Posted: 31/08/2013 at 09:32
Excellent advice from waterbutts. Additionally, you should be meticulous about clearing the fallen leaves from beneath the bushes and always burn or bin them - don't put them on the compost heap. Each Spring, put down a fresh layer of compost or manure under the bushes which will both feed them and cover any fungal spores which will be on the surface of the soil.
If you plant any more roses in the future, it would be wise to look for blackspot resistant varieties now that you know your area is susceptible to the disease.