Infected rose bushes have the potential to destroy your entire rose garden if you're not careful. There are several types of disease that cause a white substance on the stem of the rose bush. Both powdery mildew and brown canker cause white, powdery lesions on the stem. Fortunately, neither of these conditions cause the death of the rose bush if you catch them early.
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Powdery mildew causes a white substance to form on leaves and rose canes. This disease starts as a blistered surface on young leaves caused by the fungus Sphaerotheca pannosa that spreads on the wind. Unfortunately, powdery mildew is found in most growing zones, so the chance being downwind of an infected plant is fairly large. You'll need to monitor your roses carefully; treatments applied when early symptoms appear are simpler, less invasive and far more effective than later-stage treatments.
Another disease that causes a white substance to appear on the stems and leaves of roses is brown canker. While the name suggests a brown canker appearing, this disease manifests as a gray-white lesion on the stem's surface. These lesions spread over the stem and onto the leaves, then once the plant is fully infected, the lesions become brown to give the disease its distinctive name. Brown canker can kill the entire rose plant so treatment when symptoms first appear is critical.
To save the life of your rose bush, start treatment when symptoms first appear. When symptoms of powdery mildew first appear it can be washed or gently sprayed off using a garden hose. Once the disease has taken hold, commercial fungicides can be applied to control damages. Brown canker can only be controlled by removing diseased rose canes. When cutting canes infected with brown canker, the shears need to be disinfected in 10 percent diluted bleach between each cut to avoid spreading the disease.
Preventing rose diseases involves proper maintenance of the plant throughout the growing season. Planting roses further apart promotes good air flow, as does appropriate pruning. Removing any sick or diseased parts of the plant as soon as symptoms appear prevents the spread of disease to healthy parts of the plant. Finally, treating plants with a commercial fungicide prevents the spread of a disease from infected plants.