First make sure you do not have an insect infestation by checking your Hydrangeas at night with a flashlight to look for insects.

In addition, hydrangeas can be susceptible to several leaf spots the leaf spots can be common in wet seasons or in garden beds with regular irrigation that dampens the foliage. Hydrangeas and different cultivars vary in their susceptibility.

Avoid overhead watering if you have a sprinkler system and remove heavily spotted foliage during the growing season. However, this is not practical if most of the leaves are affected. Rake up fallen foliage at the end of the season. These sanitation practices will help to manage the disease from year to year. Hydrangeas prefer dappled shade. Water the hydrangea during dry periods. Water deeply and let the soil dry slightly before watering again.

In general, no fungicides are recommended as you may have to spray all season. Fungicides should be a last resort since they may impact other organisms, but they are most effective when used before symptoms appear or as an infection is just beginning to manifest. Repeated applications will be necessary to maintain protection, though details of application will vary by product and the label directions should be followed.

Thank you for your question and have a great gardening season.