Posted: 10/09/2013 at 21:58
Waterbutts´ advice is excellent.
My roses suffer from black spot (along with a myriad of fungi...) because
of my garden´s poor drainage. A solution of copper sulphate is easily
available at garden centres, and does well against this stubborn disease
(be stubborn yourself, spraying every week).
Remember the black spot´s spores remain on the damp ground ready to
spring back to the plants once it rains or the plants are watered; do not allow
any fallen (diseased) leaves there. If you find an inert sort of mulching (like
pine bark), it will help prevent this "boomerang" effect.