What does it look like?
Black spot is a common fungus disease of roses. Infected leaves have black, rounded spots on the upper surface of the leaves. The spots have feathery or fringed edges, often with a yellowish halo around the spots. In severe infections, the spots are large and run together. Eventually, the leaves turn yellow and drop. Sometimes, flower petals are also infected.
Black Spot Life Cycle
Black spot fungus overwinters on infected leaves, whether they stay on the plant or have dropped. It also remains in infected stems. In the spring, the fungus produces spores which are carried by rain splashes to infect new leaves. Spores are produced throughout the growing season, causing repeated cycles of infection. Young, rapidly growing leaves are particularly susceptible. In warm weather, the fungus can infect leaves that have been wet for as little as 7 hours. Leaf spots begin to show 4 to 5 days after the initial infection. The fungus is spread by wind-driven rain, splashed irrigation water and even on garden tools.
When is Black Spot a problem?
Roses vary widely in their susceptibility to this disease. Some cultivars and species of roses are virtually immune, while others are moderately or very susceptible to the disease. In a year with prolonged rainy weather during the summer, the most susceptible roses may lose so many leaves that the plants are unsightly and weakened. In a dry summer, even the most susceptible roses may not be greatly affected and moderately susceptible roses may show no damage. Photo: City of Victoria
How can I prevent infection?
Consider replacing your most affected roses with disease resistant cultivars or species. Many roses have been rated for resistance to black spot (and other diseases). There are colours and types for every taste: hybrid teas, floribundas, shrub roses, climbers and rugosas. Ask rose growers and garden centre staff for recommendations. Plant roses in open, sunny sites. Prune plants to ensure good air circulation through the branches. This allows leaves to dry quickly after a rain. Keep leaves dry by watering plants at the base (drip irrigation or soaker hoses work well) rather than using overhead watering. Water in the morning so that any splashed leaves dry quickly. Mulch roses in the spring to prevent over-wintering spores from being splashed up to the new leaves. What can I do to control Black Spot? During the growing season, pick off infected leaves and rake up dropped leaves. Dispose of them in household garbage or bury deeply in the soil. Don’t compost this material. In the fall, prune infected roses to within 10-15 cm of the main graft union to remove fungus overwintering within the cane. Collect all pruning's and leaf debris and dispose of this material as above. To kill over-wintering spores, spray lime-sulphur on dormant roses in late winter, before the buds break. During the growing season, sulphur fungicides can reduce the spread of infection if sprayed frequently. Spray at 7-10 day intervals and after wet weather.
Tips For A Healthy Garden
- Enrich the soil once or twice a year with compost or other organic fertilizers.
- Choose plants adapted to the conditions of sun or shade, moisture and soil acidity. If necessary, correct the drainage and acidity to suit the plants.
- Plant native plants, which are adapted to the local climate. Most are easy to care for and have few pest problems.
- Before buying plants, make sure they are healthy and free of diseases and insect pests.
- Water deeply, but infrequently, to encourage deep rooting.
- Cover the soil between plants and under shrubs with organic mulches. This insulates the soil, keeps in moisture and suppresses weeds.
- Protect and attract native beneficial insect, birds and other animals.