Rose rosette disease (RRD) is attacking Knock Out Roses. RRD manifests itself as with elongated and bright red branching. Knock Out roses and Drift roses are especially susceptible. RRD began appearing in about 2009 in Dallas.
You will see the pointed, elongated growth as shown above this is often referred to as “witch’s broom.” Your Knock Out rose will have more thorns; its buds, leaves and flowers will be distorted. The rose will be become more susceptible to mildew and black spot and generally die in two to five years.
The Knockout Rose disease is a virus carried on the wind by a winglesss mite, the eriophyid mite. Mass plantings are generally all infected. There is no treatment. The rose will be infected in May though may not be symptomic for a few months thereafter. The mites can survive a winter in the live canes. When you remove the foliage you remove the mites.
However, the cure suggested is to remove any infected plant completely. It contains the damaging mites. It has been noted that removing the damaged part of the plant often encourages the mites living within the plant to simply move lower.
The Pennsylvania nursery that introduced ‘Knockout’ roses has suggested that you prune the roses back by 2/3rds while they’re dormant in late winter to remove any overwintering mites and eggs in the bud crevices.