Treatment of black leaves can occur once you've identified the illness. Leaves with black spots caused by bacterial infections need different treatment than those with a fungal infection, and treating the wrong illness will most likely be ineffective. Mildews and molds require more in-depth treatment than spotting caused by over sunning or watering the hydrangea.
Infections from the Alternaria, Coniothyrium and Rhytisma species of fungi are among the most common causes of black leaves in hydrangeas. Depending on the specific type, these infections may produce black spots or turn whole leaves black. Most fungal problems require a multifaceted approach to rid the hydrangea and soil of problems. First, remove all affected leaves, stems and flowers to prevent further spread. Second, growers should apply an appropriate fungicide according to the packaging instructions. This process will stop the fungi from spreading to other plants, and will clean the soil so the bacteria don’t return.
Hydrangeas suffer from fewer bacterial infections than fungal infections, though some will still turn leaves black. If growing conditions are not optimal, especially if too much water is present, bacteria can overwhelm soil and leaves, creating conditions where necrotic spots will begin to turn leaves black. The most common bacterial infection is in the Pseudomonas family, though Xanthomonas and Corynebacterium are close behind. Each type can cause black leaves on a hydrangea. Most of these bacterial problems will begin in the soil and progress until the leaves have died and fallen. Cure the problem by removing and replacing contaminated soil and removing infected parts of the plant.
Over watering may be a serious problem for potted or outdoor grown hydrangeas. If water is allowed to sit at the roots, root rot may take hold. If left untreated, roots turn gray and mushy. As the disease progresses, leaves will turn dark brown or black at the edges before prematurely falling off. Hydrangeas require watering once per week, anymore than that is too much. Soil should promote good drainage to prevent bacterial growth and other problems.