Black Spot, Brown Spot, Leaf Rot or whatever else you want to call it,,, I would look at the reason for slow growing leaves and old branches, anaerobic soil conditions with old tired , less than enthusiastic roots... Gardeners spend a lot of time with their soil constantly improving the tilth...
Once you are satisfied that the tilth of the soil is adequate, then look at sun/shade and air circulation issues... What I do in my own garden when it is going to be a dry sunny day,,, I'll hose off the leaves allowing them to soak in water early in the day strenthen their defenses against the microbes that are having lunch on your bushes...
I agree with dKoester, in that you discover environmental/cultural
cause then treat...
