Everyone loves a beautiful, well-tended rose garden. Many gardeners eagerly plant roses, only to be disappointed when disease strikes and ruins their beauty. This doesn't have to be the inevitable fate of the rose, or the rose gardener. A good understanding of what conditions lead to rose disease will go a long way toward prevention and treatment of these ailments.
Roses require full sun--at least 6 hours a day. Rose diseases, all fungal, need moist conditions to thrive. The sun's heat evaporates rain and dew, keeping leaves dry.
Roses require nutrient-rich, moist, well-drained soil. Poor, compacted soil causes roses to languish. Proper soil preparation includes adding lots of organic matter to improve drainage, moisture-retention and nutrient content. Sand or gravel improves drainage. Good soil contributes to healthy roses that are less susceptible to disease.
Air circulation is crucial for good rose health. Plants which are crowded stay wet longer after a rain, encouraging the development of fungal disease. Roses should be planted so they can grow to their ultimate size without crowding each other. If used as a foundation planting, roses should be planted at least 3 feet away from the wall. If your roses are crowded, consider moving them to a more spacious location.
Regular pruning removes affected plant parts and improves circulation. Prune roses of dead, weak or diseased canes. Trim off spent flower heads which might harbor a fungus. Trim out crossed canes and canes touching the ground or crowding other canes.
There are three diseases which most commonly affect roses. Powdery mildew exhibits a fine, gray, powdery film on rose leaves. As the disease worsens, leaves curl, turn black and fall off. Black spot, the most common rose disease, shows black spots on leaves, followed by yellowing. Untreated, the leaves fall off, killing the cane. Rust shows irregular, rusty-red or brown spots on leaves. As rust progresses, the spots cover the leaves, causing them to fall off.
Treatment of all three diseases is the same. Treat affected plants by first removing all affected parts and trimming out excess growth to improve circulation. Rake leaves off the ground. Treat the plant with a fungal spray containing triforine according to label instructions. Spray leaves and flowers, top and bottom, canes and the ground underneath. Continue treatment throughout the season, even after symptoms have disappeared.