Most of my knockout roses succumbed to the witches broom. They were dug out and discarded. I believe that the mulch my landscapers used and replenished periodically contained the mite and that was the reason for my roses being infected. I told them the bushes needed to be totally destroyed, not sent someplace for recycling. I have way too much mulch to economically have it removed and replaced with new this year and I cannot be sure new mulch wouldn't carry the mite anyway. Am i right that the mulch they used could have carried the mite? The few bushes that remain unharmed are planted in an area with rock, not mulch. Also, i started a climbing rose bush that was a start from a bush that had been growing at my parents home for near 50 years. As my parents and the home are gone I dont want to lose this bush! I have seen signs if the disease on it. Is there anyway to save it? Once again, it was in a mulched area.
I don't know that you can blame the mulch. This viral infection is hitting the entire country. If you have any multiflora rose near you, remove it as it is the main host plant for the disease. This website discusses containment/treatment; most just recommend removal, including plant roots. good luck. http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/viruses/rose-rosette.aspx