Purplish-black blotches appear on the leaves, which then start to yellow and fall early. In severe cases the stems also become affected.
Cause
A fungus.
Harm
Badly affected plants may die as they are so seriously weakened they cannot withstand the winter frosts. It is very unsightly.
Treatment
Organic – This fungus over winters on fallen leaves and stem lesions, so it is essential to clear away leaves as they fall and prune out any damaged branches in the spring. It is important to keep roses as healthy as possible so they can withstand attacks. So mulch with a good 3 inch layer of well rotted farmyard manure around the base of the plants in the autumn and spring. It is also worth searching out roses that show resistance to black spot.
Chemical - spray with a fungicide. It is worth using this as a preventative rather than as a control, because once black spot has taken hold it is difficult to eradicate. So you can start spraying in the spring as the buds start to break, but before the leaves unfurl.