Most rosarians know powdery mildew symptoms. An unrelated mildew called downy mildew (Peronospora sparsa) causes much more damage to roses and has been spotted already this year in some Plant Clinic samples. The affected plants had been shipped into Illinois from out of state. The take-home message is to check rose purchases carefully for this disease.
Downy mildew symptoms on rose include lesions on the leaves, stems, and flowers. The leaf lesions appear purplish to brown, blocky, and are often accompanied by yellowing of surrounding tissues. Severe defoliation may occur as a result of infection. The symptoms are suggestive of chemical injury or possibly nutrient stress. The downy mildew fungus forms a downy mass of spores on the underside of leaf lesions. This downy growth is difficult to see without a hand lens or microscope. The fungus thrives in cool, humid (85% relative humidity) conditions. The optimum temperature for rose downy mildew is 64 degrees F. Spores form more readily in cool, humid conditions. Once conditions turn warm and dry, the disease is kept in check until the next period of cool, humid weather.
Examine new roses carefully before purchase. Do not buy plants with these lesions. The fungus affects newest growth first and is easily spread by windblown spores. If established plants are infected, there are chemicals available to control the fungus. Many options are listed in the 2003 Commercial Landscape and Turfgrass Pest Management Handbook, as well as the Home, Yard, and Garden Pest Guide. These products are listed as preventives and work best before the disease is established. Be sure to get thorough coverage of the foliage, especially the underside of leaves. Because the fungus survives on infected plant parts, remove infected tissue from the site. Place tissue directly into a plastic bag and seal it before moving through the garden.