if you live in Cornwall choose balck spot resistant varieties, as the warm damp conditions make BS flourish. Otherwise, cut off affected leaves and burn, ensure no dead leaves under plant which will carry the spores. As a last resort spray - garden centres will advise ob spray.

Follow Cornishchris's advice above, but recommend you adopt a spray regime according to instructions on the container with something like Fungus Clear Ultra for flowering plants or Roseclear Gun.