- There are three common rose leaf diseases, all of them fungal: powdery mildew (Sphaerotheca pannosa var. rosa), rust (Phragmidium mucronatum), and downy or black spot mildew (Peronospora sparsa).
- The names of the diseases accurately describe their appearance. You'll know your rose is infected if you see powdery, rust-colored or black spots forming on leaves, and spreading across the plant.
- All three diseases can completely defoliate a rose bush, weakening and eventually killing it. Remove infected leaves as soon as you spot them, and burn them to prevent the fungus spores from traveling to other leaves or even other nearby rose bushes.
- Rose leaf diseases thrive when nights are damp and days are hot and moist. Avoid overhead watering of roses, which can spread the spores and encourage disease; water instead at the base of the rose, or use an underground soaker hose or other irrigation system. Healthy roses are more resistant, so appropriate feeding, watering and pruning of your roses will also help prevent disease.You can also take care to buy roses varieties that are disease-resistant.
- Once a rose bush is infected, chemical sprays designed to fight rose fungus diseases can be used. However, these sprays can be more toxic to some roses than the disease itself; this is especially true of some heirloom or old garden roses.
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