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Purplish-black blotches appear on the leaves, which then start to yellow and fall early. In severe cases the stems also become affected.
A fungus.
Badly affected plants may die as they are so seriously weakened they cannot withstand the winter frosts. It is very unsightly.
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.