When I enlarge the photo, the white looks fuzzy, or maybe like a sprinkling of confectioner's sugar... powdery mildew seems likely. Increasing air circulation by spacing plants more widely and turning a fan on them should help... you'll find various remedies online, too... spraying with hydrogen peroxide (2 tablespoons per quart of water) is where I'd start, or maybe rubbing the solution onto the leaves with a paper towel so you get the mildew residue off at the same time.
