(Written by Sean Mullahy, U of I Plant Clinic Student Worker)
Downy mildew of roses is generally not a problem in the home garden; it mostly affects roses grown in the greenhouse environment. Most rose growers are familiar with its similar (in symptoms) cousin powdery mildew, although the two are very different and can be differentiated.
Downy mildew first appeared in the 1860’s in England, and from that point on, it was reported all throughout Europe. After making its way through Europe in the early 1900’s, it made its way over to Scandinavia and the Soviet Union. Downy mildew made its way stateside in 1880, first appearing in the Midwest. Since then, it has made its way all around the United States, and to almost every corner of the globe.
Downy mildew can be identified through a number of symptoms. Watch out for purplish red to dark brown spots developing on leaves (these will be more angular in shape) and the yellowing of leaflets. These yellow leaflets may have green tissue sections of up to 1cm2 in size. Check beneath lesions for mycelia and conidia, especially under cool humid conditions. Abscission of leaves may also be quite severe. Purple to black colored spots may also appear on stems and peduncles. Downy mildew can be differentiated from powdery mildew because the grayish spores will be produced on the underside of the leaf as opposed to the top.
For greenhouse management, lowering the humidity should be the first step, this can be done with ventilation and aeration. Also increasing temperatures to 27C / 80F will help in controlling it; as spores are killed at this temperature. In both cases, keeping the plants aerated and dry is a great idea. Prune roses to keep them open, and try to water them at the base. This will help prevent germination of the downy mildew. For field or garden grown roses, a fungicide application is recommended. This should be done in a preventative manner, when environmental conditions are favorable for developments (cool, humid). If you already see the symptoms, it’s probably too late! The growing of a resistant cultivar is as always recommended. For example, the rugosa family of roses is resistant to downy mildew.
To prevent spread and seasonal carry over, all suspected plant parts should be removed and all infected plant material should be destroyed.