Barb,

Its probably powdery mildew, take a look at the link for a picture to see if thats it.
Powdery mildew is a disease that occurs worldwide. For decades, organic gardeners had to rely on making a spray from baking soda to control the disease.

Now, instead of measuring out the baking soda and combining it with a surfactant (a "sticking" substance) of either oil or soap, gardeners need only head for their refrigerators.

A weekly spray of milk at a concentration of at least 10% (1 part milk to 9 parts water) significantly reduced the severity of powdery mildew infection on the plants by 90%.

How does milk control powdery mildew?

Scientist aren't 100% sure how milk works to control this disease. It seems that milk is a natural germicide. In addition, it contains several naturally occurring salts and amino acids that are taken up by the plant. From previous experiments using sodium bicarbonate, potassium phosphate, and other salts, researchers have found that the disease is sensitive to these salts. It is possible then, that milk boosts the plant's immune system to prevent the disease.

Worth a try :)

Here is a link that might be useful: {{gwi:149308}}

There is a organic place called GardensAlive down in southern indiana Lawrenceburg, I think, that sells organic remedies by mail order. I got some of their fungicide, it's a copper based soap product and I really liked it for some powdery mildew that I got on my tomato plants in the greenhouse. They have a website, too. www.gardensalive.com
If it makes you feel better--they taught us in the master gardener classes that the kind of mildew that gets on the phlox won't spread to other kinds of plants, just to other phlox plants. Mildews are host specific.

Sandy,

Gardens Alive is not organic, none of their products have been certified and they will not tell you what is in their products, they said that my ag agent had to write to them to request organic certification status, still waiting for a response.
Some of their products contain cottonseed oil that is not organic and I cant see how it would not contain pesticide residue.
I have "inherited" a bunch of their products but will not use them without knowing whats in them.

There are a couple of things that can be used to treat for Powdery Mildew, the milk spray mentioned by Termater, or a spray of 1 teaspoon baking soda in 1 quart of water with a few drops of a light oil added.
Both work if sprayed on susceptible plants early every 10 days to prevent PM from getting a toe hold or once it is present spray every 5 days to eliminate it.

Here is a link that might be useful: Cornell U PM Fact Shhet