Is that dusty and fairly easily wiped off? Could be Powdery Mildew. Sprays of 1 teaspoon of baking soda mixed in 1 quart of water, or a 50/50 mix of fat free milk and water, has been an effective control for many of us.

Here is a link that might be useful: Powdery Mildew

the above is the solution.. but to be clear.. it will not make the leaves new and clean ...

ken

ps: baking soda is a lot cheaper than milk