I have four varities of Astrantia, major, ruby wedding, un named and hapsen blood. Every year they put on a wonderful show but then their leaves start to develop black spots and then turn yellow and die. The hapsen blood is not as affected as the others. At the moment there is a space in the border where the foliage was but they have produced a second flush of flowers. Is there anything I can do to prevent this happening or will I just have to put up with it?