Prevention is better than cure and in the case of roses this is most definitely the case. The prevention programme starts in the autumn by clearing up all the fallen rose leaves and prunings, then burning them as they carry the spores for next years infection.
After pruning the plants should be sprayed with ‘Jeyes Fluid’ giving each plant a good soaking together with the soil around the base.
Mulching should to carried out around the plant before the new leaves appear, this stops the black spot spores in the soil being splashed back onto the new spring growth by rain drops. Add magnesium in the spring to strengthen the growth.