Certified GKH Gardening Expert
Unfortunately, your photo did not come through. Depending on where it occurs and how it looks, specifically, it could be one of two most likely illnesses.
Powdery Mildew is suspect. This is controlled, easily, with fungicide use.
These articles will help:
Downy Mildew is another suspect. This one is quite a bit more difficult to get rid of and will do much more damage to the plants.
This article will help: