Black spot on rose leaf

BLACK SPOT ON ROSE - Black spot is caused by a fungus called Diplocarpon rosae. Rose species and cultivars vary from highly susceptible to very resistant to infection.  At the Morris Arboretum, we see most of our black spot on our climbing and shrubby roses.Black spots with fringed edges appear on either leaf surface but mostly on the upper surface of the leaves.  Infected leaves turn yellow (except for the black spots) and drop from the plant.  The same type of symptoms occurs on the leaf petioles and fruit.  Flower petals can develop red spots and some distortion.  During the first year of development of the fungus, the rose canes can become infected.  The fungus will cause the canes to form slightly raised lesions, which turn from reddish purple to black.  Black spot causes plant disfigurement and weak growth, which can predispose a plant to other infectious or noninfectious problems.Some rose species are resistant to black spot.  In order to avoid this disease, choose only varieties which claim to be resistant to black spot disease.  Also, to avoid the disease in nonresistant rose species, mix the resistant and susceptible roses.  Increasing air flow around rose plants will reduce the severity of the disease by promoting rapid drying of the foliage.  The spores that spread the fungus are carried by water as it is splashed onto other areas or other plants.  Because the disease survives in fallen leaves and the canes, these parts, if diseased, should be removed and destroyed in the fall to help prevent a heavy resurgence of the disease next spring.  Protectant fungicides can be used.  Application should begin in the spring when the new leaves appear.  From then on, apply weekly well into the fall on highly susceptible varieties.