Black spots on the leaves of hydrangeas are usually caused by a fungal infection named Cercospora hydrangeae. While these blackened spots are mainly an aesthetic issue, the fungus can reduce vigor in the affected hydrangeas and cause premature leaf drop.
Black spots caused by fungus typically start toward the bottom of a plant and slowly work upward until the entire plant is affected. Spots are characterized as circular and small and, in most cases, are scattered throughout each leaf. Minimizing leaf wetness by watering only at the ground can help control the spread of this fungal disease, as can removing dead and dying foliage from the plant and around its base.