PlantLife Online

Answer

There are two main fungi that infect hydrangeas and cause spots on leaves cercospora and anthracnose. Cercospora leaf spots are scattered, small, circular spots that appear black, brown, or purple; spots are first visible on the leaves near the base of your hydrangea and work up wards. Cercospora spots are generally a quarter of an inch in diameter or smaller. Anthracnose leaf spots are circular and sunken into the leaf they appear black or brown. The dark and lighter rings of dead leaf areas can resemble a bull's-eye, the center of the spots may reach a diameter of approximately 1 inch. Once you determine the which fungal spore is the culprit you can begin to treat with the appropriate fungicide. If you are having a trouble diagnosing the spots take a damaged leaf in a plastic bag to your local garden center a good nursery should help you and point out the correct treatment. However before applying the fungicide a few steps need to be take for the best results. 

The first step before treating with a fungicide is to remove any infected foliage and flowers and leaf liter around the plant. Fallen diseased leaves are primary source of the spores which cause the fungal diseases. 

The second step would be to fertilize with a hydrangea specific fertilizer or one with higher nitrogen and give the plant sufficient water on the soil surface, avoid watering the foliage.

Third step, treat with fungicide if you feel the spread of disease requires action.