Black Foliage

Black Foliage
Dying potted roses, please help?

I have two potted rose bushes that are nearly two years old. I brought them in for the winter when the weather turned cold and they promptly started growing again, as they did last winter. However, the foliage has now died down and the stems are turning black in a way that looks like it may be a disease. They’re in an indoor room with a steady temp of about 65-70 degrees and one window giving them indirect, filtered sunlight. One appears to have some sort of bright yellow mold or algae growing on the soil near the base of the plant. What can I do to save my roses?
The pots are very large (I have to use a dolly to move them as it is, the pots are made for small trees) and the roses have been correctly watered — as I said I’ve had them for nearly two years. I’m thinking they have a disease that is causing the black stems, but I don’t know how to treat them or what to use.

bigger pot, sand in the soil, only water when very dry then water very wet then wait until dry again. also i use my coffee grounds once a month on top of the soil.