One of the most common diseases affecting roses, powdery mildew is a fungal disease caused by Sphaerotheca pannosa var. rosae that leaves a superficial, white, dusty coating on leaves, stems and sometimes the flowers. Powdery mildew tends to create a problem for gardeners in middle to late summer, reducing the strength and damaging the appearance of infected plants, but rarely killing them. The fungal spores penetrate the leaves of a rose and deprive it of moisture and nutrients. The spores themselves are unusually rich in moisture so, unlike many fungal problems that need wet conditions to take hold, powdery mildew thrives with warmth (temperatures around 70 to 80 F.) and humidity and is inhibited by rain. The disease is spread when spores are blown from one plant to the next and flourishes when some warm humidity is added to the picture.

Not surprisingly, good maintenance practices and smart gardening are the key to reducing the incidence of powdery mildew as they limit the impact of humidity around the plant. Planting in an area where air circulation is high and sun is strong is important. This fungus is present in soil, but a healthy plant is always more resistant to disease problems. Good garden hygiene - remove fallen leaf litter (where fungal spores may be present), water roses from the ground level (rather than splashing water on leaves) and in the morning only (when it has all day to burn off) - will help to limit powdery mildew as well.

The chief impact of this disease is the unattractive appearance it creates but a slow weakening of the rose will result if the plant is affected to the point of losing its ability to photosynthesize. Do your best to limit the incidence of powdery mildew through rose selection and garden hygiene and then allow minor infections to run their course. If your rose has a history of aggressive and threatening occurrences, remove the worst stems and apply the most environmentally friendly product that will be effective to the rest of the plant as soon as you notice the powdery mildew. Available products change constantly, so contact your county's cooperative extension office to find out which products, including biorational fungicides, are currently licensed and effective for home use in your area. If you need help locating your county's cooperative extension office, please contact us at the email address plantinfo@NYBG.org and we would be happy to help direct you.

For more on rose disease and other problems see our Guide to Rose Problems.

Courtesy of NYBG Plant Information Service