Black spot fungus is the most serious rose disease, characterized by black or purple spots on the leaves. The fungus quickly mutates into new strains, making it difficult to treat. It takes time to develop and breed new rose varieties, so the resistance bred into a new variety does not usually last. An infection of black spot fungus can cause the rose to fall prey to other diseases. It thrives in warm, humid weather.
Black spot fungus usually starts as a black or purple spot on the top of a leaf. The leaf may turn yellow around the black spot. Leaves often drop, and scabby black lesions appear on young rose stems. Roses look bare and straggly and blooms may be distorted. Overall growth is poor.
Black spot fungus is caused by the Diplocarpon rosae fungus. The fungus overwinters on canes and fallen leaves. The infection cycle begins in the spring, when spores are splashed onto new leaves. Roses may struggle with repeated cycles of infection, peaking in late summer. Late summer with its high temperatures and high humidity is the peak time for the fungus.
In the fall when summer blooming is complete, remove any fallen leaves from the ground. When pruning in the spring before the roses bloom, remove all stems with lesions. This will help delay the onset of disease, but fungus spores are most likely still present in the ground and will need treating. Fungicides containing difenoconazole, myclobutanil and triticonazole are effective treatments.
Once black spot appears, begin treatment immediately to minimize damage to plants. You may want to alternate between two or three chemicals to prevent the build-up of resistance to one treatment. Do not use products containing insecticides unless there is a pest problem that needs treating.
Since the fungus prefers moist, humid environments, space roses far enough apart to allow for air circulation and minimize drying time after rain. Avoid planting too close to fences and walls, which limit air circulation.
The first line of defense against black spot is to select disease-resistant varieties of roses. Cultivars can range from very resistant to very susceptible to black spot. Since black spot increases the risk of other diseases, look for roses that are also resistant to powdery mildew and rust. Some good disease-resistant roses include Nearly Wild, Henry Hudson, John Cabot and Louis Jolliet. It's also best to buy stock locally, as local retailers tend to sell varieties best suited for that particular area.