Do you clean up the fallen leaves or just leave them lay there. You have to understand how fungal diseases work. The disease is in the soil. Water splashes on the soil and carries the disease to the lowest leaves. Water splashes on those leaves and carries the spores to the next leaves and before you know it the whole bush is infected. Those infected leaves die and fall on the soil and reinfect the soil and the cycle starts all over again.

It will take some diligence on your part to get rid of the black spot and if you have humid wet conditions it will come back. Start by clearing the bed of any debris from the year before. Use the fungicide at the first sign of the disease but first pick off any leaves that are infected. Keep a close watch for further infection and pick off any infected leaves. Picking them off is a great deal better than leaving them to infect more leaves

You might want to post on the rose forum too. Some cultivars of roses are very prone to black spot, so some would suggest get rid of the plants and get new varieties. You could prune them back hard, remove all the clippings. As the other person said, try to clean up all around and under. Do you mulch? Make sure you use clean mulch this year which does not expose the soil so the soil doesn't splash up. Good drainage is important. I garden organically so don't spray my roses, but once they are pruned back, you could spray for the black spot, I am not sure what time of year is ideal for that. I think it's also important to disinfect your pruners with a bleachy solution so you are not spreading the fungus.

Thanks guys, would pre treatment with spraying mineral oil also help?

No.

What does help, some of which has already been sad:
1. Growing a rose in full sun and with excellent air circulation.
2. Growing a rose that has some tolerance to black spot.
3. Keeping the leaves as dry as possible.
4. Clean up fallen leaves.