Lack of water can also cause brown, brittle spots on leaf edges, and a few branches all on one side may die out.
25+ Hydrangea Leaves Turning Brown Spots. Wash off pests with a direct stream of water. Fungi and bacteria that cause brown spots on hydrangea leaves spread from fallen diseased leaves or pruned parts of diseased plants;
What S Wrong With My Plant Garden University Of Minnesota Extension from apps.extension.umn.edu
All hydrangeas will turn brown if they wilt too many times in hot weather. A hydrangea with brown leaf margins is likely suffering from a cultural. Usually this occurs when automated sprinkler spray isn't reaching the entire hydrangea plant.
Water splashed on the leaves when watering the plant.
Cercospora leaf spot typically appears in late summer and fall. Often, affected leaves turn yellowish green and fall off, although the plant usually survives. Water slowly and deeply to moisten the soil several inches deep. Check occasionally for pests such as aphids and scale insects, which can cause leaves to shrivel and turn brown.
