Certified GKH Gardening Expert
They can be a little finicky in container, but I would chalk this up to diseased soil, or soil that has been left wet and gone bacterial.
Vigorous fungicide treatment will be necessary, ACCORDING TO ALL LABELS ON THE PACKAGING. This article will help with fungicide use:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/ornamental/flowers/roses/container-roses-growing-roses-in-pots.htm
You may be watering the leaves instead of the soil and helping spread fungal diseases like black spot, etc.