I am surprised that no-one has answered this question. I know very little about clematis but there are various sites on the internet that will tell you, one is the following, just copy and paste the site into your browser. Otherwise I would suggest a question at the garden centre or a trip to the library!
http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/clematis-diseases

or this

http://www.clematisinternational.com/beglist.html

http://www.clematis.hull.ac.uk/new-disease-leafspot.cfm