What is the best treatment for Knock out roses that have a fungus or virus?
Virus and Fungus on Double Knock Out Roses
If you have a virus in the roses, there really is not much you can do. The viruses that I am aware of will typically stunt the growth and make markings on the foliage. Some will eventually kill the rosebush. If it is just funguses you are dealing with, then an earth friendly fungicide named Green Cure works very well to cure and keep funguses away. There are also some others on the market such as Banner Maxx or its generic form named Honor Guard. For black spot that is out of control, Mancozeb works the best.