- The Diplocarpon rosae fungus causes black spot disease in roses. The Sphaerotheca pannosa fungus causes powdery mildew infections, while the Peronospora sparsa fungus causes downy or black mildew. Several different varieties of the Phragmidium fungus cause rust infections.
- Roses with black spot disease have black spots on the leaves, ranging in size from 1/16 inch to 1/2 inch in diameter. The spots are surrounded by yellow rings. Infected leaves turn yellow and fall prematurely, resulting in defoliation. Powdery mildew infections are characterized by a powdery white growth that begins in patches but eventually covers the blossoms, leaves, shoots and stems, according to Oregon State University Extension. Downy mildew, which begins with reddish-purple or brown leaf spots, causes defoliation and dieback in young shoots. Rust infections initially appear as raised orange blotches on the undersides of leaves, but they spread to cover the entire leaf and stem.
- Planting disease-resistant cultivars may prevent fungal infections. A preventative fungicide spray applied as soon as the first leaves appear in the spring may also help. Infected leaves and stems should be removed. Disposal of all debris, including fallen leaves, helps limit the spread of mildew rust. Adequate soil drainage and proper spacing to ensure air circulate is essential.