Black spot is a rose disease caused by a fungus infection, and it can be treated by improving sanitation and using neem oil and potassium bicarbonate sprays. Downy mildew is a serious rose disease cured by practicing good gardening sanitation and using a good dormant spray. Roses also suffer from rust infection, treatable by using neem oil or by removing affected leaves.
Roses suffer from black spot during humid weather, since the fungus infection is water-borne. The fungus causes black spots on the stems and leaves, leading to dropping of leaves and weakening of the plant. A person can prevent the disease by planting varieties of roses that are resistant to black spot. Using a dormant spray containing potassium bicarbonate, lime sulfur, or neem oil helps with persistent black spot.
Downy mildew is a destructive and rapid fungal disease that affects blooms, stems and leaves of roses. Affected parts get dead tissue spots and yellow areas, and can cause death of leaves if left untreated. The disease is treated by regular pruning to improve circulation and by using a dormant spray.
Rust is a fungal disease on a rose that manifests as small rust-colored spots on the leaves’ undersides, which then spreads to the upper sides. The disease can cause defoliation. The disease is cured by practicing good gardening sanitation. Neem oil and removing affected leaves helps control the spread of the disease.