Disease

Disease happens. Unless it is Rose Rosette Disease (RRD), diseases can be lived with. Most people wanting to grow roses are generally growing them for their pleasure and use.
 I am going to include a link to Google to see what rose rosette disease looks like. RRD is a fatal, incurable disease for roses that are spread by mites. I recommend if you discover RRD it may be better to dig that rose up, place all of it in a garbage sack and throw it away. Do not compost the plant.

https://www.google.com/search?q=rose+rosette+disease&oq=rose+rosette

As for other diseases,such as black spot, powdery mildew and to some extent Downey mildew can be kept under control with basic garden hygiene. With Downey mildew there should be some fungicides involved as well.
Powdery mildew can be kept to a tolerable, if not almost eliminated by keeping the plant watered and spraying water on the affected areas.
However, wet foliage may lead to black spot. I have found that at least for me, I would rather just remove the infected leaves than spend money on fungicides.

Black spot on Graham Thomas