Caused by the fungi Diplocarpon rosea (sexual stage) or Marssonina rosae (asexual stage), black spots appear as dark brown to black rounded blemishes on potted roses. Black spots affect the canes and leaves of roses, appearing as reddish in the first year and darkening as they mature. Fortunately, prevention isn't difficult and will ensure that your roses will stay beautiful and healthy.
Choose rose species that are less susceptible to black spots. Rugosa, hybrid tea, floribunda, grandiflora, shrub and miniature roses are more resistant to fungal infections.
Place the potted roses in locations where there is good air circulation. Avoid crowding of potted plants, which can diminish air flow.
Water the bushes at soil level. Use drip irrigation to avoid wetting the leaves or water early in the day so the leaves dry quickly. Leaving the leaves too wet can promote the growth of fungi.
Place the potted roses in areas that get enough sunlight, facilitating rapid leaf-drying. Sunlight can promote rapid leaf-drying that can deter growth of fungi that cause black spots.
Isolate the infected potted roses and keep them away from the healthier ones. A black spotted leaf touching the healthier plant will contaminate the healthy plant.
Cut away the diseased areas and send the clipping to the landfill. Avoid putting the diseased leaves and stems into the compost to prevent fungus from spreading.
Disinfect the shears and gardening tools used from infected roses before cutting the healthier ones. This can prevent cross-contamination and transfer of fungi-causing organisms.
Apply fungicides when the weather is wet to protect leaves and canes from infection. Start applying fungicides in early spring and maintain the protection as needed throughout the growing season if wet conditions exist. Spraying both the upper and lower surfaces of leaves can ensure better coverage. Applying rose fungicide at root level, deep into the soil, can also help prevent black spots.