Leaves can get sun scald
Q A hydrangea, planted this year, bore very healthy mophead style flower heads, but most of the leaves developed unsightly blotches.
A Your photo shows dark blotches on the leaves, some at the edges and others at the leaf centres. Tan, dead areas appear in the middle of the large blotches. This sort of damage could be caused by sunscald or a leaf spotting fungus — or both. Fungal leaf spot infections usually begin at midsummer and become most noticeable in au- tumn. Damage is most acute in years, like this one, with frequent late summer rains.
Hydrangea roots must never go dry.
Sun scald injury can cause similarly marked leaves, and set up a plant for fungal infections in the damaged leaf areas. Plants exposed to full summer heat and sun are subject to sun scald, as are those that are newly planted or watered in full sun. Water drops on the foliage act like magnifying glasses and cause burning.