Spray roses with a good blackspot and fungus spray. On all new new growth and old, spray regulary. The treatment usually lasts seven days, but spray any new growth or leaves inbetween. This will keep it in check.
Make sure you prune spindly growth to promote stronger stems and thin out the centre of roses to allow good air circulation.
Cut off all leaves that show amy sign of blackspot, rust or mildew. It might look a bit drastic, but new leaves and shoots will grow quickly after you have done it. Also cut off any yellowing leaves and cut any stems back to healthy green. This will stop die back. Die back is one of the most common reasons for roses dying if left on check.
This year I haven't used any sprays, and have just been religiously been cutting of any sign of blackspot and my roses are floureshing. I have been growing roses for at least 35 years. I was taught this way to look after roses and it definately works.