Black spot is a fungus that only infests roses. The disease occurs in places that get a lot of rain in the summertime and have high humidity levels. Black spot first appears as dark circles on new rose leaves. The circles, which are irregularly shaped, may have yellow rings around them. The fungus kills the leaves, which then fall off the rose. A badly-infested plant may lose all its leaves, which can kill it.
Treat your infested roses with a mixture of 2 tsp. baking soda and an equal amount of summer oil, combined with 1 gallon of water. Mix the ingredients together, put the liquid into a spray bottle and spray the roses well. Apply this mixture once a week for best results.
Treat your infested roses with homemade soap spray. Mix ½ tsp. of dish soap, 1 tsp. of cooking oil and 1 qt. of water. Put the combination into a spray bottle and apply it to the roses. After the initial treatment, spray the roses again once a week until the infestation is gone.
Treat black spot with calcium sulphide or calcium polysulphide, usually sold as a concentrate and labeled "lime sulfur." Shake the container of concentrate well, mix it with water according to the directions on the packaging and spray it on your roses. Reapply according to the manufacturer's directions.
Apply a stronger chemical fungicide such as triforine, chlorothalonil or thiophanate-methyl. Follow the package directions very carefully, and wear a breathing device, protective clothing and goggles.