Clematis wilt is a common malady of almost all clematis (clemtaii? ). Firstly, clematis likes cool roots so try working in plenty of good peat moss around the root area. It likes lots of water but not wet feet... keep it well drained. Secondly, many commercial fungicides will work, but if you prefer a more eco-friendly help try baking soda... (sodium bicarbonate) will control powdery mildew and other mildews when sprayed on plants each week in a solution of 1 teaspoonful to 2 quarts of water. Additionally, any product made from Neem oil will control the fungus and is all natural... Best of luck!