Rose Rust-phragmidium tuberculatum; is a fungus that grows on living plant tissue, in this case; roses.
It is generally not considered to be a serious disease, more so now that some modern roses have been bred with a resistance to it.
This fact is worth considering when you purchase new plants.
Symptoms;
In spring; the symptoms appear as pustules filled with orange coloured spores on twisted young growths.
The upper surface of leaves are covered in yellow spots with pustules of dusty orange spores directly under these spots on the underside of the leaves.
Later in the year these pustules turn black, which can lead to premature leaf fall.
These dark sticky spores remain dormant over winter on fallen leaves, stems and supporting framework, later to be blown on the wind and re-infect the plants the following spring.
Control;
Prevention is often the best course of action.
As soon as the disease is detected prune out the infected growth to prevent the spread of spores later.
Ensure that all fallen leaves and plant debris is removed from the site.
Some suitable fungicides are available to control the problem, check with your garden centre for suitable products.