I have three beautiful clematis that grow up and over a large trellis. They grow well and are covered with buds and flowers every year. The problem is as soon as they get to their best they start dying from the bottom up. The leaves turn yellow then brown, at which point they start to fall off. I have removed the leaves as soon as I see it start to happen but it makes no difference. The roots are in the shade under slate and the tops are in sun.
Anyone have any ideas as to how to stop this happening and what could be causing it.