If your clematis is turning black or brown, it is likely suffering from a common fungus plaguing clematis. Although it may look as if your clematis is dying or dead, you can save it more easily than you think.
The typical cause of a clematis turning brown and black is the fungus Ascochyta clematidina, commonly called "clematis wilt" or "clematis leaf and stem spot." Your clematis is a victim of wilt if the stem suddenly collapses, followed by the stem and the leaves turning black. Only individual stems will turn black, not the entire plant.
Wilt does not affect the roots; the clematis will continue to develop healthy new growth despite the fungus attacking the plant. Prune the diseased stems, cutting just under the black and wilted part. If the affected stem is close to the ground, prune as close to the ground as possible or even just under the soil. The plant recovers well if the affected areas are removed.
Other reasons for clematis plants turning black and brown include spider mites, excessive heat or a hard frost. Planting or relocating your clematis to a place where it gets sun, is protected from the worst of the afternoon heat, is planted in well-drained soil and has shaded roots may help. A general-use pesticide can eliminate spider mites.