By Tanim
United States 
I have had a simular issue with Clematis discolor. When I moved it too a dryer, more sharply drained soil, the disease dissapeared entirely. This disease is certainly fungal and will destroy ever leaf on your plant. Try a dryer area with as much sun as possible. Also remove and dispose of any diseased leaves for the entire growing season.
Thanks, Calvin
7 Jun, 2017
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7 Jun, 2017