Spring rain and cool, moist conditions can increase the risk of rust and black spot in roses, according to University of California Integrated Pest Management guidance.
Rust, caused by the fungus Phragmidium disciflorum, is favored by cool, damp weather and is more common in coastal areas, though it can also appear inland during wet years. Infected plants develop small orange pustules on the undersides of leaves. The upper leaf surfaces may discolor, and leaves can drop prematurely.
Gardeners are advised to avoid overhead watering and prune back severely affected canes. During winter, leaves remaining on plants and those that have fallen should be collected and disposed of to reduce the spread of spores. Low levels of rust damage can often be tolerated without significant impact. Preventive fungicide applications are available, but frequent treatments may be required and are often unnecessary in home landscapes.
Black spot, caused by the fungus Diplocarpon rosae, produces black spots on the upper surfaces of leaves and stems. The spots typically have feathery margins, with no fungal growth visible on the undersides of leaves. Small black fruiting bodies may be present within the spots.
Because the fungus requires free water to grow, rose leaves should not remain wet for more than seven hours. Good air circulation around plants is recommended. Fallen leaves and infected plant material should be removed, and infected stems should be pruned during the dormant season. Black spot is generally not a major issue in most areas of California.
Fungicides may be used preventively if necessary, though cultural practices are often sufficient to manage the disease.