Hydrangea plants that suffer from leaf spot fungus display dots and large blotches on their leaves. If your hydrangeas have a dark dot within a splotch, you've got one of the leaf spot fungi.
The common types of leaf spot fungus that affect hydrangea are Phyllosticta, Colletotrichum and Cercospora. All have similar signs and symptoms.
Fungus in hydrangea is often caused by wet, warm weather and poor air circulation. Hydrangea plants with water sitting on their leaves or flowers are more susceptible to fungus.
Blotches on affected leaves may appear brown, grey or purple. Severely spotted leaves will turn yellow and fall; affected leaves can spread fungus to healthy leaves when they fall, intensifying the infection.
Remove any affected leaves or blossoms and sweep or rake fallen growth from the ground to remove fungus from the area. Spray with a copper fungicide for an organic treatment.
Spray hydrangea with a sulphur-based fungicide that is approved for your area. Fungicides will curtail an infection but will not cure leaves already infected with fungal leaf spot.