A fungal disease, rose blackspot is the most serious problem of roses. It overwinters on dead leaves and infected shoots. In spring the emerging leaves are infected and the cycle re-commences. It causes black or brown spots on the leaves which often turn yellow and fall off. The plant is seriously weakened after a few years.
Collect and burn affected leaves in the autumn. Spray in late March or early April, with Roseclear, or Multirose to break the cycle. Continue spraying, at least once a month, until July, and longer in a wet summer. Some varieties are more susceptible. Alternate the spray chemical to reduce the chances of resistant strains building up.