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Malirechen--cocoa shell is extremely toxic to animals, particularly dogs. Please don't use it.
I use Rose Clear, as soon as possible in the year, when the roses are coming into leaf, you need to spray several time during the season, full instructions are on the bottle. Also be very vigilant clearing up all leaves that drop. It will help but some roses are more prone than others to blackspot. I don't like using chemicals but in some case I just have to or else my roses would just have bare stems.
Roseclear is a good product but it doesn't cure blackspot, it only holds it back. As already said, it does need spraying regularly from the time that the leaves emerge for maximum effect - one or two sprays will do nothing.
Pruning - have a close look at the stems of roses now and cut out any black stains that you see on them - this is the infection waiting to move in next year. Unless you see stains there is no point to pruning roses down to the ground, it won't help. Instead, you should concentrate on removing all fallen leaves, and a good mulch also helps.
Sheila3, roses are very hungry feeders. The fact that the roses were better for a short time after feeding suggests that they need a lot more food and are very hungry.
Probably closing the stable door after the horse has bolted, but try to plant disease resistant roses. We visited Rosemoor NHS Garden in June and the rose expert there said he doesn't plant roses that are prone to disease and that some are far more disease resistant than others. The roses there were fantastic.