A. Powdery mildew on roses is one of the most widespread and destructive diseases of garden and greenhouse roses. Its name reflects the distinctive grayish-white powdery mats or patches that form on plant tissue.
Powdery mildew is caused by a fungal pathogen (Sphaerotheca pannosa var. rosae) which infects the epidermal or outer surface cells of the host plant cells. The pathogen has a high demand for the nutrients necessary for growth and spore production. It obtains its nutrition from host plant cells by means of small, root-like organs (known as haustoria) that feed within the epidermal layer of the host plant.
The fungal pathogen can infect any green tissue; thus, powdery mildew may be found on leaves, green stems, and flower parts. Young, tender growth is most susceptible. Leaves become distorted and eventually fall prematurely. Powdery mildew spores are easily spread by wind to nearby healthy plants.
Newly unfolded leaves are the most susceptible to infection. Mature leaves are more resistant to infection and usually show no symptom development or, at most, only small local lesions.
Leaves of garden roses often are attacked first on the lower surface and then later on the upper surface. First symptoms are small, raised, blister-like distortions on the leaf that may or may not be accompanied by a slight purpling and curling.
As symptoms continue to develop, much of the leaf surface becomes covered by the grayish-white mildew and the leaves are twisted or distorted. The coating of the leaf by the mildew reduces the leaf surface area available for photosynthesis.
Unopened flower buds sometimes become partially covered with mildew before the leaves show extensive symptoms. The petals are usually not affected, but the sepals can be covered with mildew. Infection of flower buds causes poor quality flower formation.
Environment plays a major role in powdery mildew development. Disease incidence is most severe under cloudy, humid conditions when days are warm and nights are cool. Day temperatures in the 80s and high night humidity provide a very favorable environment for this disease. In our Texas Upper Gulf Coast growing area, powdery mildew tend to be more of a problem during our mid-spring growing season. Once hot weather conditions prevail, powdery mildew on roses usually disappears.
Powdery mildew symptoms typically begin as discrete circular, powdery white spots. However, as these spots expand, they will coalesce or join, producing a large matt of powdery mildew. Although powdery mildew rarely kills a plant, infection reduces host vigor and lowers aesthetic value.
Although powdery mildew rarely kills a plant, infection reduces host vigor and lowers aesthetic value. Some rose varieties are more susceptible to powdery mildew than others.
Powdery mildew is best managed by using an integrated approach or combination of cultural practices including the following:
Fungicides such as triforine (Funginex) and myclobutanil (Immunox) can be used to control powdery mildew. Alternating between fungicides is recommended to reduce the development of fungicide resistance in the natural population of powdery mildew. Through coverage of the foliage (including the upper- and lower-leaf surfaces) and canes is needed.
Always read and follow all directions provided on the label of a pesticide before using. Information given above is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Texas Cooperative Extension is implied.
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