witches broom
As I was working at our community garden last weekend, I overheard fellow gardeners tsk-tsking about a certain rose bush. “Witches broom,” they pronounced and said it had to go, the entire plant. As I dug it out, I got a good look at its weird purplish mutant growth and realized its affliction looked familiar.
Yep, one of my knock-out rose bushes was similarly stricken with deformed growth, so it too had to go. Removing it took some effort–it was over 10 years old and had become well rooted.
Witches broom, or rose rosette disease (RRD), is carried by a certain spider mite that if infected will inject the disease into the rose bush cane when it feeds. There is no cure, so when a rose bush shows the tell-tale witches broom growth all over like mine did, the only option is removal. The entire bush should be placed in a trash bag and thrown away. You don’t want to dispose of it in yard waste or a compost pile because then the disease can continue to be spread.
I read that if you spot RRD early on a rose bush, one approach a gardener can try is to trace the affected cane to the base of the plant and remove it carefully. This way you might be able to stop the disease before it has spread to the entire plant. I’ll be watching my remaining rose bushes carefully.
At any rate, I now have a wide open spot for more perennials–not a problem for me!
red flannel pantry 
The mite enters the cane through fresh cuts in the woody canes. I saw this more commonly in my hybrid teas because I cut roses for display in the house. I was warned to dip my secateurs into mild bleach water between cuts to help prevent this, but I never did. Lesson learned. Roses should only be pruned when dormant, since the insects are also dormant at the same time. Why your Knockout rose was afflicted is difficult to say as usually we don’t cut them for display indoors.
I heard that the roses at the Botanical Garden were similarly affected–so sad. Thanks for your expert tip about the bleach water–good to learn!