Key Features
- Black spots on leaves
- Defoliation
- Black spots on rose hips
Symptoms
Dark spots develop on the upper leaf surface and expand up to 1/2 inch in diameter. Spots may have a yellow halo surrounding them, with severely infected leaves turning yellow prior to leaf drop. In severe cases, canes may have small purplish spots on the current years growth. Early defoliation weakens plants, resulting in cane dieback, and predisposing plants to winter damage, borers, and opportunistic pathogens.
Biology
Fungal spores develop on infected fallen leaves from the previous season and are splashed to lower leaves by rain or sprinkler irrigation. For infection to occur, the leaves must remain wet for several hours. Symptoms can become visible in a few days during warm, wet weather. Within the initial black spots, fungi can begin sporulating and infecting other leaves. This fungus tolerates a wide range of temperatures, and symptoms can continue to develop all season long if moisture is adequate. The fungus overwinters in infected canes and fallen leaves.
Management Recommendations
A number of disease resistant landscape varieties, like the 'Knock out' or 'Canadian Explorer series, are available. For susceptible varieties, in the fall and remove and destroy fallen diseased leaves by raking, followed by burning or disposal. Diseased canes should be pruned back to healthy wood in the spring prior to budbreak. Avoid overhead irrigation during the growing season, to minimize leaf wetness. If overhead irrigation cannot be avoided, water in the morning so that leaves dry quickly as the temperature increases. Avoid using chlorothalonil when controlling black spot on some landscape roses, as phytotoxicity may result.
Effective Pesticides
Active Ingredients include: Chlorothalonil, Myclobutanil, Propiconazole, Sulfur, Thiophanate-methyl, Triforine
Resources
- Not satisfied with ID? Contact the Purdue Plant and Pest Diagnostic Lab
- Sign Up for the Purdue Landscape Report
