Question
There is a powdery substance on the leaves of my rose bush.  what can I do to take care of it.

Answer
This is powdery mildew. Powdery mildew fungi belong to the group of plant pathogens called obligate parasites. Plant pathogens within this group can only grow and reproduce on or in a living host plant. S. pannosa can infect any green tissue; thus, powdery mildew may be found on leaves, green stems, and flower parts.

This fungus lives largely on the outer surface of the host plant. It has a high demand for the nutrients necessary for growth and spore production. It obtains these from host plant cells by means of small, root-like organs, called haustoria, that feed within the epidermal layer of the host plant.

Powdery mildew is one of the most widespread and destructive diseases of garden and greenhouse roses. The first historical account of powdery mildew on roses was by Theophrastis about 300 B.C., and the disease ranks among the oldest recorded plant diseases. It wasn't until 1819, however, that the causal fungus was identified.

Although powdery mildew rarely kills a plant, infection reduces host vigor and lowers aesthetic value.  

This can be prevented if the gardener understands the nature of the disease, learns to recognize early symptoms, and conscientiously applies preventative control measures.

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.

When young canes become infected, they are dwarfed and distorted. Severe infections even may kill the tips of tender young canes.

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.

Several fungicides (Table I) can be used to control powdery mildew. It is a good idea to use a different fungicide each season or alternate between different fungicides during the growing season. This is done to prevent the development of fungicide resistance in the natural powdery mildew population.

Table I. Fungicides for control of powdery mildew of roses:
   Banner MAXX? Immunex?
   Benomyl? Benlate
   Folpet? Phaltan
   Bayleton? Strike?Triadimefon
   Funginex? Triforine
   Triact? Rose Defense? Neem oil
   Pipron?
   Phyton 27? copper sulfate
   Rubigan AS?
   Cooke's?wettable, Safer?Garden Fungicide, Microthiol Special?Sulfur