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Watch for Rose Downy Mildew

May 16, 2006
This fungal disease of rose can cause total leaf drop. Dont confuse it with powdery mildew, which is easy to see, causes a white powdery growth on the leaves, and can be controlled with many products. Downy mildew is not seen in the garden very often because it is usually detected and eliminated in the nursery or garden center. Downy mildew is caused by the fungus Peronospora sparsa, which grows best in cool conditions (60 to 70F) where the humidity is about 85%, such as conditions in many greenhouses and polyhouses in the spring.

Downy mildew symptoms on rose include lesions on the leaves, stems, and flowers. The leaf lesions appear purplish to brown, are blocky, and are often accompanied by yellowing of surrounding tissues. I think this symptom looks more like chemical injury than fungal infection. Look at the image 060502, rose leaves infected with downy mildew.

Severe defoliation may occur as a result of infection. 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 but is visible on the underside of each lesion. In the lab, we place leaves in humidity chambers overnight for easy viewing of spores the next day.

So far this year, the Plant Clinic has received one suspect downy mildew rose sample. Fortunately, the plant was not infected. Still, this was a reminder that the timing and weather are ripe for infection by downy mildew. Examine new roses carefully before purchase. Do not buy plants with these lesions. The fungus affects newest growth first and is easily spread by wind-blown spores. If established plants are infected, chemicals are available to control the fungus. Many options are listed in the 2005 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.