It sounds as if your plants have been attacked by powdery mildew. This is a mildew closely related to fungus. The powder is the mildew's reproductive spores. It thrives in an acid environment in a temperature range of 60-70?F (15-21?C) with a humidity above 50%. The spores are floating in the air and there is no practical way to screen them out. Instead, try to change the environment so that conditions don't match the mildew's needs. This may require raising the temperature or lowering humidity.
There are also several safe and effective ways of controlling powdery mildew using minerals or organisms.
Most of the mildicides listed here are fairly new and they are all much less harmful than the old chemical formulations. You won't find most of them at your local store or garden shop, but they are available on the Internet. Many companies sell them there. All of these mildicides are used on edibles or herbs. Some are naturally occurring organisms and are exempt from registration. Others are registered for use on vegetable crops and are considered in the "caution" category, the category for the least dangerous registered mildicides.
AQ10: AQ10 uses a totally new method of fighting powdery mildew, a biofungicide. The active ingredient, Ampelomyces quisqualis, is a fungus that parasitizes the powdery mildew organism. It offers control over a long period of time.
