Both roses and hydrangeas are very prone to Cercospora leaf spot. I don't grow roses, but I have 115 hydrangeas on my property and fungal diseases are quite prevalent this year, and early. In Delaware, our hot and humid summers don't help at all. I don't know of any systemic treatments or I would use them. Roses are susceptible to black spot, which is far more harmful to the shrub than cercospora, and knockout roses, which were bred to be resistant to black spot, just get more cercospora. You already know about watering at the base. That's good. Here is how I am approaching this issue at my house. I am with you on the organic approach. I have a certified backyard habitat and use little if any chemicals, herbicides, pesticides etc preferring instead to use bio-controls however, hand weed, etc. I draw the line at fungus. I have too much invested in my hydrangeas. I am spraying with both Bonide "Captain Jack Copper" spray as well as "Bonide Fung-onil" which uses one of the recommended active ingredient, chlorothalonil. It is a broad spectrum fungicide. I hold one of each in my hands and spray with both. I tend to spray every week to 10 days, or the morning after a rain. I also spray the ground. In the fall, we will want to rake all of the fallen leaves under the shrub and destroy. Do not compost. Hygiene is crucial here as the fungus will overwinter on leaf tissue and return next year. I am also going to start my regimen early in 2025, when leaves begin to emerge - so sometime in April. Normally I don't see cercospora so early, I typically associate it with late summer, and some of my hydrangea are more susceptible than others. I will also spray the ground under the shrubs as well in early spring. Spraying will not remove or correct affected leaves but can substantially slow or prevent new spores from forming. Since starting my regimen early in June I feel I have controlled the spread. Fortunately, the disease is not fatal. Shrubs will still blossom but it is unsightly. If I am tenacious for the next two years, I should see a significant drop in the disease pressure going forward.