Bacterial wilt may blight the flower clusters and leaves. The disease is worse after heavy rains and hot weather. If severe, wilting and root rot occur, followed by plant death. The causal organism is Pseudomonas solanacearum, and no chemical control is available.
Bud or flower blight infects dense flower clusters in wet weather or after frost. The disease is caused by Botrytis cinerea and can be controlled with sprays of benomyl.
Several genera of fungi cause leaf spots on Hydrangea. Use a spray of basic copper sulfate.
Powdery mildews in different genera cover the undersides of leaves with light gray mold. The leaves turn brown in spots and the upper leaf surfaces stay green or turn purplish brown. Young stems and flower stalks are infected and killed. Use sprays of benomyl or sulfur.
Rust causes rusty brown pustules on the leaves. The pustules are most noticeable on the undersides of leaves. Infected leaves dry up and become brittle. Sprays of sulfur control rust.