The best thing I have found to do for black spot is to be ruthless. You
have to strip the bushs of all the older leaves that have spots. Then
you have to completely clean up and destroy the mulch around the base of
the bushs. Burn or dispose of everything, do not compost it. New healthy
leaves will grow back and you will get rid of the disease. The only
problem with this is that whatever is eating your leaves could have some
of the disease on it and spread it. If you suspect slugs or snails go
out at about 10:00 pm with a flash light and ALWAYS (day or night) get
rid of ANY slug or snail you find.

Hope this helps,
Teresa