Knock Out roses (Rosa spp.) are considered to have excellent disease resistance, but, like other roses, they can suffer from illnesses. Cultural controls are the best way to prevent and treat rose diseases, and chemical controls are available for certain diseases.
The original Knock Out rose (Rosa 'Radrazz') and other members of the Knock Out family, including Rainbow Knock Out (Rosa 'Radcor') and Double Knock Out (Rosa 'Radtko'), are hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9. Diseases can affect Knock Out roses in any region, but your specific climate can affect whether or not disease development is likely.
Treatable Fungal Diseases
Black Spot and Mildew
Although black spot is one of the most common rose diseases, it rarely bothers Knock Out roses. If Knock Outs' growing conditions are poor, however, the roses can develop some black spot symptoms. Black spot results from the fungus Diplocarpon rosae. It causes round black spots with yellow edges on the leaves before the leaves drop from the affected plant. Black spot is mostly likely to develop in growing regions with wet, warm springs.
Powdery mildew, caused by the Sphaerotheca pannosa var. rosae fungus_,_ is another common disease that rarely affects Knock Out roses. It is most problematic during mild weather with high humidity at night and in shaded locations. It looks like white, fuzzy growth on leaves, buds and stems.
Because Knock Out roses are resistant to black spot and powdery mildew, they often recover from them when their diseased leaves and branches are removed and they aren't watered from above. In severe cases, use a fungicide rated for controlling the specific disease.
Rust and Blight
The symptoms of rust and botrytis blight look very different, but both diseases are caused by a fungus that thrives in stagnant, moist air. Rust symptoms include orange areas on leaves and stems and sometimes an orange dust. The disease is caused by Phragmidium fungi. Botrytis blight is caused by Botrytis cinerea fungi, which results in hairy grayish brown growths on flower buds and flowers. It also leads to depressed areas on stems, blotches on flowers and wilting flower buds.
The control methods are similar for rust and botrytis blight. Knock Out roses have a bushy growth habit. So prune out about one-third of each plant's oldest canes every year to keep the center of the plant open. Improve air circulation by removing weeds and moving all garden plants that are close enough to touch the roses' leaves. Prune to remove infected canes and leaves, and remove all leaves that fall on the soil. If either disease worsens, treat the roses with a fungicide specified for the problem.
Not all rose diseases respond to treatment for fungal diseases, but pairing chemical and cultural controls for the ones that do can effectively eliminate the problems. Because Knock Out roses are naturally disease-resistant, they should recover quickly after treatment.
Many chemical fungicides available to treat rose diseases. Read a fungicide's label carefully to determine which chemicals control which diseases. Two chemicals that treat several rose diseases are chlorothalonil and propiconazole. Prepare and apply a fungicide on a windless day when no rain is forecast for the time period needed for the application to dry.
- Apply a product containing 29.6 percent chlorothalonil at a rate of 2 1/4 teaspoons mixed with 1 gallon of water for black spot and botrytis blight. Spray enough to cover the leaves, flowers and stems completely every seven to 14 days.
- Apply a product containing 1.55 percent propiconazole at a rate of 1 tablespoon mixed with 1 gallon of water to treat powdery mildew or 2 tablespoons mixed with 1 gallon of water to treat black spot and rust. Spray enough to cover leaves, flowers and stems completely every 10 days.
Caused by several fungi, including Botryosphaeria, Coniothyrium, Cryptosporella and Leptosphaeria, stem cankers cause dead or discolored areas on rose canes. The stems can turn any color from light brown to a dark, almost black, purplish brown.
This disease cannot be treated by a fungicide. The best way to prevent it is to avoid injuring roses' canes and to control insects, black spot and powdery mildew to maintain the plants' health. If you notice symptoms of stem canker, remove the affected canes by making a clean cut several inches below each diseased area.
Rose Rosette Virus
Probably the most destructive disease in roses, rose rosette virus spreads rapidly even among disease-resistant Knock Out roses. The disease has no cure. It is spread primarily by eriophyid mites, which are invisible to the naked eye. They are so small that wind carries them from plant to plant.
Rose rosette virus can appear in your landscape if you accidentally purchase infected plants, or if nearby wild roses, including multiflora roses (Rosa multiflora, USDA zones 5 through 8), become infected. Symptoms are characterized by abnormal growth, including a "witch's broom" shape in new shoots, distorted or deformed leaves and flower buds, spiral-shaped cane growth and excessive, pliable thorns. Infected plants die within a few years and should be removed as soon as possible after diagnosis to prevent infecting other roses.