Can I replace knockouts with more knockouts? How much soil should I remove? If knockouts are not a good choice, would drift rose be okay? If not a rose, what would be a good substitute where knockouts once thrived?
Knockout Replacements After Witches Broom
I would recommend removing as much root material as possible when removing the infected rosebushes. I also recommend spraying the entire areas plantings with a miticide that is labeled to kill the tiny eriophyid mite (Such as one called Avid) as they are thought to be the main spreader of the deadly virus. I would remove the soils in a 20 to 24 inch diameter and 16 to 18 inches deep. Replace those soils with a good bagged garden soil that has compost in it, such as one of the EKO brand mixes at Lowes or Home Depot. Replacement roses using Knock Out roses or the wonderful Drift Roses would be fine. Also take a look at the Weeks Roses Home Run series of roses.