RW mentions a specific leaf spot fungus of Daphnes in his book.
I guess good air circulation to keep the leaves as dry as possible would help, if the sprays aren't helping.
Mine are under glass, and their leaves never get wet, and have never experienced this problem myself.
There is a stunning pink 'sport' of 'Eternal Fragrance' in the pipeline
, I don't know when it will be generally available though.
I saw it in the Summer - it is fantastic.