Posted: 23/05/2014 at 13:53
Black spot is a fungus (Diplocarpon Rosae). Minor attacks will not harm the plant but the affected leaves should be removed and burned. Severe attacks will weaken the plant. There are various products you can buy to combat the fungus and one method from many years ago was to spray the ground with a solution of household disinfectant or colloidal copper spray. Many of the compounds used years ago have been discontinued but there are a fair few available in GCs such as 'Rose Clear'
Punkdoc: Peat does not necessarily prevent black spot - a book I use written by Ian G. Walls 'A-Z of Garden Pests and Problems' (1979) ISBN 0 907812 66 X recommends using Peat, spent hops, or grass mowings (providing the grass hasn't been treated with hormone weed killers) as a mulch to keep the plants healthy.
Lyn, the soil here in Bristol is towards alkaline so the amount of peat I use doesn't do much to turn the soil acidic and the roses seem quite happy. In fact a small azalea we have in the garden which does need acidic soil had to be replanted yesterday in ericacious compost because it definitely looked under the weather after OH moved it last year so I've added a fair amount of peat as a mulch around that too. Time will tell how it likes that. John H