This fungal disease thrives in high-humidity areas with ample summer rain. It attacks exclusively roses. Young leaves show black, irregular circles, sometimes surrounded with a yellow halo, then drop from the plant. Heavy infestation can defoliate a plant, thus preventing it from building up nutrient reserves. In cold-weather regions, badly infected plants can become so depleted that they may not make it through the winter.
Prevent the disease by planting resistant rose varieties. Remove and destroy all diseased foliage in fall. Some gardeners have had good luck controlling black spot with weekly applications of a baking soda and summer oil spray; to make the solution, mix 2 teaspoons baking soda and 2 teaspoons summer oil with a gallon of water. Others report success with soap sprays or sulfur.
Chemical controls include chlorothalonil, triforine, and thiophanate-methyl. Repeat applications of spray will be needed as long as the weather conditions favor the development of the fungus.