According to author and horticultural professor Jeff Gillman, who has conducted extensive research on blackspot remedies, a spray composed of one part milk and two parts water is the best answer to the disease. He says this simple solution, if applied weekly, will control blackspot as well as any synthetic fungicide, including Chlorotalonil.
Will try this for my roses this year: A tonic for black spot and powdery mildew on roses Roses, while beautiful, are often plagued with black spot or powdery mildew. I mix these ingredients into a tonic, which I spray on my roses: 2 teaspoons of baking soda and 1/2 teaspoon of liquid soap or Murphy's oil soap in 2 quarts of water. The tonic protects the roses for months.
Cider Vinegar Recipe for Black Spot - Mix 3 tablespoons of cider vinegar (I always buy Braggs as it has the mother culture) to 5 quarts ( 5 liters) of water. Add 1 teaspoon of vegetable oil and a good squirt of soft soap to make sure that your organic spray sticks to the leaves. Spray on plant early morning or evening to prevent leaf burn. Reapply as necessary, and always after it has rained, once the leaves are dry.