Just lost a knockout Rose. Died very quickly. Looked like it was drying out but plenty of water. See pic. Heard in MG class that there is an insect that carries a disease from plant to plant. Lost some other roses a year or so ago but not near this one. Thoughts for the future of other roses
We viewed your photos. We cannot say for sure what you are dealing with. This does not look like rose rosette which is spread by an insect, a mite. Some possibilities include cane borers (insect), root rots, cankers, etc.
All you can do is remove all dead and damaged areas, and prune back to healthy tissue. You should also remove all weeds below and around the plants. Mulch them with no more than 2" , but pull it back from the crown of the plant. Send us additional photos as soon as you notice symptoms.
Here is more information on pruning knockout roses http://extension.umd.edu/hgic/rose-rosenot-always-look-%E2%80%98knock-out%E2%80%99-landscape-roses