Once you have black spot, nothing will take it away, but if you remove the affected leaves religiously, (pick up any that fall, too) and spray regularly with a baking soda and water solution (1/2tsp. soda to a quart of water) it can slow down or halt the spread of the fungus. Some people swear by spraying with milk, too. (but baking soda is cheaper) Both these just change the pH on the leaf surface to make it inhospitable to the fungal spores. They both will wash off if it rains, so be sure to re-apply after rain.
The other really important things especially in humid summer areas are to make sure there is plenty of space between your plants for air movement, and be sure to water early in the morning, so that the leaves will dry. Watering in the evening is a sure invitation to fungal diseases, if you wet the leaves and they stay wet over night. If you feel you need to water in the evenings, try not to wet the leaves! A drip or soaker system that does not wet the leaves is good, or using the hose to soak the soil without wetting the foliage will help to prevent the spread of the black spot.
