Some hydrangea leaf diseases include anthracnose, powdery mildew, rust and yellow leaves. These diseases occur most commonly during the summer months and are usually treatable.
Brown, yellow or black spots on hydrangea leaves may be anthracnose. Anthracnose can be fatal to hydrangeas, so it is important to prune and destroy the diseased sections of the plant. Powdery mildew generally does not kill hydrangeas, but it causes an unpleasant powdery coating to form on the plant.
Prune and remove all affected areas. Rust is a fungal disease forming round orange spots on hydrangea leaves. Rust spores are spread through water, so avoid splashing plant leaves unnecessarily to prevent disease formation. Yellow leaves are usually caused by lack of iron. Add chelated iron to the garden to lower the soil's PH balance while making more iron available to plants.