Downy mildew on rose is present this season. It was detected in a retail operation in Illinois—before plants were sold. Quick action by the retailer prevented its spread to home gardens. This disease is one that growers need to recognize to prevent spread of a damaging disease. Downy mildew is not related to powdery mildew and does not cause the same symptoms.
Downy mildew on rose is caused by Peronospora sparsa. Symptoms include lesions on the leaves, stems, and flowers. Leaf lesions are the most obvious and appear purplish to brown, blocky, and 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 growth is difficult to see without a hand lens or microscope. The fungus thrives in cool, humid conditions (85% relative humidity). The optimal temperature for rose downy mildew is 64°F. Spores form more readily in cool, humid conditions. Once conditions turn warm and dry, the disease is kept in check until the next cool, humid period.
Examine roses carefully before purchase. Usually in Illinois, this disease is not seen outside greenhouses or production areas; but that is not guaranteed. Do not buy plants with these lesions. If established plants are infected, chemicals are available for control. Many options are listed in the 2003 Commercial Landscape & Turfgrass Pest Management Handbook, as well as the Home, Yard, & Garden Pest Guide. These products are listed as preventives and work best before the disease is established. Be sure to get thorough coverage of foliage, especially undersides of leaves. Because the fungus can survive 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.