For all bad fungus (powdery mildew, blackspot, and the killer Downey mildew, I use 3 cups cornmeal slightly dug or fluffed into the soil (so birds don’t eat it all) and watered in. In a hotter climate you might repeat 2x a year but this kills all soil spores and fixes the problem with no chemicals and no spraying. In a last ditch effort to save what was left of someone else’s potted roses, I tried this for Downey mildew. Nothing else had worked for me before, but this did and saved most of the affected roses. If you can get an agricultural grade (often sold in 30 and 50 lb sacks) it is significantly cheaper than grocery store stuff!