Answer: A fungus causes black spot on roses. Some cultural practices may be helpful in controlling it. These include planting varieties resistant to it, ensuring your plants are in a location with good air circulation, avoiding wetting the leaves when watering, and cleaning up, removing and destroying any infected leaves. Do this especially well each fall to minimize reinfection from year to year. A clean layer of organic mulch (such as shredded bark or half finished compost or chopped leaves) applied before the plants leaf out again in spring should also help prevent reinfection.
Neem oil may also be helpful. At the current time, the only neem product I have seen that contains neem oil (rather than the ingredient azadirachtin) as the active ingredient is Green Light's Rose Defense. You may find there are others if you shop around, especially mail order sources. Neem controls aphids and mites on your roses, and the oil version of neem also will control powdery mildew and black spot.
Some gardeners have had success using baking soda and water sprays to control black spot. Tests in England indicated that 1.5 tablespoons of baking soda in 1 quart of water was effective. Add a few drops of dish detergent to help it stick. You may want to try it and see for yourself. Always test something new like this on a few leaves and wait a few days to see the results before spraying all of your plants.
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