What to do about black spot on rose's leaves?

Here's an article that might help you - https://www.todayshomeowner.com/how-to-control-black-spot-fungus-disease-on-roses/

Here is a helpful article for you: https://www.thespruce.com/black-spot-on-roses-4125530

In the past I thought that the spotting was mildew from moisture. A Hort friend told me that the plant needs water! Try that!

it might be a fungus on the leaves,

First you need to determine what is causing the black spots. Sometimes I have had plants growing near the roses and they have black mold on them. I pull them up and dispose of them and pinch off every leaf or stem with the same black on it. Then make sure your roses get sunlight all day or nearly all day. Also, water only the ground around the base of the roses, i.e. a soaker hose or by hand, NOT the entire rose bush as much as possible. This should stop the black mold.