Here is what I like to do:
Option 1: Use 1/4 to 1/2 cup bleach to 1 gallon to water and use a garden spray to spray the area. This will give you just enough bleach to kill the mildew and you won't have to worry about staining your roof or killing the plants below.
Option 2: Use Shingle Shield. It is a premade mixture that you use in a garden spray that kills mildew on your roof.
When you are done look into getting a zinc strip for the peak of your roof. Shingle Shield also makes zinc strips.
When you put a zinc strip at the top of your roof, as rain hits the strip and washes down it kills the mildew it runs over. This keeps the mildew from coming back.
I hope this helps.