If you discover black spot on roses then the first thing to do is prune off the damaged part of the plant. Rake around the plant and burn any infected foliage. Don’t put the diseased plants on the compost, as the fungus will be recycled back into the garden.
Once you’ve removed the diseased parts from your rose bushes, you can use a fungicidal soap or sulphur to minimize the likelihood of further attack.
There are some popular home remedies if you prefer the organic approach.
For example, dissolve 1 teaspoon baking soda in a 250ml of water; add a few drops of liquid soap to the mix to help it cling better to the foliage and spray infected plants with rose black spot thoroughly.