Could be two things - check to see if you see ants crawling around up and down the stems, and aphids on the leaves. If not, then its probably just sooty mould, a fungal infection, you can buy sprays to treat it with. If there are ants and aphids, then you need to get rid of the aphids (the ants will then disappear) - you can either use the old dilute washing up liquid trick, or buy a spray that will treat both conditions, something like Roseclear 3, which will need to be mixed up in a sprayer of your own.