Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Downy mildew, Peronospora sparsa is a serious disease that attacks all types of roses both in the greenhouse, nursery and the landscape from potted miniatures to landscape-size roses and even shrub roses. The disease can be quite severe under cool moist conditions and will defoliate rose plants within a day or two in the greenhouse or in a polyhouse. The optimal temperature for rose downy mildew is 64F with a relative humidity of 85% or greater. Making sure your humidity, especially overnight, is below 85% is effective at preventing the disease. Sudden temperature drops at dusk in greenhouses or polyhouses can significantly increase humidity around the plant and lead to conditions that favor downy mildew.

Look for purple to black spots on the stems and flower peduncles, first symptoms are on the leaves near the top of the plant (see photos). As the spots develop, the leaf may yellow and even look like a leaf burn that may be confused with pesticide toxicity.

No mycelia appear on the upper leaf surface as with powdery mildew. Downy mildew under very humid conditions will form mycelia on the underside of the leaf under the purple to black spots. As mentioned if you are forcing roses under plastic, keep the foliage dry to prevent this disease.

The following fungicides are labeled and effective for downy mildew control on roses: Heritage, Stature, Cygnus and Fen-stop. Read the label and follow directions as indicated. A severe infection cannot be arrested by fungicides. You should consider dumping severely infected rose plants.