Thanks so much for writing about your rose! This has been a trying spring for roses!

From the photos this looks like Black Spot, but could be Cercospora leaf spot https://gardeningsolutions.ifas.ufl.edu/care/pests-and-diseases/diseases/black-spot-vs-cercospora-leaf-spot-on-roses/

And this link shows rose diseases: https://extension.umd.edu/resource/rose-identify-and-manage-problems/

Although some varieties of roses have some resistance, this spring has had so much rain that even carefully grown resistance varieties could be affected.

One first step would be to remove and discard any affective leaves and branches. Repeat this process every few days. Black spot, for example, is caused by a fungus found in the soil.

1. Since this rose is in a container, as a control consider discarding the mulch and carefully removing the soil top surface layer of soil. Replace the soil with fresh soil or compost and then mulch.

2. Strive to keep the leaves dry to prevent further infection. Some control methods are watering the soil, not the plant, thus keeping the leaves dry. (If possible, use a fan and/or move the plant to a location where it will still receive sun, with plenty of ventilation but stay dry during rains, unlikely?) The organism needs to be wet for at least 7 hours to germinate, so keeping the leaves dry can prevent further infection. 

3. Do not fertilize.

For a more complete list: https://extension.umd.edu/resource/black-spot-disease-roses/

Although Bacillus amyloliquefaciens strain D747, a biofungicide sold under different labels, is better applied (spray) as a preventative measure, it might still help a bit. And good to know about for next year.

One great local resource specific to roses is the Potomac Rose Society, some of its members are rosarians,  https://potomacrose.org/

Please keep us posted!

Thanks,

KatieMac, Master Gardener volunteer