Experts and BHG readers answer.
Black Spots on Hydrangeas
All plants can be struck with fungal and bacterial diseases like the ones you're dealing with on your Hydrangeas. It's important to give your plants a lot of breathing room; plenty of air circulation can keep the powdery mildew away. Then in the fall, remove all fallen leaves and plant debris from around the base of your Hydrangeas so the pathogens won't have a cozy place to overwinter.