Looks like Powdery Mildew to me. Unless you have had sprayed it with milk spray.
You can spray them with 30/70 MILK/WATER mixture. Or with NEEM oil(per instructions on package). Even if you don't have powdery mildew, the mentioned spray once in two weeks can help prevent .

This post was edited by seysonn on Sun, Jun 23, 13 at 3:33

Yep, it's PM.

Yep. PM. As Seysonn mentioned, a milk solution or neem can help control it. There's also a baking soda solution -- google it.

In any case, you want to spray off(with a water hose/nozzle) as many of the spores as possible before applying the solution.

Also, try to keep from touching other plants AFTER you've touched affected plants to prevent the spread of spores.

Kevin

Or you could try an actual fungicide.

PM is one of those diseases that's easier to prevent than cure.