What do you treat the ground under and around roses that have been infected with black spot?
Ground Treatment for Black Spot Spores
Here is a link that will help you out with this >> https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm
I recommend just cleaning the fallen foliage away from the base of the roses and prune thing up a bit at the bases if need be, to allow for good air movement. Keeping the ground surface clean and a bit dry will go a long way to clearing up black spot problems. When I have had bad attacks of black spot I have sprayed the roses and the ground around them with Mancozeb, it works great on black spot. I get mine from http://www.rosemania.com .
Stan the Roseman