Powdery Mildew

• Powdery mildew is a fungal disease that afflicts many species of hydrangea, depositing a white, powdery fungus across the leaves of the plant. These white spots with feathery edges first appear on the lower surface of the leaves and may include yellow and purple blotches. Although the problem is primarily aesthetic, it does diminish the growth of the foliage. The fungal spores overwinter on the hydrangea and come to life in the spring during periods of wet and humid weather.
  
  

Cercospora Leaf Spot

• Cercospora leaf spot displays as circular purple or brown spots on leaves near the base of the hydrangea. Those spots then turn tan or light gray and measure up to one-fourth of an inch in diameter, before the leaves turn green and defoliate the plant. The fungus remains on fallen leaves and that litter should be removed and destroyed to prevent further spread of the disease. Fungicides are effective if applied when the leaf spot first appears.
  
  

Anthracnose

• Anthracnose, caused by the fungus Colletotrichum gloeosporioides, usually attacks the hydrangea when extremely hot and moist conditions occur. Hydrangeas that are overly-fertilized are especially susceptible to this disease that produces sunken brown spots of 1 inch or more on the leaves and stems of the plant. Anthracnose is characterized by alternating rings of dark and light tissue that give the spots the look of a bulls-eye. Fungicides to control the disease should be applied at intervals of 10 to 14 days.
  
  

Botrytis Blight

• Botrytis blight, also called gray mold, displays on the buds and blossoms of hydrangea and is especially prevalent on big leaf hydrangeas. Tiny, water-soaked spots first afflict the petals before reddish-brown spots occur. Petals decompose quickly into a brown, mottled clump covered by a collection of fuzzy, gray mold. As diseased sections of petals fall onto leaves, spots also develop on the foliage. Fungicides help to control this blight if applied when hydrangeas first begin to bloom and should be continued throughout the growing season.