Answer: Various fungal spots may develop on hydrangea leaves. These are triggered by too much or too little water or inconsistent soil moisture. Spots also may develop when hot sun strikes wet foliage. You can't do anything about rain, but you can water the soil, not the foliage.
Fungicides such as neem oil, available at garden centers, can be effective as a foliar spray. Be sure to remove fallen, diseased leaves.
Hydrangeas are best in organically enriched, well-draining but moist soils. Good air circulation discourages fungal diseases. Some sun promotes blooms, but hot afternoon rays are no friend to mophead types; and with less sun, they require less watering.
Q&A Library Searching Tips