, fertile, well-drained soil, with the top of the rootball 8cm below ground. Avoid stressing the plant by keeping it well watered and shading the roots - try covering over the root area with slates or stones to keep it cool.
Chemical
There's no ready-made chemical treatment, but spraying the soil around the plant with a fungicide containing penconazole or myclobutanil can work.
