Witches’ Broom disease, known as Rose Rosette when it is mainly on your roses, usually attack multiflora roses and shrub roses as opposed to your more ornamental roses. It is first detected by red pigmentation on the underside of the leaf in the veins followed by an increased growth of vegetative shoots. The leaves become small and crinkled with varying shades of red and yellow. A pronounce increase in thorns will occur and your rose will just look odd, especially on one side.
The time to start looking for these signs is during the months of May through July. Smaller rose bushes are more susceptible than larger ones and shrub roses more than ornamental roses. Best advice is to dig up the entire plant and discard it as soon as you see any of these symptoms. However, if you wish to treat the rose for mites or aphids, try using Seven or Insecticidal Soap or a Horticultural Oil spray (preferably one designed for summer use). The earlier they are sprayed, the better your chances of prevention. Completely pruning out infected areas followed by spraying is the best method short of total removal.
Witches’ Broom is not a new disease but just like the terrible reviews concerning virus’ on impatiens some 4 or 5 years ago, stories are now coming out about this disease on knock-out roses as well as all shrub roses. Simply try hard to keep growing areas clean, sprinkle or dust insecticides and fungicides on the roses as well as on the ground and always buy the best, healthiest plants you can find. Weak, small plants are very susceptible to insect invasions and fungus attacks.