- Black spot (blackspot) is the most important fungal disease of roses worldwide.
- The initial symptoms start as feathery-edged, black spots on lower leaves.
- As these spots enlarge, the leaves turn yellow and drop off. The disease continues up the stems until the entire plant becomes defoliated.
- Stem lesions are less obvious but start as dark, irregular blotches that eventually become blistered.
- Stem lesions are the most important source of fungal spores for initiation of the infection cycle next season.
- The disease is caused by the fungal pathogen Diplocarpon rosae.
- Leaves are most susceptible when young and must remain moist overnight before infection can occur.
- The disease can be spread by rain, dew, irrigation, people, insects, and transport of infected plants.
- The fungus cannot live in the soil or last on pruning tools for longer than a month.
- Black spot spores can survive in fallen leaves and stem lesions over the winter and will remain active year round on the plant in mild climates.
- Black spot is a serious disease problem for rose growers in Maryland.
- Look for and plant disease-resistant roses. Rose cultivars resistant to black spot are increasingly more available, but resistance can be regionally variable.
- Sanitation is critical for black spot management.
- Removal of fallen leaves and pruning infected canes will dramatically slow initial spring infections.
- Good air circulation will reduce the incidence of black spot by promoting faster drying of leaf surfaces.
- Restrict irrigation during cloudy, humid weather.
- Most people will need to use labeled fungicide sprays every 7-14 days as the first leaves emerge in the spring through the fall for adequate control of this disease.
- It is best to spray before a rain event to protect the leaves and to alternate types of fungicides.