The solution seemed too simple, and too healthy to be effective. If you got any of this on your clothes or skin you would only get cleaner and suffer no ill effects, because it is only a soap solution. The solution is an alkaline base, which the plants thrive on and the black spot and powdery mildew hate.
Spraying on a weekly or 10 day rotation is a must. The same is true for the toxic sprays. It is recommended that you try it in a controlled area and see if you are happy with the results before you commit to an all garden test.
The simple solution is made by using bar Castile soap and water in a ration of ¼ bar to a quart of water. The soap should dissolve completely in the water and will look a silver milky color. Speed up the process by shaving the soap into the water. When the soap is dissolved you use 1/3 cup of the solution to 1 gallon of water. You don’t have to worry about too much spray on your plants as the dropping off also treats the ground where the black spot lives. This should be used up in a couple of weeks so use the extra on the soil around the plants. Castle soap can be bought in most stores.