This has come up; here is our suggestion from below: There are a few different fungal diseases that can cause spotting on hydrangea leaves, including black spot, powdery mildew, and cercospora. They will not kill the shrub, but it is good to try to control spread. Treating any of them includes removing diseased leaves from the plant and the soil. Once the soil surface is clean, put down (and maintain) a very thick layer of compost mulch to suppress disease spores, which are airborne. Then, follow instructions on how and when to apply neem oil.
This has come up; here is our suggestion from below: There are a few different fungal diseases that can cause spotting on hydrangea leaves, including black spot, powdery mildew, and cercospora. They will not kill the shrub, but it is good to try to control spread. Treating any of them includes removing diseased leaves from the plant and the soil. Once the soil surface is clean, put down (and maintain) a very thick layer of compost mulch to suppress disease spores, which are airborne. Then, follow instructions on how and when to apply neem oil.