Hydrangeas sometimes suffer irregular black, tan and brown foliage spotting. The foliage may wilt, appear distorted and defoliation may occur. The leaves and flowers of the plant look burnt. Foliage damage occurs from diseases, overexposure to intense sunlight, extended periods of drought, prolific insect feeding and over-application of fertilisers.
The fungal disease, anthracnose, caused by Colletotrichum gloeosporioides, particularly affects big leaf hydrangeas. The dark brown spots of the fungus appear somewhat circular and sunken on the leaf's surface. The lesions also may have a bull's-eye appearance. Cercospora leaf spot appears similar to anthrocnose on the plant's foliage, but heavy defoliation also occurs. Powdery mildew causes the leaves to take on a greyish, furred appearance. Botrytis blight causes the flowers to sport reddish spotting and the foliage to have greyish leaf spotting. Wilting often occurs.
Spider mites and aphids congregate on the new growth of hydrangeas and may damage the foliage. Four-lined plant bugs cause brown spotting on hydrangea foliage, according to the University of Florida IFAS Extension.
Hydrangeas do not do well in intense heat or in an exposed planting location. The plants prefer a shady, cool area. Hydrangeas require moist soil and do not tolerate drought well. The large leaves are especially susceptible to water loss, which causes the foliage to wilt in hot sunlight, even with adequate water. In intense summertime heat, hydrangeas benefit from mulch to keep their roots cooler.
Over-fertilising hydrangeas may dry out their roots, resulting in root burn and scorched-looking foliage. Extensive fertiliser use may even kill the plants. Fertilise hydrangeas once or twice during the summer months. Follow the directions on the fertiliser label to determine the correct amount to apply. Avoid fertilising after August so the plants can go dormant for the winter months, advises the Hydrangeas! Hydrangeas! website.