Hydrangea leaves may turn brown and develop a crispiness if the shrub is subjected to a number of stresses including drought conditions, salt burn and soil compaction. If salt burn is suspected, try thoroughly leaching salts out of the soil with a thorough, heavy watering. If overfertilization is suspected, reduce fertilizer applications and maintain a proper watering routine. To battle drought, apply mulch around the hydrangea to limit the amount of water lost from the soil due to evaporation.
Certain hydrangeas are susceptible to fungal rust, which causes brown or orange pustules to form on the leaves. These pustules are most noticeable on the undersides of leaves, where rubbing will release the orange spores created by the fungus. Leaves infected with rust become dry and brittle. Rust is most prevalent near the end of the growing season and is rarely fatal to the host plant. To treat fungal rust, prune infected branches, improve cultural practices and, if necessary, treat the plant with a sulfur spray.
Multiple species of the powdery mildew may cause portions of leaves to turn brown and kill young stems and flower stalks. Powdery mildew fungus can be recognized by the light-gray mold it produces on leaf undersides. Remove and destroy fallen plant debris regularly, especially in fall so that the fungus cannot overwinter near the plant. Prune out heavily infected portions and, if necessary, treat the plant with a sulfur application.
Fungal Leaf Spots
Several fungal pathogens may attack hydrangea leaves. Leaf spot symptoms consist of lesions that may be brown, depending on the specific fungus. Some degree of defoliation may occur, but damage is primarily an aesthetic concern. Prevent leaf spots by using good cultural practices, including avoiding getting leaves wet during watering. If necessary, apply fungicides to address the leaf spots.
Normal Seasonal Browning
Certain hydrangeas, as semideciduous shrubs, will have leaves turn brown and die in fall. Leaves will remain on the shrub until spring, when new growth emerges. The old leaves can be removed if they appear too unsightly. It's only cause for alarm if no new growth appears or if the new growth that emerges in spring is brown or turns brown prematurely.