If you are sure your roses have Black Spot disease, you'll need to spray them every 7 to 10 days with a rose fungicide. You can buy rose fungicides at most garden centers. Also, remove all diseased foliage and never water your plants from above. If this is a continuing problem in your garden, you might consider switching to shrub and landscape roses which are much less likely to have this problem. Hybrid teas and floribundas are often the main target of this disease.