Hydrangeas, with their attractive green leaves and clusters of flowers, grow relatively easily in most warm climates. The plants do, however, sometimes get brown spots on their leaves. These spots indicate a bacterial or fungal disease.
Cercospora leaf spot shows up as 1/4- to 1/8-inch brown or purplish spots on hydrangea leaves, and the spots eventually turn tan or gray in the center. Brown anthracnose spots are as large as an inch across and develop a bulls-eye appearance with rings of lighter and darker coloring. Bacterial leaf spot shows up as irregular reddish-brown spots that eventually spread into large patches of dead tissue.
All of these brown leaf spot diseases damage the leaves and reduce the amount of photosynthesis they can perform. Very severe infections can cause foliage drop and harm the overall health of the hydrangea plant.
Fungi and bacteria that cause brown spots on hydrangea leaves spread from fallen diseased leaves or pruned parts of diseased plants; therefore, gardeners should remove infected plant parts from the garden areas surrounding healthy plants. The Alabama Cooperative Extension Service also recommends reducing the moisture on plant foliage by watering at the roots and fungicides as ways to get rid of fungal spots. Michigan State University suggests pruning infected plant parts to prevent infections from spreading.