If you notice feathery-edged black spots on the leaves of your rose plants, you've got blackspot, a fungal disease caused by Diplocarpon rosae. The fungus thrives in warm (above 75 degrees Fahrenheit), humid weather, especially on plants that have wet leaves. The fungus rarely bothers rose gardens in dry Western areas, but is prevalent in humid, coastal areas of the United States. As the disease spreads, the leaves turn yellow and drop from the plant. If left untreated, blackspot kills rose plants and can spread to other roses in your garden.
Remove all diseased leaves by pinching off the leaves at the base of the cane with your fingers. Pick up any diseased leaves lying on the soil. Discard the leaves promptly in your trash can and wash your hands.
Mix the baking soda, horticultural oil and water in a bucket, stirring with a spoon until the baking soda is dissolved.
Pour the mixture into your sprayer. Make half a batch and pour into a spray bottle if you are only treating one or two plants.
Spray the mixture onto your infected plants, coating each leaf thoroughly. Coat the undersides of the leaves as well as the tops. Apply the mixture in the morning every seven days until no more black spots appear.