Roses can be plagued by black spot & powdery mildew. Not as much of a problem with the newer roses, which many of them have this problem breed out of them.

There is  a Rose Care Kit on the market called DEFENDER which is an insecticide and fungicide.Also sulphur is a good fungicide.

Preventative measures that can help is to water early in the am & never water over the leaves. Plant roses, if possible, in an area with good air movement.