Black spot of Roses

Black spot is the most serious disease of roses. It is caused by a fungus, Diplocarpon rosae, which infects the leaves and greatly reduces plant vigour. Expect to see leaf markings from spring, which will persist as long as the leaves remain on the plant.
The fungus is genetically very diverse and new strains arise rapidly. Unfortunately, this means that the resistance bred into new varieties usually fails to last because new strains of the fungus arise to overcome it.
Symptoms. These are variable, depending on the rose variety and the strain of the fungus.
You may see the following symptoms:
Typically, a rapidly enlarging purplish or black patch appears on the upper leaf surface, with diffuse and radiating strands of the fungus sometimes just visible.
Leaf tissues may turn yellow around the spots and the leaf often drops, even though other parts are as yet unaffected
At other times, the yellow colour does not appear, but infected leaves still drop
Sometimes, the spots remain relatively small and the leaf does not drop
Small, black, scabby lesions may also appear on young stems
Badly affected plants can shed almost all their leaves and their vigour is greatly reduced. The symptoms are so severe that, anecdotally, the disease has been blamed for a decline in the popularity of roses in UK gardens in recent decades.