If you're saying you're definitely sure its actually Black Spot and not just marks on the leaves from a different problem of some kind, then spray with a fungicide.

i had the same problem last year, took off loads of infected leaves and sprayed throughout the season and mulched but alas i see it is just as bad some leaves turning yellow others staying green but lots of black spots sorry i can`t be any help i think alot of people had this problem last year