Posted: 28/02/2014 at 09:36
Roses get black spot because air quality is good Back in the days when everyone had coal fires and the air was sulphurous and smoggy and gave everyone bad chests, the roses didn't get black spot because the sulphur helped kill the spores that cause it.
Some roses are more resistant to black spot than others, so choose those if you can. When pruning keep the centre of the bush open to allow for good air flow. If you water your roses, water the soil not the leaves.
Don't over-fertilise the roses - grow them 'hard' using slow release fertiliser such as Fish Blood and Bone, as high nitrogen fertilisers will give rise to soft lush growth which will be more susceptible to infection - and every autumn when the leaves fall, rake them up and burn them.
There are also organic sulphur-based sprays that may help prevent infection, but the trick is to apply the sulphur before the spores have landed on the leaves. See here http://www.rosemagazine.com/articles02/pages/blackspot.asp
I hope that's helpful