My knockout roses bloomed fine last summer and after our first frost at the end of November I covered all of my rose bushes with layers of straw just as I have been doing so for the last five years. I have not lost any rose bushes until this spring. I pulled out a knockout bush last week and didn't think anything about it until this evening when I went out to do some more yard work, I discovered that I lost another knockout rose bush so I decided to take a picture of it. I started to remove some of the straw to let them air out a little bit and it kind of fell over. as from the attached pictures you can see that the bottom is damp and is dark brown in color and but some of the branches are green. I would appreciate any of your thought on this, I hope it's not a disease and we did have a very cold winter last year, I am in Zone 5 if this helps!