Black spots on roses are the result of a fungus called diplocarpon rosae. The fungus initially appears as a black spot on the leaves of the rose. As the fungus spreads, the leaf turns yellow and then brown until the leaf defoliates and falls from the branch. Remedies are available to prevent the fungus from attacking your roses.
In addition to using chemical-based commercial sprays, you can use natural home remedies to treat black spots on your roses. Spray the leaves using a baking soda solution: Mix 1 tsp baking soda in 1 qt. water with 3 drops liquid soap. Spray your rose bush, covering both sides of the leaves and branches.
Use manure tea to treat the black spots on your roses: Take 1 gallon composted manure in a 5 gallon bucket and fill it with water. Mix the manure thoroughly with the water and keep it in a warm place for three days. Strain the manure solution using a cheesecloth or strainer and use the "tea" that strains out as a spray for your rose bushes.
As a preventive measure, spray your rose bush with fungicidal soap and wettable sulphur, wetting the leaves and branches thoroughly. The fungal spores of diplocarpon rosae cannot thrive in sulphur and so will not infest your rose bush. Also, since black spot spreads more in warm weather, spray your rose bush at the start of spring to prevent the fungus from appearing. Sulphur washes off with water, so reapply as necessary.
Certain fungi thrive on water, so when watering your roses direct the water to the roots so the leaves stay dry, or water in the morning so the leaves dry as the temperature rises throughout the day.
Plant small rose bushes 3 feet apart and large rose bushes 4 feet apart to encourage adequate air circulation to prevent diseases. Also avoid planting your roses near walls or structures.
Pluck and dispose any leaves infected by black spot. You do not want any black-spotted leaves becoming part of the rose bush soil or compost as it can encourage germination in your current rose bush or roses planted later.