Hydrangeas (Hydrangea spp.) are deciduous shrubs that have beautiful flowers and foliage. Fungal diseases attack hydrangea plants and have a significant impact on their appearance. Fungal diseases can be treated if you become familiar with their symptoms and identify the disease. Do not leave them untreated, as the disease will greatly reduce the health of your plant.
Cercospora Leaf Spot
Cercospora leaf spot (Cercospora hydrangea) is a fungal disease that occurs on hydrangeas in the landscape but rarely in potted hydrangeas. It will not kill the plant, but it causes heavy brown or purple spotting on the leaves and reduces the plant’s vigor. The spots typically are 1/8- to ¼-inch in diameter. Heavily spotted leaves will fall prematurely. Diseased leaves should be removed and burned, as they will carry spores and spread to healthy parts of the plant. Cercospora leaf spot can be controlled with a fungicide spray. Azoxystrobin should be sprayed, per label instructions, at the first sign of the disease and repeated at 14- to 28-day intervals. Chlorothalonil should be applied, per label instructions, when symptoms first appear on lower leaves.
Anthracnose (Colletotrichum gloeosporioides) is a fungal disease that attacks leaves and blooms of hydrangea plants. The disease favors hot, wet weather. It is spread by rainfall and moist conditions. Symptoms are large, dark-brown irregular blotches. The plant loses vigor with this disease, and affected parts should be removed and burned. Fungicides should be applied at 10- to 14-day intervals during the summer. Chlorothalonil or thiophanate-methyl should be applied, per label instructions, when symptoms first appear on lower leaves.
Botrytis blight (Botrytis cinerea) is a gray, mold fungal disease that attacks hydrangeas after several days of cloudy, humid and rainy weather. Under favorable conditions, the mold can develop overnight. Mold symptoms usually are seen on flower buds and petals, and flowers brown and die. Diseased flowers should be removed and burned. Protective fungicides will control botrytis blight, and treatment should start when flowering begins and repeated when conditions are favorable for the disease. Iprodione and thiophanate-methyl should be sprayed according to the label directions.