Our spring rain may cause Rust and/or Black Spot in our roses.
Rust, caused by the fungus Phragmidium disciflorum, is favored by cool, moist weather, such as in the coastal area of California, although it may also be a problem inland during wet years. Infected plants have small orange pustules on the leaf undersides; the upper sides of leaves may discolor, and leaves may drop. Avoid overhead watering and prune back severely affected canes. During the winter, collect and dispose of leaves remaining on the plants as well as those that have fallen off. Low levels of damage can be tolerated without significant losses. Preventive applications of fungicides can be used but frequent applications may be needed and may not be justified in garden or landscape situations.
Black spot, caused by the fungus Diplocarpon rosae, causes black spots to develop on the upper surface of leaves and stems. The spots have feathery or fibrous margins and no fungal growth on the undersides of leaves. Small black fruiting bodies are often present in spot on upper sides of leaves. The fungus requires free water to reproduce and grow, so leave should net remain we for more than 7 hours. Provide good air circulation around the plants. Remove fallen leaves and other infested material, and prune out infected stems during the dormant season. Black spot is usually not a problem in most of California. If required, fungicides can be applied preventively. ■
