I have black spots on my hydrangea leaves. What can I do ?
Black spotting can occur on Hydrangea leaves from fungal diseases.
Treatment is from cultural, and chemical approaches.
Trim the largest and oldest stems each spring, after flowering, to open up the plants and allow more air flow.
Remove any dead or dropped plant material and dispose of.
Apply 2 inches of organic compost to the soil around the plant to help smother any fungal spores.
I would use a Neem Oil mixture on the plants; mixed 1/2 teaspoon Neem Oil to 1 pint water. Spray the entire plant both sides of leaves.
Spray the Hydrangea every 7 days.
Do not save the mixture as Neem Oil breaks down, mix new each time.