To treat roses with black spots, spray the plants with baking soda, sulfur products or neem oil. Insecticidal soaps with added fungicide may help counter the problem as well. Excess quantities of the compounds may burn the plants, so be sure to use correct amounts.Continue Reading
To treat with baking soda, put 1 teaspoon baking soda in a sprayer, add 1 teaspoon liquid soap, add 1/2 gallon warm water, mix the ingredients thoroughly, and apply the mix to the plants, working on one plant at a time. Spray the plants once per week. Baking soda creates an alkaline condition on roses, so it can work as a preventive measure as well.
Sulfur can treat and prevent black spots on roses. Purchase sulfur that readily mixes with water. Sulfur is toxic and corrodes metal, so wear appropriate protective clothing, and use a plastic sprayer when working with the chemical compound.
Like sulfur, don't use neem oil to treat the roses in hot weather. Wait for at least two weeks prior to using the oil if you applied sulfur before. To prevent the condition, plant roses that are resistant to mildew and rust. Don't plant the roses closely together, prune them regularly, and cut off any infected leaves. Applying mulch around the plants may also be helpful.Learn more about Outdoor Plants & Flowers