Black spots on roses
I have no doubt your plants are suffering from Rose Black Spot (Diplocarpon rosae), a fungal disease prone to rose bushes.
Good care and maintenance of your rose bushes will help, removing the worst affected areas early in the season which also opens up the bush and allows better air flow between the stems and leaves which helps. Clear away the dead leaf litter from around the bases regularly, as old fallen leaves will be harbouring the disease.
Apply a fungicidal spray, to the remaining leaves. If you do prefer to keep things organic then I would recommend this spray ( follow this link https://www.harrodhorticultural.com/organic-2-in-1-rtu-750ml-pid8561.html#product-tabs ) . It will also tackle the main insect pests on your roses too, such as aphids and mites.
I hope this helps, and many thanks for your query.
Pest Control Expert