Knockout roses are bright, beautiful roses. They are popular not only for their large blooms and brilliant colors, but also because they are fairly low-maintenance and disease-resistant. However, just because knockout roses resist many diseases does not mean that they are completely impervious to infections. Knowing the signs and symptoms of diseases of knockout roses is the best way to keep these flowers happy and healthy in your yard for years to come.
Powdery mildew is a black, white, gray or pinkish mold that grows in humid, moist environments. It tends to be a largely aesthetic problem, but if you allow it to grow unchecked, it can weaken a plant and eventually cause the plant's demise. Remove the affected parts of the rose bush using sterile pruning technique. Dispose of them completely so that the bush does not become reinfected. Be sure to water your knockout roses using a drip hose early in the morning -- before 9am -- to avoid further infection. While fungicides will work on powdery mildew, they are usually unnecessary.
Knockout Rose Rust
Rose rust is an infection that manifests itself in orange dots on the leaves of your knockout roses. Sometimes these spots also appear on the stems. Use sterile pruning to remove affected parts of the plant. Do not drop the debris on the ground, where it can cause reinfection, but place it in a garbage bag for sanitary disposal. Trim back your bushes so that they get good air circulation and be sure to water early in the morning using a drip hose. If the infection persists, you can treat the rose rust with a lime sulfate mixture.
While some knockout rose cultivars are resistant to this fungal infection, it can spread to the plants if another rose nearby develops the problem. At first, small, black spots will develop on young leaves. Left unchecked, the spots will enlarge, develop yellow rings and spread until the leaves start to fall off. To treat black spot, you need to remove all affected foliage using sterile pruning, then remove all mulch and leaf debris from the area beneath the roses to prevent reinfection. Start watering in early morning with a drip hose to prevent black spot, and treat any infected plants with a baking soda mixture to prevent the spread of the infection.